Plesiosaurus

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Plesiosaurus (Greek: {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}), near to + {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}), lizard) is a genus of extinct, large marine sauropterygian reptile that lived during the Early Jurassic. 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, 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 Microcleidus.

DiscoveryEdit

File:Mary Anning Plesiosaurus.jpg
Letter concerning the discovery of the genus from Mary Anning

The first complete skeleton of Plesiosaurus was discovered by early paleontologist and fossil hunter Mary Anning in Sinemurian (Early Jurassic)-age rocks of the lower Lias Group in December 1823 near Lyme Regis in Dorset, England.<ref>Torrens 1995</ref><ref name=Storrs146>Storrs 1997 pp. 146</ref> Additional fossils of Plesiosaurus were found in rocks of the Lias Group of Dorset for many years,<ref>Andrew 1896</ref><ref>Lydekker 1889</ref><ref>Owen 1865</ref> "until the cessation of quarrying activities in the Lias Group, early in this [20th] century."<ref name=Storrs146/> although less complete remains were used by Henry De la Beche and William Conybeare to name the species two years earlier in 1821,<ref name=Conybeare1821>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, together with general remarks on the osteology of the Ichthyosaurus. Transactions of the Geological Society of London 5: 559–594</ref> and despite being discovered first, Conybeare's remains were not the holotype; 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 Henry De la Beche, 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, Dorset, and Lyme Regis in 1821.<ref name=Conybeare1821 /> The type species of Plesiosaurus, P. dolichodeirus, was named and described by Conybeare in 1824 on the basis of Anning's original finds.

DescriptionEdit

Skull and dentitionEdit

File:Plesiosaurus Scale.svg
Plesiosaurus with a human to scale.

Compared to other plesiosaur genera, Plesiosaurus has a small head. The skull is much narrower than long,<ref name=Storrs166>Storrs 1997 pp. 166</ref> reaching its greatest width just behind the eyes (the postorbital bar).<ref name=Storrs165>Storrs 1997 pp. 165</ref> The anterior portion is "bluntly triangular".<ref name=Storrs165/> In lateral view, the skull reaches its highest point at the rear of the skull table.<ref name=Storrs167>Storrs 1997 pp. 167</ref> "The external nostrils overlie the internal nares".<ref name=Storrs165/> They are not positioned at the tip of the snout, but farther back, nearer the eyes than the tip of the skull.<ref name=Storrs166/> Unlike the nostrils of Rhomaleosaurus,<ref>Cruickshank 1991</ref> they do not appear to be adapted for underwater olfaction.<ref name=Storrs165/> The orbits (eye sockets) are roughly circular and are positioned about halfway along the length of the skull.<ref name=Storrs165/> They face up and to the sides.<ref name=Storrs166/><ref name=Storrs167/> Just posterior to the orbits are the supratemporal fenestrae, which are about the same size as the orbits and also roughly circular.<ref name=Storrs165/> Between the four openings is the pineal foramen, and between the temporal fenestrae is a narrow sagittal ridge.<ref name=Storrs165/> As in other plesiosaurs, the pterygoids of the palate are fused to the basioccipital of the braincase,<ref name=Storrs165/> although the union is not as robust as in the pliosaurs Rhomaleosaurus and Pliosaurus.<ref name=Storrs165/><ref>Taylor and Cruickshank 1993</ref> "The palatal bones are thin, but there is no suborbital fenestra."<ref name=Storrs165/>

The two rami of the lower jaw make a "V" shape with an angle of about 45°.<ref name=Storrs166/> 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.<ref name=Storrs169>Storrs 1997 pp. 169.</ref>

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.<ref name=Storrs166/> There are 20 to 25 teeth per upper jaw tooth row,<ref name=Storrs165/> and 24 per low jaw tooth row.<ref name=Storrs166/> Up to four teeth of a lower jaw's tooth row are found in the symphyseal region.<ref name=Storrs169/>

Vertebral columnEdit

File:Conybeare Plesiosaur 1824.jpg
Illustration of the skeletal anatomy of a Plesiosaurus dolichodeirus from Conybeare's 1824 paper that described an almost complete plesiosaur skeleton found by Mary Anning in 1823

