''Plesiosaurus'' (Greek: ' ('), near to + ' ('), lizard) is a genus of extinct, large marine

sauropterygia Sauropterygia ("lizard flippers") is an extinct taxon of diverse, aquatic reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Triassic before all except for the Plesiosauria became ...

n reptile that lived during the

Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-J ...

. It is known by nearly complete skeletons from the

Lias Lias may refer to: Geology * Lias Formation, a geologic formation in France *Lias Group, a lithostratigraphic unit in western Europe * Early Jurassic, an epoch People * Godfrey Lias, British author * Mohd Shamsudin Lias (born 1953), Malaysian ...

of England. It is distinguishable by its small head, long and slender neck, broad turtle-like body, a short tail, and two pairs of large, elongated paddles. It lends its name to the order

Plesiosauria The Plesiosauria (; Greek: πλησίος, ''plesios'', meaning "near to" and ''sauros'', meaning "lizard") or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared ...

, of which it is an early, but fairly typical member. It contains only one species, the type, ''Plesiosaurus dolichodeirus''. Other species once assigned to this genus, including ''P. brachypterygius'', ''P. guilielmiimperatoris'', and ''P. tournemirensis'' have been reassigned to new genera, such as '' Hydrorion'', '' Seeleyosaurus'' and '' Occitanosaurus''.

Discovery

The first complete skeleton of ''Plesiosaurus'' was discovered by early paleontologist and fossil hunter

Mary Anning Mary Anning (21 May 1799 – 9 March 1847) was an English fossil collector, dealer, and palaeontologist who became known around the world for the discoveries she made in Jurassic marine fossil beds in the cliffs along the English Channel ...

Sinemurian In the geologic timescale, the Sinemurian is an age and stage in the Early or Lower Jurassic Epoch or Series. It spans the time between 199.3 ± 2 Ma and 190.8 ± 1.5 Ma (million years ago). The Sinemurian is preceded by the Hettangian and ...

(

)-age rocks of the lower

Lias Group The Lias Group or Lias is a lithostratigraphic unit (a sequence of rock strata) found in a large area of western Europe, including the British Isles, the North Sea, the Low Countries and the north of Germany. It consists of marine limestones, sh ...

in December 1823.Storrs 1997 pp. 146 Additional fossils of ''Plesiosaurus'' were found in rocks of the Lias Group of Dorset for many years, "until the cessation of quarrying activities in the Lias Group, early in this

0th 0th or zeroth may refer to: Mathematics, science and technology * 0th or zeroth, an ordinal for the number zero * 0th dimension, a topological space * 0th element, of a data structure in computer science * Zeroth (software), deep learning softw ...

century." although less complete remains were used by

Henry De la Beche Sir Henry Thomas De la Beche KCB, FRS (10 February 179613 April 1855) was an English geologist and palaeontologist, the first director of the Geological Survey of Great Britain, who helped pioneer early geological survey methods. He was the ...

and William Conybeare to name the species two years earlier in 1821,De la Beche, H. T. & W. D. Conybeare. (1821). Notice of the discovery of a new fossil animal, forming a link between the ''

Ichthyosaurus ''Ichthyosaurus'' (derived from Greek ' () meaning 'fish' and ' () meaning 'lizard') is a genus of ichthyosaurs from the Early Jurassic (Hettangian - Pliensbachian), with possible Late Triassic record, from Europe ( Belgium, England, Germany, ...

'' and crocodile, together with general remarks on the osteology of the ''Ichthyosaurus''. ''Transactions of the Geological Society of London'' 5: 559–594 and despite being discovered first, Conybeare's remains were not the holotype; Anning's were. ''Plesiosaurus'' was one of the first of the "

antediluvian The antediluvian (alternatively pre-diluvian or pre-flood) period is the time period chronicled in the Bible between the fall of man and the Genesis flood narrative in biblical cosmology. The term was coined by Thomas Browne. The narrative takes ...

reptiles" to be discovered and excited great interest in 19th-century England. It was so-named ("near lizard") by William Conybeare and

, to indicate that it was more like a normal reptile than ''

'', which had been found in the same rock strata just a few years earlier. ''Plesiosaurus'' is the archetypical genus of Plesiosauria and the first to be described, hence lending its name to the order. Conybeare and De la Beche coined the name for scattered finds from the Bristol region,

Dorset Dorset ( ; archaically: Dorsetshire , ) is a county in South West England on the English Channel coast. The ceremonial county comprises the unitary authority areas of Bournemouth, Christchurch and Poole and Dorset. Covering an area of , ...

, and

Lyme Regis Lyme Regis is a town in west Dorset, England, west of Dorchester and east of Exeter. Sometimes dubbed the "Pearl of Dorset", it lies by the English Channel at the Dorset–Devon border. It has noted fossils in cliffs and beaches on the Heri ...

in 1821. The

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specime ...

of ''Plesiosaurus'', ''P. dolichodeirus'', was named and described by Conybeare in 1824 on the basis of Anning's original finds.

