Systematic Descriptions

Archaeohyracidae Ameghino

This family is differentiated by the presence of enamel on all sides of the first incisor, by the unreduced condition of the lateral incisors, and by the small bulla of the mastoid. These are primitive features. Ameghino considered this family to belong to the hyracoids; but, as explained earlier, I believe them to be true Typotheria, though less specialized than the other families.

Archaeohyrax Ameghino

Archaeohyrax Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 431.

This interesting genus is known by a complete skull found by Ameghino and of which we found no duplicates. I insert a reproduction of the side view of the skull, and the dentition is shown in [fig. 24 a], and [fig. 25 a]. The dental formula is

Incisor 1 is a little larger than the other incisors. Each upper molar has a vertical groove near the anterior external margin. In each upper premolar (after the first) and molar, there is a central pit surrounded by enamel, which is opposite the internal inflexion, and in a young individual, is presumably connected with the fold. In the same way, the last three lower premolars and the lower molars each have an internal pit, adjacent to the external inflexion. With advanced age all the teeth show closed roots, another primitive feature. In spite of the closed roots, the full dentition, and the enamel on the incisor; and on account of the deep inflexions and the isolated pits, I consider this genus a specialized side line, retaining many primitive features, and expect to find the ancestor of the typotheres in some one of the related Casamayor genera.

Fig. 27. Archaeohyrax patagonicus, after Ameghino—natural size.

Ameghino described three species, A. patagonicus, which we have figured, and which has a length of 84 mm. from inc. 1 to m. 3 in both the upper and lower dentitions; A. propheticus, of the same size, but with the dental series closed; and A. concentricus of larger size, the three lower molars having a length of 38 mm.

Plagiarthrus Ameghino

Plagiarthrus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 436.

This genus is known only by the lower premolars and molars, which are permanently growing teeth, composed of two subcylindrical cylinders almost entirely separated by the external and internal folds which almost meet in the median line. On the outside, each tooth is coated with a layer of cement. When better known it may prove that this genus, so specialized in the character of the teeth, does not belong in this family.

Plagiarthrus clivus Ameghino

Fig 28. Left lower
premolars 3 and 4
and molars 1-3—
natural size.

P. clivus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 436.

This species is represented by a single specimen from the Chico del Chubut, west of Puerto Visser, which preserves pm. 3 and 4 and the molars. The characters of this, the type species, are those of the genus. The total length of the five teeth is 36 mm., and [fig. 28] shows in natural size the various individual teeth.

Hegetotheriidae Ameghino

This family includes a large variety of forms from the formations from the Deseado up to the Mt. Hermosa, but all agree in having the first upper and the first two lower incisors enlarged into strong gnawing teeth; in the reduction or absence of in. 3, the canine, and premolar 1 of the upper and lower dentitions; in having the external face of the upper molars not inflexed; in lower molar 3 being three-lobed; and in the bulla being inflated and hollow. There are in the family two series of forms, at least, the one leading to the running Hegetotherium, the other to the hopping Pachyrukhos, and the very little known form Phanophilus which may fit into one of the other series when better known.

In the Deseado the following genera are assigned to the family.

Prohegetotherium, like Hegetotherium, except that the last premolar and the molars have a vertical furrow near the external anterior margin.

Prosotherium,

upper inc. 2 and 3 and lower inc. 3 vestigal, upper pms. not molariform, molars with a deep internal fold.

Propachyrucos,

lower jaw only, similar to Pachyrukhos but have inc. 3, the canine, and pm. 1 present and only a little reduced.

Phanophilus, upper molars only, but peculiar in having a strong external medial column.

Prohegetotherium Ameghino

Prohegetotherium Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 424.

