Eurypterids, often informally called sea scorpions, are a group of extinct
arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
s that form the
order
Order, ORDER or Orders may refer to:
* Categorization, the process in which ideas and objects are recognized, differentiated, and understood
* Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...
Eurypterida. The earliest known eurypterids date to the
Darriwilian
The Darriwilian is the upper stage of the Middle Ordovician. It is preceded by the Dapingian and succeeded by the Upper Ordovician Sandbian Stage. The lower boundary of the Darriwilian is defined as the first appearance of the graptolite species '' ...
stage of the
Ordovician
The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...
period 467.3
million years ago
The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds.
Usage
Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago). ...
. The group is likely to have appeared first either during the
Early Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya.
...
or
Late Cambrian
Late may refer to:
* LATE, an acronym which could stand for:
** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia
** Local-authority trading enterprise, a New Zealand business law
** Local average treatment effect, ...
period. With approximately 250 species, the Eurypterida is the most diverse
Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ' ...
chelicerate
The subphylum Chelicerata (from New Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks, and mite ...
order. Following their appearance during the Ordovician, eurypterids became major components of marine faunas during the
Silurian
The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
, from which the majority of eurypterid species have been described. The Silurian genus ''
Eurypterus
''Eurypterus'' ( ) is an extinct genus of eurypterid, a group of organisms commonly called "sea scorpions". The genus lived during the Silurian period, from around 432 to 418 million years ago. ''Eurypterus'' is by far the most well-studied and ...
'' accounts for more than 90% of all known eurypterid specimens. Though the group continued to diversify during the subsequent
Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
period, the eurypterids were heavily affected by the
Late Devonian extinction event
The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch, which collectively represent one of the five largest mass extinction events in the history of life on Earth. The term primarily refers to a major exti ...
. They declined in numbers and diversity until becoming extinct during the
Permian–Triassic extinction event
The Permian–Triassic (P–T, P–Tr) extinction event, also known as the Latest Permian extinction event, the End-Permian Extinction and colloquially as the Great Dying, formed the boundary between the Permian and Triassic geologic periods, as ...
(or sometime shortly before) 251.9million years ago.
Although popularly called "sea scorpions", only the earliest eurypterids were
marine
Marine is an adjective meaning of or pertaining to the sea or ocean.
Marine or marines may refer to:
Ocean
* Maritime (disambiguation)
* Marine art
* Marine biology
* Marine debris
* Marine habitats
* Marine life
* Marine pollution
Military
* ...
; many later forms lived in
brackish
Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...
or
fresh water
Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...
, and they were not true
scorpion
Scorpions are predatory arachnids of the order Scorpiones. They have eight legs, and are easily recognized by a pair of grasping pincers and a narrow, segmented tail, often carried in a characteristic forward curve over the back and always end ...
s. Some studies suggest that a dual
respiratory system
The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies grea ...
was present, which would have allowed for short periods of time in terrestrial environments. The name ''Eurypterida'' comes from the
Ancient Greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic peri ...
words ('), meaning 'broad' or 'wide', and ('), meaning 'wing', referring to the pair of wide swimming appendages present in many members of the group.
The eurypterids include the largest known arthropods ever to have lived. The largest, ''
Jaekelopterus
''Jaekelopterus'' is a genus of predatory eurypterid, a group of extinct aquatic arthropods. Fossils of ''Jaekelopterus'' have been discovered in deposits of Early Devonian age, from the Pragian and Emsian stages. There are two known species: th ...
'', reached in length. Eurypterids were not uniformly large and most species were less than long; the smallest eurypterid, ''
Alkenopterus
''Alkenopterus'' is a genus of prehistoric eurypterid classified as part of the family Onychopterellidae. The genus contains two species, ''A. brevitelson'' and ''A. burglahrensis'', both from the Devonian of Germany.
Description
Like the othe ...
'', was only long. Eurypterid fossils have been recovered from every continent. A majority of fossils are from fossil sites in
North America
North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
and
Europe
Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
because the group lived primarily in the waters around and within the ancient supercontinent of
Euramerica
Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...
. Only a handful of eurypterid groups spread beyond the confines of Euramerica and a few genera, such as ''
Adelophthalmus
''Adelophthalmus'' is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of ''Adelophthalmus'' have been discovered in deposits ranging in age from the Early Devonian to the Early Permian, which makes it the longest lived of ...
'' and ''
Pterygotus
''Pterygotus'' is a genus of giant predatory eurypterid, a group of extinct aquatic arthropods. Fossils of ''Pterygotus'' have been discovered in deposits ranging in age from Middle Silurian to Late Devonian, and have been referred to several di ...
'', achieved a
cosmopolitan distribution
In biogeography, cosmopolitan distribution is the term for the range of a taxon that extends across all or most of the world in appropriate habitats. Such a taxon, usually a species, is said to exhibit cosmopolitanism or cosmopolitism. The ext ...
with fossils being found worldwide.
Morphology
Like all other
arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
s, eurypterids possessed segmented bodies and jointed appendages (limbs) covered in a
cuticle
A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...
composed of
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s and
chitin
Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...
. As in other
chelicerates
The subphylum Chelicerata (from New Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks, and mit ...
, the body was divided into two tagmata (sections); the frontal
prosoma
The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. (The terms ''prosoma'' and ''opisthosoma'' are equivalent to ''cepha ...
(head) and posterior
opisthosoma
The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma (cephalothorax). It is a distinctive feature of the subphylum Chelicerata (arachnids, horseshoe crabs and others). Although it is similar in most respects to a ...
(abdomen). The prosoma was covered by a
carapace
A carapace is a Dorsum (biology), dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tor ...
(sometimes called the "prosomal shield") on which both
compound eyes
A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which distin ...
and the
ocelli
A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-l ...
(simple eye-like sensory organs) were located.
The prosoma also bore six pairs of appendages which are usually referred to as appendage pairs I to VI. The first pair of appendages, the only pair placed before the mouth, is called the
chelicerae
The chelicerae () are the mouthparts of the subphylum Chelicerata, an arthropod group that includes arachnids, horseshoe crabs, and sea spiders. Commonly referred to as "jaws", chelicerae may be shaped as either articulated fangs, or similarly ...
( homologous to the fangs of spiders). They were equipped with small pincers used to manipulate food fragments and push them into the mouth. In one lineage, the
Pterygotidae
Pterygotidae (the name deriving from the type genus ''Pterygotus'', meaning "winged one") is a family of eurypterids, an extinct group of aquatic arthropods. They were members of the superfamily Pterygotioidea. Pterygotids were the largest kno ...
, the chelicerae were large and long, with strong, well-developed teeth on specialised
chelae
A chela ()also called a claw, nipper, or pinceris a pincer (biology), pincer-like organ at the end of certain limbs of some arthropods. The name comes from Ancient Greek , through New Latin '. The plural form is chelae. Legs bearing a chela are ...
(claws). The subsequent pairs of appendages, numbers II to VI, possessed gnathobases (or "tooth-plates") on the coxae (limb segments) used for feeding. These appendages were generally walking legs that were cylindrical in shape and were covered in spines in some species. In most lineages, the limbs tended to get larger the farther back they were. In the
Eurypterina
Eurypterina is one of two suborders of eurypterids, an extinct group of chelicerate arthropods commonly known as "sea scorpions". Eurypterine eurypterids are sometimes informally known as "swimming eurypterids". They are known from fossil deposi ...
suborder
Order ( la, ordo) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and ...
, the larger of the two eurypterid suborders, the sixth pair of appendages was also modified into a swimming paddle to aid in traversing aquatic environments.
The opisthosoma comprised 12 segments and the
telson
The telson () is the posterior-most division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment of the arthropod body, or an additional division that is not a true segment on accou ...
, the posteriormost division of the body, which in most species took the form of a blade-like shape. In some lineages, notably the
Pterygotioidea
Pterygotioidea (the name deriving from the type genus ''Pterygotus'', meaning "winged one") is a superfamily of eurypterids, an extinct group of aquatic arthropods. Pterygotioids were the most derived members of the infraorder Diploperculata and ...
Mycteroptidae
Mycteroptidae are a family of eurypterids, a group of extinct chelicerate arthropods commonly known as "sea scorpions". The family is one of three families contained in the superfamily Mycteropoidea (along with Hibbertopteridae and Drepanopte ...
, the telson was flattened and may have been used as a rudder while swimming. Some genera within the superfamily
Carcinosomatoidea
Carcinosomatoidea is an extinct superfamily of eurypterids, an extinct group of chelicerate arthropods commonly known as "sea scorpions". It is one of the superfamilies classified as part of the suborder Eurypterina.
Some carcinosomatoid genera ...
, notably ''
Eusarcana
''Eusarcana'' (meaning "true flesh") is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of ''Eusarcana'' have been discovered in deposits ranging in age from the Early Silurian to the Early Devonian. Classified as part of ...
