Radiodonta is an extinct
order of
stem-group
In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...
arthropods that was successful worldwide during the
Cambrian period. They may be referred to as radiodonts,
radiodontans,
radiodontids,
anomalocarids,
or anomalocaridids,
although the last two originally refer to the family
Anomalocarididae
Anomalocarididae (occasionally mis-spelt Anomalocaridae) is an extinct family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpo ...
, which previously included all species of this order but is now restricted to only a few species.
Radiodonts are distinguished by their distinctive frontal appendages, which are morphologically diverse and used for a variety of functions. Radiodonts included the earliest large predators known, but they also included sediment sifters and filter feeders.
Some of the most famous species of radiodonts are the
Cambrian taxa ''
Anomalocaris canadensis
''Anomalocaris'' ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of ''Anomalocaris'' were discovered in the ''Ogygopsis'' Shale of the Stephen F ...
'', ''
Hurdia victoria
''Hurdia'' is an extinct genus of hurdiid radiodont that lived 505 million years ago during the Cambrian Period. As a radiodont like ''Peytoia'' and ''Anomalocaris'', it is part of the ancestral lineage that led to euarthropods.
Description
' ...
'', ''
Peytoia nathorsti
''Peytoia'' is a genus of hurdiid radiodont that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and ''Peytoia infercambriensis'' from Poland, dating to Cambrian Stage 3. Its two front ...
'', ''
Titanokorys gainessii,
Cambroraster falcatus'' and ''
Amplectobelua symbrachiata'', the
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.
T ...
''
Aegirocassis benmoulai'' and the
Devonian ''
Schinderhannes bartelsi
''Schinderhannes bartelsi'' is a species of hurdiid radiodont (anomalocaridid) known from one specimen from the lower Devonian Hunsrück Slates. Its discovery was astonishing because previously, radiodonts were known only from exceptionally well ...
''.
Etymology
The name Radiodonta (Latin for ''radius'' "spoke of a wheel" and Greek for ''odoús'' "tooth") refers to the radial arrangement of tooth plates (oral cone) surrounding the mouth,
although these features are suggested to be absent in some radiodont species.
Definition
The original diagnosis of order Radiodonta in 1996 is as follows:
[ ]
In 2014, the clade Radiodonta was defined
phylogenetically as a clade including any taxa closer to ''
Anomalocaris canadensis
''Anomalocaris'' ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of ''Anomalocaris'' were discovered in the ''Ogygopsis'' Shale of the Stephen F ...
'' than ''
Paralithodes camtschaticus
The red king crab (''Paralithodes camtschaticus''), also called Kamchatka crab or Alaskan king crab, is a species of king crab native to the Bering Sea and Gulf of Alaska. It was introduced to the Barents Sea. It grows to a leg span of , and is ...
''.
In 2019, it was redefined
morphologically as animal bearing head carapace complex with central (H-) and lateral (P-) elements; outgrowths (endites) from frontal appendages bearing auxiliary spines; and reduced anterior flaps or bands of lamellae (setal blades) and strong tapering of body from anterior to posterior.
Description
![20210208 Radiodonta size comparison complete specimens](https://upload.wikimedia.org/wikipedia/commons/6/6d/20210208_Radiodonta_size_comparison_complete_specimens.png)
Most radiodonts were significantly larger than other
Cambrian fauna, with typical body lengths varying from 30 to 50 centimeters.
The largest described radiodont is
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.
T ...
''
Aegirocassis benmoulai'', which may have grown up to two meters long.
A nearly complete specimen of a juvenile ''Lyrarapax unguispinus'' measured only , making it among the smallest radiodont specimens known, though adults reached a length of
An isolated frontal appendage of a hurdiid with a length less than half that of the juvenile ''Lyrarapax'' is known, but it is not known whether this specimen pertains to an adult.
The largest known Cambrian radiodont was ''Laminacaris'', although known from only by frontal appendages, had an estimated body length of up to based on ''Anomalocaris''. ''Anomalocaris'' and ''Amplectobelua'' are also large ones, reached and (there was an estimation that ''Houcaris saron'' (previously ''Anomalocaris saron'') reached , but specimen used for estimating the body length no longer belongs to that species
); the Cambrian hurdiid ''Titanokorys'' approached it in size, with an estimated body length of approximately .
The body of a radiodont could be divided into two regions: head and trunk. The head is composed of only one body segment
known as the ocular somite, covered by sclerites (head carapace complex), bore arthropodized frontal appendages, ventral mouthparts (oral cone), and
stalked compound eye
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 disti ...
s. The tapering trunk is composed of multiple body segments, each associated with pairs of flaps and gill-like structures (setal blades).
Frontal appendage
The anterior structures on the head are a pair of frontal appendages which have been referred to as 'claws', 'grasping appendages', 'feeding appendages', or 'great appendages' in previous studies (the last term is discouraged since the homology between frontal appendages and the original,
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 ...
n
great appendage
Great appendages are claw-like appendages which attach to the heads of the "great appendage arthropods", a name usually refers to Megacheira, a class of extinct arthropod characterized by a pair of "short-great appendages" bearing in front of the ...
s is questionable.
). They are
sclerotized (hardened) and arthropodized (segmented), bearing ventral endites (spines) on most of their podomeres (segmental units), and the endites may bear additional rows of auxiliary spines on their anterior and posterior margins.
The frontal appendage consists of two regions: the shaft ('peduncle',
'base'
or 'promixal region'
in some studies) and the distal articulated region
(also referred to as 'claw'
). A triangular region covered by soft cuticle (arthrodial membrane) may occur on the ventral side between podomeres and provide flexibility.
Their pre-ocular and protocerebral origin suggest they are homologous to the primary antennae of
Onychophora
Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...
and the
labrum of
Euarthropoda
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, oft ...
(all arose from ocular
somite
The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals. In vertebrates, somites subdivide ...
), and not homologous with 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 similarl ...
of
Chelicerata
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 mi ...
nor the
antennae or '
great appendage
Great appendages are claw-like appendages which attach to the heads of the "great appendage arthropods", a name usually refers to Megacheira, a class of extinct arthropod characterized by a pair of "short-great appendages" bearing in front of the ...
s' of other arthropods, which are deutocerebral (arose from post-ocular somite 1).
Since the morphology of the frontal appendages, especially those of the spines, always differs between species, it is one of the most important means of species identification.
In fact, many radiodonts are only known from a handful of fossilized frontal appendages.
File:20191221 Radiodonta frontal appendage Anomalocarididae Amplectobeluidae.png, Frontal appendages of Anomalocarididae
Anomalocarididae (occasionally mis-spelt Anomalocaridae) is an extinct family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpo ...
, Amplectobeluidae
''Amplectobeluidae'' is a clade of Cambrian Radiodonta, radiodonts.
Definition
In 2014, ''Amplectobeluidae'' was defined as the most inclusive clade including ''Amplectobelua, Amplectobelua symbrachiata'' but not ''Anomalocaris, Anomalocaris ca ...
, and possibly related species
File:20191228 Radiodonta frontal appendage Tamisiocarididae Cetiocaridae.png, Frontal appendages of Tamisiocarididae
Tamisiocarididae is a family of radiodonts, extinct marine animals related to arthropods, that bore finely-spined appendages that were presumably used in filter-feeding. When first discovered, the clade was named Cetiocaridae after a speculative ...
File:20191229 Radiodonta frontal appendage Hurdiidae.png, Frontal appendages of Hurdiidae
Hurdiidae is an extinct cosmopolitan family of radiodonts, a group of stem-group arthropods, which lived during the Paleozoic Era. It is the most long-lived radiodont clade, lasting from the Cambrian period to the Devonian period.
Description ...
