Thursday, November 27, 2008
Closing in on turtle origins
I wrote in this blog in October that we can expect a more complete prototurtle. Never would I have dreamed that it would appear so quickly and that it would be even more primitive than Proganochelys or Chinlechelys. Finally something that hasn’t progressed so far down the road to turtlehood that its ancestry has been all but erased. Named Odontochelys semitestacea, it came as quite a shock to me – why? Because of all the competing hypotheses of turtle origins this guy seems to support the one I found the least convincing – that is turtles are the sister group of sauropterygians (an aquatic group of diapsid reptiles including placodonts, nothosaurs and plesiosaurs). My own musings that aetosaurs might be related to turtles now seems very unlikely indeed. It will take time for the remains of Odontochelys to be hashed over (the announcement paper is somewhat light on anatomical detail) to really determine what origin theory it supports. However I think we can now confidently rule out a the pariesaurian hypothesis. Of the synapomorphies linking pareiasaurs, or derived subclades within pareiasaurs, to turtles a good many of them are missing in Odonotochelys. These include: basal tuberae (ventral swelling of the braincase) midway between the occipital condyle and the basipterygoid processes (where the palate attaches to the braincase); acromial process on the scapula, closure of the spaces between the ribs (it is debatable wether or not any turtle actually has this condition); fewer than twenty caudasl vertebrae; and body covered in united osteoderms. Note that although it appears to support a diapsid origin for turtles the skull of Odontochelys lacks any trace of temporal openings so perhaps we can't quite rule out other anapsid sister groups just yet.
Not a part of the turtle sister group, the pareiasaur Bradysaurus. Image from wikipedia commons
Odonotchelys is yet another gem from the palaeontologically rich nation of China, this time from the marine Triassic deposits of the Guizhou Province, which are famous for their diverse Marine reptile fauna. One last note on the dating. The age of Odontochelys is given as about 220 ma based on biostratigraphy which places the unit it comes from in the Lower Carnian. As discussed recently by Bill Parker over at Chinleana the dates for the Triassic have been substantially revised of late, and if a lower Carnian age is to be upheld for Odontochelys then its absolute age is probably closer to 235 ma.
Chun Li, Xiao-Chun Wu, Olivier Rieppel, Li-Ting Wang, Li-Jun Zhao (2008). An ancestral turtle from the Late Triassic of southwestern China Nature, 456 (7221), 497-501 DOI: 10.1038/nature07533