Plesiosaurus was a moderately sized plesiosaur that grew to Template:Cvt in length.<ref>Template:Cite journal</ref><ref name=Storrs149>Storrs 1997 pp. 149</ref> There are approximately 40 cervical vertebrae (neck vertebrae), with different specimens preserving 38 to 42 cervical vertebrae.<ref name=Storrs170>Storrs 1997 pp. 170</ref> Of the rest of the vertebral column, there are a handful (four or five in the holotype specimen) of "pectoral" vertebrae from the neck-torso transition,<ref name=Storrs170/> approximately 21 dorsal or back vertebrae, three or more sacral vertebrae, and at least 28 caudal vertebrae.<ref name=Storrs171>Storrs 1997 pp. 171</ref> 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.<ref name=Storrs170/>

Ribs are found from the neck to the tail. Cervical ribs are hatchet-shaped and have two articular heads.<ref name=Storrs170/> 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."<ref name=Storrs170>Storrs 1997 pp. 170</ref> Plesiosaurus also has gastralia, also known as "belly ribs." Nine or more sets of gastralia are present between the shoulder and pelvis. Each set is composed of seven elements: a bone on the midline flanked by three lateral elements.<ref name=Storrs171/>

LimbsEdit

The shoulder girdle is only partly known but appears to be typical for plesiosaurs. It includes fused clavicles at the anterior end, scapulae (shoulder blades), and large coracoids. 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.<ref name=Storrs171/> 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 sauropterygians. Mature Plesiosaurus also have a distinctive groove along the ventral surface of the humerus. The forearm includes a flat, broad, crescent-shaped ulna and a "robust and pillar-like" radius. The wrist includes six bones.<ref name=Storrs173>Storrs 1997 pp. 173</ref> The hand paddle has five digits; the phalangeal formula is uncertain, but the count for one large individual, from "thumb" to fifth "finger", is 4-8-9-8-6.<ref name=Storrs176>Storrs 1997 pp. 176</ref>

The pelvis includes equant pubic bones, ischia,<ref name=Storrs176/> and blade-shaped ilia connecting the pelvis to the vertebral column.<ref name=Storrs178>Storrs 1997 pp. 178</ref> 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.<ref name=Storrs176/> The hindlimbs are long and narrow,<ref name=Storrs178/> and in adults, they are much smaller than the forelimbs.<ref name=Storrs176/> The thigh bones are straight. The lower hindlimb includes two roughly equal-sized bones, the robust tibia 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".<ref name=Storrs178/>

ClassificationsEdit

File:Plesiosaurus dolichodeirus.jpg
Specimen referred to Plesiosaurus dolichodeirus (NHMUK OR 36183)
File:Plesiosaurus2.jpg
Modern restoration

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 or Cretaceous species belong to Plesiosaurus. Review of the Early Jurassic species indicates that the only English species properly assigned to Plesiosaurus is P. dolichodeirus.<ref name=Storrs146/> 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 follows an analysis by Benson et al., 2012, and shows the placement of Plesiosaurus within Plesiosauria.<ref name=Bensonetal12>Template:Cite journal</ref>

PalaeobiologyEdit

Plesiosaurus fed mainly on clams and snails, and is thought to have eaten belemnites, fish and other prey as well.<ref>Template:Cite news</ref> 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 snakes. The young might have lived in estuaries 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.

PalaeoenvironmentEdit

File:Plesiosaurus sp.jpg
Tentatively referred specimen in Calgary

Unequivocal specimens of Plesiosaurus dolichodeirus are limited to the Lyme Regis area of Dorset.<ref>Storrs 1997 pp. 148</ref> It appears to be the most common species of plesiosaur in the Lias Group of England.<ref>Storrs 1997 pp. 179</ref> Plesiosaurus is best represented from the "upper part of the Blue Lias, 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 fossil zones, these rocks date to the early Sinemurian 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.<ref name=Storrs180>Storrs 1997 pp.180</ref>

See alsoEdit

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ReferencesEdit

NotesEdit

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SourcesEdit

  • 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 linksEdit

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