'', had a telson similar to that of modern
scorpion
Scorpions are predatory arachnids of the order Scorpiones. They have eight legs, and are easily recognized by a pair of grasping pincers and a narrow, segmented tail, often carried in a characteristic forward curve over the back and always end ...
s and may have been capable of using it to inject
venom
Venom or zootoxin is a type of toxin produced by an animal that is actively delivered through a wound by means of a bite, sting, or similar action. The toxin is delivered through a specially evolved ''venom apparatus'', such as fangs or a sti ...
. The coxae of the sixth pair of appendages were overlaid by a plate that is referred to as the metastoma, originally derived from a complete exoskeleton segment. The opisthosoma itself can be divided either into a "
mesosoma
The mesosoma is the middle part of the body, or tagma, of arthropods whose body is composed of three parts, the other two being the prosoma and the metasoma. It bears the legs, and, in the case of winged insects, the wings.
In hymenopterans of t ...
" (comprising segments 1 to 6) and "
metasoma
The metasoma is the posterior part of the body, or tagma, of arthropods whose body is composed of three parts, the other two being the prosoma and the mesosoma. In insects, it contains most of the digestive tract, respiratory system, and circula ...
" (comprising segments 7 to 12) or into a "preabdomen" (generally comprising segments 1 to 7) and "postabdomen" (generally comprising segments 8 to 12).
The underside of the opisthosoma was covered in structures evolved from modified opisthosomal appendages. Throughout the opisthosoma, these structures formed plate-like structures termed ( in German). These created a branchial chamber (gill tract) between preceding and the
ventral
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Th ...
surface of the opisthosoma itself, which contained the respiratory organs. The second to sixth opisthosomal segments also contained oval or triangular organs that have been interpreted as organs that aid in respiration. These organs, termed or "gill tracts", would potentially have aided eurypterids to breathe air above water, while , similar to organs in modern
horseshoe crab
Horseshoe crabs are marine and brackish water arthropods of the family Limulidae and the only living members of the order Xiphosura. Despite their name, they are not true crabs or crustaceans: they are chelicerates, most closely related to arachn ...
s, would cover the parts that serve for underwater respiration.
The appendages of opisthosomal segments 1 and 2 (the seventh and eighth segments overall) were fused into a structure termed the genital operculum, occupying most of the underside of the opisthosomal segment 2. Near the
anterior
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
margin of this structure, the genital appendage (also called the or the median abdominal appendage) protruded. This appendage, often preserved very prominently, has consistently been interpreted as part of the reproductive system and occurs in two recognized types, assumed to correspond to male and female.
Biology
Size
Eurypterids were highly variable in size, depending on factors such as lifestyle, living environment and taxonomic affinity. Sizes around are common in most eurypterid groups. The smallest eurypterid, '' Alkenopterus burglahrensis'', measured just in length.
The largest eurypterid, and the largest known arthropod ever to have lived, is ''
Jaekelopterus rhenaniae
''Jaekelopterus'' is a genus of predatory eurypterid, a group of extinct aquatic arthropods. Fossils of ''Jaekelopterus'' have been discovered in deposits of Early Devonian age, from the Pragian and Emsian stages. There are two known species: the ...
''. A chelicera from the
Emsian
The Emsian is one of three faunal stages in the Early Devonian Epoch. It lasted from 407.6 ± 2.6 million years ago to 393.3 ± 1.2 million years ago. It was preceded by the Pragian Stage and followed by the Eifelian Stage. It is named after the ...
Klerf Formation
The Klerf Formation is an Early Devonian (Emsian) formation that includes a Lagerstätte in the Northern Eifel hills, at Willwerath near Prüm, Rhineland-Palatinate, Germany. In it ''Jaekelopterus rhenaniae'', a giant eurypterid was discovered. T ...
of Willwerath,
Germany
Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...
measured in length, but is missing a quarter of its length, suggesting that the full chelicera would have been long. If the proportions between body length and chelicerae match those of its closest relatives, where the ratio between claw size and body length is relatively consistent, the specimen of ''Jaekelopterus'' that possessed the chelicera in question would have measured between , an average , in length. With the chelicerae extended, another meter (3.28 ft) would be added to this length. This estimate exceeds the maximum body size of all other known giant arthropods by almost half a meter (1.64 ft) even if the extended chelicerae are not included. Two other eurypterids have also been estimated to have reached lengths of 2.5 metres; '' Erettopterus grandis'' (closely related to ''Jaekelopterus'') and '' Hibbertopterus wittebergensis'', but ''E. grandis'' is very fragmentary and the ''H. wittenbergensis'' size estimate is based on trackway evidence, not fossil remains.
The family of ''Jaekelopterus'', the Pterygotidae, is noted for several unusually large species. Both ''
Acutiramus
''Acutiramus'' is a genus of giant predatory eurypterid, an extinct group of aquatic arthropods. Fossils of ''Acutiramus'' have been discovered in deposits of Late Silurian to Early Devonian age. Seven species have been described, five from Nor ...
'', whose largest member ''A. bohemicus'' measured , and ''
Pterygotus
''Pterygotus'' is a genus of giant predatory eurypterid, a group of extinct aquatic arthropods. Fossils of ''Pterygotus'' have been discovered in deposits ranging in age from Middle Silurian to Late Devonian, and have been referred to several di ...
,'' whose largest species ''P. grandidentatus'' measured , were gigantic. Several different contributing factors to the large size of the pterygotids have been suggested, including courtship behaviour, predation and competition over environmental resources.
Giant eurypterids were not limited to the family Pterygotidae. An isolated long fossil metastoma of the carcinosomatoid eurypterid '' Carcinosoma punctatum'' indicates the animal would have reached a length of in life, rivalling the pterygotids in size. Another giant was ''
Pentecopterus decorahensis
''Pentecopterus'' is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils have been registered from the Darriwilian age of the Middle Ordovician period, as early as 467.3 million years ago. The genus contains only one species, ...
'', a primitive carcinosomatoid, which is estimated to have reached lengths of .
Typical of large eurypterids is a lightweight build. Factors such as locomotion, energy costs in
molting
In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
and respiration, as well as the actual physical properties of the
exoskeleton
An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...
, limits the size that arthropods can reach. A lightweight construction significantly decreases the influence of these factors. Pterygotids were particularly lightweight, with most fossilized large body segments preserving as thin and unmineralized. Lightweight adaptations are present in other giant paleozoic arthropods as well, such as the giant millipede ''
Arthropleura
''Arthropleura'' () is a genus of extinct millipede arthropods that lived in what is now North America and Europe around 345 to 290 million years ago, from the Viséan stage of the lower Carboniferous Period to the Sakmarian stage of the lower P ...
'', and are possibly vital for the evolution of giant size in arthropods.
In addition to the lightweight giant eurypterids, some deep-bodied forms in the family Hibbertopteridae were also very large. A carapace from the Carboniferous of Scotland referred to the species ''Hibbertoperus scouleri'' measures wide. As ''Hibbertopterus'' was very wide compared to its length, the animal in question could possibly have measured just short of in length. More robust than the pterygotids, this giant ''Hibbertopterus'' would possibly have rivalled the largest pterygotids in weight, if not surpassed them, and as such be among the heaviest arthropods.
Locomotion
The two eurypterid suborders, Eurypterina and
Stylonurina
Stylonurina is one of two suborders of eurypterids, a group of extinct arthropods commonly known as "sea scorpions". Members of the suborder are collectively and informally known as "stylonurine eurypterids" or "stylonurines". They are known from ...
, are distinguished primarily by the morphology of their final pair of appendages. In the Stylonurina, this appendage takes the form of a long and slender walking leg, while in the Eurypterina, the leg is modified and broadened into a swimming paddle. Other than the swimming paddle, the legs of many eurypterines were far too small to do much more than allow them to crawl across the
sea floor
The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'.
The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
. In contrast, a number of stylonurines had elongated and powerful legs that might have allowed them to walk on land (similar to modern
crab
Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...
s).
A
fossil trackway
A fossil track or ichnite (Greek "''ιχνιον''" (''ichnion'') – a track, trace or footstep) is a fossilized footprint. This is a type of trace fossil. A fossil trackway is a sequence of fossil tracks left by a single organism. Over the year ...
was discovered in Carboniferous-aged fossil deposits of Scotland in 2005. It was attributed to the stylonurine eurypterid ''Hibbertopterus'' due to a matching size (the trackmaker was estimated to have been about long) and inferred leg anatomy. It is the largest terrestrial trackway—measuring long and averaging in width—made by an arthropod found thus far. It is the first record of land locomotion by a eurypterid. The trackway provides evidence that some eurypterids could survive in terrestrial environments, at least for short periods of time, and reveals information about the stylonurine gait. In ''Hibbertopterus'', as in most eurypterids, the pairs of appendages are different in size (referred to as a heteropodous limb condition). These differently sized pairs would have moved in phase, and the short stride length indicates that ''Hibbertopterus'' crawled with an exceptionally slow speed, at least on land. The large telson was dragged along the ground and left a large central groove behind the animal. Slopes in the tracks at random intervals suggest that the motion was jerky. The gait of smaller stylonurines, such as ''
Parastylonurus
''Parastylonurus'' is a genus of prehistoric eurypterid. It is classified within the Parastylonuridae family and contains three species, ''P. hendersoni'' and ''P. ornatus'' from the Silurian of Scotland and ''P. sigmoidalis'' from the Silurian ...
