This family is characterized by the teeth being hypsodont, the second upper incisor and the third lower incisor being enlarged into caniniform teeth, the upper molars complicated by the development of cristae, limbs short, feet tridactyl and semidigitigrade.
Fig. 79. A, upper and a lower molars 2 of Proadinotherium;
B, upper and b lower molars 2 of Coresodon;
C upper and c lower molars 2 of Neudon—
½ natural size.
In the Santa Cruz, the family is represented by the two genera Nesodon and Adinotherium. In the Deseado we find Proadinotherium evidently ancestral to Adinotherium and very little differentiated from it. Ameghino has described a genus, Pronesodon, which is evidently ancestral to Nesodon. I have referred Coresodon to this family because the molars of the upper and lower jaws are very close to those of Adinotherium. Ameghino has also described two genera, Nesohippus and Interhippus, based on upper molars which are very similar in pattern to Adinotherium and which I believe belong to this family, if they prove to be valid genera, of which I have some doubt, feeling that they will prove to be the deciduous upper premolars of Proadinotherium or some similar form. The genus Senodon, which Ameghino also places in this family, I feel will prove to be worn teeth of Leontinia.
Proadinotherium Ameghino
Proadinotherium Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 625.
Fig. 80. P. leptognathus, rear portion of skull—
½ natural size; shaded areas are matrix.
The dental formula is
- 3 1 4 3
- ———
- 3 1 4 3
as in Adinotherium, the chief difference being that the teeth are less hypsodont than in the Santa Cruz genus. Little is known as yet of the skeleton, but when more is known probably more distinctive characters will appear. Ameghino made two species, P. leptognathus which we also found, and P. angustidens a much smaller form.
Proadinotherium leptognathus Ameghino
P. leptognathus Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 625.
P. leptognathus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 467.
Fig. 81. Left lower jaw with
incisor 3 and molar 2—
½ natural size.
Of this species we found on the Chico del Chubut River, west of Puerto Visser, three specimens; the back of a skull as far forward as molar 2, and two lower jaws. In general, the species is very similar, even to size, to Adinotherium ovinum of the Santa Cruz.
The upper molars are strongly hypsodont, curved teeth. On the upper surface, the basin is subdivided by two strong cristae into three smaller bays. In an early stage of wear, the second crista unites with the posterior lobe, converting bay 3 into a pit. On the posterior margin of the tooth, the cingulum is developed so as to appear like a third crista, which inclosed bay 4, and when the tooth is worn, bay 4 becomes a pit also.
In my lower jaw incisor 3 is developed into a strong caniniform tush. Most of the teeth are lacking, but lower molar 2 is a strongly compressed, hypsodont tooth, surrounded by a thick layer of enamel. This tooth rises 22 mm. above the well-developed roots, and is already considerably worn down. The pillar is prominent as a strong fold in the middle of the posterior crescent. In this specimen there is no trace of the usual pit (3) indicative of the septum, but I should expect to find it in a younger specimen. The mandible broadens in front into a scoop-like anterior end, and the alveoli of the first two incisors would indicate that they were proclivous. The alveoli for the other teeth are arranged as in Adinotherium.
| Measurements | |||||
|---|---|---|---|---|---|
| Skull, width across the zygomatic arches | 148 mm. | ||||
| Skull, width across opposite m. 3 (outside) | 73 mm. | ||||
| Upper dentition, | molar 2, | length | 25 mm., | width | 13 mm. |
| Upper dentition, | molar 3, | length | 23 mm., | width | 12 mm. |
| Lower dentition, | incisor 3, | length | 13 mm., | width | 7 mm. |
| Lower dentition, | molar 2, | length | 20 mm., | width | 7½ mm. |
Proadinotherium angustidens Ameghino
P. angustidens Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 467.
This is based on a single lower tooth, which is considered either pm. 4 or m. 1, and measures 13 mm. long by 4½ mm. wide.
Pronesodon Ameghino
Pronesodon Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 626.
The genus is said to resemble Proadinotherium, but with the caniniform incisors proportionally much shorter. An associated calcaneum is shorter than that of Adinotherium and longer than that of Nesodon.
Two species are described.
Pronesodon cristatus Ameghino
P. cristatus Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 626.
P. cristatus Amegh., 1897, Bol. Inst. Geog. Argen., t. 15, p. 467.
This species is very imperfectly known, is characterized by a large external anterior style, molars said to be 15 mm. wide.
Pronesodon robustum Ameghino
P. robustum Amegh., loc. cit. above.
This is a larger species, of which the three lower molars are known, and which measure 16, 22, and 30 mm. in length respectively, while they are 9-10 mm. wide.
Coresodon Ameghino
Coresodon Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 630.
Coresodon Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 459.
Coresodon Gaudry in part, 1908, Anal. Palaeontologie, t. 1, p. 46.
