Plesiosauroidea

Plesiosauroids Temporal range: Early Jurassic - Late Cretaceous, 190–65.5Ma PreЄ Є O S D C P T J K Pg N
Reconstructed skeleton of Thalassomedon hanningtoni, an elasmosaurid
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	†Sauropterygia
Order:	†Plesiosauria
Node:	†Neoplesiosauria
Superfamily:	†Plesiosauroidea Gray, 1825
Families
see text

Plesiosauroidea (pron.: / ˈ p l iː s i ə s ɔər /; Greek: plēsios/πλησιος 'near' or 'close to' and sauros/σαυρος 'lizard') is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods. After their discovery, some plesiosauroids were somewhat fancifully said to have resembled "a snake threaded through the shell of a turtle", 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. While they were Mesozoic diapsid reptiles that lived at the same time as dinosaurs, they were not dinosaurs. Gastroliths are frequently found associated with plesiosaurs.

History of discovery

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. 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.

Many museums have plesiosauroid specimens. Notable among them is the collection of plesiosauroids in the Natural History Museum, London, which are on display in the marine reptiles gallery. Several historically important specimens can be found there, including the partial skeleton from Elston, Nottinghamshire reported by William Stukeley in 1719 which is the earliest written record of any marine reptile. Other specimens include those purchased from Thomas Hawkins in the early 19th century.

Specimens are on display in museums in the UK, including New Walk Museum, Leicester, The Yorkshire Museum, The Sedgwick Museum in Cambridge, Manchester Museum, Warwick Museum, Bristol Museum and the Dorset Museum. A specimen was put on display in Lincoln Museum (now The Collection) in 2005. Peterborough Museum holds an excellent collection of plesiosauroid material from the Oxford Clay brick pits in the area. The most complete known specimen of the long-necked plesiosauroid Cryptoclidus, excavated in the 1980s can be seen there.

Description

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.

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.

Behaviour

Unlike their pliosauroid cousins, plesiosauroids (with the exception of the Polycotylidae) were probably slow swimmers. 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}. 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.

The series Walking with Dinosaurs shows the plesiosauroid Cryptoclidus hauling out on land like a sea lion.

On 12 August 2011, researchers from the U.S. described a fossil of a pregnant plesiosaur found on a Kansas ranch in 1987. 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.

Taxonomy

The classification of plesiosauroids has varied; the following represents one version (see Evans 2012)

• Superorder SAUROPTERYGIA
  • Order PLESIOSAURIA
    • Suborder Pliosauroidea
    • Suborder Plesiosauroidea
      •  ? Leurospondylus
      • Eoplesiosaurus
      • Eretmosaurus
      • Westphaliasaurus
      • Family Plesiosauridae
        • Plesiosaurus
      • Euplesiosauria
        • Family Microcleididae
          • Hydrorion
          • Lusonectes
          • Microcleidus
          • Occitanosaurus
          • Seeleyosaurus
        • Family Cryptoclididae
          •  ? Abyssosaurus
          • Opallionectes
          • Pantosaurus
          • Picrocleidus
          • Plesiopterys
          • Scanisaurus
          • Subfamily Cryptoclidinae
            • Colymbosaurus
            • Cryptoclidus
            • Kimmerosaurus
            • Tatenectes
            • Tricleidus
          • Subfamily Muraenosaurinae
            • Muraenosaurus
            • Tremamesacleis
            • Vinialesaurus
        • Family Elasmosauridae
          • Albertonectes
          • Aphrosaurus
          • Callawayasaurus
          • Elasmosaurus
          • Eromangasaurus
          • Fresnosaurus
          • Futabasaurus
          • Hydralmosaurus
          • Hydrotherosaurus
          • Libonectes
          • Mauisaurus
          • Styxosaurus
          • Terminonatator
          • Thalassomedon
          • Tuarangisaurus
          • Wapuskanectes
          • Zarafasaura
          • Subfamily Aristonectinae
            • Aristonectes
            • Kaiwhekea
          • Dubious Elasmosaurids
            • Cimoliasaurus
            • Goniosaurus
            • Ogmodirus
            • Orophosaurus
            • Piptomerus
            • Woolungasaurus