CHAPTER V

TYPOTHERIA

In the Deseado beds this group of running and hopping animals is well represented, making about 14% of the Amherst collection, and varying in size from a little larger than a rat to larger than a sheep.

The group all have the front teeth modified into cropping or gnawing types, which grow permanently from persistent pulps; and the back teeth also growing through the whole or a large part of life, and also rootless, the crowns being variously infolded to make grinding surfaces. The skull is flattened above, and abruptly truncated behind; the cranium being large and swollen, the facial portion broad above and excavated on the sides. The orbits are centrally located, of considerable size, and unbounded behind. The tympanic bulla is swollen and may be hollow or filled with cancellous tissue. This cavity of the tympanic is continued above and expands in the upper part of the squamosum, making a swollen capsule on either side of the back of the cranium. The openings of the auditory meatus are well back and in a tubular growth of the periotic which is directed back, and upward in an entirely characteristic manner. The strong paroccipital processes project far below the base of the carnium. The concave palate is wide and carried well back behind the teeth ending in two strong pterygoid processes. The mandible is deep, especially the back portion; has a slender coronoid process, and a small rounded articular condyle which would seem to indicate a forward and backward motion of the jaws. On account of the agreement with these general features, I have placed among the _Typotheria_ the forms which Ameghino classified as _Hyracoidea_.

While agreeing in the above general features, there is great variation among the various forms. The first upper and lower incisor may be greatly enlarged or of normal size. There is a tendency for the third upper and lower incisor, the canines, and the first premolars to be reduced and disappear, and all intermediate grades are found. In the molars there is a regular tendency toward simplification; so that in the upper molars of the earlier forms there is a deep inner fold and a more moderate outer fold, either or both of which may disappear completely, though in one series the fold seems to have been accentuated instead of lost. The feet may be adapted to running or hopping.

In the Deseado and Santa Cruz material, four series of modifications may be distinguished which I have designated as families; (1) the _Archaeohyracidae_, primitive forms in which the incisors are little enlarged, with inner and outer folds on the molars, those on the inner side of the upper molars being very deep, bulla small, feet unknown; (2) _Interatheriidae_, first upper and lower incisors rooted and of moderate size, inflexions on both the inner and outer sides of the molars, bulla large, feet adapted to running; (3) _Hegetotheriidae_, incisor 1 of upper and lower dentition greatly enlarged and rootless, molars simplified, bulla large, feet adapted to running or to hopping; (4) _Eutrachytheridae_, large forms with the first upper and lower incisor enlarged and rootless, the upper molars with the inner fold developed and bifurcated, bulla large, feet unknown.

For comparison of the various genera, they are charted on page 55, the dental character being used, as but few have the skeleton known, which is especially so of the earlier genera.

CHART OF TYPOTHERIA ============================================================================ | Age |Formula|Canines | U. Molars |Last i.| Toes | | | | | | molar | | --------------+-------+-------+--------+------------+-------+--------+------ Hegetotherium | Santa |3 1 4 3|vestigal|no inner |3-lobed|cleft | | Cruz |-------| |fold | | | | |3 1 4 3| | | | | --------------+-------+-------+--------+------------+-------+--------+------ Prohegeto- |Deseado| | |no inner | | | therium | | | |fold | | | --------------+-------+-------+--------+------------+-------+--------+------ Pachyrukhos |Santa |1 0 3 3|lacking |no inner |3-lobed|not | | Cruz |-------| |fold | |cleft | | |2 0 3 3| | | | | --------------+-------+-------+--------+------------+-------+--------+------ Propachyrucos |Deseado| |slightly| |3-lobed| | | |-------|reduced | | | | | |3 1 4 3| | | | | --------------+-------+-------+--------+------------+-------+--------+------ Prosotherium |Deseado|1 0 4 3|lacking |simple |3-lobed|slightly| pm. | |-------| |inner | |cleft |simple | |2 0 4 3| |fold | | | --------------+-------+-------+--------+------------+-------+--------+------ Archaeophylus |Deseado|? 1 4 3|rather |deep inner | | | | |-------|large |slight fold,| | | | | | |outer one | | | --------------+-------+-------+--------+------------+-------+--------+------ Interatherium |Santa |3 1 4 3|vestigal|slight inner|2-lobed|slightly| | Cruz |-------| |and outer | |cleft | | |3 1 4 3| |folds | | | --------------+-------+-------+--------+------------+-------+--------+------ Protypotherium|Santa |3 1 4 3|large |deep inner |2-lobed|slightly|closed | Cruz |-------| |and outer | |cleft |series | |3 1 4 3| |folds | | | --------------+-------+-------+--------+------------+-------+--------+------ Argyrohyrax |Deseado|3 1 4 3|large |bifurcated | | | | |-------| |inner fold | | | | | | | | | | --------------+-------+-------+--------+------------+-------+--------+------ Eutrachytherus|Deseado|3 1 4 3|lacking |bifurcated |2-lobed| | | |-------| |inner fold | | | | |2 1 4 3| | | | | --------------+-------+-------+--------+------------+-------+--------+------ Isoproedrium |Deseado| | | | | | | |-------| | | | | | |? ? 4 3| | | | | --------------+-------+-------+--------+------------+-------+--------+------ Archaeohyrax |Deseado|3 1 4 3|large |deep inner |3-lobed| | | |-------| |fold, slight| | | | |3 1 4 3| |outer fold | | | --------------+-------+-------+--------+------------+-------+--------+------ Plagiarthrus |Deseado| | | |2-lobed| | | |-------| | | | | | |? ? 4 3| | | | | --------------+-------+-------+--------+------------+-------+--------+------

