Historical Papers
Baur, G. (1891). On the Relations of Carettochelys, Ramsay. American Naturalist. 1891:631-639.
This was bad news. Nothing left of the bones but the skull! But probably it was possible to determine the systematic position of the interesting animal from the photographs, which were on the way. A few days after the letter the photographs came: 1, two upper views of the entire animal ; 2, one lower view ; 3, the upper view, and 4th, the lower view of the posterior portion of the skull. To Prof. Ramsay I have to express my best thanks for his kindness and liberality.
The skull at once showed that this was no Pleurodiran ; that its nearest living relatives appeared to be the Trionychia, its very closest fossil relative the peculiar Pseudotrionyx Dollo, from the Eocene, which I always had suspected as such.
The skull is only comparable with that of the Trionychia. As in this group, we have three greatly developed, crest-like posterior processes : the supaoccipital, and on each side the squamosal. The supraoccipital process is a club-shaped and enormously developed,---More than in any other tortoise known. Of course this charachter alone would not be sufficiant to establish absoloutely the near affinity of the peculiar form with the Trionychia; such a development of the posteria portion of the skull could take place in the Pleurodira or Cryptodira just as well. But there are other characters which at once show that the form has nothing to do with the Pleurodira. Before all, the pterygoids extend behind between quadrate, basiphenoid, bassioccopital, a condition never seen in Pleurodira. Wether the pterygoids are completely seperated by the basiphenoid as in the Trionychia cannot be seen from the photographs; this question, therefore is still an open one. The quadrate is perculiar. The articular face with the lower jaw is Trionychian, not Pleurodiran; and so is the posterior end of the lower jaw. The quadrate is not completely closed behind, but only on its outer border, as in Podocnemis, for instance, but not in such a great degree. As is well known, the quadrate of the Trionychia is completely closed behind; this, of course, is a secondary condition, and there cannot be any doubt that the ancestors of the Trionychia had quadrate open behind. The quadrate of Carettochelys is exactly of such a form which we may expect in the ancestors of the Trionychia. The pterygoids resemble very much the same elements in the Trionychia. The lower jaw is rounded in front and has a short symphysis. The upper side is of the skull is very interesting. The greatest peculiarity is that the upper surface of the of the bones is granulated exactly as the shell. The dermal plates described by Ramsay do not exist ; there are no plates on the skull at all. This peculiar condition is only found in the Jurrasic Compsemys plicatulus Cope. The sutures of the bones of the upper side of the skull, which can be seen, just as the sutures of the elements of the carapace and plastron are visible, must have been taken as indications dermal plates by Prof. Ramsay.
The interorbital space is very large, the orbits being completely lateral ; the potorbital arch is about half of the interorbital space. The whole upper aspect of the skull reminds us of the Dermatemyndidæ, Staurotypidæ, Cinosternidæ ; and the arrangement of the elements is the same, the frontals being excluded from the orbits. There is no indication in the photographs of free nasal bones. The nose is projected much in front, and must have, when in fresh condition, an appearance very much like that in the Trionychia, but not so much pointed. The zygomatic arch is not elevated as in the Trionychia, but is in a line with the maxillary and quadrate, as in the Cinostnidæ, for instance.
The neck, the vertebrae of which were unfortunately not pre- served, was short; but I do not see any reason why the head could not have been retracted, as in the Chelydridæ, for instance. Nothing is known about the shoulder-girdle and the pelvis. But one thing seems to be sure : the pelvis was not coosified with the carapace and the plastron, but free. If it had been coosified with the shell, as in the Pleuridira, it probably would have been preserved with the shell. An important question is the number of Phalanges in the fourth digit ; as is well known, in all Trionychia we have more than three phalanges in the fourth digit. It looks to me, as far as I can conclude from the photographs, that in Carettochelys the number three was not surpassed.
We have now to consider the carapace and plastron. Both have been figured by Ramsay, but there was some doubt about the presence or absence of a mesoplastral element. In reguard to the carapace, I have nothing new to add. This first six pleuralia meet in the middle line behind, being separated in front by the neuralia. The seventh and eighth neuralia touch each other completely in the middle. There is only one postneural. The number of the perpheralia (marginal bones) is ten on each side, besides the single pygal. The most interesting new point to be noted in the plastron is the presence of a small distinct mesoplastral element. The structure of the plastron is best seen from the figure.
I have stated above that Pseudotrionyx is the nearest relatve of Carettochelys. Peudotrionyx was described by Dollo (6) in 1886. The portions found in the middle Eocene of Belgium consisted of the posterial part of the carapace, and the nearly complete hyo-, hypo-, and xiphiplastron of the right side. The sculpturing of the shell is the same as in Carettochelys. There is no trace of dermal scutes. The number of the peripheralia is the same as in Carettochelys. There is only one postnueral, of the same shape as in this form. There is a difference in the neuralia, however. There are seven slender neuralia in Pseudotrionyx, which are all connected with each other, seperating the first of the six pleuralia completely; the seventh pleuralia meet behind, and the eighth are entirely connected. In all the pleuralia the rib heads are well developed. If we now compare the plastron of Carettochelys with the portions preserved in Pseudotrionyx, we are struck at once by the enormous resemblence. The hyoplastra of both is are nearly identical in shape. I may call especial attention to the border connecting the hyoplastron with the endo- and epiplastron. But to conclude from Dollo's figure, it seems to me that the hyoplastron was not entirely united to these elements, but only connected with them by ligament, as in the Cinosternidæ. The most interesting point, however, is that Pseudotionyx doubtles also had a distinct mesoplastral element as Carettochelys. Dollo held on the opinion that there was a small fontanelle at the outer border of the hyo- and hypoplastron (Enchancrure naturele, reste d'une fontanelle laterale, N. Fig. I. Pl. II) Besides, he thinks that the line of the connection between carapace and plastron was very short. there cannot be any doubt, however, that Pseudotionyx showed about the same conditions as Carettochelys.