Convergent Mososaur/Ichthyosaur evolution. – [Retrospective Post]

A research team from Sweden, Canada and the US have recently published a paper in the PLoS one journal which looks set to redefine the way vertebrate palaeontologists look at a whole group of mesosoic marine reptiles. Unlike the recent reclassification and demotion of the Torosaurus genus there has been little reaction to this online.

The paper; entitled “Convergent Evolution in Aquatic Tetrapods: Insights from an Exceptional Fossil Mosasaur” describes the exceptional preservation of a mososaur specimen of the species Platecarpus tympaniticus, including significant soft-body preservation. The researchers then go on to discuss the convergent evolution that this mososaur species exhibits with other marine vertebrates.

The largest single paradigm shift in the paper is the observation that the fossil’s tail vertebrae fall into four distinct sections and that the tail bends downward approximately three fifths of the way down the tail (posterior to the animal’s rear paddles). This downward bend in the vertebral column is something that is shared with other marine reptiles including the Ichthyosaurs and Metriorhynchidae. Modern sharks show a similar – but upward – bend in their cartillaginous skeletons.

This “kink” in the tail of the animals is indicative of the existence of a caudal fin (or tailfin), like those seen in modern fish such as sharks. Mososaurs had until recently been considered to be very crocodilian in their body plan; so much so that historically the tails had routinely been straightened by overzealous palaeontologists; incidentally you can still buy a model Ichthyosaur skeleton from National Geographic which amongst other inaccuracies displays a straightened tail.

One other mososaur skeleton is known to have exhibited a caudal fin, that being the  mososaur described by Lindgren et al. in 2007 as Plotosaurus which existed later in mososaur evolution and was highly specialised. Platecarpus however is a much older animal (preceding Plotosaurus by 20 million years) and helps to demonstrate the evolutionary path taken by this taxon that led to more fish-like descendents such as Plotosaurus. The interesting part of all this has already been pointed out by Brian Switek over at the Guardian Science Blog that this is another example of marine reptiles adopting the downward tail-bend in preference to any other… which begs the question why? what caused this evolutionary preference?

Another notable conclusion of the research is that there is not only convergence in the evolution of marine reptiles and cetaceans (whales) but also in the rate of evolution, with all four major animal groups (Ichthyosauria, Metriorhynchidae, Mososauridae and Cetaceans) attaining a streamlined, piscine (fish-like) shape within the first ten million years of divergence from land antecedents.

This Story Features in PalaeoNews : Webisode 3 (18th Aug – 05th Sept)

Sources and Related Articles:

• The Mosasaur’s kinky tail (guardian.co.uk)
• Mosasaur fossil at Natural History Museum of L.A. County re-explores 85-million-year-old sea monster (eurekalert.org)
• ScienceShot: Rapid Evolution for Ancient Sea Monsters (news.sciencemag.org)