Description

Skull and dentition

Compared to other plesiosaur genera, ''Plesiosaurus'' has a small head. The skull is much narrower than long,Storrs 1997 pp. 166 reaching its greatest width just behind the eyes (the

postorbital The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some ...

bar).Storrs 1997 pp. 165 The anterior portion is "bluntly triangular". In lateral view, the skull reaches its highest point at the rear of the skull table.Storrs 1997 pp. 167 "The external nostrils overlie the internal nares". They are not positioned at the tip of the snout, but farther back, nearer the eyes than the tip of the skull. Unlike the nostrils of ''

Rhomaleosaurus ''Rhomaleosaurus'' (meaning "strong lizard") is an extinct genus of Early Jurassic ( Toarcian age, about 183 to 175.6 million years ago) rhomaleosaurid pliosauroid known from Northamptonshire and from Yorkshire of the United Kingdom. It was fi ...

'', they do not appear to be adapted for underwater olfaction. The

orbits In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

(eye sockets) are roughly circular and are positioned about halfway along the length of the skull. They face up and to the sides. Just posterior to the orbits are the supratemporal fenestrae, which are about the same size as the orbits and also roughly circular. Between the four openings is the

pineal foramen A parietal eye, also known as a third eye or pineal eye, is a part of the epithalamus present in some vertebrates. The eye is located at the top of the head, is photoreceptive and is associated with the pineal gland, regulating circadian rhyth ...

, and between the temporal fenestrae is a narrow sagittal ridge. As in other plesiosaurs, the

pterygoid Pterygoid, from the Greek for 'winglike', may refer to: * Pterygoid bone, a bone of the palate of many vertebrates * Pterygoid processes of the sphenoid bone ** Lateral pterygoid plate ** Medial pterygoid plate * Lateral pterygoid muscle * Medi ...

s of the palate are fused to the

basioccipital The basilar part of the occipital bone (also basioccipital) extends forward and upward from the foramen magnum, and presents in front an area more or less quadrilateral in outline. In the young skull this area is rough and uneven, and is joined t ...

of the braincase, although the union is not as robust as in the pliosaurs ''Rhomaleosaurus'' and ''

Pliosaurus ''Pliosaurus'' (meaning 'more lizard') is an extinct genus of thalassophonean pliosaurid known from the Kimmeridgian and Tithonian stages (Late Jurassic) of Europe and South America. Their diet would have included fish, cephalopods, and marin ...

''. "The palatal bones are thin, but there is no suborbital fenestra." Plesiosaurus Scale

The two rami of the lower jaw make a "V" shape with an angle of about 45°. The specialized region where they meet, the

symphysis A symphysis (, pl. symphyses) is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint. # A symphysis is an amphiarthrosis, a slightly movable joint. # A growing togethe ...

, is robust. The two rami are fused at the symphysis, making a pointed, shallow scoop-like shape.Storrs 1997 pp. 169. The teeth of ''Plesiosaurus'' are "simple, needle-like cones" that are "slightly curved and circular in transverse section". They are sharply pointed with fine striations running from tip to base, and point forward (procumbent). This procumbency becomes more pronounced near the leading end of the skull, where they may be only 10–15° above horizontal. There are 20 to 25 teeth per upper jaw tooth row, and 24 per low jaw tooth row. Up to four teeth of a lower jaw's tooth row are found in the symphyseal region.

Vertebral column

''Plesiosaurus'' was a moderately sized plesiosaur that grew to in length and in body mass.Storrs 1997 pp. 149 There are approximately 40 cervical vertebrae (neck vertebrae), with different specimens preserving 38 to 42 cervical vertebrae.Storrs 1997 pp. 170 Of the rest of the vertebral column, there are a handful (four or five in the

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of sever ...

specimen) of "pectoral" vertebrae from the neck-torso transition, approximately 21 dorsal or back vertebrae, three or more sacral vertebrae, and at least 28

caudal vertebra The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

e.Storrs 1997 pp. 171 Generally, the

centra Centra is a convenience shop chain that operates throughout Ireland. The chain operates as a symbol group owned by Musgrave Group, the food wholesaler, meaning the stores are all owned by individual franchisees. The chain has three different ...

of the cervical vertebrae are relatively elongated, being slightly longer than tall. The width, however, is usually greater than or equal to the length. The articular surfaces of the cervical centra are "slightly concave and kidney-shaped, with rounded, slightly rugose edges." Small holes called foramina subcentralia are found on the ventral surface of the centra. Some of the dorsals have rugose articular edges, like the cervicals; this feature is typically absent from the caudals. Ribs are found from the neck to the tail. Cervical ribs are hatchet-shaped and have two articular heads. Dorsal ribs are thick and have only one head. Sacral ribs are "short, robust, and blunt or knob-like on both ends." Caudal ribs have different morphologies depending on their location along the tail, with anterior examples being pointed and more distal examples being "broad and blunt."Storrs 1997 pp. 170 ''Plesiosaurus'' also has

gastralia Gastralia (singular gastralium) are dermal bones found in the ventral body wall of modern crocodilians and tuatara, and many prehistoric tetrapods. They are found between the sternum and pelvis, and do not articulate with the vertebrae. In thes ...

