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

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

reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

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� ...

. It is known by nearly complete skeletons from the Lias 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 or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared in the latest Triassic Period, possibly in the Rhaetian stage, about 203 million year ...

, of which it is an early, but fairly typical member. It contains only one species, the

type Type may refer to: Science and technology Computing * Typing, producing text via a keyboard, typewriter, etc. * Data type, collection of values used for computations. * File type * TYPE (DOS command), a command to display contents of a file. * ...

, ''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 '' Microcleidus''.

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, fossil trade, dealer, and palaeontologist. She became known internationally for her discoveries in Jurassic marine fossil beds in the cliffs along the English Cha ...

Sinemurian In the geologic timescale, the Sinemurian is an age (geology), age and stage (stratigraphy), stage in the Early Jurassic, Early or Lower Jurassic epoch (geology), Epoch or series (stratigraphy), Series. It spans the time between 199.5 ±0.3 annu ...

(

)-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, ...

in December 1823 near

Lyme Regis Lyme Regis ( ) is a town in west Dorset, England, west of Dorchester, Dorset, 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 ...

Dorset Dorset ( ; Archaism, archaically: Dorsetshire , ) is a Ceremonial counties of England, ceremonial county in South West England. It is bordered by Somerset to the north-west, Wiltshire to the north and the north-east, Hampshire to the east, t ...

England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...

.Storrs 1997 pp. 146 Additional fossils of ''Plesiosaurus'' were found in rocks of the Lias Group of

for many years, "until the cessation of quarrying activities in the Lias Group, early in this 0thcentury." 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'' and

crocodile Crocodiles (family (biology), family Crocodylidae) or true crocodiles are large, semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term "crocodile" is sometimes used more loosely to include ...

, 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 A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

; Anning's were. ''Plesiosaurus'' was one of the first of the " antediluvian 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 '' Ichthyosaurus'', 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,

, and

in 1821. The

type species In International_Code_of_Zoological_Nomenclature, 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 spe ...

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 bar).Storrs 1997 pp. 165 The

anterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

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

orbits In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificia ...

(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, and between the temporal fenestrae is a narrow sagittal ridge. As in other plesiosaurs, the pterygoids of the palate are fused to the basioccipital of the braincase, although the union is not as robust as in the pliosaurs ''Rhomaleosaurus'' and '' Pliosaurus''. "The palatal bones are thin, but there is no suborbital fenestra." 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, 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.Storrs 1997 pp. 149 There are approximately 40

cervical vertebrae In tetrapods, cervical vertebrae (: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In saurop ...

(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

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 vertebrae.Storrs 1997 pp. 171 Generally, the centra 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, 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 m ...

and

pelvis The pelvis (: pelves or pelvises) is the lower part of an Anatomy, anatomical Trunk (anatomy), trunk, between the human abdomen, abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also c ...

. 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, collarbone, or keybone is a slender, S-shaped long bone approximately long that serves as a strut between the scapula, shoulder blade and the sternum (breastbone). There are two clavicles, one on each side of the body. The clavic ...

s at the anterior end,

scapula The scapula (: 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 either side ...

e (shoulder blades), and large

coracoid A coracoid 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 present as part of the scapula, but this is n ...

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 The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

(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 or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

and a "robust and pillar-like"

radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...

. The wrist includes six bones.Storrs 1997 pp. 173 The hand paddle has five digits; the phalangeal formula is uncertain, but the count for one large individual, from "

thumb The thumb is the first digit of the hand, next to the index finger. When a person is standing in the medical anatomical position (where the palm is facing to the front), the thumb is the outermost digit. The Medical Latin English noun for thumb ...

" to fifth "finger", is 4-8-9-8-6.Storrs 1997 pp. 176 The pelvis includes equant pubic bones,

ischia Ischia ( , , ) is a volcanic island in the Tyrrhenian Sea. It lies at the northern end of the Gulf of Naples, about from the city of Naples. It is the largest of the Phlegrean Islands. Although inhabited since the Bronze Age, as a Ancient G ...

, and blade-shaped ilia connecting the pelvis to the vertebral column.Storrs 1997 pp. 178 The

acetabulum The acetabulum (; : acetabula), also called the cotyloid cavity, is a wikt:concave, concave surface of the pelvis. The femur head, head of the femur meets with the pelvis at the acetabulum, forming the Hip#Articulation, hip joint. Structure The ...

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 (; : tibiae or tibias), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two Leg bones, bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outsi ...

and the semilunate-shaped fibula. 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. 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 Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

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 language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

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

Palaeobiology

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

sea snake Sea snakes, or coral reef snakes, are Elapidae, elapid snakes that inhabit Marine (ocean), marine environments for most or all of their lives. They belong to two subfamilies, Hydrophiinae and Sea krait, Laticaudinae. Hydrophiinae also includes ...

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 environm ...

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.

Palaeoenvironment

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.Storrs 1997 pp. 179 ''Plesiosaurus'' is best represented from the "upper part of the

Blue Lias The Blue Lias is a formation (stratigraphy), 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 ...

, the 'Shales with Beef,' and the lower Black Ven Marls" the latter of which form part of the Charmouth Mudstone; using the Lias Group

ammonite Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...

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 The Rhaetian is the latest age (geology), age of the Triassic period (geology), Period (in geochronology) or the uppermost stage (stratigraphy), stage of the Triassic system (stratigraphy), System (in chronostratigraphy). It was preceded by the N ...

or early

Hettangian The Hettangian is the earliest age and lowest stage of the Jurassic Period of the geologic timescale. It spans the time between 201.3 ± 0.2 Ma and 199.3 ± 0.3 Ma (million years ago). The Hettangian follows the Rhaetian (part of the Triass ...

rocks.Storrs 1997 pp.180

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 Sir Richard Owen (20 July 1804 – 18 December 1892) was an English biologist, comparative anatomy, comparative anatomist and paleontology, palaeontologist. Owen is generally considered to have been an outstanding naturalist with a remarkabl ...

, 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