'', was probably faster and more precise.
The functionality of the eurypterine swimming paddles varied from group to group. In the Eurypteroidea, the paddles were similar in shape to oars. The condition of the joints in their appendages ensured their paddles could only be moved in near-horizontal planes, not upwards or downwards. Some other groups, such as the Pterygotioidea, would not have possessed this condition and were probably able to swim faster. Most eurypterines are generally agreed to have utilized a rowing type of propulsion similar to that of crabs and
water beetle
A water beetle is a generalized name for any beetle that is adapted to living in water at any point in its life cycle. Most water beetles can only live in fresh water, with a few Marine (ocean), marine species that live in the intertidal zone or l ...
s. Larger individuals may have been capable of underwater flying (or
subaqueous flight
Aquatic locomotion or swimming is biologically propelled motion through a liquid medium. The simplest propulsive systems are composed of cilia and flagella. Swimming has evolved a number of times in a range of organisms including arthropods, fis ...
) in which the motion and shape of the paddles are enough to generate
lift
Lift or LIFT may refer to:
Physical devices
* Elevator, or lift, a device used for raising and lowering people or goods
** Paternoster lift, a type of lift using a continuous chain of cars which do not stop
** Patient lift, or Hoyer lift, mobile ...
, similar to the swimming of
sea turtle
Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhead, ...
s and
sea lion
Sea lions are pinnipeds characterized by external ear flaps, long foreflippers, the ability to walk on all fours, short and thick hair, and a big chest and belly. Together with the fur seals, they make up the family Otariidae, eared seals. ...
s. This type of movement has a relatively slower acceleration rate than the rowing type, especially since adults have proportionally smaller paddles than juveniles. However, since the larger sizes of adults mean a higher
drag coefficient
In fluid dynamics, the drag coefficient (commonly denoted as: c_\mathrm, c_x or c_) is a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment, such as air or water. It is used in the drag equ ...
, using this type of propulsion is more energy-efficient.
Some eurypterines, such as '' Mixopterus'' (as inferred from attributed fossil trackways), were not necessarily good swimmers. It likely kept mostly to the bottom, using its swimming paddles for occasional bursts of movements vertically, with the fourth and fifth pairs of appendages positioned backwards to produce minor movement forwards. While walking, it probably used a gait like that of most modern insects. The weight of its long abdomen would have been balanced by two heavy and specialized frontal appendages, and the
center of gravity
In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weight function, weighted relative position (vector), position of the distributed mass sums to zero. Thi ...
might have been adjustable by raising and positioning the tail.
Preserved fossilized eurypterid trackways tend to be large and heteropodous and often have an associated telson drag mark along the mid-line (as with the Scottish ''Hibbertopterus'' track). Such trackways have been discovered on every continent except for South America. In some places where eurypterid fossil remains are otherwise rare, such as in
South Africa
South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...
and the rest of the former supercontinent
Gondwana
Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...
, the discoveries of trackways both predate and outnumber eurypterid body fossils. Eurypterid trackways have been referred to several ichnogenera, most notably ''
Palmichnium
''Palmichnium'' ("palm trace") is an ichnofossil genus, interpreted as a eurypterid trackway. It has been found by many places around the world, such as Australia, Canada, United States or Wales.
Its trackways consist of three or four subcircular ...
'' (defined as a series of four tracks often with an associated drag mark in the mid-line), wherein the holotype of the ichnospecies ''P. kosinkiorum'' preserves the largest eurypterid footprints known to date with the found tracks each being about in diameter. Other eurypterid ichnogenera include '' Merostomichnites'' (though it is likely that many specimens actually represent trackways of crustaceans) and '' Arcuites'' (which preserves grooves made by the swimming appendages).
Respiration
In eurypterids, the respiratory organs were located on the ventral body wall (the underside of the opisthosoma). , evolved from opisthosomal appendages, covered the underside and created a gill chamber where the "gill tracts" were located. Depending on the species, the eurypterid gill tract was either triangular or oval in shape and was possibly raised into a cushion-like state. The surface of this gill tract bore several
spinule Spinules are small spines or thorns (vertebral columns) that are part of biological and manmade structures. The word originates from the Latin word and is often used in botany and zoology.
The presence or absence of spinules, and their shape, can ...
s (small spines), which resulted in an enlarged surface area. It was composed of spongy tissue due to many
invagination
Invagination is the process of a surface folding in on itself to form a cavity, pouch or tube. In developmental biology, invagination is a mechanism that takes place during gastrulation. This mechanism or cell movement happens mostly in the vegetal ...
s in the structure.
Though the is referred to as a "gill tract", it may not necessarily have functioned as actual gills. In other animals, gills are used for oxygen uptake from water and are outgrowths of the body wall. Despite eurypterids clearly being primarily aquatic animals that almost certainly evolved underwater (some eurypterids, such as the pterygotids, would even have been physically unable to walk on land), it is unlikely the "gill tract" contained functional gills when comparing the organ to gills in other invertebrates and even fish. Previous interpretations often identified the eurypterid "gills" as homologous with those of other groups (hence the terminology), with gas exchange occurring within the spongy tract and a pattern of branchio-cardiac and dendritic veins (as in related groups) carrying oxygenated blood into the body. The primary analogy used in previous studies has been horseshoe crabs, though their gill structure and that of eurypterids are remarkably different. In horseshoe crabs, the gills are more complex and composed of many lamellae (plates) which give a larger surface area used for gas exchange. In addition, the gill tract of eurypterids is proportionally much too small to support them if it is analogous to the gills of other groups. To be functional gills, they would have to have been highly efficient and would have required a highly efficient circulatory system. It is considered unlikely, however, that these factors would be enough to explain the large discrepancy between gill tract size and body size.
It has been suggested instead that the "gill tract" was an organ for breathing air, perhaps actually being a
lung
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...
, plastron or a pseudotrachea. Plastrons are organs that some arthropods evolved secondarily to breathe air underwater. This is considered an unlikely explanation since eurypterids had evolved in water from the start and they would not have organs evolved from air-breathing organs present. In addition, plastrons are generally exposed on outer parts of the body while the eurypterid gill tract is located behind the . Instead, among arthropod respiratory organs, the eurypterid gill tracts most closely resemble the pseudotracheae found in modern
isopods
Isopoda is an order of crustaceans that includes woodlice and their relatives. Isopods live in the sea, in fresh water, or on land. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed limbs on the thorax, an ...
. These organs, called pseudotracheae, because of some resemblance to the tracheae (windpipes) of air-breathing organisms, are lung-like and present within the
pleopods
The decapod ( crustaceans such as a crab, lobster, shrimp or prawn) is made up of 20 body segments grouped into two main body parts: the cephalothorax and the pleon (abdomen). Each segment may possess one pair of appendages, although in variou ...
(back legs) of isopods. The structure of the pseudotracheae has been compared to the spongy structure of the eurypterid gill tracts. It is possible the two organs functioned in the same way.
Some researchers have suggested that eurypterids may have been adapted to an amphibious lifestyle, using the full gill tract structure as gills and the invaginations within it as pseudotrachea. This mode of life may not have been physiologically possible, however, since water pressure would have forced water into the invaginations leading to
asphyxiation
Asphyxia or asphyxiation is a condition of deficient supply of oxygen to the body which arises from abnormal breathing. Asphyxia causes generalized hypoxia, which affects primarily the tissues and organs. There are many circumstances that can i ...
. Furthermore, most eurypterids would have been aquatic their entire lives. No matter how much time was spent on land, organs for respiration in underwater environments must have been present. True gills, expected to have been located within the branchial chamber within the , remain unknown in eurypterids.
Ontogeny
Like all arthropods, eurypterids matured and grew through static developmental stages referred to as
instar
An instar (, from the Latin '' īnstar'', "form", "likeness") is a developmental stage of arthropods, such as insects, between each moult (''ecdysis''), until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or ass ...
s. These instars were punctuated by periods during which eurypterids went through
ecdysis
Ecdysis is the moulting of the cuticle in many invertebrates of the clade Ecdysozoa. Since the cuticle of these animals typically forms a largely inelastic exoskeleton, it is shed during growth and a new, larger covering is formed. The remna ...
