River Monsters, Cretaceous Style: A Freshwater Plesiosaurian from Victoria

The tragedy of extinction (necessary evil that it is) is that we’ll never (barring the physicists getting their act together and building a time machine – sort it out guys) get to see all those incredible animals in the flesh, moving around, eating, sleeping, mating, fighting and just doing whatever it was that they did. One of the groups that I feel most dismayed about never being able to see is the extinct marine reptiles.  This group consisted of animals such as the ichthyosaurs, mosasaurs, nothosaurs, thalattosaurs, thalattosuchians and the group which is the focus of this post, the plesiosaurs. All these reptile groups made the transition from being terrestrial animals to secondarily aquatic reptiles at various points during the Mesozoic era but from the reptiles, only turtles and snakes and iguanas have modern representatives in the seas today.

The plesiosaurs first appeared in the Late Triassic (Taylor & Cruickshank 1993) and had become widespread by the Jurassic. Plesiosaurs are distinguished by having a unique body plan which consists of a short, stiff trunk, short tail, all four limbs modified into flippers and highly variable neck length and skull size (Benson et al. 2013). Although there were also intermediate body shapes, the group tend to be fall into either the long necked, small skulled plesiosaurs or the short necked, large skulled pliosaurs. In addition to being adaptable morphologically, they were also adaptive ecologically too, as they are also known from freshwater localities including some from Australia.

Skeletal drawings of the two man body plans of the plesiosaurids: A, the long necked, small skulled plesiosaur and B, the short necked, large skulled pliosaur. Image from Springer Images.
Skeletal drawings of the two man body plans of the plesiosaurids: A, the long necked, small skulled plesiosaur and B, the short necked, large skulled pliosaur. Image from Springer Images.

Freshwater plesiosaurs are known from two localities in Victoria, Australia. These are the Otways and Flat Rocks localities, both of which are from the same formation (Eumarella Formation) and represent a time in the Early Cretaceous when the two localities were part of the same depositional basin. Southeastern Australia in the Early Cretaceous was joined to Australia (although they were in the process of rifting apart) and situated within the Antarctic Circle. The area was a flood plain that was filled with braided river channels that would have burst their banks when the spring thaw floods passed through each year. These flood waters carried with them the remains of animals that had perished in the waters and been carried downstream and it’s their fossils that we search for at these localities today (see here and here for previous posts on each locality).

In a new paper to be published in the next issue of the journal Alcheringa, Roger Benson and colleagues describe a freshwater plesiosaurian tooth (NMV P198945) from the Eumarella Formation in Victoria, Australia. The interesting thing about the tooth is that it is different to any other plesiosaur teeth that have been found at these localities previously. It is much larger than all other plesiosaur teeth discovered; it has a length of 45.1 mm, whereas the other teeth from that locality are between 10-30 mm in length. It has ridges on its enamel which are spaced apart from each other and stop short of the crown apex (the top of the tooth), differing from the smaller teeth from the locality, which possess ridges that are closer together and continue closer to the apex of the crown.

Figure showing NMV P198945 in A, C, undetermined, B, labial, and D, lingual views. Image from Benson et al. 2013
Figure showing NMV P198945 in A, C, undetermined, B, labial, and D,
lingual views. Image from Benson et al. 2013

So what does this new tooth mean for our interpretation of the palaeoecosystem at this locality? Well, this is the first unambiguous evidence of a multitaxic (multiple species) freshwater plesiosaur assemblage known anywhere in the world.  In freshwater river systems today there are equivalents in the form of river dolphins from China and the Amazon. Whilst the Chinese dolphins might actually be extinct, the Amazon dolphins partition food and habitat to avoid directly competing with each other. A similar thing may have been happening in the Early Cretaceous of Victoria with these freshwater plesiosaurs. The estimated size of the animal that this tooth belonged to is a seriously impressive 4-5 m, a true river monster if there ever was one! If those physicists ever do get round to that time machine, I hope they let Jeremy Wade use to do a palaeo special episode for this beastie. Jeremy, you might need a bigger boat…

(For those of you who don’t know who Jeremy Wade is, he is the host of a show on Nat Geo called River Monsters, definitely worth checking out!)

References

Roger B.J. Benson , Erich M.G. Fitzgerald , Thomas H. Rich & Patricia Vickers-Rich (2013): Large freshwater plesiosaurian from the Cretaceous (Aptian) of Australia, Alcheringa: An Australasian Journal of Palaeontology, DOI:10.1080/03115518.2013.772825

Taylor, M. A. and Cruickshank, A. R. I. 1993. A plesiosaur from the Linksfield erratic (Rhaetian, Upper Triassic) near Elgin, Morayshire. Scottish Journal of Geology, 29, 191-196.

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