While all of small size, numerically the rodents make about a third of our collection, the number of genera and species being, however, relatively small. All are hystricomorphs with the pattern on the crowns of the teeth relatively simple. While the incisors are typically rodent-like, permanently growing teeth, the molars are all rooted, some being entirely brachydont, others beginning to show hypsodont features.
So far as yet known, the rodents make their first appearance in South America, in this Deseado formation. Were they, as Ameghino thought, developed there from such a form as Propolymastodon or Promysops of the Casamayor formation? Or did they migrate into Patagonia from some other section? For the former proposition to be convincing to me, it would require more complete material of the forms suggested than now exists.[20] Other groups of hystricomorphs occur in the Theridomyidae of the European Oligocene, and from the Oligocene of the Fayum.[21] Either the old world forms are descended from the South American forms, or vice-versa. The two African lower jaws are very much like those of Cephalomys, and my feeling is that the Patagonian forms are derived from some immigrant reaching that section before Deseado times.
The Deseado genera are not widely different from each other, but it is evident that they are the representatives of at least two families, and my expectation is that other families will be found eventually to be already represented.
Our material does not permit the discussion of the skeleton or even of the skull as a whole, for the specimens occur only as isolated jaws, palates, or even as isolated teeth. In a few cases, the upper and lower dentitions are associated, but in no case was skeleton material clearly associated with the teeth. The remains look very much like such as are often found today in the western United States under a hawk’s nest or below the roosting place of owls. I think most of our specimens passed, before burial, through the stomach of birds or carnivores.
Ameghino puts most of the forms in the family Cephalomidae, which he considers ancestral to Hystricomorpha in general. I feel, however, that it is better to assign the Deseado genera to the families which have persisted until recent times, as Scott and Ameghino, in another place, have done. There are six living families, four of which Scott found already represented in the Santa Cruz. Two of these clearly may be continued back into the Deseado, the Erethizontidae, and the Chinchillidae, nothing as yet having been found to represent the Santa Cruz families Cavidae and Octodontidae.
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 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. |
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. |
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.
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. |
A lower jaw, with premolar 4 and molars 1 and 2, from the Deseado beds on the Chico del Chubut River, west of Puerto Visser, indicates a genus of hystricomorph rodents not previously reported. The lower molars suggest those of Perimys, but premolar 4 is similar to that of Scotaeumys from the Santa Cruz. Scotamys differs, however, from Scotaeumys, in that its molars do not have the third lobe found in the Santa Cruz genus. I have, therefore, made a new genus which appears to be ancestral to Scotaeumys.
Fig. 125. Lower premolar
4—molar 2, × 4/1.
This, the type species of the above genus, is based on specimen 3063, a lower jaw with the incisor, premolar 4 and the first two molars. The incisor is fairly large and heavy, the anterior face slightly convex, and the anterio-posterior diameter greater than the transverse diameter. Premolar 4 has a deep external fold, dividing the crown into an anterior and posterior lamina, the former being then subdivided by another external fold, making the tooth three-lobed. Just internal to the median fold is a tiny pit, apparently the last vestige of an internal fold. Each molar consists of two laminae separated by a deep external fold, around the inner end of which the laminae are connected by a narrow bar. In the present condition of wear there is no indication of secondary furrows. The premolar is smaller than the molars.
| Measurements Specimen 3063 | ||
|---|---|---|
| Lower dentition, in. 1 to pm. 4 | 8 mm. | |
| Lower dentition, premolar 4 length | 3 mm., | width 2.5 mm. |
| Lower dentition, molar 1 length | 2.5 mm., | width 2.5 mm. |
| Lower dentition, molar 2, length | 2.75 mm., | width 3 mm. |
| Height of mandible under pm. 4 | 5.5 mm. | |
Fig. 126. Left mandible external side, × 4/1.
One set of lower teeth found by the Amherst party shows a simplicity of pattern found in no other genus of South America; and this is, therefore, named Litodontomys. The teeth are brachydont, the premolar and the molars each being divided into two laminae by an external and an internal fold, the distinctive generic feature being in that this fold is narrowest at the margin of the tooth and expands internally. In connection with the expanded folds, the ends of the laminae are curved toward each other, so that in a worn specimen they would meet on the margins of the tooth, and leave the folds to appear as pits. No indication of a furrow is evident on either lamina.
