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

The giant pineal 'eye' of Platycyclops

The answer to the the little photo quiz was as the title suggests the relatively enormous pineal foramen of the oudenodontid dicynodont Platycyclops - so kudos to Matt. Quite why these dicynodonts have such enormous pineal openings is an unanswered question. Modern mammalian pineal glands are buried deep in our grossly inflated brains but are still used to regulate day/night cycles and seasonal cycles. In mammals it is influenced indirectly by exposure to light via signal that originate from the retina. In other vertebrates with a pineal foramen direct exposure of the pineal itself triggers the pineal gland to secrete its regulatory hormones. Pinealocytes have a strong resemblance to retinal cells and it is certainly tempting to speculate that the pineal organ of Platycyclops and related dicynodonts had crude image forming abilities. The forward tilt of the opening certainly gives the impression of a third 'eye'.
Googling around for information turned up one other little factoid, the name Platycyclops Broom 1932(the dicynodont) is a junior homonym of Platycyclops Sars 1914 (a copepod crustacean). As far as I know no-one has proposed a replacement name for the dicynodont.

Picture of the Day: long tailed widow bird

Just a quick post while I'm on leave - I'll be revealing the mystery orifice soon, so keep guessing - one of you is very close. We took a family outing to the rhino and lion park in ‘The cradle of humankind’ a world heritage area that includes the famous Sterkfontein and Swartkrans caves where several australopithecines have been found. While there I managed to get this shot of a breeding male long-tailed widow bird (Euplectes progne), the epitome of an elaborate sexual display that is a major handicap to its owner. Non-breeding males are drab, brownish, sparrow-like birds with tails of normal length.

What is it?

Yes, another round of 'what is it?' I'm off on leave for a week. I hope I can post something in the during this time but in case I don't have a go at guessing the identity of this osteal orifice. Clue: it has nothing to do with any sort of sauropodomorph (for once). Oh the scale bar represents 2 cm.

Defending the indefensible

A couple of weeks back there was a fresh round of bashing on poor little Pantydraco. Bashing the name, not the dinosaur that is. It is a matter of some embarassment to me that I should be tied to what is widely regarded as the worst dinosaur name ever. I take pride in the names I craft, and I don't think I'm too bad at it (Dracovenator, Antetonitrus, Nanalania are some faves) even if I do say so myself.
So here is some of the backstory behind the name. Firstly: it was not my invention! That particular dubious honour goes to Peter Galton. When I first named the species caducus I found some characters that linked it with what I was calling (and still call) Thecodontosaurus antiquus, so thought that the wisest course of action was to name a new species in that genus. However as I found out more about Thecodontosaurus antiquus (and I regard all of the English cave fill sauropodomorphs as one taxon) more and more differences with the Welsh ‘Thecodontosaurus ‘ caducus started to show up. For instance the ischial shafts of T. antiquus are an unusal flattened ovoid cross-sectional shape whereas those of ‘T.’ caducus have the classic triangular cross section seen in most other basal sauropodomorphs. Significantly support for a monophyletic Thecodontosaurus had diminished to the point that it was no longer recovered in all of the most parsimonious trees of my improved cladistic analyses (as more characters were added and more scorings were based on first hand observations) . So I had come to the conclusion that it was time to erect a new genus for ‘T.’ caducus. I even had a tentative name thought up – Cambrambulus – the Welsh wanderer. However I was not quick enough and at the 2005 SVP Peter Galton told me that he was close to submitting an MS giving caducus that infamous generic name.
Naturally I felt rather attached to the first dinosaur species that I named and wanted to remain associated with the generic name (which after all is the most frequently cited name in dinosaur circles). So I told Peter about my plans and we agreed to write the paper together, even though our reasons for naming the new genus were different. Peter felt that Thecodontosaurus was a dubious name because he thought that the type specimen of T. antiquus (the type species of the genus) was indeterminate. I agree that it isn’t the best and that it has no single derived character that cannot be seen in other sauropodomorph taxa. However I am still unconvinced that there are two morphs in the original Durdham Downs quarry, or that Asylosaurus is all that distinct. Taken collectively the Thecodontosaurus antiquus sample is unique and diagnosable. The utility of bone bed taxa is itself the subject for another post but to put it simply I’m all for using them in the right circumstances. In this case the surviving scanty sample from Durdham Downs is backed up by a large sample from Tytherington . This is another set of sauropodomorph bones from a fissure fill in the English south-west. Here there is absolutely no sign of two morphs and all of the recovered bones are virtually identical with those from Durdham Downs.
So there we are, the Pantydraco paper is very much a compromise - melding two different viewpoints but agreeing that little caducus needed a new generic title. Perhaps I should have pushed Peter to change the name, but being my meek and mild self, didn't do so. In anycase I didn't think it was quite that bad at the time,although yes, I was quite aware of the conotations (I guess thats a little bit of my juvenile sense of humor showing though).
Lastly for all those who absolutely hate the name - remember it is correctly pronounced 'Pant - uh - dray - co' which is not quite so bad as 'panty'.

From the galleries of the BPI: The Cape Giant Zebra

This specimen is a partial set of jaws of Equus capensis, the so called cape giant zebra, from Makapansgat, the most northerly australopithecine site in South Africa. Actually, although robust these equids are not so giant, being about the size of a big modern horse. Fossils of this robust equuid are widespread throughout South Africa, with the type coming from close to Cape Town, way down in the southwest.
It is often said that Africa escaped the megafaunal extinctions of the late Pleistocene but there is a definite set of large African mammal species that clearly did not make it through to the present. These include the giant buffalo Pelorovis, other bovids like Megalotragus, and supposedly Equus capensis. But if Charles Churcher is right reports of E. capensis' demise are greatly exaggerated. It s apparently alive and well in the form of.... Grevy's Zebra.


Image from wikimedia commons

Apparently the teeth (which are what most extinct Equus taxonomy is based on)of E. capensis do not differ in significant ways from those of E. grevyi and a bunch of east and northern african fossil equiids (e.g. E. oldowayensis).
Grevy's zebra is now restricted to East Africa and cannot be found anywhere near Suth Africa. So if it doesn't represent an actual extinction it does represent a drammatic range contraction.

reference

Churcher CS (2006)Distribution and history of the Cape zebra (Equus capensis) in the Quaternary of Africa. Transactions of the Royal Society of South Africa 61:89-95

What was it?

That ugly mystery bone is out now. What a disaster! when lifting it from its jacket, an undetected puddle of glue had soaked through the sepparator and firmly welded the underside to the jacket (lesson for the future - make sure there are several layers of sepparator between the bone and the jacket). The remarkable thing about the bones from this quarry is that although the compact outer bone is well-preserved the interior is like compressed flour. Once a break is started the whole bone tends to crumble to powder, much like the unravelling of a woollen jumper once the stiching comes undone in one place. Consequently the finished product is not one of our lab's proudest moments and I won't be showcasing it here. Nonetheless two things are apparent 1) it is a busted ischial peduncle from an ilium - exposed in medial view in the photo. 2) this part alone is about the same size as a middling Massospondylus, so although Rutger was halfway right he doesn't get full marks. The diagram below indicates which part of the ilium the bone is, and should give the astute clues to where I think its relationships lie (incidentally it is from the same site as the sauropod caudal I described in the South African Journal of Science (2004, vol. 100. 504-506)