CHAPTER VIII

NESODONTIDAE

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.

[Illustration]

[Illustration: 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.

[Illustration: 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.

[Illustration: 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.

[Illustration: 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.

[Illustration: 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.

[Illustration: Fig. 84. I. phorcus. “Upper molar 3”—natural size, after Ameghino.]

[Illustration: Fig. 85. I. deflexus, “Upper molar 1”—natural size, after Ameghino.]

[Illustration: 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.