The Deseado Formation of Patagonia

by Frederic Brewster Loomis

Fig. 102. Upper dentition of Astrapothericulus iheringi—
½ natural size.

Of the upper dentition, Ameghino figures only the first molar and the canine. I have given Ameghino’s figure of the upper dentition of Astrapothericulus, to indicate what this would be like, for the variation is only slight. The canine is a great tush, not unlike the incisor-tush of Pyrotherium, oval in cross section with the greater diameter from front to back. The first and second premolars have disappeared. Premolars 3 and 4 are greatly reduced. The molars are very like those of Asmodeus, large brachydont grinders, composed of an outer wall, and an anterior and posterior lobe. The external cingulum is a trace only, and the internal cingulum is developed in varying degrees. The basin is deep and subdivided by a crista which rises from the external wall, and as the surface is worn off unites with the anterior lobe, cutting off a small pit. Behind the posterior lobe is a small basin, bounded in the rear by a second crista from the rear end of the external wall, which, as the tooth is worn down, unites with the posterior lobes, cutting off a small posterior pit, suggestive of that of homalodontotheres.

The three lower incisors are expanded at their ends into thick shovel-like crowns, each with a strong crescentic cingulum on the posterior face, and with a shallow furrow on both the front and back faces. Relatively the incisors are much larger and longer than in Astrapotherium.

The lower canine is flattened on the upper face, so that its cross section is close to semicircular making a typical permanently growing rooting implement. This tooth is relatively shorter and smaller than in Astrapotherium.

Premolars 1 and 2 are wanting, a long diastema occupying the interval between the canine and pm. 3. Premolar 3 is greatly reduced in size, and in my specimen has fallen out, being represented by a small alveolus. I judge that in old individuals it falls out. The fourth premolar and the molars are typically those of Toxodontia. The young show two plump crescents, with a low plump pillar, situated near the anterior horn of the posterior crescent, which pillar, as the tooth wears, unites with the anterior horn.

The scapula is a remarkably heavy and elongated bone, greatly arched where it lay over the ribs. The spine is high and heavy, with the upper margin developed into a thick ridge like a banister rail, which is prolonged in front to, or a little beyond, the level of the glenoid fossa, this distal portion being expanded into a broad plate more than half as wide as the widest portion of the blade of the scapula. The glenoid fossa is relatively small, oval in outline, and with the long axis parallel to the long axis of the body. The anterior margin of the articular surface is reflexed, apparently to come in contact with the base of the greater tuberosity of the humerus. This glenoid cavity is only large enough to actually cover about half of the head of the humerus, and fits so that, in a position of rest, the glenoid covered the outer part of the humeral head, and only articulated on the inner part of the humerus head when the limb was bent inward. The blade of the scapula is narrow, with the proximal end prolonged and ending in a thick rugose mass. The anterior and posterior margins are rugose and thickened, the great thickness of the proximal end being due to the convergence of these thickened margins and the heavy spine. Lastly, this thick proximal end is peculiar in having on its posterior side a large rugose cavity, which was apparently to receive muscular attachments.

For such a heavy animal, the humerus is extraordinarily long and slender. The sessile head is strongly compressed from side to side, very convex, and much larger than the glenoid fossa, its articular surface extending onto the base of the greater tuberosity. This tuberosity is heavy and thick, but does not project above the head. The powerful deltoid ridge extends from the tuberosity two-thirds of the way down the shaft. The shaft is unusually slender. Distally it expands laterally to make the two large epicondyles, of nearly equal size. The trochlea is relatively narrow, the internal surface being the narrower, and rising to a high margin; while the external portion is wider, rounded, and has a low margin. The supratrochlear fossa is moderately deep, the anconeal fossa somewhat deeper, but there is no connecting foramen.

Gaudry[17] figures a radius and ulna, both relatively long bones, and closely apposed; so that there was no possibility of a rotary motion of the forearm. The proximal end of the radius is expanded, so that its articular surface is in contact with the full width of the humeral trochlea on the anterior side. Below, the bone contracts to a moderately slender shaft, and then expands distally into a heavy club-like distal end. The ulna has a short but heavy olecranon process, with a prominent coronoid process. The sigmoid notch is shallow, but the articular surface expands on both sides, so that it covers the full width of the humeral trochlea on the posterior side. Distally the ulna is not so heavy as the radius.

Under the name Pyrotherium romeri, Ameghino[18] figures a carpus and metacarpus, which Tournier[19] however assigns to Parastrapotherium, probably P. herculeum; and figures a carpus and metacarpus of the same type, but smaller, which he attributes to Parastrapotherium. I, however, can not see how such a small foot can belong to so large an animal, and feel that, until evidence of direct association is given, it is best not to consider these feet as belonging to Parastrapotherium, but rather to Pyrotherium.

Of the femur I have only the distal end, which, however, corresponds completely with the one figured by Gaudry. It is a long bone, slightly shorter than the humerus, with a small head, set on a short and poorly outlined neck. The greater trochanter is wide and rugose, rising to about the same height as the head. The lesser trochanter is not distinguishable. About the middle of the shaft there is a powerful third trochanter, which continues as a narrow ridge upward to the greater trochanter, and downward in a similar narrow ridge almost to the outer condyle. At the proximal end the shaft is greatly flattened, but in the central and lower parts becomes almost circular in section. The two condyles are set wide apart, project considerably behind the posterior face of the shaft, and are only slightly convex. The trochlea is of moderate width, short, and shallow.