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.
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.
Fig. 93. Premolar 4 to molar 3—
½ natural size, after Ameghino.
| 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.
Fig. 94. Humerus,
anterior side—
⅕ natural size.
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.
Fig. 96. Upper end of radius,
ulnar side—
⅕ natural size.
Fig. 97. Left tibia,
posterior side—
⅕ natural size.
Fig. 98. Astragulus,
dorsal aspect—
½ natural
size,
after Ameghino.
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.
Fig. 100. Upper and lower incisors, canines, and
premolars—½ natural size, after Ameghino.
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. |