Oral cone
![20210520 Radiodonta oral cone](https://upload.wikimedia.org/wikipedia/commons/e/e8/20210520_Radiodonta_oral_cone.png)
The mouth is on the ventral side of the head, behind the attachment point of frontal appendages and is surrounded by a ring of tooth plates, forming the mouthpart known as oral cone ('jaws' in previous studies
). 3 or 4 tooth plates might be enlarged, giving the oral cone a triradial (e.g. ''
Anomalocaris
''Anomalocaris'' ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of ''Anomalocaris'' were discovered in the ''Ogygopsis'' Shale of the Stephen F ...
'') or tetraradial (e.g.
Hurdiidae
Hurdiidae is an extinct cosmopolitan family of radiodonts, a group of stem-group arthropods, which lived during the Paleozoic Era. It is the most long-lived radiodont clade, lasting from the Cambrian period to the Devonian period.
Description ...
, ''
Lyrarapax
'' Lyrarapax'' is a radiodont genus of the family Amplectobeluidae that lived in the early Cambrian period 520 million years ago. Its neural tissue indicates that the radiodont frontal appendage is protocerebral, resolving parts of the arthropod ...
'') appearance.
The inner margin of tooth plates have spikes facing towards the mouth opening. Additional rows of internal tooth plates may occur in some
hurdiid
Hurdiidae is an extinct cosmopolitan family of radiodonts, a group of stem-group arthropods, which lived during the Paleozoic Era. It is the most long-lived radiodont clade, lasting from the Cambrian period to the Devonian period.
Descripti ...
genera.
Detail reconstruction of some
amplectobeluid oral cones are speculative, but they possibly did not present a typical radial arrangement.
Head sclerites, eyes and trunk
![20210516 Radiodonta head sclerites](https://upload.wikimedia.org/wikipedia/commons/f/f2/20210516_Radiodonta_head_sclerites.png)
Three head sclerite (
carapace) complex formed by a central H-element (anterior sclerite or head shield) and a pair of P-elements (lateral sclerites) cover the dorsal and laterovental surface of the animal's head.
The P-elements may connect to each other as well as the H-element by a narrow anterior extension (P-element neck or 'beak').
The head sclerites are small and ovoid in
Anomalocarididae
Anomalocarididae (occasionally mis-spelt Anomalocaridae) is an extinct family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpo ...
and
Amplectobeluidae
''Amplectobeluidae'' is a clade of Cambrian Radiodonta, radiodonts.
Definition
In 2014, ''Amplectobeluidae'' was defined as the most inclusive clade including ''Amplectobelua, Amplectobelua symbrachiata'' but not ''Anomalocaris, Anomalocaris ca ...
,
but often enlarged in
Hurdiidae
Hurdiidae is an extinct cosmopolitan family of radiodonts, a group of stem-group arthropods, which lived during the Paleozoic Era. It is the most long-lived radiodont clade, lasting from the Cambrian period to the Devonian period.
Description ...
, corresponded to their distinct body shapes (streamlined in Anomalocarididae/Amplectobeluidae but often compact in Hurdiidae).
The head bore two
stalked compound eye
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 disti ...
s, which may have had mobility,
and are located between the gaps formed by the posterior regions of the H-element and P-elements.
Contrary to the original diagnosis, the division of body segments (segmental boundaries) can be visible externally
and no known member of Radiodonta (except the putative radiodont ''
Cucumericrus
''Cucumericrus decoratus'' is a species of putative radiodont known from a few poorly preserved specimens. Only fragments of trunk cuticle and corresponded appendages had been revealed, while important radiodont features such as frontal appendag ...
''
) is known to have pediform trunk appendages (legs).
The trunk has numerous body segments (
somites
The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals. In vertebrates, somites subdivide i ...
), tapering from anterior to posterior, with the anterior 3 or 4 segments significantly constricted into a neck region.
File:20210914 Radiodonta body flaps lobes structures variations.png, Variations of radiodont body flaps
File:Radiodonta body flaps movement.gif, The movement of radiodont flapping appendages
File:20200103 Radiodonta gnathobase‐like structures.png, Ventral view of a generalized GLS-bearing radiodont, showing gnathobase‐like structures (GLSs) associated with reduced anterior flaps
The trunk appendages were fin-like body flaps ('lateral flaps' or 'lobes' in some studies), usually one pair of ventral flaps per body segment, each slightly overlapping the one more anterior to it, but additional, non-overlapping sets of small dorsal flaps may occur in some
Hurdiid
Hurdiidae is an extinct cosmopolitan family of radiodonts, a group of stem-group arthropods, which lived during the Paleozoic Era. It is the most long-lived radiodont clade, lasting from the Cambrian period to the Devonian period.
Descripti ...
species.
The flaps may have numerous vein-like structures (referred to as 'strengthening rays',
'flap rays',
'tranverse rods',
'transverse lines' or 'veins'
). The flaps on the neck region (referred to as 'reduced flaps',
'neck flaps',
'head flaps',
'anterior flaps'
or 'differentiated flaps'
) are significantly reduced. In some species, jaw-like feeding appendages called gnathobase-like structures (GLSs) arose from each of the bases of their reduced neck flaps.
Numerous elongated blade-like extensions (referred to as lanceolate blades or lamellae
) arranged in a row, forming bands of
gill
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...
-like structures known as setal blades, covered the dorsal surface of each body segment.
At least in ''
Aegirocassis
''Aegirocassis'' is an extinct genus of radiodont arthropod belonging to the family Hurdiidae that lived 480 million years ago during the early Ordovician. It is known by a single species, ''Aegirocassis benmoulai''. Van Roy initiated scientifi ...
'', each of the lanceolate blades are covered in wrinkles.
The ventral flaps may be homologous to the endopod of the biramous limbs of euarthropods and lobopodous limbs (lobopods) of gilled
lobopodia
The lobopodians, members of the informal group Lobopodia (from the Greek, meaning "blunt feet"), or the formally erected phylum Lobopoda Cavalier-Smith (1998), are panarthropods with stubby legs called lobopods, a term which may also be used as ...
ns, and the dorsal flaps and setal blades may be homologous to the exite and gill-bearing dorsal flaps of the former taxa.
The trunk may end either with a tail fan compose of 1 to 3 pairs of blades,
a pair of long furcae,
an elongated terminal structure,
or a featureless blunt tip.
File:20220724 Hurdiidae.png, ''Stanleycaris
''Stanleycaris'' is an extinct, monotypic genus of hurdiid radiodont from the middle Cambrian (Miaolingian). The type species is ''Stanleycaris hirpex''. ''Stanleycaris'' was described from the Stephen Formation near the Stanley Glacier and Bu ...
'', ''Hurdia
''Hurdia'' is an extinct genus of hurdiid radiodont that lived 505 million years ago during the Cambrian Period. As a radiodont like ''Peytoia'' and ''Anomalocaris'', it is part of the ancestral lineage that led to euarthropods.
Description
' ...
'', ''Aegirocassis
''Aegirocassis'' is an extinct genus of radiodont arthropod belonging to the family Hurdiidae that lived 480 million years ago during the early Ordovician. It is known by a single species, ''Aegirocassis benmoulai''. Van Roy initiated scientifi ...
'', ''Peytoia
''Peytoia'' is a genus of hurdiid radiodont that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and ''Peytoia infercambriensis'' from Poland, dating to Cambrian Stage 3. Its two front ...
'' and ''Cambroraster
''Cambroraster'' is an extinct monotypic genus of hurdiid radiodont, dating to the middle Cambrian, and represented by the single formally described species ''Cambroraster falcatus''. Hundreds of specimens were found in the Burgess Shale, and de ...