From the foregoing chart and the comparative figures of the upper and lower dentitions, the variety and at the same time the homogeneity of the _Typotheria_ is evident. The gnawing front teeth resemble those of rodents, especially in the genera where the enamel is lacking on all but the front face, but this is entirely a parallelism and there is no evident phylogenetic relationship. As to affinities with the _Hyracoidea_, Sinclair[12] has carefully balanced them and finds so little in common between the two groups that he makes them a separate suborder. I find certain features in common, like the lophiodont dentition with the tendency toward hypsodont incisors, the inflation of the tympanic and the extension of this up into the periotic region, and the general arrangement of the basicranial foramena. On the other hand, there are also numerous features in common with the Toxodonts, and several peculiar to the group, so that I would feel that all the Notungulates are descended from the _Hyracoidea_, and this group has developed its peculiarities in South America, retaining however a little more of the hyracoid aspect.

[Footnote 12: Princeton Expeditions Reports, Vol. VI, p. 7, 1909.]

The _Archaeohyracidae_ are the most primitive of the Deseado forms, but as all the families are already separated before this time the Deseado genera can not be considered as the ancestral ones, though they seem to have retained more of the primitive features.

The _Interatheriidae_ represent an offshoot line of development in which the incisors are not much enlarged and the infoldings of the teeth remain. The genus _Archaeophylus_ seems to be directly ancestral to the Santa Cruz genera _Interatherium_ and _Protypotherium_. In the family _Hegetotheriidae_ there is a strong tendency for the incisors to develop into very large gnawing teeth, while the lateral incisors, the canine and the first premolar, tend to drop out, and the molars become more simplified. _Propachyrucos_ seems to represent a hold over of the most primitive type of these. The _Prohegetotherium_ and _Hegetotherium_ have retained the less specialized feet and less advanced type of teeth, while _Prosotherium_ has tended to the development of the hopping mode of locomotion, which is attained in _Pachyrukhos_ later. There thus seem to be two series inside of this family. When the material is better known, it may be best to separate the two series. The _Eutrachytheridae_ have retained the complexity of molars united with a permanently growing incisor. They seem also to have developed into a series of comparatively large forms, which, as they have advanced, have developed a bifurcated fold on the inner side of the upper molars, which in its complete development makes the upper molars three-lobed, as is seen in the typical _Typotherium_, representing the end of the series up in the Pampean formation. These relationships may be expressed graphically as in fig. 26.

ADAPTATIONS

Most striking of all the typothere peculiarities, is the development of the first upper and lower incisor into permanently growing teeth, having the enamel reduced to the anterior side only, making thus a self-sharpening tooth similar to that of rodents. Such teeth are characteristic of gnawing forms and would indicate that the form lived, in at least a considerable part, on bark and twigs. In the eating of such food and breaking up the wood cells for the contained protoplasm and starch, an immense amount of chewing is involved, followed by a rapid wear of the molars. This is met, as is characteristic in rodents and grass eaters, by the development of first high-crowned, then permanently growing molars. In acquiring the permanently growing tooth, some of the irregularities of the crown are lost, others which are deep-seated enough to affect the tooth even to the root are maintained, so that especially the external and internal infoldings become a persistent part of the tooth, having been impressed into the dental papilla. A further supplement to the resistant character of the teeth is seen in the development, in the most advanced types, of a cement layer on the outside of the molars, a feature apparently also a part of permanently growing roots.

The feet are generally those of a running type, but a single phylum has acquired the hopping habit.

The above features seem to indicate a more special adaptation than grass feeding. From the aspect of the whole Deseado fauna, we would seem to be dealing with the inhabitants of an arid area, where bushes have, in part at least, replaced the grass. The typotheres seem to me to represent a part of the fauna which lived by gnawing the bark and eating the twigs and leaves of bushes. This does not preclude the eating of grass also, but I do not see how they would have developed all their peculiarities by eating grass alone. The rodents are of such insignificant size that they could hardly have monopolized this food supply, and the typotheres seem to have adjusted themselves to, and occupied the place of rabbits on our western plains; but went even farther in developing in great numbers and varieties.

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

3 1 4 3 -------. 3 1 4 3

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.

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

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_,

1 0 4 3 -------, 2 0 4 3

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

_Propachyrucos_,

-------, 3 1 4 3

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