Interatheriidae
The family is characterized by the incisors not being enlarged, by upper and lower premolars and molars being inflexed on both the inner and out sides, and by the inflated mastoid bulla being filled with cancellous tissue. The only genus referable to this family, from the Deseado beds, is Archaeophylus.
Archaeophylus Ameghino
Archaeophylus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 423.
The genus is based on a skull which preserved most of the upper dentition except the incisors, and which shows the characteristics of the family in their inception.
Archaeophylus patrius Ameghino
Fig. 45.
Archaeophylus patrius
after Ameghino,
right upper dentition—
natural size.
A. patrius Amegh., loc. cit.
We found no specimen of this interesting type, but I give a diagram of Ameghino’s type, which shows the characteristics of the species and genus. The length of the premolar-molar series is 27 mm.
Eutrachytheridae Ameghino
The family consists of larger forms than the other families, and is characterized by the upper molars having the inflexion bifurcated, so that the teeth are at least incipiently three-lobed. I would place in the family the genera, Eutrachytherus, Argyrohyrax, and Isopoedrium.
Eutrachytherus Ameghino
Trachytherus Amegh., 1889, Act. Acad. Nac. Cienc. Cordoba, t. VI, p. 918.
Trachytherus Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 623.
Eutrachytherus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 429.
This genus was first called Trachytherus, and when this was found to be preoccupied, changed to Eutrachytherus. Ameghino gives the dental formula as
- 3 0 4 3
- ———,
- 2 1 4 3
but in my specimen which is a comparatively young individual, I find a small alveolus for the upper canine, which modifies the formula to
- 3 1 4 3
- ———,
- 2 1 4 3
In the first upper dentition found, incisors 2 and 3 were represented by alveoli only, but our specimen shows these teeth, and we find that they have enamel only on the outer face, making this genus more specialized in this respect than any of the other typotheres. The upper molars have the deep inner inflexion bifurcated, which makes the tooth three-lobed, a character which grows more marked the older the individual is. The premaxillae are high and bring the snout well forward. The nasals are lacking, but the bounding bones show them to have been unusually broad and flat. The maxilla extends up to the nasals, and bounds the lower border of the orbit, and projects backward in a heavy overhanging zygomatic process. The lachrymal bone is large externally, with a large but low lachrymal tubercle just below which is found the lachrymal duct, opening just in front of the margin of the orbit. The frontals are short and wide, extending outward over the orbit in a strong postorbital process which bounds half of the rear of the orbit. The parietals, meeting medianly, rise in a strong sagittal crest. Unfortunately the back part of the cranium is lacking. From another specimen, which contained the brain cast, it is clear that the bulla was much inflated and hollow, and that there was an inflation in the upper part of the squamosum, as in Prosotherium, etc.
Fig. 46. E. spegazzinianus—½ natural size.
One specimen with the facial portion badly weathered, but retaining enough to identify the species as E. spegazzinianus, preserved the brain case, so that it could be prepared out.
The most striking feature of this brain is its relatively large size, E. spegazzinianus being an animal about the size of a sheep, and the brain is as large as that of the sheep, which is in strong contrast to what would be expected of an Oligocene form. Compared with the herbivorous Oligocene oreodont, Eucrotaphus, an animal of approximately the same size, this brain is half again as large in every way. A second striking feature is the short compact character of the brain, the forebrain extending only a short distance in front of the exit of the optic nerves, and extending backward so as to cover most all of the cerebellum. Thirdly, the cerebral surface is considerably convoluted, comparable to the convolution of a pig’s brain. These features would indicate a specialization of the nervous system, approximating that of the skeleton, and would indicate that this group had advanced in intelligence and activity beyond the grade of nervous development which is apparent in the contemporaries of the Typotheria.
The relatively small olfactory lobes are entirely beneath the frontal lobes and are seen only on the side view, but as there is a large hippocampal lobe behind them, it would only seem proper to attribute to these animals a well-developed sense of smell. The frontal lobes are unexpectedly large, and are not clearly bounded off from the parietal lobes. The occipital lobes are also well developed and make a large portion of the backward extension of the cerebrum. The large size of this area, together with the fact that the optic nerves are large, indicates a good visual development. The temporal lobes are also large and extend well down on either side. And, finally, below all the others, come the swollen hippocampal lobes which complete this large cerebrum.
