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Re: Eunotosaurus and turtles



I do not know if there are features that indicate Eunotosaurus is less
related to turtles than to procolophonids, parieasaurs,
sauropterygians or archosaurs,

Well, *E.* hasn't been found close to the turtles in any phylogenetic analysis ever, though it hasn't been in many. It has so far only come out as either some very basal "parareptile"/"anapsid"/proganosaur, or as the sister-group of Millerettidae which also occupies such a position. Fairly far away from procolophonoids and pareiasaurs.


1- The widened ribs. Something also striked me as similar is that
there are 8 expanded ribs in each side in Eunotosaurus, while in
turtles the carapace is formed by 8 pleural plates/ribs!!!

Nine ribs take part in the carapace in *Proganochelys*, but only eight in *Odontochelys*. (This is not counting the first dorsal rib in *P.*, which is sutured or fused to the carapace but does not lie in the same plane as the other dorsal ribs and has a free distal end.)


Although this counts as just a similar apomorphy, the fact that
there is the same number of expanded ribs seems as much coincidential!!

*E.* has 10 dorsal ribs, of which all are broadened along at part of their length; the 10th is fused to the vertebra and apparently more or less conical. *Proganochelys*, however, officially has 10 dorsal ribs, of which the last 9 participate in the carapace; I think the so-called 8th cervical rib, which is much longer and taller than the other cervical ribs and lies behind the scapula (see below), should also be counted as a dorsal one. The first full-length dorsal rib is the 3rd one in *E.* but the officially 1st one in *P.* and apparently *Odontochelys* and the officially 2nd one in other turtles.


2- These dorsal ribs are said in Romer 56' to articulate with the
vertebra via a single attachment, as in turtles (I suppose it would be
the capitulum, as it seems to be low in the centrum).
3- Finally, the dorsals of Eunotosaurus are quite elongate, just as in
turtles!!!

All of these are correlated to restricted or lost mobility of the back: few longer instead of many shorter vertebrae, and less mobile ribs.


Eunotosaurus also has osteoderms,

No. Watson (1914) believed he saw some, but Cox (1969) says they were just discolorations in the matrix, and no osteoderms have been reported since.


Also, *E.* really does have broadened ribs. It's much like uncinate plates gone wild: there is a very low "keel" near the front edge of the plate, and the broadening extends all the way to the rib head. *Odontochelys* is different: the broad part does not extend all the way to the rib head, and the much more prominent "keel" is about in the middle of the plate, sometimes not parallel to its edges; this also seems to hold for *Proganochelys* (fig. 79 of Gaffney 1990). Finally, if you can, have a look at figs. 75 and 76 of Gaffney (1990), which shows a ventral view of the carapace of *Proganochelys*, fig. 91, a caudoventral view, and fig. 94, which offers cranial and caudal views, as well as fig. 104, which shows the type specimen -- a natural shell endocast without any bone --, and fig. 133, which shows a longitudinal section through the shell.. For part of their length, the ribs are vertical plates that stand out from the carapace, like in *Chinlechelys* (*), and especially the first and last few ribs lift off from the carapace completely for a smaller part of their length (the 9th and 10th actually for a greater absolute length than the 1st, which, see above, is not officially considered part of the carapace). I conclude that the costal and the neural plates meet, and the ribs and the vertebrae meet some distance under them; this is more easily compatible with the hypothesis that the costal plates are dermal bones than with the hypothesis that they are broadenings of the ribs.

(*) Gaffney (1990:122f.) talks of "the comparatively deep shaft of the first thoracic rib", "rib ridges", and "transverse dorsal plate"s of ribs, and mentions (p. 123): "The thoracic ribs, discussed above, are continuous with laterally directed ridges that decrease in height distally. Theses ridges, however, are much better developed in *Proganochelys* than in most other turtles because they are distinct for nearly half the distance to the lateral margin of the shell."

(Incidentally, fig. 76 shows the pits for the articulation of the scapulae on the carapace -- in front of the first dorsal rib. It's actually even in front of the last so-called cervical rib, as seen in fig. 83. Everyone in choir: the shoulder girdle lies inside the carapace, but in front of the ribcage. It also lies in front of the ribcage in *Eunotosaurus*... and thus in front of what is functionally a carapace...)

but these seem to be present in most of the taxa
considered as related to turtles (parieasaurs, placodonts).

Not in *Paraplacodus*, which is apparently the sister-group of the rest of Placodontia, and not in Sauropterygia (sister-group of Placodontia) either. On the other hand, there are rumors that some procolophonoids had osteoderms.


Christopher Barry Cox (1969): The problemantic Permian reptile *Eunotosaurus*, Bulletin of the BM(NH), Geology, 18(5), 165 -- 196
Eugene S. Gaffney (1990): The comparative osteology of the Triassic turtle *Proganochelys*, Bulletin of the AMNH 194, all 263 pages
David Meredith Something Watson (1914): *Eunotosaurus africanus* Seeley, and the ancestry of the Chelonia, Proceedings of the Zoological Society of London, 1914: 1011 -- 1020. I don't have access to this one, it's cited by Cox.