(molting of the cuticle) after which they underwent rapid and immediate growth. Some arthropods, such as insects and many crustaceans, undergo extreme changes over the course of maturing. Chelicerates, including eurypterids, are in general considered to be direct developers, undergoing no extreme changes after hatching (though extra body segments and extra limbs may be gained over the course of
ontogeny
Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the stu ...
in some lineages, such as
xiphosura
Xiphosura () is an order of arthropods related to arachnids. They are more commonly known as horseshoe crabs (a name applied more specifically to the only extant family, Horseshoe crab, Limulidae). They first appeared in the Hirnantian (Late Ordo ...
ns and
sea spider
Sea spiders are marine arthropods of the order Pantopoda ( ‘all feet’), belonging to the class Pycnogonida, hence they are also called pycnogonids (; named after ''Pycnogonum'', the type genus; with the suffix '). They are cosmopolitan, fou ...
s). Whether eurypterids were true direct developers (with hatchlings more or less being identical to adults) or hemianamorphic direct developers (with extra segments and limbs potentially being added during ontogeny) has been controversial in the past.
Hemianamorphic direct development has been observed in many arthropod groups, such as
trilobite
Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...
s,
megacheira
Megacheira ("great hands") is an extinct class of predatory arthropods that possessed a pair of great appendages, hence the class name as well as the common name "great appendage arthropods". Their taxonomic position is controversial, with stud ...
ns, basal
crustacean
Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...
s and basal
myriapods
Myriapods () are the members of subphylum Myriapoda, containing arthropods such as millipedes and centipedes. The group contains about 13,000 species, all of them terrestrial.
The fossil record of myriapods reaches back into the late Silurian, a ...
. True direct development has on occasion been referred to as a trait unique to
arachnid
Arachnida () is a class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegaroon ...
s. There have been few studies on eurypterid ontogeny as there is a general lack of specimens in the fossil record that can confidently be stated to represent juveniles. It is possible that many eurypterid species thought to be distinct actually represent juvenile specimens of other species, with paleontologists rarely considering the influence of ontogeny when describing new species.
Studies on a well-preserved fossil assemblage of eurypterids from the
Pragian
The Pragian is one of three faunal stages in the Early Devonian Epoch. It lasted from 410.8 ± 2.8 million years ago to 407.6 ± 2.8 million years ago. It was preceded by the Lochkovian Stage and followed by the Emsian Stage. The most important La ...
Wyoming
Wyoming () is a U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the south ...
, composed of multiple specimens of various developmental stages of eurypterids ''Jaekelopterus'' and '' Strobilopterus'', revealed that eurypterid ontogeny was more or less parallel and similar to that of extinct and extant xiphosurans, with the largest exception being that eurypterids hatched with a full set of appendages and opisthosomal segments. Eurypterids were thus not hemianamorphic direct developers, but true direct developers like modern arachnids.
The most frequently observed change occurring through ontogeny (except for some genera, such as ''Eurypterus'', which appear to have been static) is the metastoma becoming proportionally less wide. This ontogenetic change has been observed in members of several superfamilies, such as the Eurypteroidea, the Pterygotioidea and the Moselopteroidea.
Feeding
No fossil gut contents from eurypterids are known, so direct evidence of their diet is lacking. The eurypterid biology is particularly suggestive of a carnivorous lifestyle. Not only were many large (in general, most predators tend to be larger than their prey), but they had
stereoscopic vision
Stereopsis () is the component of depth perception retrieved through binocular vision.
Stereopsis is not the only contributor to depth perception, but it is a major one. Binocular vision happens because each eye receives a different image becaus ...
(the ability to perceive depth). The legs of many eurypterids were covered in thin spines, used both for locomotion and the gathering of food. In some groups, these spiny appendages became heavily specialized. In some eurypterids in the Carcinosomatoidea, forward-facing appendages were large and possessed enormously elongated spines (as in ''Mixopterus'' and ''
Megalograptus
''Megalograptus'' is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of ''Megalograptus'' have been recovered in deposits of Katian (Late Ordovician) age in North America. The genus contains five species: ''M. alveolatu ...
''). In
derived
Derive may refer to:
* Derive (computer algebra system), a commercial system made by Texas Instruments
* ''Dérive'' (magazine), an Austrian science magazine on urbanism
*Dérive, a psychogeographical concept
See also
*
*Derivation (disambiguatio ...
members of the Pterygotioidea, the appendages were completely without spines, but had specialized claws instead. Other eurypterids, lacking these specialized appendages, likely fed in a manner similar to modern horseshoe crabs, by grabbing and shredding food with their appendages before pushing it into their mouth using their chelicerae.
Fossils preserving
digestive tracts
The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...
have been reported from fossils of various eurypterids, among them ''Carcinosoma'', ''Acutiramus'' and ''
Eurypterus
''Eurypterus'' ( ) is an extinct genus of eurypterid, a group of organisms commonly called "sea scorpions". The genus lived during the Silurian period, from around 432 to 418 million years ago. ''Eurypterus'' is by far the most well-studied and ...
''. Though a potential anal opening has been reported from the telson of a specimen of '' Buffalopterus'', it is more likely that the
anus
The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...
was opened through the thin cuticle between the last segment before the telson and the telson itself, as in modern horseshoe crabs.
Eurypterid coprolites discovered in deposits of Ordovician age in Ohio containing fragments of a trilobite and eurypterid ''Megalograptus ohioensis'' in association with full specimens of the same eurypterid species have been suggested to represent evidence of
cannibalism
Cannibalism is the act of consuming another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded in more than 1,500 species. Human cannibalism is well documented, b ...
. Similar coprolites referred to the species '' Lanarkopterus dolichoschelus'' from the Ordovician of Ohio contain fragments of
jawless fish
Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animals, cyclostomes ...
and fragments of smaller specimens of ''Lanarkopterus'' itself.
Though apex predatory roles would have been limited to the very largest eurypterids, smaller eurypterids were likely formidable predators in their own right just like their larger relatives.
Reproductive biology
As in many other entirely extinct groups, understanding and researching the reproduction and sexual dimorphism of eurypterids is difficult, as they are only known from fossilized shells and carapaces. In some cases, there might not be enough apparent differences to separate the sexes based on morphology alone. Sometimes two sexes of the same species have been interpreted as two different species, as was the case with two species of '' Drepanopterus'' (''D. bembycoides'' and ''D. lobatus'').
The eurypterid prosoma is made up of the first six exoskeleton segments fused together into a larger structure. The seventh segment (thus the first opisthosomal segment) is referred to as the ''metastoma'' and the eighth segment (distinctly plate-like) is called the ''operculum'' and contains the genital aperature. The underside of this segment is occupied by the genital operculum, a structure originally evolved from ancestral seventh and eighth pair of appendages. In its center, as in modern horseshoe crabs, is a genital appendage. This appendage, an elongated rod with an internal duct, is found in two distinct morphs, generally referred to as "type A" and "type B". These genital appendages are often preserved prominently in fossils and have been the subject of various interpretations of eurypterid reproduction and sexual dimorphism.
Type A appendages are generally longer than those of type B. In some genera they are divided into different numbers of sections, such as in ''Eurypterus'' where the type A appendage is divided into three but the type B appendage into only two. Such division of the genital appendage is common in eurypterids, but the number is not universal; for instance, the appendages of both types in the family Pterygotidae are undivided. The type A appendage is also armed with two curved spines called (lit. 'fork' in Latin). The presence of in the type B appendage is also possible and the structure may represent the unfused tips of the appendages. Located between the
dorsal
Dorsal (from Latin ''dorsum'' ‘back’) may refer to:
* Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism
* Dorsal, positioned on top of an aircraft's fuselage
* Dorsal co ...
and ventral surfaces of the associated with the type A appendages is a set of organs traditionally described as either "tubular organs" or "horn organs". These organs are most often interpreted as
spermatheca
The spermatheca (pronounced plural: spermathecae ), also called receptaculum seminis (plural: receptacula seminis), is an organ of the female reproductive tract in insects, e.g. ants, bees, some molluscs, oligochaeta worms and certain other ...
e (organs for storing
sperm
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
), though this function is yet to be proven conclusively. In arthropods, spermathecae are used to store the
spermatophore
A spermatophore or sperm ampulla is a capsule or mass containing spermatozoa created by males of various animal species, especially salamanders and arthropods, and transferred in entirety to the female's ovipore during reproduction. Spermatophores ...
received from males. This would imply that the type A appendage is the female morph and the type B appendage is the male. Further evidence for the type A appendages representing the female morph of genital appendages comes in their more complex construction (a general trend for female arthropod genitalia). It is possible that the greater length of the type A appendage means that it was used as an
ovipositor
The ovipositor is a tube-like organ used by some animals, especially insects, for the laying of eggs. In insects, an ovipositor consists of a maximum of three pairs of appendages. The details and morphology of the ovipositor vary, but typical ...