'' are all examples of hurdiid
Hurdiidae is an extinct cosmopolitan family of radiodonts, a group of stem-group arthropods, which lived during the Paleozoic Era. It is the most long-lived radiodont clade, lasting from the Cambrian period to the Devonian period.
Descripti ...
radiodonts.
File:20210626 Anomalocaris.png, ''Anomalocaris
''Anomalocaris'' ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of ''Anomalocaris'' were discovered in the ''Ogygopsis'' Shale of the Stephen F ...
'' is a member of the anomalocarididae
Anomalocarididae (occasionally mis-spelt Anomalocaridae) is an extinct family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpo ...
family, which at one point included all radiodonts, but now only includes a few genera.
File:20210912 Amplectobeluidae.png, ''Amplectobelua
''Amplectobelua'' (meaning "embracing beast") is an extinct genus of late Early Cambrian amplectobeluid radiodont, a group of stem arthropods that mostly lived as free-swimming predators during the first half of the Paleozoic Era.
Anatomy
...
'' and ''Lyrarapax
'' Lyrarapax'' is a radiodont genus of the family Amplectobeluidae that lived in the early Cambrian period 520 million years ago. Its neural tissue indicates that the radiodont frontal appendage is protocerebral, resolving parts of the arthropod ...
'' are representatives of the amplectobeluidae
''Amplectobeluidae'' is a clade of Cambrian Radiodonta, radiodonts.
Definition
In 2014, ''Amplectobeluidae'' was defined as the most inclusive clade including ''Amplectobelua, Amplectobelua symbrachiata'' but not ''Anomalocaris, Anomalocaris ca ...
which is a very inclusive family of mainly Chinese radiodonts.
File:20191228 Radiodonta frontal appendage Tamisiocarididae Cetiocaridae.png, ''"Anomalocaris" briggsi'' and ''Tamisiocaris
''Tamisiocaris'' (from Latin ''tamisium'', sieve, and Greek ''karis'', crab, shrimp) is a radiodont genus initially only known from frontal appendages from the Buen Formation in Sirius Passet. Further specimens revealed that the frontal appenda ...
'' are examples of the family tamisiocarididae
Tamisiocarididae is a family of radiodonts, extinct marine animals related to arthropods, that bore finely-spined appendages that were presumably used in filter-feeding. When first discovered, the clade was named Cetiocaridae after a speculative ...
which were exclusively suspension feeding
Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...
radiodonts from the Cambrian.
Internal structures
![20191107 Radiodonta brain and digestive system](https://upload.wikimedia.org/wikipedia/commons/5/59/20191107_Radiodonta_brain_and_digestive_system.png)
Traces of
muscles
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of musc ...
,
digestive system and
nervous system
In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes ...
were described from some radiodont fossils. Pairs of well-developed muscles were connected to the ventral flaps located at the lateral cavities of each body segment.
Between the lateral muscles is a sophisticated digestive system, formed by a widening of the foregut and hindgut, both connected by a narrow midgut associated with six pairs of gut divercula (digestive glands).
Compared to the three-segmented
brain
A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a ve ...
s of
euarthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, oft ...
s and two-segmented brains of
onychophoran
Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...
s, the brain of radiodonts is composed of only one brain segment originating from the ocular somite, the protocerebrum. The nerves of the frontal appendages and compound eyes arose from the anterior and lateral regions of the brain.
Posterior to the brain was a pair of apparently unfused
ventral nerve cord
The ventral nerve cord is a major structure of the invertebrate central nervous system. It is the functional equivalent of the vertebrate spinal cord. The ventral nerve cord coordinates neural signaling from the brain to the body and vice versa, i ...
s which ran through the animal's neck region.
Paleoecology
Physiology
![Cambroraster](https://upload.wikimedia.org/wikipedia/commons/1/1b/Cambroraster.jpg)
Radiodonts were interpreted as
nektonic
Nekton or necton (from the ) refers to the actively swimming aquatic organisms in a body of water. The term was proposed by German biologist Ernst Haeckel to differentiate between the active swimmers in a body of water, and the passive organisms t ...
or
nektobenthic animals, with their morphology suggesting an active swimming lifestyle. The muscular, overlapping ventral flaps may have propelled the animal through the water, possibly by moving in a wave-like formation resembling modern
rays
Ray may refer to:
Fish
* Ray (fish), any cartilaginous fish of the superorder Batoidea
* Ray (fish fin anatomy), a bony or horny spine on a fin
Science and mathematics
* Ray (geometry), half of a line proceeding from an initial point
* Ray (gra ...
and
cuttlefish.
Pairs of dorsal flaps, which make up a tail fan in some species, may have helped steering and/or stabilizing the animal during locomotion.
In ''
Anomalocaris
''Anomalocaris'' ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of ''Anomalocaris'' were discovered in the ''Ogygopsis'' Shale of the Stephen F ...
'', morphology of the tail fan even suggests it could rapidly change its swimming direction efficiently. On the other hand, some hurdiids have features significantly specialized for a nektobenthic lifestyle, such as ''
Cambroraster
''Cambroraster'' is an extinct monotypic genus of hurdiid radiodont, dating to the middle Cambrian, and represented by the single formally described species ''Cambroraster falcatus''. Hundreds of specimens were found in the Burgess Shale, and de ...
'' with its dome-like H-element similar to the
carapace of a
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 ar ...
.
Bands of setal blades with wrinkling lanceolate blades may have increased the surface area, suggesting they were
gills
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...
, providing the animal's
respiratory
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 gre ...
function.
Abundance of the remains of scleritzed structures such as disarticulated frontal appendages and head sclerite complexes, suggest that mass
moulting events may have occurred among radiodonts,
a behavior which also has been reported in some other
Cambrian arthropods such as
trilobites
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 ...
.
Diet
Radiodonts had diverse feeding strategies, which could be categorized as
raptorial
The term ''raptorial'' implies much the same as ''predatory'' but most often refers to modifications of an arthropod's foreleg that make it function for the grasping of prey while it is consumed, where the gripping surfaces are formed from the o ...
predators
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 th ...
, sediment sifters, or suspension,
filter feeders
Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...
.
For example, raptorial predators like ''
Anomalocaris
''Anomalocaris'' ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of ''Anomalocaris'' were discovered in the ''Ogygopsis'' Shale of the Stephen F ...
'' and
Amplectobeluids might have been able to catch agile prey by using their raptorial frontal appendages; the latter even bore a robust endite for holding prey like a
pincer Pincer may refer to:
* Pincers (tool)
*Pincer (biology), part of an animal
*Pincer ligand
In chemistry, a transition metal pincer complex is a type of coordination complex with a pincer ligand. Pincer ligands are chelating agents that binds tig ...
.
With the smaller head carapace complex and large surface of arthrodial membranes, frontal appendages of these taxa had greater flexibility.
Stout frontal appendages of sediment sifters like ''
Hurdia
''Hurdia'' is an extinct genus of hurdiid radiodont that lived 505 million years ago during the Cambrian Period. As a radiodont like ''Peytoia'' and ''Anomalocaris'', it is part of the ancestral lineage that led to euarthropods.
Description
' ...
'' and ''
Peytoia
''Peytoia'' is a genus of hurdiid radiodont that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and ''Peytoia infercambriensis'' from Poland, dating to Cambrian Stage 3. Its two front ...
'' have serrated endites with mesial curvature, which could form a basket-like trap for raking through sediment and passing food items towards the well-developed oral cone.
Endites of frontal appendages from suspension/filter feeders like ''
Tamisiocaris
''Tamisiocaris'' (from Latin ''tamisium'', sieve, and Greek ''karis'', crab, shrimp) is a radiodont genus initially only known from frontal appendages from the Buen Formation in Sirius Passet. Further specimens revealed that the frontal appenda ...
