The Deseado Formation of Patagonia

The systematic arrangement of the South American ungulates is of such a nature that scarcely two students of these forms have agreed. I feel that the Pyrotheridae are proboscideans as did Ameghino, but there my agreement ends. The other varied groups I believe have a common ancestry, their great divergencies being due to adaptations to the greatly varied characters of the country they occupied. In spite of the great variation they have certain features in common so that I agree with those who have developed the term Notungulata to include them all.

From what source they originally came is not clear, but it seems to me that these notungulates have more in common with what we know of the African fauna of the Fayum than with any other fauna; so that my feeling would be that these two faunas had a common ancestry at least, and possibly the South American ungulates are derived from the African. The lophiodont upper dentition, the bicrescentric lower molars with a “pillar” in the posterior crescent, the development of the tympanic bulla with the extension of the inflated cavity up into the squamosal bone, the development of the post-tympanic portion of the squamosum, and the general arrangement of the basicranial foramena indicate in my mind that these notungulates have all risen from the same stock, and that that stock had much in common with the hyracoids.

I should therefore arrange the various groups as follows.[11]

NOTUNGULATA

LITOPTERNA

This order of South American ungulates is less abundantly represented in the Deseado formation than in the Santa Cruz, but most of the genera of this latter formation have representatives in the Deseado so that they seem to have diverged still earlier.

By Scott the order is divided into two families, the Proterotheriidae and the Macrauchenidae, the less known Adiantidae being placed under the latter family until better known. I feel that I should prefer to retain the Adiantidae for the present, until they can be shown to be subordinate to another family, so that in this paper the three families are retained. The striking features of the two larger families may be best brought out by a comparison of their chief features as follows.

Proterotheriidae Ameghino

In the Deseado, this family is scantily represented as compared with the rich fauna, both as to species and numbers of individuals in the Santa Cruz, but of the four chief genera of the Santa Cruz, three have been found, though the remains are very fragmentary. They are the genera Eoprototherium, belonging to the Prototherium series, Deuterotherium belonging to the Thaotheriumseries, and Notodiaphorus representing the Diadiaphorus series.

The following table will give what is known in comparing the two series.

Eoproterotherium Ameghino

Eoproterotherium Amegh., 1904, Anal. Mus. Nac. B. A., ser. 3, t. 3, p. 441.

The genus is founded on single teeth of the upper molar series, which, except for size, are very like those of Proterotherium. Limbs, etc., are unknown, so that this genus is simply a carrying back of the Proterotherium line into the Deseado. We found no teeth of this form, but one species has been described, E. inaequifacies, of which I reproduce Ameghino’s figure compared with Proterotherium, which shows this species to have the metaconule better developed.

Notodiaphorus gen. nov.

The basis of this genus is particularly a hind limb found associated which is much less developed than the Santa Cruz genus Diadiaphorus to which it is most nearly related. These two genera are unique in having the ectal facet on the astragulus developed in two planes so that it appears as a deep notch. In the case of the new genus the toes are almost equal in size, giving us a stage in the development of this three-toed form which is much more primitive than the well-known Santa Cruz genus.

Notodiaphorus crassus sp. nov.

The specimen selected as type is number 3287 of the Amherst Collection, consisting of a complete pes, tarsus, lower end of the tibia, and the femur, from the Deseado on the Chico del Chubut River, west of Puerto Visser. Beside this, there are seven other specimens, mostly parts of hind limbs, but others having also the lower end of the humerus, the radius and ulna, metacarpals, and some phalanges. The species is distinguished by its large size, being larger than the species of the Santa Cruz, and, at the same time, the three toes of both the pes and the manus are subequal in size.

The distal end of the humerus associated indirectly with this species is moderately heavy, with fair-sized epicondyles, and no entepicondylar foramen. The supratrochlear fossa is moderately deep, the anconeal very deep, the two being connected by a small foramen, as is typical for this family. The trochlearis, slightly oblique to the long axis of the shaft, has a simple pulley-like articular end without ridges of division, the internal border being narrower and higher than the external.

The radius and ulna were from another specimen which, however, was associated with a typical astragulus. The two bones are long, slender, strongly curved, and in contact with each other throughout their entire length, so that there could have been no rotary movement of the forearm. The radius is a slender bone with the proximal articular facet relatively small, the facet being slightly concave, of ovoid outline and with the transverse diameter the greater. There is but a tiny band-like facet for the ulna situated on the posterior side near the inner margin. Distally, the radius widens into a heavy end with a rugose area on the outer side for contact with the ulna, and with two distal facets, a larger for the scaphoid, and a smaller for the lunar, the two being separated by a low ridge.

The ulna is heavier above, with a strong backwardly directed olecranon process. The sigmoid notch makes almost a semicircle, the articular surface being broad and extending well onto either side of the bone. The facets for the radius are tiny. The distal end of this bone is wanting.

