Half shelled turtle fossil found, according to reports in Nature News, ScienceNOW and BBC News 26 Nov 2008 and Nature vol. 456, p497, 26 Nov 2008.
Turtles have a two part shell – a lower shell, called a plastron, that forms a shield on its underside, and an upper shell, called a carapace, which covers the back. Palaeontologists in China have found a fossil turtle that has a plastron, but not a carapace.
The fossil was found in the Nanpanjiang Trough Basin in south western China and is dated at 220 million years old, making it the oldest known fossil turtle. The 40 cm fossil turtle had long forelimbs similar to living turtles, but unlike living turtles, had a long tail and teeth. It has been named Odontochelys semitestacea, meaning half-shelled turtle with teeth.
Chun Li, from the Institute of Vertebrate Palaeontology and Palaeoanthropology at the Chinese Academy of Sciences, one of the scientists who studied the fossil, claims it is “the missing link of turtle evolution”. He told Nature News: “We found how the turtle shell formed. It is not derived from a fusion of osteoderms. It is the first fossil evidence to show that the turtle may have originated from a water rather than land environment.”
However, not all palaeontologists agree with this interpretation. Robert Reisz and Jason Head, palaeontologists at the University of Toronto, Canada, suggest the lack of a carapace is an adaptation to living in the sea. Reisz commented in a Nature podcast: “The paper suggests there is a disconnect between the evolution of two parts of the shell. This is an interesting idea but we disagree. Because what you see in turtles that live in marine environments today is that their shell is reduced. So another interpretation of the fossil could be that this form of the Chinese fossil is actually a specialised condition, so that this shell may actually be in the processing of reducing.” In response to this suggestion, Oliver Rieppel of the Field Museum in Chicago, Illinois, one of the team who reported the fossil in Nature, commented: “If you look at aquatic turtles alive today that have reduced carapace, none match the patterns seen in the fossil. Our fossil looks like the embryonic pattern.”
Editorial Comment: The debate between the two groups of evolutionary scientists is good illustration of the difference between actual evidence and the interpretation of the evidence. The evidence is the same in both cases – a turtle fossil with a plastron, but no carapace. The idea that it represents a turtle in the process of gaining a shell or losing a shell is an interpretation based on prior beliefs. We could add a third view, since this fossil is different to other known turtles because of the teeth and tail, it could be a previously unknown species, and there were once more kinds of turtles than exist now.
The absence of the carapace could be a developmental defect where the upper shell failed to grow. Rieppel’s comment about it being similar to the embryonic pattern of modern turtles would support this idea. The degenerate or new species concepts fit well into the Biblical history of the world, which tells us animals were created as fully functional creatures in different kinds. Since then the world has become a harsher place causing many animals to die out or suffer disease and degeneration.
Evidence News 12 December 2008
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