(used to deposit eggs). The different types of genital appendages are not necessarily the only feature that distinguishes between the sexes of eurypterids. Depending on the genus and species in question, other features such as size, the amount of ornamentation and the proportional width of the body can be the result of sexual dimorphism. In general, eurypterids with type B appendages (males) appear to have been proportionally wider than eurypterids with type A appendages (females) of the same genera.
The primary function of the long, assumed female, type A appendages was likely to take up spermatophore from the substrate into the
reproductive tract
The reproductive system of an organism, also known as the genital system, is the biological system made up of all the anatomical sex organs, organs involved in sexual reproduction. Many non-living substances such as fluids, hormones, and pherom ...
rather than to serve as an ovipositor, as arthropod ovipositors are generally longer than eurypterid type A appendages. By rotating the sides of the operculum, it would have been possible to lower the appendage from the body. Due to the way different plates overlay at its location, the appendage would have been impossible to move without muscular contractions moving around the operculum. It would have been kept in place when not it use. The on the type A appendages may have aided in breaking open the spermatophore to release the free sperm inside for uptake. The "horn organs," possibly spermathecae, are thought to have been connected directly to the appendage via tracts, but these supposed tracts remain unpreserved in available fossil material.
Type B appendages, assumed male, would have produced, stored and perhaps shaped spermatophore in a heart-shaped structure on the dorsal surface of the appendage. A broad genital opening would have allowed large amounts of spermatophore to be released at once. The long associated with type B appendages, perhaps capable of being lowered like the type A appendage, could have been used to detect whether a substrate was suitable for spermatophore deposition.
Evolutionary history
Origins
Until 1882 no eurypterids were known from before the Silurian. Contemporary discoveries since the 1880s have expanded the knowledge of early eurypterids from the
Ordovician
The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...
period. The earliest eurypterids known today, the megalograptid ''Pentecopterus'', date from the
Darriwilian
The Darriwilian is the upper stage of the Middle Ordovician. It is preceded by the Dapingian and succeeded by the Upper Ordovician Sandbian Stage. The lower boundary of the Darriwilian is defined as the first appearance of the graptolite species '' ...
stage of the Middle Ordovician, 467.3
million years ago
The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds.
Usage
Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago). ...
. There are also reports of even earlier fossil eurypterids in deposits of Late
Tremadocian
The Tremadocian is the lowest stage of Ordovician. Together with the later Floian Stage it forms the Lower Ordovician Epoch. The Tremadocian lasted from to million years ago. The base of the Tremadocian is defined as the first appearance of the ...
(Early Ordovician) age in
Morocco
Morocco (),, ) officially the Kingdom of Morocco, is the westernmost country in the Maghreb region of North Africa. It overlooks the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to ...
, but these have yet to be thoroughly studied.
''Pentecopterus'' was a relatively derived eurypterid, part of the megalograptid family within the carcinosomatoid superfamily. Its derived position suggests that most eurypterid clades, at least within the eurypterine suborder, had already been established at this point during the Middle Ordovician. The earliest known stylonurine eurypterid, ''
Brachyopterus
''Brachyopterus'' is a genus of prehistoric eurypterid of the family Rhenopteridae. It is one of the earliest known eurypterids, having been recovered from Middle Ordovician deposits in Montgomeryshire, Wales.Dunlop, J. A., Penney, D. & Jekel, D. ...
'', is also Middle Ordovician in age. The presence of members of both suborders indicates that primitive stem-eurypterids would have preceded them, though these are so far unknown in the fossil record. The presence of several eurypterid clades during the Middle Ordovician suggests that eurypterids either originated during the Early Ordovician and experienced a rapid and explosive radiation and diversification soon after the first forms evolved, or that the group originated much earlier, perhaps during the
Cambrian
The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...
period.
As such, the exact eurypterid time of origin remains unknown. Though fossils referred to as "primitive eurypterids" have occasionally been described from deposits of Cambrian or even
Precambrian
The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...
age, they are not recognized as eurypterids, and sometimes not even as related forms, today. Some animals previously seen as primitive eurypterids, such as the genus '' Strabops'' from the Cambrian of
Missouri
Missouri is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Ranking List of U.S. states and territories by area, 21st in land area, it is bordered by eight states (tied for the most with Tennessee ...
, are now classified as aglaspidids or strabopids. The aglaspidids, once seen as primitive chelicerates, are now seen as a group more closely related to trilobites.
The fossil record of Ordovician eurypterids is quite poor. The majority of eurypterids once reportedly known from the Ordovician have since proven to be misidentifications or
pseudofossil
Pseudofossils are inorganic objects, markings, or impressions that might be mistaken for fossils. Pseudofossils may be misleading, as some types of mineral deposits can mimic lifeforms by forming what appear to be highly detailed or organized str ...
s. Today only 11 species can be confidently identified as representing Ordovician eurypterids. These taxa fall into two distinct ecological categories; large and active predators from the ancient continent of
Laurentia
Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, although ...
, and
demersal
The demersal zone is the part of the sea or ocean (or deep lake) consisting of the part of the water column near to (and significantly affected by) the seabed and the benthos. The demersal zone is just above the benthic zone and forms a layer of ...
(living on the
seafloor
The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'.
The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
) and basal animals from the continents
Avalonia
Avalonia was a microcontinent in the Paleozoic era. Crustal fragments of this former microcontinent underlie south-west Great Britain, southern Ireland, and the eastern coast of North America. It is the source of many of the older rocks of Wester ...
and Gondwana. The Laurentian predators, classified in the family Megalograptidae (comprising the genera '' Echinognathus'', ''Megalograptus'' and ''Pentecopterus''), are likely to represent the first truly successful eurypterid group, experiencing a small radiation during the Late Ordovician.
Silurian
Eurypterids were most diverse and abundant between the Middle Silurian and the Early Devonian, with an absolute peak in diversity during the Pridoli epoch, 423 to 419.2 million years ago, of the very latest Silurian. This peak in diversity has been recognized since the early twentieth century; of the approximately 150 species of eurypterids known in 1916, more than half were from the Silurian and a third were from the Late Silurian alone.
Though stylonurine eurypterids generally remained rare and low in number, as had been the case during the preceding Ordovician, eurypterine eurypterids experienced a rapid rise in diversity and number. In most Silurian fossil beds, eurypterine eurypterids account for 90% of all eurypterids present. Though some were likely already present by the Late Ordovician (simply missing from the fossil record so far), a vast majority of eurypterid groups are first recorded in
strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
of Silurian age. These include both stylonurine groups such as the
Stylonuroidea
Stylonuroidea is an extinct superfamily of eurypterids, an extinct group of chelicerate arthropods commonly known as "sea scorpions". It is one of four superfamilies classified as part of the suborder Stylonurina.
Stylonuroidea, which lived from ...
Mycteropoidea
Mycteropoidea is an extinct superfamily of eurypterids, an extinct group of chelicerate arthropods commonly known as "sea scorpions". It is one of four superfamilies classified as part of the suborder Stylonurina. Mycteropoids have been recovere ...
as well as eurypterine groups such as the Pterygotioidea, Eurypteroidea and
Waeringopteroidea
Waeringopteridae is a family of eurypterids, an extinct group of aquatic arthropods. The Waeringopteridae is the only family classified as part of the superfamily Waeringopteroidea, which in turn is classified within the infraorder Diploperculat ...
.
The most successful eurypterid by far was the Middle to Late Silurian ''Eurypterus'', a
generalist
A generalist is a person with a wide array of knowledge on a variety of subjects, useful or not. It may also refer to:
Occupations
* a physician who provides general health care, as opposed to a medical specialist; see also:
** General pract ...
, equally likely to have engaged in
predation
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...
or
scavenging
Scavengers are animals that consume dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a herbivorous feeding be ...
. Thought to have hunted mainly small and soft-bodied invertebrates, such as
worm
Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes (though not always).
Worms vary in size from microscopic to over in length for marine polychaete wor ...
s, species of the genus (of which the most common is the type species, ''E. remipes'') account for more than 90% (perhaps as many as 95%) of all known fossil eurypterid specimens. Despite their vast number, ''Eurypterus'' are only known from a relatively short temporal range, first appearing during the Late Llandovery epoch (around 432 million years ago) and being extinct by the end of the Pridoli epoch. ''Eurypterus'' was also restricted to the continent
Euramerica
Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...
(composed of the
equator
The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...
ial continents Avalonia,
Baltica
Baltica is a paleocontinent that formed in the Paleoproterozoic and now constitutes northwestern Eurasia, or Europe north of the Trans-European Suture Zone and west of the Ural Mountains.