'' and ''
Aegirocassis
''Aegirocassis'' is an extinct genus of radiodont arthropod belonging to the family Hurdiidae that lived 480 million years ago during the early Ordovician. It is known by a single species, ''Aegirocassis benmoulai''. Van Roy initiated scientifi ...
'' have flexible, densely-packed auxiliary spines, which could filter out organic components such as
mesozooplankton and
phytoplankton down to 0.5mm.
Frontal appendages of ''
Caryosyntrips'', which are unusual for radiodonts in having the direction of endite-bearing surfaces opposing one another and may have been able to manipulate and crush prey in a scissor-like slicing or grasping motion.
Oral cones of radiodonts may have been used for suction and/or biting.
Together with the great variety of frontal appendages in different species of radiodonts, differentiation of oral cones between species suggests preferences of different diets as well.
For example, the triradial oral cone of ''
Anomalocaris
''Anomalocaris'' ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of ''Anomalocaris'' were discovered in the ''Ogygopsis'' Shale of the Stephen F ...
'' with irregular, tuberculated toothplates and a small opening may have been adapted to small and nektonic prey,
; while the rigid tetraradial oral cones of ''
Peytoia
''Peytoia'' is a genus of hurdiid radiodont that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and ''Peytoia infercambriensis'' from Poland, dating to Cambrian Stage 3. Its two front ...
'', ''
Titanokorys
''Titanokorys'' is a genus of extinct hurdiid radiodont (a grouping of primitive stem arthropods which lived during the early Paleozoic) that existed during the mid Cambrian. It is the largest member of its family from the Cambrian, with a b ...
'', ''
Hurdia
''Hurdia'' is an extinct genus of hurdiid radiodont that lived 505 million years ago during the Cambrian Period. As a radiodont like ''Peytoia'' and ''Anomalocaris'', it is part of the ancestral lineage that led to euarthropods.
Description
' ...
'', and one isolated oral cone attributed to ''
Cambroraster
''Cambroraster'' is an extinct monotypic genus of hurdiid radiodont, dating to the middle Cambrian, and represented by the single formally described species ''Cambroraster falcatus''. Hundreds of specimens were found in the Burgess Shale, and de ...
'' with a larger opening and sometimes additional tooth plates may have been capable to consume larger food items relative to their body size and probably benthic or endobenthic prey.
Classification
Taxonomic affinities
File:20220213 Opabiniidae Opabiniids.png, The opabiniids ''Opabinia
''Opabinia regalis'' is an extinct, stem group arthropod found in the Middle Cambrian Burgess Shale Lagerstätte (505 million years ago) of British Columbia. ''Opabinia'' was a soft-bodied animal, measuring up to 7 cm in body length, and it ...
'' (top) and ''Utaurora
''Utaurora'' is an extinct genus of opabiniid, which were bizarre stem-arthropods closely related to true arthropods and radiodonts; the type species is ''U. comosa''. The animal's fossils come from the Cambrian of Utah. This genus is so far ...
'' (bottom)
File:20210310 Kylinxia zhangi.png, The radiodont/opabiniid
Opabiniidae is an extinct family of bizarre marine stem- arthropods. Its type and best-known genus is ''Opabinia''. It also contains ''Utaurora''. Opabiniids closely resemble radiodonts, but their frontal appendages were basally fused into a pr ...
-euarthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, oft ...
intermediate ''Kylinxia
''Kylinxia'' is a genus of extinct arthropod described in 2020. It was described from six specimens discovered in Yu'anshan Formation ( Maotianshan Shales) in southern China. The specimens are assigned to one species ''Kylinxia zhangi.'' Dated t ...
''
File:20220218 Erratus sperare diagrammatic reconstruction.png, A diagrammatic reconstruction of the basal deuteropod ''Erratus
''Erratus'' is an extinct genus of marine arthropod from the Cambrian of China. Its type and only species is ''Erratus sperare''. ''Erratus'' is likely one of the most basal known arthropods, and its discovery has helped scientists understand the ...
''
File:20221119 Mieridduryn bonniae diagrammatic reconstruction.png, ''Mieridduryn
''Mieridduryn'' is a genus of extinct dinocaridid arthropod that lived during the Middle Ordovician of what is now the United Kingdom. This animal was described in 2022 based on a singular fossil found in Castle Bank, a Burgess shale type lag ...
'' is a panarthropod
Panarthropoda is a proposed animal clade containing the extant phyla Arthropoda, Tardigrada (water bears) and Onychophora (velvet worms). Panarthropods also include extinct marine legged worms known as lobopodians ("Lobopodia"), a paraphylet ...
from the Middle 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.
Th ...
that shares traits with both opabiniids and radiodonts
Most
phylogenetic
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
analyses suggest that radiodonts, alongside
opabiniids (''
Opabinia
''Opabinia regalis'' is an extinct, stem group arthropod found in the Middle Cambrian Burgess Shale Lagerstätte (505 million years ago) of British Columbia. ''Opabinia'' was a soft-bodied animal, measuring up to 7 cm in body length, and it ...
'' and ''
Utaurora
''Utaurora'' is an extinct genus of opabiniid, which were bizarre stem-arthropods closely related to true arthropods and radiodonts; the type species is ''U. comosa''. The animal's fossils come from the Cambrian of Utah. This genus is so far ...
''
), are
stem-group
In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...
arthropods just basal to
deuteropoda
Deuteropoda or "upper stem group arthropods" is a proposed clade of arthropods whose members are distinguished by an anatomical reorganization of the head region, namely the appearance of a differentiated first appendage pair (the ' deutocerebr ...
,
a clade including upper stem (e.g.
fuxianhuiids and bivalved arthropods) and crown
Euarthropoda
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, oft ...
(e.g.
Artiopoda
The Artiopoda is a grouping of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilo ...
,
Chelicerata
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 mi ...
and
Mandibulata
Mandibulata, termed "mandibulates", is a clade of arthropods that comprises the extant subphyla Myriapoda (millipedes and others), Crustacea and Hexapoda (insects and others). Mandibulata is currently believed to be the sister group of the clade ...
).
This interpretation is supported by numerous arthropod groundplan found on radiodonts and opabiniids, such as
stalked compound eye
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 disti ...
s,
digestive glands,
trunk appendages forming by dorsal and ventral elements (precursor of arthropod biramous appendages).
Compared to opabiniids, which possess posterior mouth opening and fused frontalmost appendages (comparable to euarthropod posterior-facing
labrum/hypostome complex),
radiodonts on the other hand featured euarthropod-like dorsal sclerite (H-element) and arthropodization (frontal appendages) on their head regions,
alongside
cuticularized gut termini.
The fact that both radiodonts and opabiniids lack exoskeleton on their trunk region suggests that the origin of compound eyes and arthropodization (segmented appendages) precede arthrodization (full set of trunk exoskeleton) in the arthropod stem lineage.
The constricted neck region with feeding appendicular structures of some radiodont may also shed light on the origin of the sophisticated arthropod head, which was formed by the fusion of multiple anterior body segments.
Basal deuteropods that possess a mixture of radiodont/opabiniid characters like ''
Kylinxia
''Kylinxia'' is a genus of extinct arthropod described in 2020. It was described from six specimens discovered in Yu'anshan Formation ( Maotianshan Shales) in southern China. The specimens are assigned to one species ''Kylinxia zhangi.'' Dated t ...
'' and ''
Erratus
''Erratus'' is an extinct genus of marine arthropod from the Cambrian of China. Its type and only species is ''Erratus sperare''. ''Erratus'' is likely one of the most basal known arthropods, and its discovery has helped scientists understand the ...