The femur belongs to the type specimen which is about 5% larger than the other specimens. This bone is long and rather slender, with the greater trochanter rising well above the head, which is rounded, on a short neck, and has the ligamentary pit on the posterior margin. The thick, rugose, greater trochanter bends in over the head at its upper end. The lesser trochanter is relatively small, and prolonged into a ridge. Unfortunately the third trochanter is broken off in my specimen. The digital fossa is extremely large and deep. Proximally the shaft is flattened, but becomes rounded distally. Just above the condyles there is a deep rugose pit for the plantaris muscle, and on the anterior side the suprapatellar fossa is well marked. The condyles are placed a trifle obliquely; the internal one being shorter and with a rounded articular face, the external condyle being longer, and with a flattened articular face which slopes obliquely inward.

Of the tibia, only the distal end is preserved. This indicates a rather slender bone, with a shallow, fairly wide concavity for the external astragular trochlea, and a narrower and deeper concavity for the internal astragular trochlea. On the internal side of the tibia there is a rugose surface for the fibula.

An isolated lower end of a fibula indicates a slender bone, enlarged distally where it comes in contact with the tibia. The fibula carries on its inner face a moderately large facet for the external side of the astragulus, and on the distal end a wider one for contact with the calcaneum.

The tarsus is compactly built, wider than that of Diadiaphorus, because the external digits are not as much reduced. This especially shows in the greater development of the cuboid and the mesocuneiform, but in other features it is similar to that of its descendant.

The astragulus is a very characteristic bone. The trochlea is asymetrical, the external condyle rising higher than the internal, and the median groove being wide and shallow. On the nearly vertical outer face of the astragulus, there is a semicircular band-like facet for the fibula. The trochlea extends well around the top of the bone, allowing a wide movement of the foot. The neck of the astragulus is long and wide, carrying a broad flattened head, with its convex facet for the navicular, covering the entire end. On the plantar side are the most marked features. The ectal facet is in two planes, the anterior portion being bent down to nearly right angles with the posterior, which seems to be characteristic of this Diadiaphorus series. The sustentacular facet also is characteristic, being gently rounded and extending clear to the navicular facet on the head, in Diadiaphorus becoming actually confluent with the navicular facet. Just at the edge of this sustentacular facet is a tiny surface where the astragulus rubs on the cuboid, the only case, as far as I am aware, where this occurs in any Litopterna.

The calcaneum is long and slender, the tuber being but slightly enlarged, its sustentacular facet being a broad oval surface, while the ectal facet is in two planes to correspond to that on the astragulus. The facet for the cuboid is at the distal end, but is unusually oblique, its inner margin sloping up almost to the sustentacular facet. It is this slope which brings the cuboid in contact with the astragulus.

The navicular is broad and low, with a prominent hook behind. On its upper face there is only the broad facet for the astragulus head; on the lower face are three facets, externally, a large, more or less triangular area, for the ectocuneiform; medianly a smaller similar facet for the mesocuneiform; and on the internal side, sloping up onto the internal face, a small facet for the reduced endocuneiform. On the external face of this bone there is a tiny beveled facet for the cuboid.

The endocuneiform is a large scale-like ossicle articulating on the lateral internal face of the navicular, and overlapping markedly the inner surface of Metatarsus II.

The mesocuneiform is considerably reduced in size, carrying a broad flat facet on the upper surface for the navicular, and a shallow saddle-like one below for Mt. II, which is entirely carried by this bone.

The ectocuneiform is considerably larger than the mesocuneiform, resting above on the navicular, and carrying below the whole of Mt. III. On its inner side are two facets which rub against the upper end of Mt. II.

The cuboid is a nodular bone, its upper surface occupied by the facet for the calcaneum, the lower face occupied by the facet for Mt. IV, while on the external side there is a tiny beveled facet for the vestige of Mt. V, and with a small boss on the inner surface which carries two tiny facets, the upper one for the ectocuneiform and the lower for the navicular. On this same inner side, near the top there is a second small boss, which carries a tiny facet to rub on the astragulus, and below that a second tiny facet for the navicular.

The pes consists of three digits, with a vestige of Mt. V. Of the developed digits, the median one is the largest, but the two lateral digits are only a little smaller and were functional, so that this form was truly three-toed, comparable in the digital reduction to Mesohippus.

Mt. II is flattened above but soon broadens into a rounded shaft of considerable length, on the end of which is the articular trochlea, with the carina extending onto both the upper and lower surface, being, however, higher on the lower surface. Proximally this bone is overlapped by the endocuneiform, is carried by the small mesocuneiform, and also articulates on the inner side of the ectocuneiform. Mt. III is also compressed at the upper end, broadens below, and carries an articular trochlea similar to that of Mt. II, except that the carina does not extend so far onto the upper surface. Like Mt. II, Mt. IV is carried high on the tarsus, and therefore, though nearly as long as Mt. III, it does not have the same effective length. Proximally it articulates entirely on the cuboid; distally it has a trochlea similar to that of Mt. II, the carina extending onto the dorsal surface. While Mt. V is lacking, it is clearly indicated that a vestige of it should have been present, as there is a tiny articular surface for it on the cuboid, and a rugose surface on the outside of Mt. IV.

