The upper members of the limbs are longer than the lower, which is common to many massive animals. The humerus is tremendously flattened from front to back, even more so than in any of the animals used for comparison, though flattening is a feature of them and of the elephants the most so. With the flattening, the deltoid ridge is prolonged enormously making a crest along the outer side of the bone, which at the lower end rises in a prominent process, as in elephants (also in Diprotodon but in this case the rest of the bone is very different). In addition to this, the supinator ridge is prolonged upward until it almost meets the deltoid, ending in a sharp spur at the top. This spur is more marked in Pyrotherium than in elephants, although they show the same development of the supinator ridge.
The femur has the head much higher than the greater trochanter, which is a feature common to elephants, Diprotodon, Arsinotherium, etc., so that it must be looked upon as an adaptation. The third trochanter has disappeared, and in elephants, it is lost in the advanced forms, remaining however as a trace in Palaeomastodon.
The tibia is very short and massive and hardly gives any suggestions of relationship, except that it is not fused with the fibula at the upper end, in which it is in strong contrast to the toxodonts.
While in the [table of comparisons] numbers 1, 2, 3, 4, 8, 9, 10, 14, 17, 19, 20, and 21 may be, in part, or wholly, interpreted as adaptations, and alone would not be at all conclusive of relationship to elephants, numbers 1, 5, 6, 7, 8, 11, 12, 13, 15, 18, and 21 point toward the elephants as the close relatives of the Pyrotheria. In the first series of points there are none which mitigate against associating these two groups, while if the attempt is made to associate Pyrotherium with any group other than Proboscidea there are strong points, and a number of them, which would prevent this association. As a result of the foregoing, together with a feeling which continued handling of the specimens has given me, I can come to no other conclusion than that the Pyrotheria should be placed under Proboscidea.
In his Linea Filogenetica de los Proboscideos, Ameghino assigns to this suborder, or at least puts into the phylogenetic tree, a considerable number of forms from the Casamayor beds, all of them genera with bunodont molars, usually known by but one or two teeth, such as Asmithwoodwardi, Nephracodus, Cephanodus, Paulogervaisia, and the better known genera, Carloameghinia, and Didolodus, all of which he makes ancestral to Pyrotherium. So far as known, however, these forms show none of the peculiarities of the Pyrotherium skull or dentition, so that it is difficult for me to see any reason for including them even in the suborder. The genus Carlozittelia, from the upper Casamayor, is in a different position, having an enlarged upper incisor (found isolated) and molars of the bilophodont type. I should include this in the family Pyrotheridae and none of the others.
Pyrotheriidae Ameghino
All the forms assigned to this family are supposed to be closely related to Pyrotherium and to have much the same structure. Ameghino has proposed the following genera, Pyrotherium, Parapyrotherium, Richardowenia, Archaeolophus, Propyrotherium.
Parapyrotherium is based on a small molar and a tush which Ameghino first described as Pyrotherium planum, later elevating the species to a genus, designated as Parapyrotherium, differentiated by the transverse crests being low and the valley at either end being blocked by an intertubercular ridge. Gaudry considered that this genus represented either the milk teeth of P. romeri, or a variation of that species. I can not see the basis of a new genus in the material.
The genus Richardoweni is based on half of a molar, which has the transverse crest interrupted in the middle. Too little is known of this form to base a valid genus or even to associate it with Pyrotherium.
Archaeolophus is founded on a small tush and part of an upper molar, also inadequate material for a genus. It is probably Pyrotherium.