The thick core of Baltica, the East European Craton, is mo ...
and Laurentia), which had been completely colonized by the genus during its merging and was unable to cross the vast expanses of ocean separating this continent from other parts of the world, such as the southern supercontinent Gondwana. As such, ''Eurypterus'' was limited geographically to the coastlines and shallow inland seas of Euramerica.
During the Late Silurian the pterygotid eurypterids, large and specialized forms with several new adaptations, such as large and flattened telsons capable of being used as rudders, and large and specialized chelicerae with enlarged pincers for handling (and potentially in some cases killing) prey appeared. Though the largest members of the family appeared in the Devonian, large two meter (6.5+ ft) pterygotids such as ''Acutiramus'' were already present during the Late Silurian. Their ecology ranged from generalized predatory behavior to
ambush predation
Ambush predators or sit-and-wait predators are carnivorous animals that capture or trap prey via stealth, luring or by (typically instinctive) strategies utilizing an element of surprise. Unlike pursuit predators, who chase to capture prey us ...
and some, such as ''Pterygotus'' itself, were active apex predators in Late Silurian marine ecosystems. The pterygotids were also evidently capable of crossing oceans, becoming one of only two eurypterid groups to achieve a
cosmopolitan distribution
In biogeography, cosmopolitan distribution is the term for the range of a taxon that extends across all or most of the world in appropriate habitats. Such a taxon, usually a species, is said to exhibit cosmopolitanism or cosmopolitism. The ext ...
.
Devonian
Though the eurypterids continued to be abundant and diversify during the Early Devonian (for instance leading to the evolution of the pterygotid ''Jaekelopterus'', the largest of all arthropods), the group was one of many heavily affected by the
Late Devonian extinction
The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch, which collectively represent one of the five largest mass extinction events in the history of life on Earth. The term primarily refers to a major exti ...
. The extinction event, only known to affect marine life (particularly trilobites,
brachiopod
Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...
s and
reef
A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic processes— deposition of sand, wave erosion planing down rock out ...
-building organisms) effectively crippled the abundance and diversity previously seen within the eurypterids.
A major decline in diversity had already begun during the Early Devonian and eurypterids were rare in marine environments by the Late Devonian. During the
Frasnian
The Frasnian is one of two faunal stages in the Late Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Givetian Stage and followed by the Famennian Stage.
Major reef-building was under way during th ...
stage four families went extinct, and the later
Famennian
The Famennian is the latter of two faunal stages in the Late Devonian Epoch. The most recent estimate for its duration estimates that it lasted from around 371.1 million years ago to 359.3 million years ago. An earlier 2012 estimate, still used b ...
saw an additional five families going extinct. As marine groups were the most affected, the eurypterids were primarily impacted within the eurypterine suborder. Only one group of stylonurines (the family
Parastylonuridae
The Parastylonuridae are a family of eurypterids, an extinct group of chelicerate arthropods commonly known as "sea scorpions". The family is one of two families contained in the superfamily Stylonuroidea (along with Stylonuridae), which in tu ...
) went extinct in the Early Devonian. Only two families of eurypterines survived into the Late Devonian at all (
Adelophthalmidae
Adelophthalmidae (the name deriving from the type genus ''Adelophthalmus'', meaning "no obvious eyes") is a family of eurypterids, an extinct group of aquatic arthropods. Adelophthalmidae is the only family classified as part of the superfamily ...
and Waeringopteridae). The eurypterines experienced their most major declines in the Early Devonian, during which over 50% of their diversity was lost in just 10 million years. Stylonurines, on the other hand, persisted through the period with more or less consistent diversity and abundance but were affected during the Late Devonian, when many of the older groups were replaced by new forms in the families Mycteroptidae and Hibbertopteridae.
It is possible that the catastrophic extinction patterns seen in the eurypterine suborder were related to the emergence of more derived fish. Eurypterine decline began at the point when jawless fish first became more developed and coincides with the emergence of
placoderms
Placodermi (from Greek πλάξ 'plate' and δέρμα 'skin', literally 'Plate (animal anatomy), plate-skinned') is a Class (biology), class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devoni ...
(armored fish) in both North America and Europe.
Stylonurines of the surviving hibbertopterid and mycteroptid families completely avoided competition with fish by evolving towards a new and distinct ecological niche. These families experienced a radiation and diversification through the Late Devonian and Early Carboniferous, the last ever radiation within the eurypterids, which gave rise to several new forms capable of "sweep-feeding" (raking through the substrate in search of prey).
Carboniferous and Permian
Only three eurypterid families—Adelophthalmidae, Hibbertopteridae and Mycteroptidae—survived the extinction event in its entirety. These were all freshwater animals, rendering the eurypterids extinct in marine environments. With marine eurypterid predators gone,
sarcopterygian
Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includi ...
fish, such as the
rhizodonts
Rhizodontida is an extinct group of predatory tetrapodomorphs known from many areas of the world from the Givetian through to the Pennsylvanian - the earliest known species is about 377 million years ago (Mya), the latest around 310 Mya. Rhizodo ...
, were the new apex predators in marine environments. The sole surviving eurypterine family, Adelophthalmidae, was represented by only a single genus, ''
Adelophthalmus
''Adelophthalmus'' is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of ''Adelophthalmus'' have been discovered in deposits ranging in age from the Early Devonian to the Early Permian, which makes it the longest lived of ...
''. The hibbertopterids, mycteroptids and ''Adelophthalmus'' survived into the Permian.
''Adelophthalmus'' became the most common of all late Paleozoic eurypterids, existing in greater number and diversity than surviving stylonurines, and diversified in the absence of other eurypterines. Out of the 33 species referred to ''Adelophthalmus'', 23 (69%) are from the Carboniferous alone. The genus reached its peak diversity in the Late Carboniferous. Though ''Adelophthalmus'' had already been relatively widespread and represented around all major landmasses in the Late Devonian, the amalgamation of
Pangaea
Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...
into a global supercontinent over the course of the last two periods of the Paleozoic allowed ''Adelophthalmus'' to gain an almost worldwide distribution.
During the Late Carboniferous and
Early Permian 01 or '01 may refer to:
* The year 2001, or any year ending with 01
* The month of January
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Music
* '01 (Richard Müller album), 01'' (Richard Müller album), 2001
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* 01 (Urban ...
''Adelophthalmus'' was widespread, living primarily in brackish and freshwater environments adjacent to coastal plains. These environments were maintained by favorable climate conditions. They did not persist as climate changes owing to Pangaea's formation altered depositional and vegetational patterns across the world. With their habitat gone, ''Adelophthalmus'' dwindled in number and had already gone extinct by the
Leonardian
In the geologic timescale, the Kungurian is an age or stage of the Permian. It is the latest or upper of four subdivisions of the Cisuralian Epoch or Series. The Kungurian lasted between and million years ago (Ma). It was preceded by the Artins ...
stage of the Early Permian.
Mycteroptids and hibbertopterids continued to survive for some time, with one genus of each group known from Permian strata: '' Hastimima'' and '' Campylocephalus'' respectively. ''Hastimima'' went extinct during the Early Permian, as ''Adelophthalmus'' had, while ''Campylocephalus'' persisted longer. A massive incomplete carapace from Permian deposits in Russia represents the sole fossil remains of the species ''C. permianus'', which might have reached in length, while originally considered to be Late Permian in age other sources suggest a Kungurian-Roadian age for the specimen. A fragment of eurypterid cuticle, given the species name '' Woodwardopterus freemanorum'' is known from the terminal Permian of Australia, which represents the youngest known eurypterid. No eurypterids are known from fossil beds higher than the Permian. This indicates that the last eurypterids died either in the catastrophic extinction event at its end or at some point shortly before it. This extinction event, the
Permian–Triassic extinction event
The Permian–Triassic (P–T, P–Tr) extinction event, also known as the Latest Permian extinction event, the End-Permian Extinction and colloquially as the Great Dying, formed the boundary between the Permian and Triassic geologic periods, as ...
, is the most devastating
mass extinction
An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. It ...
recorded, and rendered many other successful Paleozoic groups, such as the trilobites, extinct.
History of study
The first known eurypterid specimen was discovered in the Silurian-aged rocks of
New York
New York most commonly refers to:
* New York City, the most populous city in the United States, located in the state of New York
* New York (state), a state in the northeastern United States
New York may also refer to:
Film and television
* '' ...
, to this day one of the richest eurypterid fossil locations.
Samuel L. Mitchill
Samuel Latham Mitchill (August 20, 1764September 7, 1831) was an American physician, naturalist, and politician who lived in Plandome, New York.
Early life
Samuel Mitchill was born in Hempstead in the Province of New York, the son of Robert M ...
described the specimen, discovered near Westmoreland in Oneida county in 1818. He erroneously identified the fossil as an example of the fish ''
Silurus
''Silurus'' is a genus of catfishes native to Europe and Asia.
