Thursday, November 27, 2008

Closing in on turtle origins


ResearchBlogging.org
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

6 comments:

Mike Taylor said...

I hate to give away how ignorant I am in a public forum, but what the heck. I am told that Odontochelys suggests an aquatic origin for turtles; but if so, that implies that tortoises re-evolved a terrestrial foot. Doesn't that seem a little unlikely? (Or, maybe I should say "a little unparsimonious", which is what scientists are meant to say instead of "unlikely".)

220mya said...

Mike;

I don't think it is very unlikely at all. Even if Odontochelys is aquatic/marine, and I have some doubts, its clear that a wide variety of more derived stem-turtles were terrestrial. The interrelationships near the base of the crown of turtles are quite contentious, which makes inferring the basal ecology of crown turtles problematic. My understanding though is that basal testudinoids do not have a specialized foot like tortoises do.

The general rule is that unlikely does not mean impossible. Structures and complex adaptations often evolve multiple times. There are studies that show viviparity has evolved many times separately in lizards, and that the switch from metamorphosis to direct development has gone back and forth many times in certain clades of lissamphibians. See for example:

Wiens, J.J., C.A. Kuczynski, W.E. Duellman, and T.W. Reeder. 2007. Loss and re-evolution of complext life cycles in marsupial frogs - does ancestral trait reconstruction mislead?. Evolution 61(8):1886-1899. DOI: 10.1111/j.1558-5646.2007.00159.x

220mya said...

A fossil turtle specialist I know says: "tortoises evolved from an aquatic testudinoid ancestor in the early Cenozoic." So, tortoises evolved any terrestrial specializations independently of stem turtles or other terrestrial turtle clades.

Zachary said...

The paper didn't really address HOW the authors closed in on the sauropterygian sister group origin. Even the supplamentary material failed to offer the characters used in the analysis. The skull is very anapsid-like, and I just don't see the diapsid specialties, much less the sauropterygian synapomorphies, in the skeleton!

If you're going to compare the group to sauropterygians, you may as well also compare them to thalattosaurs too, which as far as I'm aware are just outside Sauropterygia proper.

Nick Gardner said...

Zach, I hate to have to track after you like this, however, you are being misleading. The authors did not include a complete character list, that is true. However (!), they did indicate what their matrix was based on (e.g. - Rieppel and Reisz 1999*). They explained which characters they altered, which ones they added, character scorings that they changed, and and the scorings for Odontochelys (the only taxa they added, as far as I can tell from their description).

* Tip: Google search for the following string: "The origin and early evolution of turtles" filetype:pdf

Nick Gardner said...

"BTW, could someone please convince Adam Yates to allow name/URL comments at Dracovenator? I'd love to comment there, but I'm certainly not going to the trouble of getting a Google/Blogger ID or an OpenID just for that purpose."

http://chinleana.blogspot.com/2008/11/rethinking-turtle-origins-odontochelys.html?showComment=1227886920000#c8940802981286581183