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.