The Deseado Formation of Patagonia

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

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.

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 m. 1, found on the Chico del Chubut, west of Puerto Visser, by Waldo Shumway.

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.

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

• 1 0 4 3
• ———
• 2 0 4 3

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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