RODENTIA
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.
[Footnote 20: I have a lower jaw of _Propolymastodon_ which, though
not complete in front, gives me no suggestion that the incisor was
rodent-like, and I am inclined to think that the incisor associated
with the type of _P. carlo-zitelli_ is a mistake.]
[Footnote 21: Osborn, Bul. Amer. Mus. Nat. Hist., Vol. 24, p. 265.]
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_.
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 LOWER PM. 4
TO M. 3 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.
[Illustration: Fig. 118. Right upper premolar, molar series × 4/1.]
[Illustration: 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 SPECIMEN AMEGHINO’S
3109 3099 TYPE
A SMALL A LARGE
INDIVIDUAL INDIVIDUAL
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 SPECIMEN AMEGHINO’S
3089 3058 TYPE
A SMALL A LARGE
INDIVIDUAL INDIVIDUAL
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.
[Illustration: Fig. 120. Right palate showing premolar, molar series;
external furrows appear as pits on molars 2 and 3, × 4/1.]
[Illustration: Fig. 121. Left mandible, external side, × 4/1.]
MEASUREMENTS SPECIMEN AMEGHINO’S
3091 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 AMEGHINO’S
3005 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.
[Illustration: 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.]
[Illustration: Fig. 123. _C._ prosus, left upper premolar, molar
series, × 4/1.]
[Illustration: 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 AMEGHINO’S
3009 TYPE
Upper premolar 4 to molar 3 8.5 mm.
Lower premolar 4 to molar 3 9.5 mm.
=Scotamys= gen. nov.
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_.
=Scotamys antiquus= sp. nov.
[Illustration: 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.
[Illustration: Fig. 126. Left mandible external side, × 4/1.]
=Litodontomys= gen. nov.
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.
=Litodontomys chubutensis= sp. nov.
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.
[Illustration: 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.
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= Ameghino
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.
=Asteromys prospicuus= Ameghino
A. prospicuus Amegh. loc. cit. above.
[Illustration: Fig. 128. Right upper premolar 4—molar 3, × 4/1.]
[Illustration: 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= Ameghino
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.
=Eosteiromys medianus= Ameghino
[Illustration: 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.