HOMALODONTOTHERIA
The forms making up the _Homalodontotheria_ are characterized by a
dentition which is clearly a derivative of that of _Toxodontia_, but
is distinguished by the teeth being brachydont, by the canines being
the teeth which tend to become tush-like, though not advancing to a
marked degree. But the distinctive feature of the suborder is found in
the feet, which are clawed, the ungual phalanges being deeply cleft;
and further, the animals seem to have walked on the sides of the foot,
suggesting the _Ancylopoda_; but there does not seem to have been a
phylogeretic relationship, rather it is a case of parallel development.
Most of the forms found are of considerable size, and they are
relatively scarce in all the formations.
The representatives of the group in the Deseado all belong to the genus
_Asmodeus_, which seems to be directly ancestral to the Santa Cruz
genus _Homalodontotherium_, which seems to be the last representative
of the series, no specimens, referable to the suborder having been
found in later beds.
=Asmodeus= Ameghino
Asmodeus Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 643.
Asmodeus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 476.
The formula is
3 1 4 3
-------,
3 1 4 3
the upper incisors have pits in the crowns; the canines are moderately
enlarged; the upper premolars and molars consist of an external wall,
with an anterior and posterior lobe, the lower premolars and molars are
typically like those of toxodonts. Two species have been distinguished,
a larger, _A. osborni_, and a smaller, _A. scotti_. Our collection
contains seven specimens, all of which should apparently be assigned to
_A. osborni_.
=Asmodeus osborni= Ameghino
A. osborni Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 644.
A. osborni Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 478.
Homalodontotherium osborni Gaudry, 1906, Anal. Palaeontologie, t. 1, p.
11.
The type of this species is a calcaneum and astragulus, to which
Ameghino later assigned the rear part of a mandible with pm. 4 and the
three molars; also a milk dentition, this last I think wrongly, for it
is too small. I should interpret this specimen as deciduous inc. 2 to
deciduous pm. 4, plus permanent molar 1, in which case the permanent
molar corresponds to that of _A. scotti_ and it is not necessary to
discuss “the remarkable bicuspid canine,” as Ameghino does. Gaudry had
some of this material, upper molars, the lower end of the humerus, the
ulna, calcaneum and astragulus, and he referred the genus as the same
as _Homaladontotherium_. With this last, I can not agree. We found the
three upper molars, the lower end of the humerus, part of the radius,
the tibia, and two phalanges, all on the Chico del Chubut, west of
Puerto Visser.
[Illustration: Fig. 92. Molars 1-3 of the left side—½ natural size.]
While brachydont, the external faces of the molars are high, and each
has a tiny cingulum along the base of the crown. There is also a strong
cingulum around the anterior, internal, and posterior faces of the
crown, which on the posterior margin flares out, making a marked and
characteristic ridge. The grinding surface, with its external wall and
two transverse lobes, is very similar to the molar of a rhinoceros.
When the tooth wears down, the inclosed basin becomes a large pit.
Between the posterior lobe and the flaring cingulum on the posterior
margin, there is also a small posterior bay, which, in an old tooth,
will also appear as a pit, but being shallow, it does not last long.
The lower molars, as figured by Ameghino, are of the same type as those
of the toxodonts, consisting of two crescents with the pillar in the
middle of the posterior crescent, but the crescents and pillar are very
plump; so that with wear they form broad grinding surfaces; and the
bays, instead of becoming pits, first appear as notches, then disappear
entirely. Each premolar and molar has a cingulum on the internal and
external sides.
[Illustration: Fig. 93. Premolar 4 to molar 3—½ natural size, after
Ameghino.]
MEASUREMENTS, SPECIMEN 3179
Upper dentition, molar 1, length 46 mm., width 50 mm.
Upper dentition, molar 2, length 51 mm., width 55 mm.
Upper dentition, molar 3, length 50 mm., width 51 mm.
Lower dentition, from Ameghino’s measurements
Lower dentition, premolar 4, length 28 mm., width 23 mm.
Lower dentition, molar 1, length 34 mm., width 24 mm.
Lower dentition, molar 2, length 46 mm., width 24 mm.
Lower dentition, molar 3, length 76 mm., width 23 mm.
Only the distal end of the scapula has been found; and this shows a
shallow glenoid cavity, which is much longer in the antero-posterior
direction, than in the transverse. The spine rises close above the rim
of the glenoid, and is unusually heavy.
[Illustration: Fig. 94. Humerus, anterior side—⅕ natural size.]
[Illustration: Fig. 95. Ulna anterior side—⅕ natural size, after
Gaudry.]
