Chinchillidae
In the Deseado, this family is represented by the genera Cephalomys, Scotamys, and possibly Litodontomys. Cephalomys is very abundant and seems to be ancestral to Perimys of the Santa Cruz; Scotamys is relatively rare but seems to be ancestral to Scotaeumys; while Litodontomys is also rare and as far as I can see without a successor.
Cephalomys Ameghino
Cephalomys Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 494.
This is the common genus of the Deseado, over three-fourths of the specimens of rodents found belonging to one of its three species. Its dental characters mark it clearly. All the premolars and molars are rooted, though the crown is incipiently hypsodont, as much so as in any rodent of this period. The incisors are moderately large with the anterior face slightly convex, and the antero-posterior diameter comparing with the transverse diameter as 3 does to 2. The interval between the incisor and premolar 4 is moderate, indicating a short snout.
Each lower molar consists of two transverse laminae separated from each other by an internal and an external infolding, both of which approach the median line but do not meet, a narrow, longitudinal bar separating the folds and connecting the anterior and posterior laminae. On the inner side, the posterior lamina has a furrow extending to the middle of the tooth, but only sinking into the crown about a fourth of its height, so that, with wear, it appears first as a bay, later as a pit, and finally disappears. In general it will be found only on molar 3, and may be wanting there on old individuals. On an unworn tooth, there occurs, on the inner side of the anterior lamina, a rudimentary pit corresponding to the one on the posterior lamina, but of much less depth, so that it is only occasionally seen, and that only on a very slightly worn tooth. The premolar differs from the foregoing in having a small median column on the anterior face of the anterior lamina.
In three cases we found the deciduous fourth premolar ([see fig. 119A]), a complicated tooth, consisting primarily of three laminae in which furrows have developed until there are four folds or furrows on the internal side, separating five crests; while on the external side there are three furrows and four crests. Ameghino’s figure of this tooth in C. prosus has four laminae running clear across the tooth. I think the difference is due to his having an unworn deciduous premolar whereas mine are all worn considerably.
At first glance, the upper teeth appear strikingly different, resembling those of Perimys to which genus they are probably ancestral. Each molar consists of two laminae, separated by a deep internal fold which extends almost to the external margin. On little worn teeth each lamina shows, on the external side, a shallow furrow extending to about the middle of the tooth, but these furrows early become pits and then disappear with further wear, being preserved on not over a fourth of our specimens. The fourth upper premolar consists of two laminae, but in this case, the separating fold is on the external side and extends nearly to the internal margin, so that this tooth appears to be reversed in its position in the jaw. As in the molars, there is, on the external side of either lamina, a furrow, the one in the anterior lamina shallow and seldom seen, that in the posterior lamina deep and present in all but the most worn teeth.
While the upper and lower molars appear so different they may be readily derived from such a tooth as the lower molar, as both have the two laminae and separating furrows in common. In the upper molars, however, the internal fold is prolonged until the external fold is merely indicated or lacking. On upper premolar 4, on the contrary, it is the external fold which is prolonged. The furrows in the external portions of the laminae of the upper molars correspond to those on the internal portions of the same laminae of the lower teeth, reversed, as is typical of all teeth.
Ameghino distinguished three species of Cephalomys, which are based primarily on size, the other characters which he gave being inconstant. We found these three and no others.
| Upper pm. 4 to m. 3 | Lower pm. 4 to m. 3 | |
|---|---|---|
| C. arcidens | 13-14 mm. | 14-15 mm. |
| C. plexus | 9.5 mm. | 10.5 mm. |
| C. prosus | 8.5 mm. | 9.5 mm. |
Cephalomys arcidens Ameghino
C. arcidens Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 494.
This, the type species, is by far the commonest of the rodents, in fact of all the species in the Deseado, and we found forty-seven specimens on the Chico del Chubut River, west of Puerto Visser. In the species there is considerable variation in size for a rodent, but as there are intermediate specimens all the way between the extremes, and as the variation is mostly in the size of the fourth premolar, it does not seem proper to separate the material into more than one species.
Fig. 118. Right upper premolar,
molar series × 4/1.
Fig. 119. Left lower premolar, molar series;
A, deciduous premolar 4;
B, a little worn molar 1;
C, series about half worn down;
a.l, anterior lamina;
p.l. posterior lamina; i.f., internal fold;
e.f., external fold; p.g., furrow in
posterior lamina; × 4/1.