The type is number 3086 of the Amherst collection, from the Deseado beds on the Chico del Chubut River, west of Puerto Visser, and consists of the lower premolar-molar series.
Fig. 127. Right lower
premolar 4
—molar 3, × 4/1.
Premolar 4 is elongated, the anterior lamina being considerably longer than it is wide, whereas the laminae of the other teeth are wider than they are long.
The following measurements with the figure give the specific details.
| Measurements Specimen 3063 | ||
|---|---|---|
| Lower dentition, | premolar 4, to molar 3 | 10.5 mm. |
| Lower dentition, | premolar 4, length | 3.5 mm. |
| Lower dentition, | molar 1, length | 2. mm. |
| Lower dentition, | molar 2, length | 2.5 mm. |
| Lower dentition, | molar 3, length | 2.5 mm. |
| Lower dentition, | width of molars | 2. mm. |
ERETHIZONTIDAE
Asteromys Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 495.
The genus contains small forms, with brachydont teeth. The upper molars consist of two laminae, separated by an internal and an external fold, and each lamina having, on the internal half, a deep furrow, which is but little shallower than the median fold; so that the outer side shows three furrows, folds, or pits, more or less completely separating four lobes; while on the inner side of the tooth there are but two lobes. On the lower teeth the external median fold is deep, while the internal median fold is shallower, usually appearing as a pit. Both the anterior and posterior laminae are subdivided by wide internal furrows which extend to the median line.
These characters associate the genus with Acaremys of the Santa Cruz, from which genus it is not easy to separate Asteromys; but as we know only the teeth from the Deseado beds, it is probable that, when the skull is found, larger differences will be recognized. Asteromys appears to be the direct ancestor to Acaremys.
The species are all tiny, the following three being distinguished by Ameghino.
| Length of Lower Molar Series |
|
|---|---|
| A. punctus (Bol. Inst. Geog. Argen., 1897, t. 18, p. 495) | 12 mm. |
| A. annectens (Bol. Acad. Nac. Cienc. Cordoba, 1902, t. 17, p. 37) | 11 mm. |
| A. prospicuus (Bol. Inst. Geog. Argen., 1897, t. 18, p. 495) each molar | 1.6 to 1.8 mm. |
Of these three we found only the last.
A. prospicuus Amegh. loc. cit. above.
Fig. 128. Right upper premolar
4—molar 3, × 4/1.
Fig. 129. Left lower
molar 2, × 4/1.
The species is rare, only three specimens turning up in our collections. The upper molars are as described in the generic discussion, but premolar 4 is simpler than the molars, the posterior lamina being small and without any sort of furrow. In the upper molars the anterior lamina is larger than the posterior, and the anterior furrow wider than the posterior. The following measurements, with the figures, indicate the character of the species.
| Upper dentition, | premolar 4 to molar 3 | 8.75 mm. |
| Upper dentition, | premolar 4, length | 2. mm. |
| Upper dentition, | each molar, length | 2.3 mm. |
| Upper dentition, | each molar, width | 2. mm. |
| Lower dentition, | molar 2, length | 2.75 mm. |
| Lower dentition, | molar 2, width | 1.75 mm. |
Eosteiromys Amegh., 1902, Bol. Acad. Nac. Cienc., Cordoba, t. 17, p. 110.
The genus was established by Ameghino for forms similar to Steiromys of the Santa Cruz, but antedating that time. I confess I can see but little difference between Steiromys and Eosteiromys, but the latter is as yet known only from isolated teeth and as in general it would be expected that there should be a generic difference, we may let this genus stand representing rather a prophecy than the facts as yet known.
The upper teeth are brachydont, the crown being on the same general plan as in the foregoing genus, i. e., it is divided into an anterior and posterior lamina by a deep external median fold and by a shorter oblique internal median fold. The anterior lamina is subdivided by two external furrows, a lesser anterior and a larger posterior; while the posterior lamina is subdivided by a single external furrow; so that this tooth has four folds, furrows, or pits on the external side separating five lobes; while on the inner side there is but the one oblique fold separating two lobes.
Fig. 130. Right
upper molar,
2 × 4/1.
? E. medianus Amegh., 1902, Anal. Mus. Nac. Buenos Aires, ser. 3, t. 2, p. 129.
The genus is based on a single, though entirely characteristic, upper molar. We found just one tooth of the species, also an upper molar. It is described above, under the genus. The measurements are: Upper molar 2, length 4 mm., width 5 mm.