In Depth
In 1937 a fossil collector named Allan Bennison discovered mosasaur fossils in California’s Mount Diablo Range. These fossils were first described by Charles Camp in 1942 and published as Kolposaurus; however this name is actually preoccupied as a synonym for Nothosaurus, hence the naming revision to Plotosaurus in 1951.
The original fossils were of more than one individual, with one being quite a bit larger than the other. Whereas the type species P. bennisoni reached nine meters, the second species , P. tuckeri reached thirteen meters long, just a bit short of giants such as Tylosaurus and Mosasaurus, though possibly equal to Hainosaurus. However the future of P. tuckeri is in doubt since a 2008 review by Lindgren, Caldwell and Jagt concluded that P. tuckeri should actually be treated as a synonym to the type species P. bennisoni.
Plotosaurus represents a mosasaur that is significantly better adapted to swimming than most of its known relatives. The body is more streamlined with a fairly rigid forward body that would have stabilised the swimming pattern of Plotosaurus reducing turbulence and subsequent drag. Skin impressions for Plotosaurus have also revealed specially adapted scales that have been interpreted as being analogous to those of sharks in that they were capable of producing vortices as they moved through the water to reduce drag even further.
The limbs shaped into flippers were also streamlined to help reduce drag through the water, and in addition to this the skull of Plotosaurus was very long but also slender. Other interesting features of the skull are the openings for large eyes, and when you combine these with the rigidity of the forward body, you get a marine reptile that is possibly analogous to the opthalmosaurid ichthyosaurs that lived earlier. These fast swimming and deep diving ichthyosaurs do not seem to be present at the end of the Cretaceous, so this quite a tantalising idea that mosasaurs like Plotosaurus were developing to fill an ecological void left by them. Another feature of Plotosaurus are the large nasal openings that facilitated a faster transfer of fresh air into the lungs. This could be either to cope with a faster and hence more energetic style of swimming, or deep diving trips beneath the surface where Plotosaurus would have to of held its breath but replace the oxygen depleted air in its lungs quickly when back on the surface.
Plotosaurus was not the only mosasaur named by Charles Camp in 1942 (even if it was as Kolposaurus), another genus from the Moreno Formation called Plesiotylosaurus was also named by him. Other marine reptiles from the Moreno Formation include the plesiosaurs Fresnosaurus, Hydrotherosaurus, Morenosaurus and Aphrosaurus. One candidate for a possible close relative of Plotosaurus is Eremiasaurus.
Further Reading
– Plotosaurus, a new generic name for Kolposaurus Camp, preoccupied. – Journal of Paleontology 25:822 – Charles L. Camp – 1951. – A fishy mosasaur: the axial skeleton of Plotosaurus (Reptilia, Squamata) reassessed. – Lethaia 40:153-160. – J. Lindgren, J. W. M. Jagt & M. W. Caldwell – 2007. – New data on the postcranial anatomy of the California mosasaur Plotosaurus bennisoni (Camp, 1942) (Upper Cretaceous: Maastrichtian), and the taxonomic status of P. tuckeri (Camp, 1942). – Journal of Vertebrate Paleontology 28(4):1043-1054. – J. Lindgren, J. W. M. Jagt & M. W. Caldwell – 2008. – Skin of the Cretaceous mosasaur Plotosaurus: implications for aquatic adaptations in giant marine reptiles – Biology Letters – J. Lindgren, C. Awlmark, M. W. Caldwell & A. R. Fiorillo – 2009.