This little known genus is characterized by the upper molars having an external furrow near the anterior margin of the tooth. Otherwise it is similar to Hegetotherium. Ameghino described a species where the external surface of the bones was sculptured “like reptiles.” I do not see how, with the arrangement of the muscles usual to mammals, the sculpture could be similar to that of reptiles, and feel that this is due to conditions of weathering. We did not find this species, but did find a form which resembled it in general, but differed in being smaller and with the external furrow less developed.

Prohegetotherium sculptum Ameghino

P. sculptum Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 424.

This species is characterized by the deep external furrow on the upper molars. The measure given is 34 mm. for the length of the three upper molars.

Prohegetotherium shumwayi sp. nov.

Founded on a portion of the right maxilla, carrying pm. 2 to 4 and m. 1, found on the Chico del Chubut, west of Puerto Visser, by Waldo Shumway.

Fig. 29. H. Shumwayi—
natural size.

The teeth are simple, with but a shallow external furrow near the anterior margin of the premolars and molar. A film of cement covers each tooth and extends to the top of the crown. The form is smaller than P. sculptum.

Measurements
Upper premolar 3,length 6 mm.,width 3¼ mm.
Upper premolar 4,length 7 mm.,width 3½ mm.
Upper molar 1,length 7 mm.,width 3½ mm.

Prosotherium Ameghino

Prosotherium Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 426.

In founding this genus, Ameghino says that lower pm. 1 is lacking, but our specimens show it present as a vestige, and also show no trace of lower inc. 3 against which Ameghino puts a question mark, making the formula

as given above. The upper molars are similar to those of Pachyrukhos except that they have an inner fold which has been lost in Pachyrukhos. The premolars are unlike the molars. Lower molar 3 is three-lobed. The description of the skeleton is given under the specific description of P. garzoni, and this shows a remarkable resemblance to the skeleton of Pachyrukhos, throughout, so that I have no doubt but that Prosotherium is the ancestor of Pachyrukhos, the changes in the teeth proceeding in the line of simplification which seems to be general in this order, and is in general characteristic of forms in which the teeth become rootless.

Ameghino described four species, P. garzoni, P. triangulidens, P. robustum, and P. quartum, the last two of which differ so little from P. triangulidens, that I can not consider them as independent species.

Prosotherium garzoni Ameghino

P. garzoni, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 426.

This, the most abundant species of typotheres, occurs in our collection from the Chico del Chubut, west of Puerto Visser, fifteen times; and in one case the major part of a skeleton was found, consisting of the skull and jaws, vertebrae of each type, ribs, most of the fore limb, the pelvis and a hind limb.

The animal as a whole is smaller than P. triangulidens by about 12%, and is of lighter build. The skull is relatively light and narrow, especially in the rear, where the swollen hollow capsules of the squamosum bones come within ten millimeters of meeting medianly, whereas, in other species, they are twice as far apart. These hollow capsules are in this species the most marked, and in this genus even more developed than in Pachyrukhos. The lachrymal bone is larger externally than usual, the lachrymal duct opening about four millimeters in front of the margin of the orbit, and continuing to the margin by an open groove. In P. triangulidens, the duct is inside the orbit. The heavy maxilla makes a strong process for the zygomatic arch, extending fully half way back along this arch. The short, but fairly stout jugale has but a short contact with the maxilla.

Fig. 30. Left upper dentition;
left lower dentition—
natural size.

Fig. 31. Left upper dentition
No. 3083, showing
deciduous premolars.

In the dentition, the premolar and molar teeth are covered with a thin film of cement, which is thicker on the outside of the upper teeth and on the inner side of the lower teeth. On the opposite sides of these teeth this film is so thin that it is often in part worn off.

Specimen 3083 preserves three of the deciduous premolars. Pm. 2 is simple and could readily be taken for the corresponding permanent premolar, except that it is, as are all the deciduous premolars, rooted. Deciduous premolars 3 and 4, on the other hand, have a marked inflexion on the inner side, giving them the appearance of permanent molars. The series measures 31 mm. of which the deciduous premolars occupy just half.

