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 The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

before all except for the

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 ...

became extinct at the end of that period. The plesiosaurs would continue to diversify until the end of the

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

. Sauropterygians are united by a radical adaptation of their

pectoral girdle The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists of ...

, adapted to support powerful flipper strokes. Some later sauropterygians, such as the

pliosaur Pliosauroidea is an extinct clade of plesiosaurs, known from the earliest Jurassic to early Late Cretaceous. They are best known for the subclade Thalassophonea, which contained crocodile-like short-necked forms with large heads and massive toot ...

s, developed a similar mechanism in their pelvis. Uniquely among reptiles, sauropterygians moved their tail vertically like modern cetaceans and

sirenians The Sirenia (), commonly referred to as sea-cows or sirenians, are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The Sirenia currently comprise two distinct f ...

Origins and evolution

The earliest sauropterygians appeared about 247

million years ago The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds. Usage Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago) ...

(Ma), at the start of the

Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma and ...

: the first definite sauropterygian with exact stratigraphic datum lies within the Spathian division of the

Olenekian In the geologic timescale, the Olenekian is an age in the Early Triassic epoch; in chronostratigraphy, it is a stage in the Lower Triassic series. It spans the time between Ma and Ma (million years ago). The Olenekian is sometimes divided i ...

era in South China. Early examples were small (around 60 cm), semi-aquatic lizard-like animals with long limbs (

pachypleurosaur left, 220px, '' Pachypleurosaurus'' Pachypleurosauria is an extinct clade of primitive sauropterygian reptiles that vaguely resembled aquatic lizards, and were limited to the Triassic period. They were elongate animals, ranging in size from , w ...

s), but they quickly grew to be several metres long and spread into shallow waters (

nothosaur Nothosaurs (order Nothosauroidea) were Triassic marine sauropterygian reptiles that may have lived like seals of today, catching food in water but coming ashore on rocks and beaches. They averaged about in length, with a long body and tail.F. ...

s). The Triassic-Jurassic extinction event wiped them all out except for the plesiosaurs. 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 ...

, these diversified quickly into both long-necked small-headed plesiosaurs proper, and short-necked large-headed

s. Originally, it was thought that plesiosaurs and pliosaurs were two distinct superfamilies that followed separate evolutionary paths. It now seems that these were simply morphotypes in that both types evolved a number of times, with some pliosaurs evolving from plesiosaur ancestors, and vice versa.

Classification

Classification of sauropterygians has been difficult. The demands of an aquatic environment caused the same features to evolve multiple times among reptiles, an example of

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...

. Sauropterygians are

diapsid Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years a ...

s, and since the late 1990s, scientists have suggested that they may be closely related to

turtle Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...

s. The bulky-bodied, mollusc-eating

placodont Placodonts ("Tablet teeth") are an extinct order of marine reptiles that lived during the Triassic period, becoming extinct at the end of the period. They were part of Sauropterygia, the group that includes plesiosaurs. Placodonts were generall ...

s may also be sauropterygians, or intermediate between the classic eosauropterygians and turtles. Several analyses of sauropterygian relationships since the beginning of the 2010s have suggested that they are more closely related to archosaurs (birds and crocodilians) than to

lepidosaur The Lepidosauria (, from Greek meaning ''scaled lizards'') is a subclass or superorder of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata includes snakes, lizards, and amphisbaenians. Squamata contains over 9,000 species ...

s (lizards and snakes). The cladogram shown hereafter is the result of an analysis of sauropterygian relationships (using just fossil evidence) conducted by Neenan and colleagues, in 2013. The cladogram shown below follows the most likely result found by an analysis of turtle relationships using both fossil and genetic evidence by M.S. Lee, in 2013. This analysis resolved Sauropterygia as a paraphyletic assemblage of stem turtles. In cladistic analysis of 2015, Sauropterygia placed within Pantestudines:

Size and ecology

Each morphotype filled a specific ecological role. The large pliosaurs, such as ''

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 ...

'', ''

Liopleurodon ''Liopleurodon'' (; meaning 'smooth-sided teeth') is an extinct genus of large, carnivorous marine reptile belonging to the Thalassophonea, a clade of short-necked pliosaurid plesiosaurs. ''Liopleurodon'' lived from the Callovian Stage of the Mi ...

'', ''

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 ...

'', ''

Kronosaurus ''Kronosaurus'' ( ; meaning "lizard of Kronos") is a potentially dubious genus of extinct short-necked pliosaur. With an estimated length of , it was among the largest pliosaurs, and is named after the leader of the Greek Titans, Kronos. It ...

'' and '' Brachauchenius'', were the superpredators of the Mesozoic seas, measuring 7 to 12 meters in length, and filled a similar ecological role to that of

killer whale The orca or killer whale (''Orcinus orca'') is a toothed whale belonging to the oceanic dolphin family, of which it is the largest member. It is the only extant species in the genus ''Orcinus'' and is recognizable by its black-and-white ...

s today. The long-necked plesiosaurs included Plesiosauridae, Cryptoclididae, and Elasmosauridae. Some lineages of long-necked plesiosaurs evolved progressively longer and more flexible necks, reaching 13 meters in total length by the late Cretaceous. With their small heads in proportion to their neck length and body mass, long-necked plesiosaurs were limited to eating relatively small fish, which they probably snared in their tooth-lined jaws with rapid lunges of their long necks.

References

External links

Unit 220: 100: Lepidosauromorpha
''Palaeos''. July 15, 2003. Retrieved January 19, 2004.
A review of the Sauropterygia
Adam Stuart Smith. ''The Plesiosaur Directory''. Retrieved April 17, 2006.

- lists every species and synonyms. Retrieved February 26, 2006 {{Authority control Chordate superorders Early Triassic first appearances Maastrichtian extinctions