The cerebellum is small having neither considerable width or height, and being overlapped by the cerebrum. The optic nerves are represented by large projections leaving the twixt-brain well forward under the forebrain. The medulla is not clearly marked except to show that it, too, was of fair size.
Fig. 47. Cast of the brain; A, from above; B, from the side—natural size. Cer, cerebellum; F, frontal lobe; H, hippocampal lobe; Oct, occipital lobe; Op, optic nerve; T, temporal lobe; x, cast of cavity in squamosal bone; Na, case of interior of nasal cavity.
Just behind the hippocampal lobes, and connecting with the cerebellum appear two striking projections. These represent the two large cavities in the upper part of the squamosum which are so characteristic of typotheres. The two large cavities clearly opened into the brain case by a broad connection which is especially wide at the lower ends. I find no traces of a connection with the bulla as described by Sinclair for Pachyrukhos. Further down are two small knobs apparently also representing cavities in the squamosum, and also connected with the brain case. In considering the brain these should be overlooked; but they doubtless represent some nervous function to which I have as yet no clue.
Ameghino considered that Eutrachytherus was the connecting link between Archaeohyrax and Typotherium. I feel that this genus is too highly specialized to be a connecting form, though it doubtless belongs to the series which ends in Typotherium; and such a form as Argyrohyrax is more likely to be the really ancestral form.
Two species are described, E. spegazzinianus, and E. conturbatus, which is about 15% smaller. Our collection offers a third species, E. grandis, which is nearly 50% larger than the first named species.
Eutrachytherus spegazzinianus Ameghino
Trachytherus spegazzinianus Amegh., 1889, Act. Acad. Nac. Cienc. Cordoba, t. VI, p. 919.
Trachytherus spegazzinianus Lydekker, 1894, Anal. Mus. La Plata, pt. 3, p. 2.
Trachytherus spegazzinianus Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 622.
Eutrachytherus spegazzinianus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 428.
This species was founded on the anterior part of a skull with the full upper dentition. My specimen differs from Ameghino’s in having a tiny alveolus for the upper canine, the difference being due to my specimen being younger.
The upper dentition is very characteristic. Incisor 1 is a powerful, deep-set, curved gnawing tooth, with a heavy layer of enamel on the anterior face, and none on the other sides; and is moderately beveled in the rear as a result of wear. The second and third incisors are much smaller, each having enamel on the outer face only, and with a marked tendency to become vestigal. The suture of the premaxilla comes to the base of inc. 3. There follows a short diastema, then a tiny alveolus for the canine, closely followed by the first premolar which is also small. The second premolar shows no inflexion. Beginning with the third premolar there is a strong inner inflexion, which in the fourth premolar and molars is bifurcated. The molars are considerably larger than the premolars, the second being the largest of the series. With each successive molar, the inflexion is wider, so that in m. 3 the tooth is divided into three lobes of nearly equal size. All premolars and molars are rootless, curved, and set so deep in the jaw that they almost meet in the median line. A typical molar measures 50 mm. in length, of which only 7-8 mm. project above the border of the jaw. All the back teeth are covered with a thick coating of cement.
Fig. 48. E. spegazzinianus, right upper dentition—
natural size.
The arrangement of the teeth of the lower jaw is shown in [fig. 25 h]., after Ameghino. The first and second incisors are greatly enlarged. Incisor 3 is lacking, and the canine vestigal. Pm. 1 is small and single-lobed, the succeeding premolars and molars being large and divided into two lobes by a deep external, and a shallow internal infolding.
Eutrachytherus conturbatus Ameghino
Trachytherus conturbatus Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 623.
Eutrachytherus conturbatus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 429.
This species is founded on upper teeth, which are said to be relatively smaller anteriorly, and actually smaller throughout, by about 15% than are those of the preceding species. Molar 1 measures 17 mm. long by 9 mm. wide.
Eutrachytherus grandis sp. nov.
This species is based on upper molars 1 and 2 of the left side, from the Deseado beds, of the Chico del Chubut, west of Puerto Visser. The teeth are typically those of the genus and differ only from other species in their large size, being some 50% larger than the corresponding teeth of E. spegazzinianus. Each is covered with a layer of fully half a millimeter of cement.