Species
There are currently 20 recognized species in this genus:
* ''Silurus aristotelis'' Garman, 1890 (Aristotle's catfish)
* ''Silurus asotus'' Linnaeus, 1758 (Amur catfish)
* ''S ...
'', likely due to the strange,
catfish
Catfish (or catfishes; order Siluriformes or Nematognathi) are a diverse group of ray-finned fish. Named for their prominent barbels, which resemble a cat's whiskers, catfish range in size and behavior from the three largest species alive, ...
-like appearance of the carapace. Seven years later, in 1825, James E. DeKay examined the fossil and recognized it as clearly belonging to an arthropod. He thought the fossil, which he named ''Eurypterus remipes'', represented a crustacean of the order
Branchiopoda
Branchiopoda is a class of crustaceans. It comprises fairy shrimp, clam shrimp, Diplostraca (or Cladocera), Notostraca and the Devonian ''Lepidocaris''. They are mostly small, freshwater animals that feed on plankton and detritus.
Description
M ...
, and suggested it might represent a missing link between the trilobites and more derived branchiopods. The name ''Eurypterus'' derives from Greek ( 'broad, wide') and ( 'wing').
In 1843,
Hermann Burmeister
Karl Hermann Konrad Burmeister (also known as Carlos Germán Conrado Burmeister) (15 January 1807 – 2 May 1892) was a German Argentine zoologist, entomologist, herpetologist, botany, botanist, and coleopterologist. He served as a professor at ...
published his view on trilobite taxonomy and how the group related to other organisms, living and extinct, in the work ''Die Organisation der Trilobiten aus ihren lebenden Verwandten entwickelt''. He considered the trilobites to be crustaceans, as previous authors had, and classified them together with what he assumed to be their closest relatives, ''Eurypterus'' and the genus '' Cytherina'', within a clade he named "Palaeadae". Within Palaeadae, Burmeister erected three families; the "Trilobitae" (composed of all trilobites), the "Cytherinidae" (composed only of ''Cytherina'', an animal today seen as an
ostracod
Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typic ...
) and the Eurypteridae (composed of ''Eurypterus'', then including three species).
The fourth eurypterid genus to be described (following ''Hibbertopterus'' in 1836 and ''Campylocephalus'' in 1838, not identified as eurypterids until later), out of those still seen as taxonomically valid in modern times, was ''Pterygotus'' (), described by
Louis Agassiz
Jean Louis Rodolphe Agassiz ( ; ) FRS (For) FRSE (May 28, 1807 – December 14, 1873) was a Swiss-born American biologist and geologist who is recognized as a scholar of Earth's natural history.
Spending his early life in Switzerland, he rec ...
in 1839. ''Pterygotus'' was considerably larger in size than ''Eurypterus'' and when the first fossils were discovered by quarrymen in
Scotland
Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...
they were referred to as "
Seraph
A seraph (, "burning one"; plural seraphim ) is a type of celestial or heavenly being originating in Ancient Judaism. The term plays a role in subsequent Judaism, Christianity, and Islam.
Tradition places seraphim in the highest rank in Chr ...
ims" by the workers. Agassiz first thought the fossils represented remains of fish, only recognizing their nature as arthropod remains five years later in 1844.
In 1849, Frederick M'Coy classified ''Pterygotus'' together with ''Eurypterus'' and ''
Belinurus
''Belinurus'' is an extinct genus of arthropods belonging to the order Xiphosura. It is part of the family Belinuridae, in the infraorder Belinurina. There used to be a longstanding academic controversy on whether ''Belinurus'' (König, 1820), ...
'' (a genus today seen as a xiphosuran) within Burmeister's Eurypteridae. M'Coy considered the Eurypteridae to be a group of crustaceans within the order
Entomostraca
Entomostraca is a historical subclass of crustaceans, no longer in technical use. It was originally considered one of the two major lineages of crustaceans (the other being the class Malacostraca), combining all other classes—Branchiopoda, Cepha ...
, closely related to horseshoe crabs. A fourth genus, ''
Slimonia
''Slimonia'' is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of ''Slimonia'' have been discovered in deposits of Silurian age in South America and Europe. Classified as part of the family Slimonidae alongside the relat ...
'', based on fossil remains previously assigned to a new species of ''Pterygotus'', was referred to the Eurypteridae in 1856 by David Page.
Jan Nieszkowski's ''De Euryptero Remipede'' (1858) featured an extensive description of ''Eurypterus fischeri'' (now seen as synonymous with another species of ''Eurypterus'', ''E. tetragonophthalmus''), which, along with the monograph ''On the Genus Pterygotus'' by
Thomas Henry Huxley
Thomas Henry Huxley (4 May 1825 – 29 June 1895) was an English biologist and anthropologist specialising in comparative anatomy. He has become known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution.
The storie ...
and
John William Salter
John William Salter (15 December 1820 – 2 December 1869) was an England, English natural history, naturalist, geologist, and palaeontologist.
Salter was apprenticed in 1835 to James De Carle Sowerby, and was engaged in drawing and engraving ...
, and an exhaustive description of the various eurypterids of New York in Volume 3 of the ''Palaeontology of New York'' (1859) by James Hall, contributed massively to the understanding of eurypterid diversity and biology. These publications were the first to fully describe the whole anatomy of eurypterids, recognizing the full number of prosomal appendages and the number of preabdominal and postabdominal segments. Both Nieszkowski and Hall recognized that the eurypterids were closely related to modern chelicerates, such as horseshoe crabs.
In 1865, Henry Woodward described the genus ''
Stylonurus
''Stylonurus'' is a genus of prehistoric eurypterid of the family Stylonuridae. The genus contains three species: ''Stylonurus powriensis'' from the Devonian of Scotland, ''Stylonurus shaffneri'' from the Devonian of Pennsylvania and ''Stylonurus ...
'' (named and figured, but not thoroughly described, by David Page in 1856) and raised the rank of the Eurypteridae to that of order, effectively creating the Eurypterida as the taxonomic unit it is seen as today. In the work ''Anatomy and Relations of the Eurypterida'' (1893),
Malcolm Laurie
Malcolm Laurie FRSE FLS (27 February 1866 – 16 July 1932) was a Scottish zoologist and palaeontologist.
Biography
He was born in Brunstane House south of Portobello, Edinburgh on 27 February 1866, the son of Simon Somerville Laurie an ...
added considerably to the knowledge and discussion of eurypterid anatomy and relations. He focused on how the eurypterids related to each other and to trilobites, crustaceans, scorpions, other arachnids and horseshoe crabs. The description of ''Eurypterus fischeri'' by Gerhard Holm in 1896 was so elaborate that the species became one of the most completely known of all extinct animals, so much so that the knowledge of ''E. fischeri'' was comparable with the knowledge of its modern relatives (such as the
Atlantic horseshoe crab
The Atlantic horseshoe crab (''Limulus polyphemus''), also known as the American horseshoe crab, is a species of marine and brackish chelicerate arthropod. Despite their name, horseshoe crabs are more closely related to spiders, ticks, and scorp ...
). The description also helped solidify the close relationship between the eurypterids and other chelicerates by showcasing numerous homologies between the two groups.
In 1912,
John Mason Clarke
John Mason Clarke (April 15, 1857 – May 29, 1925) was an American teacher, geologist and paleontologist.
__TOC__
Early career
Born in Canandaigua, New York, the fifth of six children of Noah Turner Clarke and Laura Mason Merrill, he attended ...
and
Rudolf Ruedemann
Rudolf Ruedemann (October 16, 1864–June 18, 1956) was a German American paleontologist, widely known as an expert in graptolites, enigmatic fossil animals. He worked at the New York State Museum for over 40 years, including a decade as Stat ...
published ''The Eurypterida of New York'' in which all eurypterid species thus far recovered from fossil deposits there were discussed. Clarke and Ruedemann created one of the first phylogenetic trees of eurypterids, dividing the order into two families; Eurypteridae (distinguished by smooth eyes and including ''Eurypterus'', ''
Anthraconectes
''Adelophthalmus'' is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of ''Adelophthalmus'' have been discovered in deposits ranging in age from the Early Devonian to the Early Permian, which makes it the longest lived of ...
'', ''Stylonurus'', '' Eusarcus'', '' Dolichopterus'', '' Onychopterus'' and ''Drepanopterus'') and Pterygotidae (distinguished by faceted eyes and including ''Pterygotus'', ''Erettopterus'', ''Slimonia'' and ''
Hughmilleria
''Hughmilleria'' is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of ''Hughmilleria'' have been discovered in deposits of the Silurian age in China and the United States. Classified as part of the basal family Hughmille ...