The mandible is deep, especially the posterior portion; has a very slender coronoid process; and a slightly rounded articular condyle, which is a little longer than wide, so that it would seem to allow a forward and backward motion of the lower jaw.

Fig. 32. Left mandible—natural size.

The vertebrae are considerably crushed, but have in each case the characteristics of the corresponding vertebra of Pachyrukhos.

Of the humerus, the head and distal ends are preserved, indicating a rather long and slender bone, very like that of Pachyrukhos. About three-fourths of the ulna is present, and it is also long and slender, with a wide articular facet for the radius, which is entirely separate from that for the humerus. Two metacarpals show the same elongation of the limb, and the two phalanges preserved indicate a small front foot.

Fig. 33. Left side of pelvis—natural size.

Fig. 34. Left femur—
natural size.

Fig. 35.
Patella—
natural size.

Fig. 36. Tibia and fibula—
natural size.

The pelvis is elongated, slender and lightly built, indicating the same characteristics in the whole hind limb. The femur has a small rounded head on a well marked neck. It is excessively long, longer than that of Pachyrukhos, and also straighter. It is further distinguished by the third trochanter being swung onto the back side of the bone. The tibia and fibula are separate throughout their entire length, in which this genus is in strong contrast to Pachyrukhos, where these two bones are fused, both distally and proximally.

Fig. 37. Calcaneum,
astragulus and cuboid,
from upper side—
natural size.

Fig. 38. Astragulus
from below—
natural size.

The astragulus is also quite characteristic, the trochlear surface being entirely on the dorsal surface, and the condylar ridges being relatively low and flat. This trochlear surface is far from being symmetrical, the inner ridge being much flatter and lower than the outer. The head of the astragulus is rounded, on a long neck, and directed obliquely inward. The fibular facet for the fibula is crescent-shaped and vertical except that the small proximal end of the crescent flares out. The outline of the sustentacular facet is that of an acute ovoid, and is situated mostly on the neck of this bone. The ectal facet is roughly rectangular in outline, strongly concave, and is separated from the sustentacular facet by a deep groove.

The calcaneum is of moderate size, has a narrow fibular facet, a broad ectal facet, and a moderately large sustentacular one. The facet for the cuboid is slightly concave, and occupies the whole of the distal end of the calcaneum.

Fig. 39. Right foot—natural size.

The metatarsals are moderately long and rather heavy, not quite as long and slender as those of Pachyrukhos. The phalanges are also shorter and slightly heavier than those of Pachyrukhos. We found four proximal and four of the second series, all associated, which probably indicates the full number of the toes. The ungual phalanges are proximally narrow and high, then expand toward the tip, developing into marginal expansions. There is but a trace of a cleft in the end of these ungual phalanges.

Measurements
Skull, greatest length99  mm.
Upper dentition, length inc. 1 to m. 355  mm.
Upper dentition, length pm. 1 to m. 331  mm.
Upper dentition, incisor 1, width6½ mm.
Upper dentition, molar 1, length6  mm.
Upper dentition, molar, width4½ mm.
Mandible, greatest length82  mm.
Lower dentition, length inc. 1 to m. 3  53  mm.
Lower dentition, length pm. 1 to m. 332  mm.
Lower dentition, molar 1, length6½ mm.
Lower dentition, molar, width3  mm.
Third metacarpus, greatest length28  mm.
Pelvis, length front to back83  mm.
Femur, greatest length (computed)93  mm.
Femur, diameter of middle of shaft9  mm.
Tibia, greatest length90  mm.
Astragulus, length14  mm.
Astragulus, width11  mm.
Calcaneum, length25½ mm.
Metatarsus III, length32  mm.
First phalanx of digit III, length12  mm.
Ungual phalanx of digit III, length9  mm.