In general, the form has relatively plump teeth, relatively heavier and thicker than in the other species. Usually the fourth premolar is but little larger than the molars, but, in this character, there is considerable variation. The following measurements give the range of size on the upper jaws:
| Specimen 3109 a Small Individual | Specimen 3099 a Large Individual | Ameghino’s Type | |
|---|---|---|---|
| Upper premolar 4 to m. 3 | 12.5 mm. | 13.5 mm. | 13.5 mm. |
| Upper premolar 4, length | 3.5 mm. | 4.5 mm. | |
| Each molar, length | 3 mm. | 3 mm. | |
| Each molar, width | 2.75 mm. | 3 mm. |
The lower jaw is low, heavy and rather short, the posterior part of the ramus being very thin, while the portion carrying the teeth is thick and heavy. A strong ridge extends along the inner side from just behind molar 3 to the base of the symphysis. As in the upper dentition, there are smaller and larger forms.
| Specimen 3089 a Small Individual | Specimen 3058 a Large Individual | Ameghino’s Type | |
|---|---|---|---|
| Lower premolar 4 to m. 3 | 14 mm. | 15 mm. | 14.5 mm. |
| Lower incisor 1 to pm. 4 | 7 mm. | 9 mm. | 7.8 mm. |
| Height of mandible under pm. 4 | 7 mm. | 8 mm. | 7 mm. |
| Length of deciduous pm. 4 | 5.5 mm. |
Cephalomys plexus Ameghino
C. plexus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 494.
In general, this species is similar to the foregoing, but is smaller in size and slenderer in proportions. Both the upper and lower fourth premolar tend to be considerably larger than the molars. The species was not nearly as abundant as C. arcidens, occurring but sixteen times in our collection.
Fig. 120. Right palate showing premolar,
molar series; external furrows appear as
pits on molars 2 and 3, × 4/1.
Fig. 121. Left mandible, external side, × 4/1.
| Measurements | Specimen 3091 | Ameghino’s Type |
|---|---|---|
| Upper dentition, pm. 4 to m. 3 | 9. mm. | 9.5 mm. |
| Upper dentition, pm. 4, length | 2.75 mm. | |
| Upper dentition, each molar length | 2.1 mm. | |
| Upper dentition, each molar width | 2.25 mm. |
The lower jaw is slender, the incisor being relatively both smaller and slenderer than in C. arcidens; the back part of the ramus light and thin, the coronoid process being a tiny spur, and the articular condyle of small size, and on a level with the teeth.
| Measurements | Specimen 3005 | Ameghino’s Type |
|---|---|---|
| Lower dentition, pm. 4 to m. 3 | 10 mm. | 10.5 mm. |
| Lower dentition, in. 1 to pm. 4 | 6 mm. | 6.5 mm. |
| Lower dentition, pm. 4, length | 3.5 mm. | |
| Lower dentition, each molar, length | 2.5 mm. | |
| Lower dentition, each molar, width | 2 mm. | |
| Height of mandible under pm. 4 | 5 mm. | 4.8 mm. |
Fig. 122. C. plexus, left lower
premolar, molar series;
A, of young individual;
B, of old individual;
int., internal side;
ext., external side, × 4/1.
Fig. 123. C. prosus,
left upper premolar,
molar series, × 4/1.
Fig. 124. C. prosus,
premolar 4 and
molar 2, × 4/1.
Cephalomys prosus Ameghino
C. prosus Amegh., 1902, Bol. Acad. Nac. Cienc. Cordoba, t. 17, p. 37.
This is the tiniest species of the genus, and least frequently found, probably because on account of the small size it was more frequently destroyed before burial, and also because it is hard to find such tiny specimens; so that the sixteen which we found would hardly represent the real proportion of the species in the fauna.
The jaws are not only small, but also slender and delicately built, with the premolar about the same size or slightly larger than the molars. The drawings represent the proportions accurately so I will give but a few measurements.
| Specimen 3009 | Ameghino’s Type | |
|---|---|---|
| Upper premolar 4 to molar 3 | 8.5 mm. | |
| Lower premolar 4 to molar 3 | 9.5 mm. |