Monday, 24 September 2012

Plesiosuchus vs. Dakosaurus... Fight!

Last Tuesday (18.09.12) a paper was published in PLOS ONE on the metriorhynchid crocodile Dakosaurus, resurrecting the name Plesiosuchus manselii for a large specimen that had been synonymised with Dakosaurus maximus in the late 19th Century (Lydekker 1888). There is not necessarily any reason that this name change would be picked up by popular science, but there are a number of implications made by the authors (Young et al. 2012 a) which made it incredibly interesting. So much so that when given the opportunity to blog on the same subject matter after Lorna Steel's (Young et al. 2012 b) talk at SVPCA Oxford I decided to hold back until the paper came out...

Metriorhynchid crocodiles were a group of thalattosuchian crocodylomorphs with a temporal range (time span) from the Middle Jurassic to the Lower Cretaceous. The group was made up of fully marine crocodiles which were highly specialized for their marine environment. Fossil evidence shows that much like other secondarily marine tetrapods (four legged land animals) [i.e. ichthyosaurs, whales and dolphins] metriorhynchids had tale flukes, like those of sharks. In addition to this clear specialization, the metriorhynchids had hydrofoil-like forelimbs, large salt-glands and lost their armoured plates. These animals filled several niches in the marine environment, they developed numerous cranial and dental morphologies and varied in size greatly. Within the Metriorhynchidae two major radiations are persistently recovered by cladistic analyses; the Metriorhynchinae and the Geosaurinae. The members of the Geosaurinae are usually interpreted as being super-predators and among them, Dakosaurus is interpreted by Young et al. (2012 a) to be macrophagous: "an animal that could feed upon prey items of similar body size" (Young et al. 2012 a). Plesiosuchus is also interpreted to be a macrophagous predator, but differences in anatomy and dental morphology led the authors to suggest a different niche for this creature and to reject Lydekker's (1888) assertions that Plesiosuchus was synonymous with Dakosaurus. This renewed view of diversity is supported by a cladistic analysis (fig. 26 Young et al. 2012 a) [Fig. 1].
Fig. 1 (fig. 26 of Young et al. 2012) cladistic analysis of crocodylomorphs, showing Plesiosuchus (bold) separate from Dakosaurus (bold). Used with the permission of authors.

By comparing Dakosaurus and Plesiosuchus with modern killer whales and false killer whales, the research team have suggested that the smaller Dakosaurus is a possible suction feeder, or predated upon abrasive prey such as sharks, whereas Plesiosuchus predated upon other large marine reptiles. This interpretation is supported by evidence of the animals relative size, their tooth wear and the maximum gape of their jaws. It seems that the smaller Dakosaurus (~4.5 m) could tear up its prey and was not limited on the size of the prey that it could capture and consume. Distinctive wear patterns on the mesial and distal carinae (front and back cutting edges) of the teeth show that the teeth were shearing, capable of ripping chunks of flesh off their prey. Whereas Plesiosuchus (~6.8 m) was rather more limited by the size of its head, making it more specialized, but nonetheless a large predator sharing affinities with killer whales, which eat other marine mammals. Either way, both would have been fearsome predators of the Late Jurassic seas. However, despite Dakosaurus' incredibly efficient bite, it seems that it may have fallen victim to the larger, veracious Plesiosuchus.

Art work by Dmitry Bogdanov (fig. 30 of Young et al. 2012)

 See also:
The original paper
The NHM article

Lydekker, R. 1888. Catalogue of the Fossil Reptilia and Amphibia in the British Museum (Natural History), Cromwell Road, S.W., Part 1. Containing the Orders Ornithosauria, Crocodilia, Dinosauria, Squamata, Rhynchocephalia, and Proterosauria. London: British Museum of Natural History. 309 p.

Young, M.T. Brusatte, S.L. de Andrade, M.B. Desojo, J.B. Beatty, B.L. Steel, L. Fernández, M.S. Sakamoto, M. Ruiz-Omeñaca, J.I. Schoch, R.R. 2012 a. The Cranial Osteology and Feeding Ecology of the Metriorhynchid Crocodylomorph Genera Dakosaurus and Plesiosuchus from the Late Jurassic of Europe. PLoS ONE 7(9): e44985. doi:10.1371/journal.pone.0044985 

Young, M. Brusatte, S. de Andrade, M. Desojo, J. Beatty, B. Steel, L. Fernández, M. Sakamoto, M. Ruiz-Omeñaca, J. & Schoch, R. 2012 b. Comparative cranial osteology and feeding mechanics of two Late Jurassic macrophagous metriorhynchids from Europe. Programme and Abstracts, SVPCA, 60th Annual Symposium on Vertebrate Palaeontology and Comparative Anatomy. p.22

No comments:

Post a Comment