, also known as "belly ribs." Nine or more sets of gastralia are present between the

shoulder The human shoulder is made up of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone) as well as associated muscles, ligaments and tendons. The articulations between the bones of the shoulder mak ...

and pelvis. Each set is composed of seven elements: a bone on the midline flanked by three lateral elements.

Limbs

The shoulder girdle is only partly known but appears to be typical for plesiosaurs. It includes fused

clavicle The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the r ...

s at the anterior end,

scapula The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eith ...

e (shoulder blades), and large

coracoid A coracoid (from Greek κόραξ, ''koraks'', raven) is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is prese ...

s. The scapulae and coracoids both contribute to the glenoids (arm sockets). A pair of oval holes called pectoral fenestrae are found midway along the scapular/coracoid contacts. The forelimbs are elongate and relatively narrow compared to those of most plesiosaurs. The humerus (upper arm bone) has distinctive curvature, which appears to be a retained primitive feature among

ns. Mature ''Plesiosaurus'' also have a distinctive groove along the ventral surface of the humerus. The forearm includes a flat, broad, crescent-shaped

ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...

and a "robust and pillar-like"

radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...

. The wrist includes six bones.Storrs 1997 pp. 173 The hand paddle has five digits; the

phalangeal formula The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones ...

is uncertain, but the count for one large individual, from " thumb" to fifth "finger", is 4-8-9-8-6.Storrs 1997 pp. 176 The pelvis includes equant

pubic bones In vertebrates, the pubic region ( la, pubis) is the most forward-facing (ventral and anterior) of the three main regions making up the coxal bone. The left and right pubic regions are each made up of three sections, a superior ramus, inferior r ...

ischia Ischia ( , , ) is a volcanic island in the Tyrrhenian Sea. It lies at the northern end of the Gulf of Naples, about from Naples. It is the largest of the Phlegrean Islands. Roughly trapezoidal in shape, it measures approximately east to west ...

, and blade-shaped ilia connecting the pelvis to the vertebral column.Storrs 1997 pp. 178 The acetabulum is formed by surfaces on the pubic bones and ischia. Similar to the pectoral girdle, there is a pair of holes between the ischia and pubic bones. The hindlimbs are long and narrow, and in adults, they are much smaller than the forelimbs. The thigh bones are straight. The lower hindlimb includes two roughly equal-sized bones, the robust

tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...

and the semilunate-shaped

fibula The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity i ...

. There are six bones in the ankle. The foot paddle includes five digits. Like the hand, the phalangeal formula is uncertain, but is at least 3-7-9-8-7 from innermost to outer "toe".

Classifications

''Plesiosaurus'' has historically been a

wastebasket taxon Wastebasket taxon (also called a wastebin taxon, dustbin taxon or catch-all taxon) is a term used by some taxonomists to refer to a taxon that has the sole purpose of classifying organisms that do not fit anywhere else. They are typically defined ...

. This is due in part to few anatomical or taxonomic studies of the relevant fossils. Uncritical taxonomic work resulted in hundreds of species representing most of the world and most of the Mesozoic being assigned to ''Plesiosaurus.'' None of the younger

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

species belong to ''Plesiosaurus''. Review of the Early Jurassic species indicates that the only English species properly assigned to ''Plesiosaurus'' is ''P. dolichodeirus''. Several other European Early Jurassic species have been assigned to new genera. ''P. brachypterygius'', ''P. guilielmiimperatoris'' and ''P. tournemirensis'', for example, were assigned to the new genera '' Hydrorion'', '' Seeleyosaurus'' and '' Occitanosaurus''. The following

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...

follows an analysis by Benson ''et al.'', 2012, and shows the placement of ''Plesiosaurus'' within Plesiosauria.

Stratigraphy

Unequivocal specimens of ''Plesiosaurus dolichodeirus'' are limited to the Lyme Regis area of Dorset. It appears to be the most common species of plesiosaur in the Lias Group of England. ''Plesiosaurus'' is best represented from the "upper part of the

Blue Lias The Blue Lias is a geological formation in southern, eastern and western England and parts of South Wales, part of the Lias Group. The Blue Lias consists of a sequence of limestone and shale layers, laid down in latest Triassic and early Jurassi ...

