The Deseado Formation of Patagonia

by Frederic Brewster Loomis

Still other forms like Amblypoda and Arsinotherium have been suggested as having characters in common with Pyrotherium, and it is clear that, with such a variety of forms, some of the characters must be parallelisms due to a common adaptation, and only one of these varied groups can be the one to which Pyrotherium is related. For myself, I have made comparisons with the Amblypoda and Arsinotherium, and feel that such features as the massive limbs, shape of individual bones, etc., are due simply to the fact that all these are massive animals. In the case of Arsinotherium, there are some characters which are also common to hyracoids and elephants, like the position of various basicranial foramena, the prolongation backward of the jugal and the shape of the palatines. My conclusion is that Pyrotherium is related to the proboscideans, and came from the same stock which gave rise to hyracoids, elephants and Arsinotherium. I think further that Pyrotherium belongs definitely to the proboscidean line.

Referring back to the foregoing table. The development of tushes may be an adaptive character; but in the elephants it is inc. 2 of the upper and inc. 2 of the lower jaw which are so developed. In Pyrotherium, in the upper dentition, it is also inc. 2 which makes the tush, and inc. 1 is enlarged as in Moeritherium, and, so far as we know, has not been reduced in later forms as it was in the elephant line. In the lower jaw we have no final evidence which will show whether it is inc. 1 or inc. 2 which makes the tush; but the lower tush bites against upper inc. 2 and I have considered it to be incisor 2.

The loss of the teeth behind the tushes is a character to be expected in the development of tushes and gives no data. The bilophodont character of the back teeth has occurred many times in the animal kingdom and while it may be the inheritance of the early elephants it can not be used as an argument.

The position of the nasal opening looks very much like that of elephants, but again is coincident with the development of a proboscis. However, this has not occurred a great number of times in the animal kingdom, and where it has, it takes a variety of forms of modification. In Pyrotherium, the modification is of the type in elephants, and elephants only.

A very striking feature is the development of the dental region downward so that the basicranial axis is bent upward, making an angle of about 140 degrees. There are other cases of the bending of the basicranial axis; but in the other ungulates it is a bend downward, the reverse of what we find here and in elephants. To adjust the posterior part of the nasal chamber to this, the pterygoids and the alisphenoids are developed into great wing-like plates on either side. I find this modification of the basicranial axis and of the palatal, pterygoid and alisphenoid bones in no other group but the elephants. In Palaeomastodon it has been developed to a degree so that the angle is about 155 degrees.

The back of the palatine bones is also characteristic, for these begin as narrow pointed bones and behind the last molar expand into wide plates, just as in Palaeomastodon (and in no other groups), having the postpalatine foramen opposite or behind the last molar.

The post-tympanic region of the squamosum is modified so that this process unites with the anterior squamosal region to crowd out more or less completely the tympanic bone where it should surround the auditory meatus. This feature is common to the elephants, the hyracoids, and the toxodonts, so that I consider it a primitive feature indicative of the ultimate common ancestry of these groups. The tympanic bulla can be compared with that of elephants closely, and has much in common with that of toxodonts, but in this last group the tympanic is much more highly developed.

The premaxilla bone in Pyrotherium is crowded out, so that it makes no part of the palate, which is a character of elephants, and in contrast to toxodonts or other groups which have been mentioned. There are two antorbital openings as in elephants, and a feature not common, though not unknown. On either side of the brain case are cellular spaces with intercellular lamellae, which are so characteristic of elephants; a confirmatory feature, though in itself not conclusive.

The foramena on the base of the cranium are similar to those on the base of the cranium of elephants, though there are some variations, as for instance, the exoccipital foramen, is isolated in Pyrotherium, but fused with the posterior lacerum foramen in elephants, and other slight variations in position; but, on the whole, the foramena of Pyrotherium are much closer to those of elephants than of any other group. There is also much in common with toxodonts and with hyracoids, as would be expected if they have a common ancestry. There is no suggestion of a marsupial arrangement as would be necessary if related to Diprotodon.

The atlas of Pyrotherium is peculiar in having a marked hypophysis which is unusual, but is a feature of the atlas of Palaeomastodon and Moeritherium. The axis is peculiar in that the odontoid is flattened on the upper side and very short and wide. In this the form is unique. The continuation of the articular surface on the lower side of the odontoid with the articular surfaces of the ant. cotyles is a feature also of elephants. The remaining cervicals are greatly shortened almost to plates, which is elephantine again, though this short neck is approximated by Diprotodon, some Amblypods and Arsinotherium, so that it must be in general looked upon as an adaptive feature, though in its detail it shows again an elephant character.