Plesiosauroidea

Plesiosauroids
Temporal range: Early Jurassic - Late Cretaceous, 190–66 Ma
Reconstructed skeleton of Thalassomedon hanningtoni, an elasmosaurid
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: †Sauropterygia
Order: †Plesiosauria
Clade: †Neoplesiosauria
Superfamily: †Plesiosauroidea
Gray, 1825
Families

see text

Plesiosauroidea (/ˈpliːsiəsɔər/; Greek: plēsios/πλησιος 'near' or 'close to' and sauros/σαυρος 'lizard') is an extinct clade of carnivorous plesiosaur marine reptiles. They have the longest neck to body ratio of any reptile. Plesiosauroids are known from the Jurassic and Cretaceous periods. After their discovery, some plesiosauroids were said to have resembled "a snake threaded through the shell of a turtle",[1] although they had no shell.

Plesiosauroidea appeared at the Early Jurassic Period (late Sinemurian stage) and thrived until the K-Pg extinction, at the end of the Cretaceous Period. The oldest confirmed plesiosauroid is Plesiosaurus itself, as all younger taxa were recently found to be pliosauroids.[2] While they were Mesozoic diapsid reptiles that lived at the same time as dinosaurs, they did not belong to the latter. Gastroliths are frequently found associated with plesiosaurs.[3]

History of discovery

Autograph letter concerning the discovery of Plesiosaurus, from Mary Anning.

The first complete plesiosauroid skeletons were found in England by Mary Anning, in the early 19th century, and were amongst the first fossil vertebrates to be described by science. Plesiosauroid remains were found by the Scottish geologist Hugh Miller in 1844 in the rocks of the Great Estuarine Group (then known as 'Series') of western Scotland.[4] Many others have been found, some of them virtually complete, and new discoveries are made frequently. One of the finest specimens was found in 2002 on the coast of Somerset (England) by someone fishing from the shore. This specimen, called the Collard specimen after its finder, was on display in Taunton Museum in 2007. Another, less complete, skeleton was also found in 2002, in the cliffs at Filey, Yorkshire, England, by an amateur palaeontologist. The preserved skeleton is displayed at Rotunda Museum in Scarborough.

Description

Dolichorhynchops, a short-necked, long-jawed plesiosauroid, National Museum of Natural History, Washington D.C., USA.

Plesiosauroids had a broad body and a short tail. They retained their ancestral two pairs of limbs, which evolved into large flippers.

It has been determined by teeth records that several sea-dwelling reptiles, including plesiosauroids, had a warm-blooded metabolism similar to that of mammals. They could generate endothermic heat to survive in colder habitats.[5]

Evolution

Plesiosauroids evolved from earlier, similar forms such as pistosaurs. There are a number of families of plesiosauroids, which retain the same general appearance and are distinguished by various specific details. These include the Plesiosauridae, unspecialized types which are limited to the Early Jurassic period; Cryptoclididae, (e.g. Cryptoclidus), with a medium-long neck and somewhat stocky build; Elasmosauridae, with very long, inflexible necks and tiny heads; and the Cimoliasauridae, a poorly known group of small Cretaceous forms. According to traditional classifications, all plesiosauroids have a small head and long neck but, in recent classifications, one short-necked and large-headed Cretaceous group, the Polycotylidae, are included under the Plesiosauroidea, rather than under the traditional Pliosauroidea. Size of different plesiosaurs varied significantly, with an estimated length of Trinacromerum being three meters and Mauisaurus growing to twenty meters.

Relationships

Muraenosaurus, a cryptoclidid
Styxosaurus, an elasmosaurid
Dolichorhynchops, a polycotylid

The following cladogram follows an analysis by Benson & Druckenmiller (2014).[6]

Plesiosauroidea

Plesiosaurus

Eretmosaurus

Westphaliasaurus

Microcleididae

Plesiopterys


Cryptoclidia

Cryptoclididae


Xenopsaria

Elasmosauridae


Leptocleidia

Leptocleididae

Polycotylidae








Behavior

Restoration of a Plesiosaurus dolichodeirus pair, one catching a fish.

Unlike their pliosauroid cousins, plesiosauroids (with the exception of the Polycotylidae) were probably slow swimmers.[7] It is likely that they cruised slowly below the surface of the water, using their long flexible neck to move their head into position to snap up unwary fish or cephalopods. Their four-flippered swimming adaptation may have given them exceptional maneuverability, so that they could swiftly rotate their bodies as an aid to catching prey.

Contrary to many reconstructions of plesiosauroids, it would have been impossible for them to lift their head and long neck above the surface, in the "swan-like" pose that is often shown {Everhart, 2005}.[8] Even if they had been able to bend their necks upward to that degree (which they could not), gravity would have tipped their body forward and kept most of the heavy neck in the water.

On 12 August 2011, researchers from the U.S. described a fossil of a pregnant plesiosaur found on a Kansas ranch in 1987.[9] The plesiosauroid, Polycotylus latippinus, has confirmed that these predatory marine reptiles gave birth to single, large, live offspring - contrary to other marine reptile reproduction which typically involves a large number of small babies. Before this study, plesiosauroids had sometimes been portrayed crawling out of water to lay eggs in the manner of sea turtles, but experts had long suspected that their anatomy was not compatible to movement on land. The adult plesiosaur measures 4 m long and the baby is 1.5 m long.[10]

References

  1. ↑ Everhart, Mike (2005-10-14). "A Snake Drawn Through the Shell of a Turtle". Oceans of Kansas Paleontology. Retrieved 2010-06-10 External link in |work= (help)
  2. ↑ Hilary F. Ketchum and Roger B. J. Benson (2011). "A new pliosaurid (Sauropterygia, Plesiosauria) from the Oxford Clay Formation (Middle Jurassic, Callovian) of England: evidence for a gracile, longirostrine grade of Early-Middle Jurassic pliosaurids". Special Papers in Palaeontology. 86: 109–129. doi:10.1111/j.1475-4983.2011.01083.x.
  3. ↑ "Occurrence of Gastroliths in Mesozoic Taxa," in Sanders et al. (2001). Page 168.
  4. ↑ p 339 Trewin, N. H.(ed)2002 The Geology of Scotland. The Geological Society, London
  5. ↑ "Warm-blooded marine reptiles at the time of the dinosaurs". Sciencedaily.com. 2010-06-15. doi:10.1126/science.1187443. Retrieved 2011-08-15.
  6. ↑ Benson, R. B. J.; Druckenmiller, P. S. (2013). "Faunal turnover of marine tetrapods during the Jurassic-Cretaceous transition". Biological Reviews: n/a. doi:10.1111/brv.12038.
  7. ↑ Massare, J. A. 1988. "Swimming capabilities of Mesozoic marine reptiles: Implications for method of predation". Paleobiology 14(2): 187-205.
  8. ↑ Henderson, D. M. 2006. "Floating point: a computational study of buoyancy, equilibrium, and gastroliths in plesiosaurs", Lethaia, 39 pp. 227–244.
  9. ↑ F. R. O’Keefe1,*, L. M. Chiappe2. "Viviparity and K-Selected Life History in a Mesozoic Marine Plesiosaur (Reptilia, Sauropterygia)". Sciencemag.org. Retrieved 2011-08-15.
  10. ↑ by Anthony King. "Ancient sea dragons had a caring side". Cosmosmagazine.com. Retrieved 2011-08-15.

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