Re: Eunotosaurus and turtles

[David Marjanovic <david.marjanovic@gmx.at>]

> 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.