Thursday, 25 June 2015

Another Transitional Fossil Turtle!

Grandfather turtle, Pappochelys
Photo credit: Rainer Schoch. Source:
A Middle Triassic stem-turtle and the evolution of the turtle body plan : Nature : Nature Publishing Group

These examples of what creationists tell us don't exist - transitional fossils - are getting so frequent that I'm wondering whether it's worth writing yet another blog about yet another one being found, or whether it would be simpler to just publish a weekly list of the latest.

But, having almost certainly just shed any creationists who might have had the courage to read the blog at all with that shocking title, I'll carry on, as those interested in truth might find it interesting.

This example, published in Nature yesterday, is of a 240 million year-old fossil from Germany which is probably an early stem species of the turtles. It is undoubtedly a lizard-like reptile but has some features which look a lot like early turtle features - namely the beginnings of a carapace and the supporting ribs.

There are other transitional fossils showing the evolution of turtles from lizards, such as an unmistakeably transitional 220 million year-old specimen from China which had a partial shell, and an African one showing tentative suggestions of transition, but this new one fits neatly between these two both chronologically and morphologically.

The origin and early evolution of turtles have long been major contentious issues in vertebrate zoology. This is due to conflicting character evidence from molecules and morphology and a lack of transitional fossils from the critical time interval. The ~220-million-year-old stem-turtle Odontochelys from China has a partly formed shell and many turtle-like features in its postcranial skeleton. Unlike the 214-million-year-old Proganochelys from Germany and Thailand, it retains marginal teeth and lacks a carapace. Odontochelys is separated by a large temporal gap from the ~260-million-year-old Eunotosaurus from South Africa, which has been hypothesized as the earliest stem-turtle. Here we report a new reptile, Pappochelys, that is structurally and chronologically intermediate between Eunotosaurus and Odontochelys and dates from the Middle Triassic period (~240 million years ago). The three taxa share anteroposteriorly broad trunk ribs that are T-shaped in cross-section and bear sculpturing, elongate dorsal vertebrae, and modified limb girdles. Pappochelys closely resembles Odontochelys in various features of the limb girdles. Unlike Odontochelys, it has a cuirass of robust paired gastralia in place of a plastron. Pappochelys provides new evidence that the plastron partly formed through serial fusion of gastralia. Its skull has small upper and ventrally open lower temporal fenestrae, supporting the hypothesis of diapsid affinities of turtles

Since this transition was taking place when all the main landmasses formed the single super-continent, Pangea, finding these related transition species in now widely separated places such as South Africa, China and Central Europe, is what we would expect.

It seems also that the close association between water and turtles is a long one. This fossil was found in an ancient dried up lake bed in Germany and one suggestion for why Pappochelys began to evolve thickened ribs could have been to provide ballast and so improve buoyancy in what was an aquatic or semiaquatic lizard, as well as adding to the protection the developing line of hard, shell-like scales on its underside that were to eventually form the ventral half of the carapace, were providing.

If confirmed, this finding also resolves the debate about how turtles relate to other reptiles. Are they closer to the lizards and snakes or to the dinosaurs and their modern descendants, birds? This fossil supports the former hypothesis.

So, not so much a new transitional fossil as a transition between two other transitional fossils. Those gaps so beloved of creationists just keep getting smaller and smaller as even the gaps between the former gaps are being filled.

'via Blog this'

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1 comment :

  1. So the old hypothesis that the Testudines were descended from an ancestrally anapsid stem reptile has been discarded? That solves a bunch of problems, especially the ones that arise if turtles diverged before the synapsid-diapsid split. A secondarily anapsid turtle lineage diverging from the lepidosaurs early cleans up the phylogeny even more.


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