Fig. 49. E. grandis, molars of the left side—
natural size.
| Measurements | ||
|---|---|---|
| Upper molar 1, | length 29 mm., | width 21 mm. |
| Upper molar 2, | length 30½ mm., | width 18 mm. |
Argyrohyrax Ameghino
Argyrohyrax Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 435.
The genus is distinguished by the full dentition in closed series, and by the upper molars having a deep internal inflexion which is bifurcated, making the teeth at least incipiently three-lobed. Incisor 1 is relatively somewhat wider than in the preceding genus. It seems to me that it is from such a genus that Typotherium arose, by the reduction of the lateral incisors, the canine, and the first two premolars, and by the increase of the bifurcated internal fold. Three species have been described, A. proavus, the type species, A. acuticostatus, of a little smaller size, and A. proavunculus of still smaller size.
Argyrohyrax proavus Ameghino
A. proavus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 436.
This species occurs three times in the Amherst collection. The best specimen has pm. 1, 3, 4, and m. 1 and 2 of the upper jaw. The species is characterized by a narrow furrow near the anterior external margin of the premolars and molars; and by pm. 3 and 4 and the molars having a deep internal bifurcated inflexion, which tends to make these teeth three-lobed.
Fig. 50. Right upper dentition, the outline teeth
after Ameghino—natural size.
The genus is known only by the upper dentition, and while I did not find any associated lower teeth, I believe that some one of the genera known only by the lower dentition, like Plagiarthrus, is that lower dentition.
| Measurements | |||
|---|---|---|---|
| Upper dentition, | length inc. 1 to m. 3 (@ Ameghino) | 71 mm. | |
| premolar 1, | length 7 mm., | width 4 mm. | |
| premolar 3, | length 7 mm., | width 6 mm. | |
| premolar 4, | length 8 mm., | width 6 mm. | |
| molar 1, | length 8 mm., | width 6½ mm. | |
Argyrohyrax proavunculus Ameghino
A. proavunculus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 436.
The species is simply said to be much smaller than the preceding, which, by the measurements, would figure out about 27%. The measurements given are pm. 1 to m. 3, 33 mm.
Argyrohyrax acuticostatus Ameghino
A. acuticostatus Amegh. 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p. 361.
This species is described as differing from A. proavus in being somewhat smaller, but I find it about the same size. The specific character is in the teeth being more compressed, and in the anterior vertical margin of the upper molars being developed in the form of a very salient crest.
Isoproedrium Ameghino
Proedrium Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 623.
Proedrium Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 431.
Isoproedrium, Amegh., 1903, Anal. Soc. Cient. Argen., t. 56, p. 18.
This genus is based on a toothless mandibular symphysis of large size, on which the second incisor is indicated as larger than the first. We found one lower jaw, which, though imperfect, confirms the above as a genus and adds the following for generic characters; premolars with a shallow inflexion on the inner and outer sides, the molars with a shallow inner, and a narrow deep outer inflexion, heavy layer of cement on the premolars and molars, enamel reduced on the anterior internal, and the posterior internal corners of at least the premolars.
Isoproedrium solitarium Ameghino
loc. cit. above.
Fig. 51. Right mandible, dotted line indicated alveolus—
natural size.
I have assigned the specimen shown in [fig. 51] to this species. Alveoli of the first and second incisors show inc. 2 considerably larger than inc. 1. About the third incisor and the canine my specimen shows nothing, being broken down in that region. Pm. 1 is represented by a moderately large alveolus, 10 mm. long. Pm. 2 is 13 mm. long by 10 mm. wide, and slightly constricted medianly as far as the enamel is concerned, the furrow in either side being filled with cement. Pm. 3 is 18 mm. long by 12 wide and similar. There is only an alveolus for pm. 4, which is 23 mm. long. M. 1 is incomplete, but was about 23 mm. long by 10½ mm. wide and is distinguished by the deep outer inflexion. Each tooth present is covered with a heavy layer of enamel nearly a millimeter thick; and each of the teeth is unique in that the enamel is wanting on the anterior internal and the posterior internal corners of the teeth.
CHAPTER VI
Toxodontia
The toxodonts of the Deseado are much more varied than those of the Santa Cruz, and less so than those of the Casamayor; the teeth less hypsodont than in the Santa Cruz, and more hypsodont than in the Casamayor; are smaller than those of the Santa Cruz, and larger than those from the Casamayor. It is a group of heavy, short-limbed, nonadaptive animals, which, as time and competition progressed, gradually diminished in numbers and variety.
