After spending the whole week in hospital (it wasn't just tonsilitis after all) Matthew will probably be coming home tomorrow ..... and one of palaeontology's holy grails - a basal stem turtle has been found and published.
I imagine most of my readers know that pinning down the closest relatives of turtles and the origins of their bizarre morphology has been one of the most recalcitrant problems in tetrapod evolution.
While the lack of holes in the cheek region of the skull suggests that they are an offshoot of the anapsid reptiles, with procolophonoids or pareiasaurs being the main contenders, more than one large scale morphological cladistic analysis has found that turtles are members of the Diapsida (derived reptiles with two pairs of holes in the temporal region of the skull). Molecular work also supports a diapsid origin for turtles but tends to place them close to, or within, the Archosauria (crocodilians, dinosaurs, birds and their kin)whereas the morphological work tends to align them with the Lepidosauromorpha (lizards, snakes and kin).
Sadly the new turtle described by Walter Joyce and colleagues in Proceedings of the Royal Society is too fragmentary to really speak to this vexatious issue. But it does tell us quite a bit about how their most distinctive feature - their shells - evolved and lets us know that the answers are out there (given the recent spate of excellent discoveries in the Chinle and its equivalents I'm sure a more complete proto-turtle won't be be a long time coming).
But first a little of the who, what where and when. The new fossil is called Chinlechelys tenertesta (the thin shelled turtle from the Chinle)and the known remains consist of fragments of shell with attached underlying parts of the vertebrae and ribs and isolated osteoderms found in the Bull Canyon Formation of New Mexico. The name is taken from the Chinle Formation, which is a little odd since most stratigraphers and palaeontologists would not to place the Bull Canyon Formation in the Chinle Formation (or Group according to some), prefering instead to place it in the Dockum Group. The age of this part of the Dockum is probably mid-late Norian of the Late Triassic, somewhere between 210 and 219 million years old.
There are other equally old turtles in Europe, Greenland and South America but none so primitive in shell design as Chinlechelys. For starters its shell is unusually thin but more importantly it still has rather individualised ribs, that although joined to the inside of the shell, are not fully subsumed into it.
The telltale fragment - a piece of shell with an individualised rib beneath it.Modified from Joyce et al. 2008
This is important because it helps falsify the model that the shell is a modified ribcage alone and supports the hypothesis that the shell is a composite structure formed from the agglomeration of trunk vertebrae, ribs and several rows of osteoderms (bony armour in the skin). Intriguingly modern developmental studies have suggested the former hypothesis because it appears that the shell grows as part of the endoskeleton (that is it is made of replacement bones that are preformed in cartilage and derived from scleritomic mesodermal cells. Developmental studies can yeild powerfull evolutionary data but I think the takehome message here is that developmental pathways can and do evolve. Palaeontology definately still has a place at the high table of evolutionary studies!
Hypothetical steps in the evolution of the turtle shell following the osteoderm hypothesis. From Joyce et al. 2008
Bonus musing: If the molecular data are correct it is perhaps no co-incidence that some pseudosuchian, or crurotarsan - whatever you preference, archosaurs have rather turtle-like carapaces. Aetosaurs even have a plastron of plate-like osteoderms.
Paratypothorax, an aetosaur. Compare to hypothetical stage II above. Image from www.dinotime.de
WG Joyce, SG Lucas, TM Scheyer, AB Heckert, AP Hunt (2008). A thin-shelled reptile from the Late Triassic of North America and the origin of the turtle shell Proceedings of the Royal Society B DOI: 10.1098/rspb.2008.1196
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