, the 'Shales with Beef,' and the lower Black Ven Marls" the latter of which form part of the

Charmouth Mudstone The Charmouth Mudstone Formation is a geological formation in England. It preserves fossils dating back to the early part of the Jurassic period (Sinemurian–Pliensbachian). It forms part of the lower Lias Group. It is most prominently exposed at ...

; using the Lias Group ammonite fossil zones, these rocks date to the early

stage. Some other ''Plesiosaurus'' fossils are from later Sinemurian rocks. The oldest specimen may be a skull thought to come from late Rhaetian or early Hettangian rocks.Storrs 1997 pp.180

Paleoecology

''Plesiosaurus'' fed mainly on clams and snails, and is thought to have eaten

belemnite Belemnitida (or the belemnite) is an extinct order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous. Unlike squid, belemnites had an internal skeleton that made up the cone. The parts are, from the arms-most ...

s, fish and other prey as well. Its U-shaped jaw and sharp teeth would have been like a fish trap. It propelled itself by the paddles, the tail being too short to be of much use. Its neck could have been used as a rudder when navigating during a chase. ''Plesiosaurus'' gave live birth to live young in the water like

sea snake Sea snakes, or coral reef snakes, are elapid snakes that inhabit marine environments for most or all of their lives. They belong to two subfamilies, Hydrophiinae and Laticaudinae. Hydrophiinae also includes Australasian terrestrial snakes, wher ...

s. The young might have lived in

estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environmen ...

before moving out into the open ocean. It has been postulated that the long neck of ''Plesiosaurus'' would have been a hindrance when trying to speed up, any bend in the neck creating turbulences. If that is the case then ''Plesiosaurus'' would have had to keep its neck straight to achieve good acceleration, something that would make hunting difficult. For this reason it may be possible that these animals would actually lie in wait for prey to come close instead of trying to pursue them.

References

Notes

Sources

* Andrews, C. W. 1896. "On the structure of the plesiosaurian skull". ''Quarterly Journal of the Geological Society'', London, 52, 246–253. * Brown, D. S. 1981. "The English Upper Jurassic Plesiosauroidea (Reptilia) and a review of the phylogeny and classification of the Plesiosauria". ''Bulletin of the British Museum (Natural History): Geology'', 35, (4), 253–347. * Cruickshank, A. R. I.; Small, P. G.; and Taylor, M. A. 1991. "Dorsal nostrils and hydrodynamically driven underwater olfaction in plesiosaurs". ''Nature'', 352, 62–64. *Lydekker, R. 1889. Catalogue of the fossil Reptilia and Amphibia in the British Museum (Natural History), Part II. Containing the Orders Ichthyopterygia and Sauropterygia. British Museum (Natural History) * Richard Owen, Fossil Reptili of the Liassic Formations, pt iii. (Monogr. Palaeont. Soc., 1865) * Persson, P. O. 1963. A revision of the classification of the Plesiosauria with a synopsis of the stratigraphical and geographical distribution of the group. Lunds Universitets Årsskrift, N. F. Avd. 2. 59, 1–59. * Storrs, G. W. 1991. "Anatomy and relationships of ''Corosaurus alcovensis'' (Diapsida: Sauropterygia) and the Triassic Alcova Limestone of Wyoming". ''Bulletin of the Peabody Museum of Natural History'', 44, 1–151. * Storrs, G. W. and Taylor, M. A. 1996. "Cranial anatomy of a new plesiosaur genus from the lowermost Lias (Rhaetian/Hettangian) of Street, Somerset, England". ''Journal of Vertebrate Paleontology'', 16, (3), 403–420. * Storrs, G. W. 1997. "Morphological and taxonomic clarification of the genus ''Plesiosaurus''". 145–190. In Callaway, J. M and Nicholls, E. L. (eds.). ''Ancient Marine Reptiles''. Academic press. London. *Taylor, M. A. and Cruickshank, A. R. I. 1993. Cranial anatomy and functional morphology of Pliosaurus brachyspondylus (Reptilia: Plesiosauria) from the Upper Jurassuc of Westbury, Wiltshire. Philosophical Transactions of the Royal Society of London, Series B, 341, 399–418. * Torrens, Hugh 1995. "Mary Anning (1799–1847) of Lyme; 'The Greatest Fossilist the World Ever Knew'". ''The British Journal for the History of Science'', 25 (3): 257–284

External links

Genus Plesiosaurus
– The Plesiosaur Directory

– Palaeos

– Mikko's Phylogeny Archive {{Taxonbar, from=Q857081 Early Jurassic plesiosaurs of Europe Plesiosauroids Fossil taxa described in 1821 Taxa named by William Conybeare Sauropterygian genera Taxa named by Henry De la Beche