The lower half of the humerus is present, and characterized by very
wide epicondyles, a shallow supratrochlear fossa, a moderately deep
anconeal fossa, no foramen, and a wide shallow trochlea. The ulna,
according to Gaudry, is a long, heavy, nearly straight bone, with a
shallow sigmoid notch, and with a large olecranon process which is not
bent backward to any marked degree. The proximal end of the radius
has a broad doubly curved articular surface to fit the full width of
the humeral trochlea. Its ulna facet is a short broad area just below
the margin of the bone, and would indicate little or no rotary motion
of the fore arm. Most of the shaft is lacking but what is present
indicates a very slender bone.
[Illustration: Fig. 96. Upper end of radius, ulnar side—⅕ natural size.]
[Illustration: Fig. 97. Left tibia, posterior side—⅕ natural size.]
[Illustration: Fig. 98. Astragulus, dorsal aspect—½ natural size, after
Ameghino.]
[Illustration: Fig. 99. _A_, Ungual phalanx, No. 3; _B_, Ungual
phalanx, No. 5—½ natural size.]
The tibia is also a rather light bone of moderate length, and is
strongly curved inward, the inner margin being especially concave.
On the wide proximal end, the inner condyle is concave, the outer
convex, the two being separated by a prominent bifid spine. The shaft
is slender, with a deep groove down the anterior face especially
at the upper end, while on the posterior face, there is a large
interosseus crest, which starts just below and external to the
spine, and extends in a sigmoid curve three-fourths of the length
of the shaft, ending on the internal border. Distally the tibia is
flattened antero-posteriorly, and the internal margin extends as a wide
process down to the level of the navicular face of the astragulus.
The articular facet for the astragulus is a rectangular depression,
being about half as wide in the antero-posterior direction as in the
transverse. This facet is only slightly concave and the inner and outer
portions are not separated by an inter-trochlear ridge. The fibula has
not been found, but the tibia shows no indication of its having been
fused to it.
Ameghino has figured the astragulus as very low, with the trochlea
flattened, the internal condyle being wider and flatter, while the
external condyle is narrower and somewhat raised. The trochlea is
peculiar in that its proximal margin is deeply notched by a depression
in which there is a large perforation. The neck is prolonged and
carries a large convex head articulating with the navicular only. The
measurements given are, length 116 mm., width 75 mm.
Gaudry figures a calcaneum, showing a long narrow tuber, and the facet
for the fibula as a wide shelf which projects strongly on the external
side. The size as given by Ameghino is 240 mm. long, by 120 mm. wide.
I have two associated ungual phalanges, one of which corresponds
to that figured by Ameghino as the third. It is high, laterally
compressed, has a very rugose surface on either side, and a deep cleft
in the end. This is 68 mm. long. The second ungual is very asymetrical,
also laterally compressed, and with the point curved inward. I take
it to be the fifth. The tibia, the tarsus, and the phalanges strongly
suggest that this animal walked on the side of its foot.
=Asmodeus scotti= Ameghino
A. scotti Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 643.
A. scotti Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 477.
This species is not represented in our collection, but I reproduce
Ameghino’s figure of the type, and of the milk dentition. Unfortunately
his type figure is from the side and does not give all the desired
information.
[Illustration: Fig. 100. Upper and lower incisors, canines, and
premolars—½ natural size, after Ameghino.]
[Illustration: Fig. 101. Milk incisors, canine, and premolars and
permanent m. 1—½ natural size, after Ameghino.]
In the upper dentition, the small incisors, pitted on the crown,
increase regularly in size toward the rear; and each has an external
cingulum around the base. The canine is about twice the size of the
adjacent incisor, and also has an external cingulum. The premolars
increase regularly in size and also have at least an external cingulum.
Figure 101 shows a dentition which Ameghino described as the milk set
of _A. osborni_. At the same time he remarks the unusual character of
the deciduous canine in being two-cusped. I think this set of teeth
should be interpreted as deciduous inc. 2 to deciduous pm. 4, plus the
permanent molar 1. With such an interpretation, we find the incisors
normal, the canine normal though not as large as in the permanent set,
and the two-cusped tooth is the first milk premolar. The last tooth
in the series is considerably different from the premolars and is
evidently permanent molar 1, which is about the size and character of
this tooth in _A. scotti_, much too small to belong to _A. osborni_.
This set of milk teeth differ from the permanent teeth in that the
premolars do not have the anterior, inner and posterior cingulum,
characteristic of the permanent dentition.
The following measurements are taken from Ameghino:
Upper dentition, inc. 1 to pm. 4 104 mm.
Upper dentition, premolar 2, length 18 mm., width 25 mm.
Upper dentition, premolar 3, length 20 mm., width 28 mm.
Upper dentition, premolar 4, length 23 mm., width 35 mm.
Upper dentition, molar 1, length 28 mm., width 39 mm.
Upper dentition, molar 2, length 37 mm., width 44 mm.
Upper dentition, molar 3, length 50 mm., width 48 mm.