'', may represent intermediate forms between radiodonts, opabiniids and other euarthropods.
File:20191217 Siberiida Siberion Megadictyon Jianshanopodia.png, The siberiid lobopodians '' Siberion'' (upper left), '' Megadictyon'' (bottom center) and ''Jianshanopodia
''Jianshanopodia decora'' is a Cambrian lobopodian. Its frontal, grasping appendages bear wedge-shaped plates. Its limbs branch, instead of being tipped with claws as many lobopods' are. It has a sediment-filled gut surrounded by serially repea ...
'' (upper right)
File:20210730 Gilled lobopodians Pambdelurion Kerygmachela.png, The ' gilled lobopodians' ''Pambdelurion
''Pambdelurion'' is an extinct genus of panarthropod from the Cambrian aged Sirius Passet site in northern Greenland. Like the morphologically similar ''Kerygmachela'' from the same locality, ''Pambdelurion'' is thought to be closely related to ...
'' (left) and ''Kerygmachela
''Kerygmachela kierkegaardi'' is a gilled lobopodian from the Cambrian Stage 3 aged Sirius Passet Lagerstätte in northern Greenland. Its anatomy strongly suggests that it, along with its relative '' Pambdelurion whittingtoni'', was a close rela ...
'' (right)
File:20191028 Megacheirans Leanchoilia Haikoucaris Yohoia Fortiforceps.png, 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 ...
or 'great appendage arthropods', a 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 ...
of possible stem-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 ...
previously thought to be radiodont's close relative
Taxa just basal to the radiodont, opabiniid and euarthropod branch are '
gilled lobopodians' like ''
Pambdelurion
''Pambdelurion'' is an extinct genus of panarthropod from the Cambrian aged Sirius Passet site in northern Greenland. Like the morphologically similar ''Kerygmachela'' from the same locality, ''Pambdelurion'' is thought to be closely related to ...
'' and ''
Kerygmachela
''Kerygmachela kierkegaardi'' is a gilled lobopodian from the Cambrian Stage 3 aged Sirius Passet Lagerstätte in northern Greenland. Its anatomy strongly suggests that it, along with its relative '' Pambdelurion whittingtoni'', was a close rela ...
'', which occasionally united under the class
Dinocaridida
DinocarididaGreek, "Terrible crabs" – sometimes informally spelt Dinocarida, but the second 'id' is linguistically correct – see is a proposed fossil taxon of basal arthropods that flourished in the Cambrian period with occasional Ordovici ...
with opabibiids and radiodonts.
They have body flaps, digestive glands, large (presumely compound) eyes and specialized frontal appendages like the former taxa, but their frontal appendages are not arthropodized nor fused, eyes sessile, gill-like structures less prominent, and certainlly bore lobopod underneath each of their flaps.
Taxa even basal to 'gilled lobopodians' are siberiids like ''
Megadictyon'' and ''
Jianshanopodia
''Jianshanopodia decora'' is a Cambrian lobopodian. Its frontal, grasping appendages bear wedge-shaped plates. Its limbs branch, instead of being tipped with claws as many lobopods' are. It has a sediment-filled gut surrounded by serially repea ...
'',
a group of
lobopodia
The lobopodians, members of the informal group Lobopodia (from the Greek, meaning "blunt feet"), or the formally erected phylum Lobopoda Cavalier-Smith (1998), are panarthropods with stubby legs called lobopods, a term which may also be used as ...
ns bore robust frontal appendages and digestive glands, but no body flaps. Such intermediate forms between lobopodian and radiodont/euarthropod suggest that the total-group Arthropoda arose from a
paraphyletic lobopodian grade, alongside the other two extant
panarthropod
Panarthropoda is a proposed animal clade containing the extant phyla Arthropoda, Tardigrada (water bears) and Onychophora (velvet worms). Panarthropods also include extinct marine legged worms known as lobopodians ("Lobopodia"), a paraphylet ...
phyla
Tardigrada
Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbä ...
and
Onychophora
Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...
.
Previous studies may suggest radiodonts as a group other than stem-arthropods, such as a hitherto unknown
phylum;
cycloneuralia
Cycloneuralia is a clade of ecdysozoan animals including the Scalidophora (Kinorhynchans, Loriciferans, Priapulids) and the Nematoida ( nematodes, Nematomorphs). It may be paraphyletic, or may be a sister group to Panarthropoda. Or perhaps P ...
n worms undergone
convergent with arthropods (based on the cycloneuralian-like radial mouthparts);
stem
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 ...
euarthropods alongside
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 also known as great appendage arthropods (based on the similarity between radiodont frontal appendages, megacheiran great appendages and
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 similarl ...
); or ''
Schinderhannes bartelsi
''Schinderhannes bartelsi'' is a species of hurdiid radiodont (anomalocaridid) known from one specimen from the lower Devonian Hunsrück Slates. Its discovery was astonishing because previously, radiodonts were known only from exceptionally well ...
,'' which resolved as a
hurdiid
Hurdiidae is an extinct cosmopolitan family of radiodonts, a group of stem-group arthropods, which lived during the Paleozoic Era. It is the most long-lived radiodont clade, lasting from the Cambrian period to the Devonian period.
Descripti ...
radiodont in recent analyses,
as a species more closely related to euarthropods than other radiodonts (based on some putative euarthropod-like features found on ''Schinderhannes'').
However, neither each of them were supported by later investigations. The radial mouthparts are not cycloneuralian-exclusive and more likely present result of convergent evolution or
ecdysozoan
plesimorphy, since they also have been found in
panarthropods
Panarthropoda is a proposed animal clade containing the extant phyla Arthropoda, Tardigrada (water bears) and Onychophora (velvet worms). Panarthropods also include extinct marine legged worms known as lobopodians ("Lobopodia"), a paraphyletic gr ...
such as
tardigrade
Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbä ...
and some
lobopodia
The lobopodians, members of the informal group Lobopodia (from the Greek, meaning "blunt feet"), or the formally erected phylum Lobopoda Cavalier-Smith (1998), are panarthropods with stubby legs called lobopods, a term which may also be used as ...
ns; radiodonts lacking definitive euarthropod features such as trunk
tergites
A ''tergum'' (Latin for "the back"; plural ''terga'', associated adjective tergal) is the dorsal ('upper') portion of an arthropod segment other than the head. The anterior edge is called the 'base' and posterior edge is called the 'apex' or 'mar ...
and multiple head appendages,
and the megacheiran great appendages were considered to be deutocerebral, which could be non-homologous to the radiodont protocerebral frontal appendages;
putative euarthropod characters found on the single ''Schinderhannes'' fossil is questionable and may present other radiodont-like structures.
Interrelationships
Traditionally, all radiodont species have been placed within one family,
Anomalocarididae
Anomalocarididae (occasionally mis-spelt Anomalocaridae) is an extinct family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpo ...
,
hence the previous common name 'anomalocaridid'
and it was still occasionally used to refer the whole order even after reclassification.
Since the reassignment done by Vinther ''et al.'' 2014, most of the radiodont species were reclassified within three new families:
Amplectobeluidae
''Amplectobeluidae'' is a clade of Cambrian Radiodonta, radiodonts.
Definition
In 2014, ''Amplectobeluidae'' was defined as the most inclusive clade including ''Amplectobelua, Amplectobelua symbrachiata'' but not ''Anomalocaris, Anomalocaris ca ...
,
Tamisiocarididae
Tamisiocarididae is a family of radiodonts, extinct marine animals related to arthropods, that bore finely-spined appendages that were presumably used in filter-feeding. When first discovered, the clade was named Cetiocaridae after a speculative ...