To make the similarity of Prosotherium with Pachyrukhos clearer, I have restored Prosotherium, [figure 40], from which it will be seen that this genus is also a hopping form with a plantigrade hind foot and a semidigitigrade front foot. In general it compares very closely with Pachyrukhos, but the limbs are shorter and the grade of specialization is not quite as high. It is, however, very evidently the ancestor of Pachyrukhos.

Fig. 40. Restoration of Prosotherium garzoni—
⅓ natural size. (Half tone.)

Prosotherium triangulidens Ameghino

P. triangulidens Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 427.
P. robustum, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 427.
P. quartum Amegh., 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p. 371.

This species is similar to P. garzoni except for size, the forms running about 12% larger, and being heavier built. In this same line the upper and lower molars are relatively wider and heavier. The top of the skull also is wider. I have drawn carefully the skull and dentition so that the detail can be seen from the figures. Beside triangulidens, Ameghino described P. robustum, which, as far as I can see, differs only in being about 5% larger, which is well within individual variation, so I have considered it as a synonym. The same is the case with P. quartum, which Ameghino distinguishes as being about the size of P. robustum, and having lower pm. 1 present. The latter character we found also characteristic of P. garzoni, so only size remains and I do not consider less than 10% enough by itself to make a species.

Measurements
Skull, length110  mm.
Upper dentition, length inc. 1 to m. 3  57  mm.
Upper dentition, length pm. 1 to m. 335  mm.
Upper dentition, incisor 1, width8  mm.
Upper dentition, molar 1, width4½ mm.
Six specimens from Chico del Chubut

Propachyrucos Ameghino

Propachyrucos Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 425.

The genus is based on lower jaws, in which the characters of the premolars, the molars and the first two incisors, resemble those of Pachyrukhos; but in this genus the third incisor, the canine, and the first premolar are retained and but little reduced. Ameghino has described two species, P. smithwoodwardi, and P. aequilatus.

Fig. 41. Top view of the skull, palatal view—natural size.

Propachyrucos smithwoodwardi Ameghino

P. smithwoodwardi, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 425.

We did not find this species, but I reproduce Ameghino’s figure of it, natural size. The length of the dentition from inc. 1 to m. 3 is 41 mm., height of mandible under m. 1 is 12 mm.

Fig. 42. P. smithwoodwardi after Ameghino,
right mandible—natural size.

Propachyrucos aequilatus Ameghino

P. aequilatus, Amegh., 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p. 371.

This species is based on the anterior lobe of each lower molar, being longer than the posterior. In size, molars 1 to 3 measure 24 mm.[13]

Phanophilus Ameghino

Phanophilus, Amegh., 1903, Anal. Soc. Cienc., Argen., t. 56, p. 202.

This genus is based on isolated upper molars, characterized as similar to Protypotherium, but having a pronounced median vertical column, instead of a groove on the external face of the upper molars, a character unique among typotheres. The position of the genus with this scant information is uncertain. One species is described, P. dorsatus.

Phanophilus dorsatus Ameghino

P. dorsatus, Amegh., loc. cit. p. 202.

In our collection, two isolated upper molars of this unusual form occur, corresponding in size and pattern to the one described by Ameghino. The external column, as seen by [fig. 42], is narrow and high. A single tooth measures 5½ mm. from front to back, and 3¾ mm. in width.

Fig. 43. External
view of molar 1—
natural size.

Fig. 44. Milk dentition, genus and species?—
natural size.

Specimen 3142 gen. and sp.?

This specimen is the mandible with the milk dentition. The molars present suggest Prosotherium triangulidens, but inc. 3 and the canine are present, and the first two incisors are not enlarged, so it would seem to represent a genus which I have not been able to identify. Molar 1 is bilobed and similar to that of Prosotherium. The deciduous premolars are all present, all rooted, and all remarkable for their great antero-posterior elongation. Roots of the incisors and canine are present, that of the canine being the largest, and those of the incisors being about equal in size. This would suggest such a form as Protypotherium, were this genus represented in the Deseado beds.