In Depth
In the early days of plesiosaur evolution back in the Jurassic, there were two main groups that were successful enough to become two of the main kinds of marine reptiles of the Mesozoic. These were long necked plesiosaurs such as Plesiosaurus and Cryptoclidus, and short necked pliosaurs (technically a sub group of plesiosaurs) such as Pliosaurus and Liopleurodon. For a time these two groups were very successful but by the later stages of the Mesozoic, both groups had waned with the short necked pliosaurs becoming particularly rare. It was now that a new kind of plesiosaur called a polycotylid emerged to fill a void left by the disappearance of other marine reptiles such as ichthyosaurs that resulted in a body shape reminiscent of the dwindling pliosaurs.
Polycotylus is the type genus of the Polycotylidae, the family group of these new kinds of plesiosaurs that appeared in the Cretaceous. Polycotylus had a body similar to those of its ancestors, though the key differences between them are that Polycotylus had a much shorter neck but longer jaws. Whereas the long-necked plesiosaurs used their necks to gain a reach advantage when hunting prey like fish, Polycotylus relied more upon speed and manoeuvrability to chase down its prey. Despite the shorter neck however, Polycotylus still retained a reasonably large number of neck vertebrae. While this might have offered increased flexibility for the neck, later polycotlids would develop similarly proportioned necks but with a reduced number of vertebrae.
In the 1980s a specimen of the type species P. latipinnis (later classified as LACM 129639) was found along with a foetus that was about forty percent the length of its mother. This revealed two things, the first being that plesiosaurs did give birth to live young instead of laying eggs. This was not a huge revelation since most palaeontologists were already of the opinion that plesiosaurs gave birth to live young because of the difficulties they would have faced when moving without the buoyancy of water to support their body weight. Additionally their earlier relatives the nothosaurs are also known to have given birth to live young with such genera as Lariosaurus and Keichousaurus. The ichthyosaurs are also known to have given birth to live young, and even some modern reptiles such as sea snakes and even the land living viviparous lizard (Zootoca vivipara) give birth to live young instead of laying eggs.
The second is that plesiosaurs like Polycotylus almost certainly employed a K-strategy survival method when raising young. What this means is that only a very small number of young are raised by an individual throughout their lifetime, but the parent makes a long term investment in raising and protecting the young so that they have a better chance of making it to adulthood. This is a stark contrast to the r-strategy method usually employed by reptiles we know today where a large number of young are raised but are not given extended care and protection by the parents. Not many young born this way survive to adulthood, but the sheer numbers involved ensure the survival of the species.
As already mentioned, Polycotylus is the type genus of the Polycotylidae, and other plesiosaurs that are members of this include Dolichorhynchops, Edgarosaurus, Eopolycotylus, Palmulasaurus and Trinacromerum amongst many others.
Further Reading
– On some reptilian remains. – The American Journal of Science, series 2 48:278. – E. D. Cope – 1869. – Notes sur les reptiles fossiles [Notes on fossil reptiles]. – Bulletin de la Soci�t� G�ologique de France, 3e s�rie 4:435-444. – H. -E. Sauvage – 1876. – On the cranial anatomy of the polycotylid plesiosaurs, including new material of Polycotylus latipinnis, Cope, from Alabama. – Journal of Vertebrate Paleontology 24 (2): 326–34. – F. R. O. Keefe – 2004. – Plesiosaurs from the Upper Cretaceous (Cenomanian-Turonian) Tropic Shale of southern Utah, Part 2: Polycotylidae. – Journal of Vertebrate Paleontology. 27 (1): 41–58. – L. B. Albright III, D. D. Gillette & A. L. Titus – 2007. -A New Species of the Plesiosaur Genus Polycotylus from the Upper Cretaceous of the Southern Urals”. Paleontological Journal. 50 (5): 494–503. – A New Species of the Plesiosaur Genus Polycotylus from the Upper Cretaceous of the Southern Urals. – Paleontological Journal. 50 (5): 494–503. – V.M. Efimov, I.A. Meleshin & A.V. Nikiforov – 2016. – Ontogeny of Polycotylid Long Bone Microanatomy and Histology. – Integrative Organismal Biology. 1. – F. R. O’Keefe, P M Sander, T Wintrich & S Werning – 2019.