(formerly
Cetiocaridae), and
Hurdiidae
Hurdiidae is an extinct cosmopolitan family of radiodonts, a group of stem-group arthropods, which lived during the Paleozoic Era. It is the most long-lived radiodont clade, lasting from the Cambrian period to the Devonian period.
Description ...
.
Including Anomalocarididae, the four recent radiodont families may form the clade
Anomalocarida
Radiodonta is an extinct Order (biology), order of stem-group arthropods that was successful worldwide during the Cambrian period. They may be referred to as radiodonts, radiodontans, radiodontids, anomalocarids, or anomalocaridids, although the ...
.
The original description of the order Radiodonta included ''
Anomalocaris
''Anomalocaris'' ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of ''Anomalocaris'' were discovered in the ''Ogygopsis'' Shale of the Stephen F ...
'', ''
Laggania
''Peytoia'' is a genus of hurdiid radiodont that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and '' Peytoia infercambriensis'' from Poland, dating to Cambrian Stage 3. Its two fro ...
'' (later known as ''
Peytoia
''Peytoia'' is a genus of hurdiid radiodont that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and ''Peytoia infercambriensis'' from Poland, dating to Cambrian Stage 3. Its two front ...
''), ''
Hurdia
''Hurdia'' is an extinct genus of hurdiid radiodont that lived 505 million years ago during the Cambrian Period. As a radiodont like ''Peytoia'' and ''Anomalocaris'', it is part of the ancestral lineage that led to euarthropods.
Description
' ...
'', ''
Proboscicaris'', ''
Amplectobelua
''Amplectobelua'' (meaning "embracing beast") is an extinct genus of late Early Cambrian amplectobeluid radiodont, a group of stem arthropods that mostly lived as free-swimming predators during the first half of the Paleozoic Era.
Anatomy
...
'', ''
Cucumericrus
''Cucumericrus decoratus'' is a species of putative radiodont known from a few poorly preserved specimens. Only fragments of trunk cuticle and corresponded appendages had been revealed, while important radiodont features such as frontal appendag ...
'', and ''
Parapeytoia''.
However, ''
Proboscicaris'' is now regarded as a junior synonym of ''
Hurdia
''Hurdia'' is an extinct genus of hurdiid radiodont that lived 505 million years ago during the Cambrian Period. As a radiodont like ''Peytoia'' and ''Anomalocaris'', it is part of the ancestral lineage that led to euarthropods.
Description
' ...
'', and ''
Parapeytoia'' is considered to be a
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 ...
n.
Due to the limited discovery, The position of ''
Cucumericrus
''Cucumericrus decoratus'' is a species of putative radiodont known from a few poorly preserved specimens. Only fragments of trunk cuticle and corresponded appendages had been revealed, while important radiodont features such as frontal appendag ...
'' within Radiodonta is unclear, as it was either unselected by
phylogenetic
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
analysis
or resolved in a polytomy with Radiodonta and
Euarthropoda
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, oft ...
.
File:20210207 Cucumericrus decoratus trunk appendage.png, One of the poorly-known body parts (trunk appendage) of ''Cucumericrus
''Cucumericrus decoratus'' is a species of putative radiodont known from a few poorly preserved specimens. Only fragments of trunk cuticle and corresponded appendages had been revealed, while important radiodont features such as frontal appendag ...
decoratus''; this species may not represent a true radiodont.
File:20191228 Radiodonta frontal appendage Anomalocaris briggsi.png, Frontal appendage of ''"Anomalocaris" briggsi''. Despite the suggested generic name, this radiodont is unlikely to be a species of genus ''Anomalocaris
''Anomalocaris'' ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of ''Anomalocaris'' were discovered in the ''Ogygopsis'' Shale of the Stephen F ...
'' based on phylogenetic analysis.
The first in-depth phylogenetic analysis of Radiodonta was conducted by Vinther ''et al''. in 2014,
followed by a handful of subsequest studies with more or less modified results.
In most analysis, ''
Caryosyntrips'' is the basal-most genus, but resolved in a polytomy with other radiodonts and Euarthropoda (alongside ''
Cucumericrus
''Cucumericrus decoratus'' is a species of putative radiodont known from a few poorly preserved specimens. Only fragments of trunk cuticle and corresponded appendages had been revealed, while important radiodont features such as frontal appendag ...
'' if included
). With the exclusion of ''Caryosyntrips'' and ''Cucumericrus'', the
monophyly
In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...
of Radiodonta is widely supported,
with a few results suggest possible
paraphyly (either the Anomalocarididae+Amplectobeluidae clade or Hurdiidae
sister
A sister is a woman or a girl who shares one or more parents with another individual; a female sibling. The male counterpart is a brother. Although the term typically refers to a family, familial relationship, it is sometimes used endearingly to r ...
to Euarthropoda).
Putative
synapomorphies of monophyletic Radiodonta including tripartite head sclerite complex and differentiated neck region.
The genus ''
Anomalocaris
''Anomalocaris'' ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of ''Anomalocaris'' were discovered in the ''Ogygopsis'' Shale of the Stephen F ...
'' in a broader sense always found to be
polyphyletic
A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...
, usually with ''"Anomalocaris" kunmingensis'' and ''"Anomalocaris" briggsi'' resolved as a member of
Amplectobeluidae
''Amplectobeluidae'' is a clade of Cambrian Radiodonta, radiodonts.
Definition
In 2014, ''Amplectobeluidae'' was defined as the most inclusive clade including ''Amplectobelua, Amplectobelua symbrachiata'' but not ''Anomalocaris, Anomalocaris ca ...
and
Tamisiocarididae
Tamisiocarididae is a family of radiodonts, extinct marine animals related to arthropods, that bore finely-spined appendages that were presumably used in filter-feeding. When first discovered, the clade was named Cetiocaridae after a speculative ...
respectively.
Interrelationship of Amplectobeluidae is uncertain, as the amplectobeluid affinities of ''
Lyrarapax
'' Lyrarapax'' is a radiodont genus of the family Amplectobeluidae that lived in the early Cambrian period 520 million years ago. Its neural tissue indicates that the radiodont frontal appendage is protocerebral, resolving parts of the arthropod ...
'' and ''
Ramskoeldia
''Ramskoeldia'' is a genus of amplectobeluid radiodont described in 2018. It was the second genus of radiodont found to possess gnathobase-like structures (abbreviated as GLS) and an atypical oral cone after '' Amplectobelua''. It was discovered ...
'' were occasionally questioned.
Monophyly of the speciose family
Hurdiidae
Hurdiidae is an extinct cosmopolitan family of radiodonts, a group of stem-group arthropods, which lived during the Paleozoic Era. It is the most long-lived radiodont clade, lasting from the Cambrian period to the Devonian period.
Description ...
was recovered by most analysis and well-supported by several synapomorphies (e.g. distal articulated region of frontal appendage with proximal 5 podomeres bearing subequal endites
), with Tamisiocarididae often suggested to be its sister group.
* Radiodonta
** ?''
Cucumericrus
''Cucumericrus decoratus'' is a species of putative radiodont known from a few poorly preserved specimens. Only fragments of trunk cuticle and corresponded appendages had been revealed, while important radiodont features such as frontal appendag ...
''
(putative radiodont
)
** ''
Caryosyntrips''
**
Anomalocarida
Radiodonta is an extinct Order (biology), order of stem-group arthropods that was successful worldwide during the Cambrian period. They may be referred to as radiodonts, radiodontans, radiodontids, anomalocarids, or anomalocaridids, although the ...
*** ''
Paranomalocaris''
(placed within Anomalocarididae by some studies.
)
*** ''
Laminacaris
''Laminacaris'' is a genus of extinct stem-group arthropods ( Radiodonta) that lived during the Cambrian period. It is monotypic with a single species ''Laminacaris chimera'', the fossil of which was described from the Chengjian biota of China ...