''). Both families were considered to be descended from a common ancestor, ''Strabops''. In line with earlier authors, Clarke and Ruedemann also supported a close relationship between the eurypterids and the horseshoe crabs (united under the
class
Class or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used differentl ...
Merostomata) but also discussed alternative hypotheses such as a closer relation to arachnids.
Classification
Historically, a close relationship between eurypterids and xiphosurans (such as the modern Atlantic horseshoe crab) has been assumed by most researchers. Several homologies encourage this view, such as correlating segments of the appendages and the prosoma. Additionally, the presence of plate-like appendages bearing the "gill tracts" on appendages of the opisthosoma (the ) was cited early as an important homology. In the last few decades of the nineteenth century, further homologies were established, such as the similar structures of the compound eyes of ''Pterygotus'' and horseshoe crabs (seen as especially decisive as the eye of the horseshoe crab was seen as possessing an almost unique structure) and similarities in the ontogeny within both groups. These ontogenetical similarities were seen as most apparent when studying the nepionic stages (the developmental stage immediately following the embryonic stage) in both groups, during which both xiphosurans and eurypterids have a proportionally larger carapace than adults, are generally broader, possess a distinct ridge down the middle, have a lesser number of segments which lack differentiation and have an underdeveloped telson.
Due to these similarities, the xiphosurans and eurypterids have often been united under a single class or subclass called Merostomata (erected to house both groups by Henry Woodward in 1866). Though xiphosurans (like the eurypterids) were historically seen as crustaceans due to their respiratory system and their aquatic lifestyle, this hypothesis was discredited after numerous similarities were discovered between the horseshoe crabs and the arachnids. Some authors, such as
John Sterling Kingsley
John Sterling Kingsley (1854–1929) was an American professor of biology and zoology.
Early life
John Kingsley was born on 7 April 1854 in Cincinnatus, New York son of Lewis and Julia A. (née Kingman) Kingsley.Twentieth Century Biographical ...
in 1894, classified the Merostomata as a sister group to the Arachnida under the class "Acerata" within a subphylum "Branchiata". Others, such as
Ray Lankester
Sir Edwin Ray Lankester (15 May 1847 – 13 August 1929) was a British zoologist.New International Encyclopaedia.
An invertebrate zoologist and evolutionary biologist, he held chairs at University College London and Oxford University. He was th ...
in 1909, went further and classified the Merostomata as a subclass within the Arachnida, raised to the rank of class.
In 1866,
Ernst Haeckel
Ernst Heinrich Philipp August Haeckel (; 16 February 1834 – 9 August 1919) was a German zoologist, naturalist, eugenicist, philosopher, physician, professor, marine biologist and artist. He discovered, described and named thousands of new sp ...
classified the Merostomata (containing virtually only the Eurypterida) and Xiphosura within a group he named Gigantostraca within the crustaceans. Though Haeckel did not designate any taxonomic rank for this clade, it was interpreted as equivalent to the rank of subclass, such as the Malacostraca and Entomostraca, by later researchers such as John Sterling Kinsgsley. In subsequent research, Gigantostraca has been treated as synonymous with Merostomata (rarely) and Eurypterida itself (more commonly).
A phylogenetic analysis (the results presented in a cladogram below) conducted by James Lamsdell in 2013 on the relationships within the Xiphosura and the relations to other closely related groups (including the eurypterids, which were represented in the analysis by genera ''Eurypterus'', ''Parastylonurus'', '' Rhenopterus'' and '' Stoermeropterus'') concluded that the Xiphosura, as presently understood, was
paraphyletic
In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
(a group sharing a
last common ancestor
In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...
but not including all descendants of this ancestor) and thus not a valid phylogenetic group. Eurypterids were recovered as closely related to arachnids instead of xiphosurans, forming the group
Sclerophorata
Dekatriata is a clade of planatergan chelicerates including the groups Arachnida, Chasmataspidida, Eurypterida and additionally two stem-genera '' Winneshiekia'' and ''Houia
''Houia'' is an extinct genus of dekatriatan, a clade of chelicera ...
within the clade
Dekatriata
Dekatriata is a clade of planatergan chelicerates including the groups Arachnida, Chasmataspidida, Eurypterid
Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest ...
(composed of sclerophorates and chasmataspidids). Lamsdell noted that it is possible that Dekatriata is synonymous with Sclerophorata as the reproductive system, the primary defining feature of sclerophorates, has not been thoroughly studied in chasmataspidids. Dekatriata is, in turn, part of the
Prosomapoda
Prosomapoda is a clade of euchelicerates including the groups Xiphosura (horseshoe crabs) and Planaterga (a group comprising bunodids, pseudoniscids, chasmataspidids, eurypterids and arachnids), as well as several basal synziphosurid genera ...
, a group including the Xiphosurida (the only monophyletic xiphosuran group) and other stem-genera.
Internal relationships
The internal classification of eurypterids within the Eurypterida is based mainly on eleven established characters. These have been used throughout the history of eurypterid research to establish clades and genera. These characters include: the shape of the prosoma, the shape of the metastoma, the shape and position of the eyes, the types of prosomal appendages, the types of swimming leg paddles, the structure of the doublure (the fringe of the dorsal exoskeleton), the structure of the opithosoma, the structure of the genital appendages, the shape of the telson and the type of ornamentation present. It is worth noting that not all of these characters are of equal taxonomic importance. They are not applicable to all eurypterids either; stylonurine eurypterids lack swimming leg paddles entirely. Some characters, including the prosoma and metastoma shapes and the position and shapes of the eyes, are seen as important only for the distinction between different genera. Most superfamilies and families are defined based on the morphology of the appendages.
The most important character used in eurypterid taxonomy is the type of prosomal appendages as this character is used to define entire suborders. General leg anatomy can also be used to define superfamilies and families. Historically, the chelicerae were considered the most important appendages from a taxonomical standpoint since they only occurred in two general types: a eurypterid type with small and toothless pincers and a pterygotid type with large pincers and teeth. This distinction has historically been used to divide the Eurypterida into the two suborders Eurypterina (small chelicerae) and "Pterygotina" (large and powerful chelicerae). This classification scheme is not without problems. In Victor Tollerton's 1989 taxonomic revision of the Eurypterida, with suborders Eurypterina and Pterygotina recognized, several clades of eurypterids today recognized as stylonurines (including hibbertopterids and mycteroptids) were reclassified as non-eurypterids in the new separate order "Cyrtoctenida" on the grounds of perceived inconsistencies in the prosomal appendages.
Modern research favors a classification into suborders Eurypterina and Stylonurina instead, supported by phylogenetic analyses. In particular, pterygotid eurypterids share a number of homologies with derived eurypterine eurypterids such as the adelophthalmids, and are thus best classified as derived members of the same suborder. In the Stylonurina, the sixth pair of appendages is represented by long and slender walking legs and lack a modified spine (referred to as the podomere 7a). In most eurypterids in the Eurypterina, the sixth pair of appendages is broadened into swimming paddles and always has a podomere 7a. 75% of eurypterid species are eurypterines and they represent 99% of all fossil eurypterid specimens. Of all eurypterid clades, the Pterygotioidea is the most species-rich, with over 50 species. The second most species-rich clade is the Adelophthalmoidea, with over 40 species.
The cladogram presented below, covering all currently recognized eurypterid families, follows a 2007 study by O. Erik Tetlie. The stylonurine suborder follows a 2010 study by James Lamsdell, Simon J. Braddy and Tetlie. The superfamily "Megalograptoidea", recognized by Tetlie in 2007 and then placed between the
Onychopterelloidea
Onychopterellidae are an extinct family of eurypterids. The family is the only family classified as part of the superfamily Onychopterelloidea. Genera included are ''Alkenopterus'', ''Onychopterella'' and ''Tylopterella''.Dunlop, J. A., Penney, D ...
and Eurypteroidea, has been omitted as more recent studies suggest that the megalograptids were members of the superfamily Carcinosomatoidea. As such, the phylogeny of the Carcinosomatoidea follows a 2015 study by Lamsdell and colleagues.
Nepidae
Nepidae is a family of exclusively aquatic Heteropteran insects in the order Hemiptera. They are commonly called water scorpions for their superficial resemblance to scorpions, due to their raptorial forelegs and the presence of a long slender p ...
—an unrelated family of insects, commonly known as "water scorpions".
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Cottidae
The Cottidae are a family of fish in the superfamily Cottoidea, the sculpins. It is the largest sculpin family, with about 275 species in 70 genera.Kane, E. A. and T. E. Higham. (2012)Life in the flow lane: differences in pectoral fin morphology ...
—family of fishes in which some members contain "sea scorpion" in their common name.
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Evolutionary history of life
The history of life on Earth traces the processes by which living and fossil organisms evolved, from the earliest #Origins of life on Earth, emergence of life to present day. Earth formed about 4.5 billion years ago (abbreviated as ''Ga'', fo ...