The plesiosaurs were a group of marine reptiles that appeared in the Middle Triassic, about 230 million years ago and went extinct in the KT extinction at the end of the Cretaceous, 65 million years ago. They are thought to have evolved from Nothosaurs, a group of semi-aquatic marine reptiles. The plesiosaurs are thought to have been fully aquatic from the outset. The term sauropterygians is used to include the nothosaurs and the plesiosuars, plus the pachypleurosaurs; lizard-like aquatic reptiles. The sauropterygians are thought to be related to the squamates - snakes and lizards.
A reconstruction of Pistosuarus, generally considered to be the earliest plesiosaur. It does not appear to be well suited to life on land.
The plesiosaurs are split into two taxonomic groups, the plesiosaurs proper and the pliosaurs (plus a few early species of uncertain affinities). The pliosuars were aquatic reptiles with short necks, elongated heads and powerful jaws, with many conical teeth (ideal for seizing large prey). They ranged from 4 m to 25 m in length and appeared in the Late Triassic, persisting till the end of the Cretaceous.
Reconstruction of a pliosaur attacking a mosasaur (large Mesozoic marine lizard) by paleo-artist Dan Varner.
The plesiosaurs proper are divided into five families, the Elasmosaurs, the Plesiosuarids, the Polycotylids, the Cimoliasaurs, and the Cryptoclids.
The elasmosaurs appeared in the Late Triassic; they had very elongate necks and small heads. The earliest species were about 3 m long, but some later species reached 14 m in length.
The plesiosaurids currently include the single genus Plesiosaurus, the genus after which the whole group has been named. Since almost all plesiosaurs have started in this group then been moved out it is quite likely that this group will be further revised. Plesiosaurus was a long-necked, small headed form, though not as extreme as the Elasmosaurs. It is from the Lower Jurassic.
The polycotylids are a Cretaceous group with long snouts and short necks, superficially resembling the pliosaurs.
Reconstruction of Polycotylus by illustrator Carl Buell.
The cimoliasuars are probably an artificial group. The group includes a number of incomplete skeletons of plesiosaurs from the Jurassic and Cretaceous, as well as one (dubious) Palaeogene specimen. Cimoliasaurs tend to have intermediate neck and snout lengths.
The cryptoclidids are a group of medium sized plesiosaurs with long necks and broad, flat heads. They ranged from the Middle Jurassic to the end of the Cretaceous.
A paper in the 12 August 2011 edition of the journal Science Robin O'Keefe of the Department of Biology at Marshall University and Luis Chiappe of the Natural History Museum of Los Angeles County describes for the first time a specimen of a plesiosaur which is apparently pregnant. If correct this settles a long standing dilemma for plesiosaur specialists. Plesiosaurs are clearly not not well adapted to land dwelling, so scientists had long suspected that they might be able to reproduce without venturing onto land. However no evidence of this had ever been found (unlike in ichthyosaurs, where the phenomenon is well documented), and not all reptiles have the potential to bear live young.
All crocodiles, most turtles and some birds and squamates (snakes and lizards) have temperature-dependent sex determination. This system uses an outside factor, temperature, to determine sex; above a certain temperature embryos will develop as one sex, bellow it as the other. This works fine for an egg-laying reptile, particularly one with a large brood size where there will be a temperature difference within the nest, but a large marine reptile is very likely to maintain a constant body temperature, which would result in all embryos developing as the same sex - not a good survival strategy.
Mammals, birds and most squamates use chromosomal sex determination. Under this system a species has sex two chromosomes. One chromosome is present in all individuals, the other only in individuals of one sex. Thus mammals have X and Y chromosomes; females have two X chromosomes males an X and a Y. Everyone inherits an X chromosome from their mother, and either an X or a Y chromosome from their father. All mammals develop as females, unless they have a Y chromosome.
This works well for mammals; the Y chromosome leads to the production of hormones that modify a potentially female animal to become male, but not so well for birds. Birds have sex chromosomes referred to as Z and W. A bird with two Z chromosomes in male, one with a Z and a W is female. In birds the presence of a W chromosome leads to the production of hormones that turn a potentially male individual female. This prevents the emergence of live births in birds; a baby bird developing inside its mother would be exposed to female hormones and develop as a female regardless of its chromosomes.
The sauropterygans (nothosaurs and plesiosaurs) are thought to be closely related the squamates, where chromosomal sex determination is the commonest method of sex determination, so it would not be surprising if this was the case in plesiosaurs.
In 2009 a paper in the Zoological Journal of the Linnean Society detailed a long-term study of sex-determination in squamates by Martina Pokorná and Lukáš Kratochvíl of the Department of Ecology at Charles University in Prague, which strongly indicated that the ancestral state in squamates was temperature dependent sex-determination, and that chromosomal sex determination has evolved several times within the group. Both the XY and ZW systems of sex determination are present within the squamates, and are likely to have evolved separately from a non-chromosomal system. This implies that the common ancestor of the sauropterygans and squamates used a temperature dependent sex determination system; the most that can be said is that the groups clearly had the potential to develop chromosomal sex determination.
The pregnant specimen described by O'Keefe and Chiappe (LACM 129639) was discovered in Kansas in the 1980s, and has been in the collection of the Natural History Museum of Los Angeles County ever since. It is not immediately obvious that this is a pregnant specimen; the fetus is not intact could potentially be the last meal of the adult rather than its young, though the pregnancy theory is supported by the absence of more common food material such as belemnites (Mesozoic molluscs) etc.
In addition LACM 129369 is a Polycotylus latippinis from the very end of the Cretaceous. P. latippinis is a highly derived species, not thought to be ancestral to any other form of plesiosaur, so it cannot be used to directly infer that other species bore live young. It would clearly have been useful to a plesiosaur, and it is considered likely that the ability arose in a nothosaur ancestor of the plesiosaurs. However this remains just as much a theory as it did before the re-examination of specimen LACM 129369.
A reconstruction of Polycotylus as a live-birthing animal, by science illustrator Stephanie Abramowicz.