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Ben Brooks
reproduction of a painting of a mososaur but Charles R. Knight (1897) | via wikimedia commons.
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.
Shark Skeleton Diagram illustrating the up-bend in the vertebral column, Re-drawn from Gottfried, Compagno, and Bowman 1996 (via /www.elasmo-research.org/)
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?
Illustration of Platecarpus tympaniticus, including downward "kink" in the tail vertebrae. Image from figure 8 of Lindgren J, Caldwell MW, Konishi T, Chiappe LM (2010), PLoS ONE 5(8): e11998. doi:10.1371/journal.pone.0011998
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:
Triops Cancriformis, Image from Wikipedia
Researchers from Glasgow University have discovered colonies of the small crustacean Triops cancriformis in mud from around the scottish nature reserve at Caerlaverock, Scotland.
Triops are members of the order notostraca and cancriformis first evolved approximately 220 million years ago in the late Triassic and haven’t changed in morphology much since, as shown by fossil examples, leading some to describe them as living fossils.
Fossil Triops cancriformis, image via AnswersinGenesis (sadly)
The creatures were discovered after the research team had dried out and then re-wetted a set of mud samples from the area. Triops cancriformis is in fact an endangered species as before now it had historically only ever been found in two locations, the first being a single pond in the New Forest, Hampshire, England, and the second in the Caerlaverock nature reserve which was discovered some six years ago.
Now however it appears that thanks to the animal’s unusual lifestyle, there may in fact be many colonies in and around those which are already known. The Triops life-cycle includes an egg stage, which is resitant to extended periods of drought and as such allows the creatures to survive even when the pools in which they live are ephemeral (seasonal or otherwise temporary). They can even re-establish populations if only one individual survives because they can reproduce by a process called autogamy (or self-fertilisation) as they are hermaphroditic organisms with both male and female reproductive organs.
This Story Features in PalaeoNews : Webisode 1 (28th July – 3rd Aug)
Image via Wikipedia
John Scannella and Jack Horner, two researchers at the Museum of the Rockies, Montana have recently published a paper in the Journal of Vertebrate Paleontology entitled “Torosaurus Marsh, 1891, is Triceratops Marsh, 1889 (Ceratopsidae: Chasmosaurinae): synonymy through ontogeny” which has caused a firestorm in the internet community thanks to the mainstream media headline writers not reading the articles they are headlining. We must forgive them however as they are out to sell papers and make money, not tell the truth!
The paper argues that the two genuses (Torosaurus and Triceratops) which have been separate entities for over a century are in fact the juvenile and adult growth stages of the same taxon – but why?
The discovery centres around the study of over 50 specimens of the two genuses and both macro-scale comparative anatomy and micro-scale histology (study of the bone structure) of the skulls of the animals.
The big hitting piece (or should that be most obvious) of evidence is the hypothesised opening up of the parietal fenestre found in Torosaurus which are not included in the usual image of Triceratops with a short, solid shield. The parietal fenestre are the holes visible in the shield at the back of the head. On closer examination, many of the available Triceratops specimens showed signs of an incipient fenestre forming initially at the margin of the shield and then “migrating” toward their later place in Torosaurus as the shield grew around them, the shape of the thinned region being near circular and expanding with growth.
Proposed Ontological sequence from Triceratops (a-i) through to Torosaurus (j,k) scale bars 10cm (from fig. 3 of Scannella & Horner, 2010)
Other evidence that bolsters the researchers’ hypothesis includes the fact that up until this point, no juvenile, or even infantile skulls of Torosaurus have been confirmed. Inclusion of the Triceratops juveniles and the proposed growth sequence does accommodate this observation.
What are the implications of this research? well first off this isn’t the first case of an ontogenic synonymy (two or more species being found to be one), not even within the one formation from which Triceratops is known, with a citation in the paper for another article by Horner and others regarding other Hell Creek Formation dinosaurs. (unfortunately I couldn’t find an online copy so cannot give details).
The research also means that there is more support for a decline in the diversity of the dinosauria in the mid to late cretaceous, as there is now one less species surviving to the end of the cretaceous and coming face to face with the dinosaur’s bolide nemesis.
What this article does illustrate though is how careful palaeontologists have to be when it comes to their research, as we only have the fossils or trace fossils to work with, and because of the extreme rarity of fossilisation we cannot be guaranteed a complete ontological sequence of skeletons. When this is combines with events such as the great bone rush (both Torosaurus and Triceratops were described by O.C. Marsh during this time.) it can be easy to pidgeon hole specimens to such an extent that you end up splitting one into two!
As to the name change, because Torosaurus was named two years after Triceratops and taxonomy is done according to the Linnean system, the first name is kept. Hence Triceratops gets to live on, while Torosaurus is no longer a valid genus name.
This Story Features in PalaeoNews : Webisode 1 (28th July – 3rd Aug)
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A Rag on Bones 