''
(placed within Amplectobeluidae by some studies.
)
*** ''
Houcaris
''Houcaris'' is a genus of tamisiocarididid radiodonts known from Cambrian Series 2 of China and the United States. It contains two species, ''Houcaris saron'' and ''Houcaris magnabasis'', both of which were originally named as species of the re ...
'' (either placed within Anomalocarididae,
Amplectobeluidae
or Tamisiocarididae.
)
*** ''
Innovatiocaris
''Innovatiocaris'' (meaning "innovation crab") is a genus of radiodont of uncertain family from the early Cambrian Chengjiang Lagerstätte of Yunnan Province, China. The genus contains two named species, ''I. maotianshanensis'', known from a near ...
''
***
Anomalocarididae
Anomalocarididae (occasionally mis-spelt Anomalocaridae) is an extinct family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpo ...
**** ''
Anomalocaris
''Anomalocaris'' ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of ''Anomalocaris'' were discovered in the ''Ogygopsis'' Shale of the Stephen F ...
'' (in a broader sense, some species may placed within the other families.
)
**** ''
Lenisicaris''
***
Amplectobeluidae
''Amplectobeluidae'' is a clade of Cambrian Radiodonta, radiodonts.
Definition
In 2014, ''Amplectobeluidae'' was defined as the most inclusive clade including ''Amplectobelua, Amplectobelua symbrachiata'' but not ''Anomalocaris, Anomalocaris ca ...
**** ''
Lyrarapax
'' Lyrarapax'' is a radiodont genus of the family Amplectobeluidae that lived in the early Cambrian period 520 million years ago. Its neural tissue indicates that the radiodont frontal appendage is protocerebral, resolving parts of the arthropod ...
''
(position questioned by some studies.
)
**** ''
Amplectobelua
''Amplectobelua'' (meaning "embracing beast") is an extinct genus of late Early Cambrian amplectobeluid radiodont, a group of stem arthropods that mostly lived as free-swimming predators during the first half of the Paleozoic Era.
Anatomy
...
''
**** ''
Ramskoeldia
''Ramskoeldia'' is a genus of amplectobeluid radiodont described in 2018. It was the second genus of radiodont found to possess gnathobase-like structures (abbreviated as GLS) and an atypical oral cone after '' Amplectobelua''. It was discovered ...
''
(position questioned by some studies.
)
**** ''"Anomalocaris" kunmingensis'' (non-''Anomalocaris'' species await to be renamed.
Labeled as ''Amplectobelua kunmingensis'' by Vinther et al. 2014.
)
***
Tamisiocarididae
Tamisiocarididae is a family of radiodonts, extinct marine animals related to arthropods, that bore finely-spined appendages that were presumably used in filter-feeding. When first discovered, the clade was named Cetiocaridae after a speculative ...
**** ''
Tamisiocaris
''Tamisiocaris'' (from Latin ''tamisium'', sieve, and Greek ''karis'', crab, shrimp) is a radiodont genus initially only known from frontal appendages from the Buen Formation in Sirius Passet. Further specimens revealed that the frontal appenda ...
''
**** ''"Anomalocaris" briggsi'' (non-''Anomalocaris'' species await to be renamed.
)
***
Hurdiidae
Hurdiidae is an extinct cosmopolitan family of radiodonts, a group of stem-group arthropods, which lived during the Paleozoic Era. It is the most long-lived radiodont clade, lasting from the Cambrian period to the Devonian period.
Description ...
****''
Peytoia
''Peytoia'' is a genus of hurdiid radiodont that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and ''Peytoia infercambriensis'' from Poland, dating to Cambrian Stage 3. Its two front ...
''
**** ''
Schinderhannes
Johannes Bückler (c.1778 – 21 November 1803) was a German outlaw who orchestrated one of the most famous crime sprees in German history. He has been nicknamed Schinderhannes and Schinnerhannes in German and John the Scorcher, John the Flaye ...
''
**** ''
Hurdia
''Hurdia'' is an extinct genus of hurdiid radiodont that lived 505 million years ago during the Cambrian Period. As a radiodont like ''Peytoia'' and ''Anomalocaris'', it is part of the ancestral lineage that led to euarthropods.
Description
' ...
''
**** ''
Aegirocassis
''Aegirocassis'' is an extinct genus of radiodont arthropod belonging to the family Hurdiidae that lived 480 million years ago during the early Ordovician. It is known by a single species, ''Aegirocassis benmoulai''. Van Roy initiated scientifi ...
''
**** ''
Stanleycaris
''Stanleycaris'' is an extinct, monotypic genus of hurdiid radiodont from the middle Cambrian (Miaolingian). The type species is ''Stanleycaris hirpex''. ''Stanleycaris'' was described from the Stephen Formation near the Stanley Glacier and Bu ...
''
**** ''
Pahvantia''
**** ''
Ursulinacaris
''Ursulinacaris'' is a genus of hurdiid radiodont from the Cambrian of North America. It contains one known species, ''Ursulinacaris grallae''. It was described in 2019, based on fossils of the frontal appendages discovered in the 1990s and the ...
''
**** ''
Cambroraster
''Cambroraster'' is an extinct monotypic genus of hurdiid radiodont, dating to the middle Cambrian, and represented by the single formally described species ''Cambroraster falcatus''. Hundreds of specimens were found in the Burgess Shale, and de ...
''
**** ?''
Zhenghecaris'' (putative radiodont
)
**** ''
Cordaticaris''
**** ''
Buccaspinea''
**** ''
Titanokorys
''Titanokorys'' is a genus of extinct hurdiid radiodont (a grouping of primitive stem arthropods which lived during the early Paleozoic) that existed during the mid Cambrian. It is the largest member of its family from the Cambrian, with a b ...
''
History
File:Anomalocaris canadensis grasping claw, Burgess Shale.jpg, Frontal appendage of ''Anomalocaris canadensis
''Anomalocaris'' ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of ''Anomalocaris'' were discovered in the ''Ogygopsis'' Shale of the Stephen F ...
''
File:Peytoia nathorsti Laggania cambria oral cone, Burgess Shale.jpg, Oral cone of ''Peytoia nathorsti
''Peytoia'' is a genus of hurdiid radiodont that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and ''Peytoia infercambriensis'' from Poland, dating to Cambrian Stage 3. Its two front ...
''
File:Hurdia victoria USNM PAL 57718.jpg, H-element of ''Hurdia
''Hurdia'' is an extinct genus of hurdiid radiodont that lived 505 million years ago during the Cambrian Period. As a radiodont like ''Peytoia'' and ''Anomalocaris'', it is part of the ancestral lineage that led to euarthropods.
Description
' ...
victoria''
File:USNM PAL 57490.jpg, Paired frontal appendages from an unnamed hurdiid
Hurdiidae is an extinct cosmopolitan family of radiodonts, a group of stem-group arthropods, which lived during the Paleozoic Era. It is the most long-lived radiodont clade, lasting from the Cambrian period to the Devonian period.
Descripti ...
radiodont
The history of radiodonts is complex. Incomplete specimens pertaining to different body parts of the same species had historically been interpreted as belonging to different species and even different phyla.
[ Prior to their recognition as a group, radiodont specimens had been assigned to five different phyla: Porifera, ]Cnidaria
Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.
Their distinguishing feature is cnidocytes, specialized cells that ...
, Echinodermata
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...
, Annelida, and Arthropoda
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, ...