The ancestral type must be sought in some such a form as Henricosbornia, where the upper molars are brachydont, have the four primary cusps distinct, and the connecting crests of small size, and a cingulum moderately developed on the front and rear sides. Progress is in the line of enlarging the crests, so that, in the later forms, the two external cusps are united to make a wall; and the anterior external and the anterior internal cusps are united into the large anterior lobe; while the posterior external and the posterior internal cusps unite to make the posterior lobe. These may remain relatively simple as in Rhynchippidae[14]; or with this simple arrangement of the cusps, the cingulum may be developed into a platform around the anterior, inner, and posterior sides of the molars, as in the Isotemnidae; or, with relatively simple molars, the incisors may be specialized into caniniform-like teeth as in the Leontinidae; or secondary processes (or cristae) may develop from the wall, making the complicated teeth characteristic of Nesodontidae.
For convenience in discussing the modifications of the toxodont tooth, I have, throughout, used the nomenclature illustrated in [fig. 52], taking one of the most complicated to show the ultimate development. In the upper tooth there is, first, the external wall, from which springs the anterior lobe, always the larger lobe, and composed of the protocone and paracone of Osborn. In the rear is a smaller narrower posterior lobe, composed of the hypocone, the metacone, and the metaconule of Osborn. Between these is a large basin, which may be subdivided by two cristae into secondary bays, referred to as bays 1, 2 and 3, while the cristae are in the same way referred to as cristae 1 and 2. In some genera, the cristae are entirely wanting, in others incipient. When fully developed, they are most marked in young individuals and, as the tooth is worn, appear progressively shorter. Behind the posterior lobe, there is a variable bay, number 4 which is bounded behind by crista 3, which is apparently a development of the posterior cingulum. This last crista and bay may or may not be present.
Fig. 52. A, upper molars of Coresodon;
B, lower molars, somewhat worn—
natural size.
The lower molars of toxodonts are all on the same plan, each tooth being composed of two crescents, the anterior and posterior. The ends of these crescents are referred to as the anterior, median and posterior horns. The bay in the anterior crescent is simple and usually disappears with the wear of the tooth without making a pit. In the centre of the posterior crescent is the pillar or posterior tubercle which Scott has found to be characteristic of these South American Ungulates. It is, to my mind, the same as the mesostylid of the Fayum hyracoids. Between the pillar and the median horn, I find a narrow vertical ridge, which I have termed the septum; and which tends to unite with the pillar inclosing a small bay, usually seen in worn teeth as a pit. The bay between the septum and the median horn is designated bay 2, and this quite generally appears in a worn tooth as a pit (2). The bay between the septum and pillar is designated bay 3, and is usually seen as a tiny pit, which however does not extend as deep into the crown as the other pits and is usually lost when the tooth is about half worn off. The bay between the pillar and the posterior horn is numbered 4, and is usually open, though in a worn tooth it also may appear as a pit. The effect of wear is shown by comparing B and C in [fig. 52], the latter being the same tooth sectioned a little below the middle. I find in studying a lower molar of Coresodon that bay 3 becomes a pit after some 6 mm. are worn off, while bays 2 and 4 remain open until some 10 mm. are worn off when they also become pits. Pit 3 will disappear when 12 mm. are worn off, but pits 2 and 4 run to the base of the crown.
The various genera of the Toxodontia in the Deseado I would divide into four families as follows:
Rhynchippidae: molars brachydont, secondary cristae lacking or little developed, none of the incisors caniniform, limbs slender, feet digitigrade, digits 3-3.
Leontinidae: molars brachydont, secondary cristae lacking or little developed, upper inc. 2 and lower inc. 3 developed into caniniform teeth, limbs heavy, feet digitigrade, digits 3-3 (according to Gaudry).
Isotemnidae: molars brachydont, secondary cristae more or less developed, crowns contracted at the top, congulum more or less developed into a platform, skeleton unknown.
Nesodontidae: molars hypsodont, secondary cristae highly developed, upper inc. 2 and lower inc. 3 caniniform, limbs heavy, feet digitigrade, digits 3-3.