In this genus, the pattern of the upper molars is essentially the same as in Proadinotherium, and they are of the same hypsodont character, and with roots. I can now find only the fact that in Coresodon the teeth are more compressed and somewhat more hypsodont, as a feature by which to distinguish this genus from Proadinotherium. Gaudry figures the front of a lower jaw under the name Coresodon which lacks the caniniform incisors. I have doubted the association, but should it prove correct, then this genus would be markedly different in that respect. Two species have been described, C. scalpridens, and C. cancellatus, both of which I consider the same.
Coresodon scalpridens Ameghino
C. scalpridens Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 630.
C. scalpridens Amegh., 1897, Bol. Inst., Geog. Argen., t. 18, p. 459.
C. cancellatus Amegh., 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p. 374.
Fig. 82. Sections of second lower molar;
A, top; B, 4 mm. down; C, 10 mm. down;
D, 18 mm. down—natural size.
Of this species we found two specimens, one containing the three lower molars, the other the second lower molar only. In establishing C. cancellatus, Ameghino says it is of the same size as C. scalpridens, but distinguished by the basin in the upper molars being narrower, the internal fold not being bifurcated, and by the absence of islets of enamel. All these features seem to me to be the results of wear.
Fig. 83. Molars 1 to 3—natural size.
While the pattern of the upper molars is the same as in Proadinotherium, these teeth are about as wide as they are long. The lower molars, however, are more compressed, with the anterior crescent occupying about a third of the tooth, and having in the early stages a deep pit, which disappears when the tooth is worn down. In the middle of the basin of the posterior crescent is a large pillar, and between this and the median horn of the crescent is a tiny septum, which early unites with the pillar, leaving a tiny pit (3) which soon disappears entirely. In fact, in an old tooth, the second and fourth bays, having become pits, may even be lost also.
| Measurements | ||
|---|---|---|
| Upper dentition, | molar 1, length | 19 mm. @ Ameghino. |
| Upper dentition, | molar 1, width | 12 mm. @ Ameghino. |
| Upper dentition, | m. 1 to m. 3, length | 52 mm. @ Ameghino. |
| Lower dentition, | premolar 2, length | 13 mm. @ Ameghino. |
| Lower dentition, | premolar 3, length | 13 mm. @ Ameghino. |
| Lower dentition, | premolar 4, length | 17 mm. @ Ameghino. |
| Lower dentition, | molar 1, length 18 mm., width | 7 mm. |
| Lower dentition, | molar 2, length 19 mm., width | 7 mm. |
| Lower dentition, | molar 3, length 20 mm., width | 7 mm. |
Interhippus Ameghino
Interhippus Amegh., 1904, Anal. Mus. Nac.
Buenos Aires, ser. 3, t. 3, p. 183.
Interhippus Amegh., 1904, Anal. Soc. Cienc. Argen., t. 56, p. 34 of reprint.
This genus was established on isolated teeth which closely resemble those of this family, though the genus was placed among the Rhynchippidae by Ameghino. The teeth described as molars are much elongated and have the cristae greatly developed, and in one species there is a style rising about the middle of the inner side of the molar. Another feature emphasized as characteristic of this and the next genus is, that the crowns are expanded much wider than the roots. While there is not yet enough direct evidence to prove it, I feel that this and the next genus will prove to be deciduous teeth, of either Proadinotherium or some related genus. Two species of this genus have been described, both from the upper Deseado.
I. phorcus Amegh., loc. cit. above.
This species is characterized by its size, the last upper molar (so called) measuring 16 mm. long by 14 mm. wide.
I. deflexus Amegh., 1904, Anal. Mus. Nac. B. A., ser. 3, t. 3, p. 183.
This species is based on a worn tooth designated as molar 1 (probably d. pm. 3) 14 mm. long by 19 mm. wide.
Fig. 84. I. phorcus.
“Upper molar 3”—
natural
size,
after Ameghino.
Fig. 85. I. deflexus,
“Upper molar 1”—
natural
size,
after Ameghino.
Fig. 86. N. insulatus,
“Upper molar 1”—
natural
size,
after Ameghino.
Nesohippus Ameghino
Nesohippus Amegh., 1904, Anal. Soc. Cienc.
Argen., t. 56, p. 34 of reprint.
Nesohippus Amegh., 1904, Anal. Mus. Nac. B. A., ser. 3, t. 3, p. 218.
This genus is described as very like the foregoing, but differs in having a strong perpendicular style on the anterior external face of the upper molars. As in the preceding genus, the crown is considerably expanded above the roots. I feel that this genus will also prove to be the milk teeth of some one of the genera of this family. One species is described, based on a single tooth.
N. insulatus Amegh., 1904, loc. cit. under the genus.
The species is just as described under the genus, the last upper molar measuring 24 mm. long by 16 mm. wide; given as from the upper Deseado.