.[
The first known radiodont specimens were collected from the trilobite beds of ]Mount Stephen
Mount Stephen, , is a mountain located in the Kicking Horse River Valley of Yoho National Park, km east of Field, British Columbia, Canada. The mountain was named in 1886 for George Stephen, the first president of the Canadian Pacific Railw ...
by Richard G. McConnell of the Geological Survey of Canada
The Geological Survey of Canada (GSC; french: Commission géologique du Canada (CGC)) is a Canadian federal government agency responsible for performing geological surveys of the country, developing Canada's natural resources and protecting the e ...
in 1886[ or 1888.][ These specimens were named '']Anomalocaris canadensis
''Anomalocaris'' ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of ''Anomalocaris'' were discovered in the ''Ogygopsis'' Shale of the Stephen F ...
'' in 1892 by GSC paleontologist Joseph Whiteaves.[ Whiteaves interpreted the specimens, now known to be isolated frontal appendages, as the abdomen of a ]phyllocarid
Phyllocarida is a subclass of crustacean
Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipe ...
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 can ...
. Additional radiodont specimens were described in 1911 by Charles Walcott.[Walcott, C. D. 1911a]
Middle Cambrian holothurians and medusae
Cambrian geoogy and paleontology II. Smithsonian Miscellaneous Collections, 57: 41-68. He interpreted an isolated oral cone, which he named ''Peytoia nathorsti
''Peytoia'' is a genus of hurdiid radiodont that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and ''Peytoia infercambriensis'' from Poland, dating to Cambrian Stage 3. Its two front ...
'', as a jellyfish, and a poorly-preserved but relatively complete specimen, which he named ''Laggania cambria'', as a holothurian. In 1912 Walcott named ''Hurdia
''Hurdia'' is an extinct genus of hurdiid radiodont that lived 505 million years ago during the Cambrian Period. As a radiodont like ''Peytoia'' and ''Anomalocaris'', it is part of the ancestral lineage that led to euarthropods.
Description
' ...
victoria'' and ''H. triangulata'' based on isolated H-elements, which he interpreted as the carapaces of crustaceans.[WALCOTT, C. D. 1912]
Middle Cambrian Branchiopoda, Malacostraca, Trilobita and Merostomata
Smithsonian Miscellaneous Collections, 57: 145–228. Isolated frontal appendages of ''Peytoia'' and ''Hurdia'', collectively known as "Appendage F" in Briggs 1979, were all identified as those of ''Sidneyia
''Sidneyia'' is an extinct arthropod known from fossils found from the Early Cambrian-age Maotianshan Shales to the Mid Cambrian Burgess Shale formation of British Columbia. 144 specimens of ''Sidneyia'' are known from the Greater Phyllopod be ...
'' at that time. A ''Hurdia'' P-element was named ''Proboscicaris'' in 1962, and interpreted as the carapace of a bivalved arthropod.
The Geological Survey of Canada initiated a revision of Burgess Shale fossils in 1966, overseen by Cambridge University
The University of Cambridge is a Public university, public collegiate university, collegiate research university in Cambridge, England. Founded in 1209 and granted a royal charter by Henry III of England, Henry III in 1231, Cambridge is the world' ...
paleontologist Harry B. Whittington
Harry Blackmore Whittington FRS (24 March 1916 – 20 June 2010) was a British palaeontologist who made a major contribution to the study of fossils of the Burgess Shale and other Cambrian fauna. His works are largely responsible for the conce ...
.[ This revision would ultimately lead to the discovery of the complete radiodont body plan. In 1978, ]Simon Conway Morris
Simon Conway Morris (born 1951) is an English palaeontologist, evolutionary biologist, and astrobiologist known for his study of the fossils of the Burgess Shale and the Cambrian explosion. The results of these discoveries were celebrated in ...
recognized that the mouthparts of ''Laggania'' were ''Peytoia''-like, but he interpreted this as evidence that it was a composite fossil made up of a ''Peytoia'' jellyfish and a sponge.[ In 1979, ]Derek Briggs
Derek Ernest Gilmor Briggs (born 10 January 1950) is an Irish palaeontologist and taphonomist based at Yale University. Briggs is one of three palaeontologists, along with Harry Blackmore Whittington and Simon Conway Morris, who were key in t ...
recognized that the fossils of ''Anomalocaris'' were appendages, not abdomens, but interpreted them as walking legs alongside "Appendage F".[ It was not until 1985 that the true nature of the fossils of ''Anomalocaris'', ''Laggania'', and ''Peytoia'' was recognized, and they were all assigned to a single genus, ''Anomalocaris''.] Subsequently, it was recognized that ''Anomalocaris'' was a distinct form from the other two, resulting in a split into two genera, the latter of which was variously named ''Laggania'' and ''Peytoia'' until it was determined that both represent the same species and ''Peytoia'' had priority. It was later recognized that some of the fossils assigned to these taxa belonged to another form, which was recognized as bearing a carapace made up of ''Hurdia'' and ''Proboscicaris'' elements. Finally, in 2009, these specimens were redescribed as ''Hurdia''. Even after these recognitions, partial misidentifications (e.g. oral cone and frontal appendages of ''Peytoia'' had been assigned to ''Anomalocaris'' and ''Hurdia'', respectively) had been revealed by subsequent studies as well.
The taxon Radiodonta itself was coined in 1996 by Desmond Collins, after it was established that ''Anomalocaris'' and its kin represented a distinctive lineage with arthropod affinities rather than a hitherto unknown phylum.[ Collins also established the class ]Dinocarida
DinocarididaGreek, "Terrible crabs" – sometimes informally spelt Dinocarida, but the second 'id' is linguistically correct – see is a proposed fossil taxon of basal arthropods that flourished in the Cambrian period with occasional Ordovici ...
to contain the order Radiodonta as well as the Opabiniidae
Opabiniidae is an extinct family of bizarre marine stem-arthropods. Its type and best-known genus is '' Opabinia''. It also contains '' Utaurora''. Opabiniids closely resemble radiodonts, but their frontal appendages were basally fused into a ...
, which he recognized as distinct due to its lacking the distinctive oral cone structure of radiodonts. Radiodonta was first given a phylogenetic definition in 2014. Radiodonta was originally viewed as containing a single family, Anomalocarididae
Anomalocarididae (occasionally mis-spelt Anomalocaridae) is an extinct family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpo ...
, but it was divided into four families in 2014: Amplectobeluidae
''Amplectobeluidae'' is a clade of Cambrian Radiodonta, radiodonts.
Definition
In 2014, ''Amplectobeluidae'' was defined as the most inclusive clade including ''Amplectobelua, Amplectobelua symbrachiata'' but not ''Anomalocaris, Anomalocaris ca ...
, Anomalocarididae, Cetiocaridae, and Hurdiidae
Hurdiidae is an extinct cosmopolitan family of radiodonts, a group of stem-group arthropods, which lived during the Paleozoic Era. It is the most long-lived radiodont clade, lasting from the Cambrian period to the Devonian period.
Description ...
. The name Cetiocaridae did not conform to the International Code of Zoological Nomenclature and so was renamed Tamisiocarididae
Tamisiocarididae is a family of radiodonts, extinct marine animals related to arthropods, that bore finely-spined appendages that were presumably used in filter-feeding. When first discovered, the clade was named Cetiocaridae after a speculative ...
in 2019.[
Until the ]2010s
File:2010s collage v21.png, From top left, clockwise: Anti-government protests called the Arab Spring arose in 2010–2011, and as a result, many governments were overthrown, including when Libyan dictator Muammar Gaddafi was killed; Crimea is ...
, radiodonts were typically considered to be uniformly large apex predators, but discoveries of new species over the course of that decade led to a considerable increase in the known ecological and morphological diversity of the group.
References
{{Taxonbar, from=Q3291852
*
Cambrian Series 2 first appearances
Dinocarida
Early Devonian extinctions
Prehistoric arthropod orders