The phalanges are long and have the articular ends swollen somewhat as in camels. The phalanges of the first row are nearly equal in size, each with the proximal trochlea deeply notched for the carina of the metatarsus; and with the distal trochlea simple, though slightly concave from side to side, and reflexed well onto the dorsal surface. The phalanges of the second row are shorter and simpler, and somewhat depressed distally. The ungual phalanges are flattened from top to bottom, of moderate size, somewhat longer than wide, and without any indications of a cleft.

Deuterotherium Ameghino

Deuterotherium Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 633.

Deuterotherium Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 452.

This genus was first founded on a clacaneum and a bit of the mandibular symphysis, to which were added, later, both the upper and lower premolar and molar teeth. As far as it is known, it is distinguished by the upper molars lacking the metaconule entirely, and being approximately like those of Thaotherium. The dental formula is given by Ameghino as

• 1 0 4 3
• ———,
• 2 0 4 3

the same as Thaotherium. But one species has been described.

Deuterotherium distichum Ameghino

We did not find this species, but the teeth assigned to it are very characteristic, and so I reproduce Ameghino’s figure of them. The species is distinguished by its size primarily. The following are the chief measurements given.

Upper dentition, pm.  3 to m. 3, length  50 mm.
Lower dentition, inc. 1 to m. 3, length  80 mm.

Macrauchenidae
(= Mesorhinidae Amegh.)

This family is distinguished, first, by the complete dental series in which none of the anterior teeth are developed into tushes; by the nasals being shortened, apparently in connection with the development of a proboscis; by its long neck; and by its feet being permanently tridactyl, all the three toes being equally developed. In the Deseado it is infrequent, but to it Ameghino has assigned two genera; Protheosodon, which he describes as similar to Theosodon, but which I find much nearer to the Casamayor representatives of this family, such as Lambdaconus, though it doubtless belongs to the series which is represented in the Santa Cruz by Theosodon. He has also made a second genus, Conioptotherium, which represents a large Macrauchenid, equal in size to Theosodon. This genus is based on the calcaleum and astragulus and seems to be rare.

Protheosodon Ameghino

Protheosodon, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 453.
Protheosodon, Amegh., 1904, Anal. Mus. Nac. B.A., ser. 3, t. 3, p. 421.

This genus was founded on an upper second molar and the fourth premolar. I figure m. 2, and it will be seen that they represent a form little specialized, resembling in the low crowns, plump cusps, and presence of both protoconule and metaconule, the Casamayor types, such as Lambdaconus or Didolodus, rather than the advanced type like the Santa Cruz genus, Theosodon. We found a specimen with the lower jaws complete and with the hind limb complete, which, I am confident, is the same form, though I can not duplicate any tooth, for we found no upper teeth; but in size they agree with Protheosodon, also in the primitive character; and, were one from the lower teeth to postulate the upper, they would be just such as Ameghino has described under the name Protheosodon. Therefore I have assigned my material to this genus and species. It adds to the genus characters the fact that this form had a shorter back, relatively as well as actually, than Theosodon; that the hind limb, at least, was much heavier and also shorter than that of Theosodon, especially in the metatarsal region where relatively the elements are only about half as long. The pes is of the same character as in Theosodon, but again relatively much shorter. I believe in Protheosodon we have to do with a form intermediate between Lambdaconus and Theosodon, and nearer to the former.

Protheosodon coniferus Ameghino

P. coniferus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 453.

Ameghino has described two upper teeth. Specimen No. 3001 of the Amherst Collection from the Chico del Chubut River, west of Puerto Visser, adds to this the knowledge of twelve vertebrae (seven dorsal and five lumbar), the lower dentition complete, the left hind limb complete, and the right hind limb complete except for the femur. In general, the animal is about ⅗ the size of Theosodon garrettorum, but in parts varies from this as follows. The lower jaw is ⅗, the vertebrae are ⅖ in length, the hind leg is ⅗ in length but ¾ in diameter of bones, while the metatarsus is only ⅓ in length. This makes an almost plantigrade form of heavy, clumsy proportions.

Of the upper dentition we know only what Ameghino has given us. The molar is distinguished by the presence of both the protoconule and metaconule, by the development of the posterior cingulum and by the presence of three external styles.

Measurements

Upper premolar 4, length 12 mm., width 15 mm.
Upper molar 2, length 14 mm., width 17 mm.

In the lower dentition, none of the teeth are reduced, and all are in a continuous series, except that there is a small diastema either side of pm. 1. The incisors are simple, compressed teeth, with but a trace of a cingulum. The canine is incisiform and a trifle larger than the incisors. Premolar 1 is also incisiform, and is isolated by a small diastema on either side. The second premolar is longer and wider than the first, and begins to show molariform characters, the anterior portion being composed of a high compressed cusp, the posterior portion by a low crescent on which but one cusp is fully developed. The third premolar is composed of two complete crescents, and has the “pillar” already developed opposite the posterior end of the back crescent. In fact, the tooth is molariform, except as to the tiny extra cusp found on the molars. Premolar 4 is more completely molariform consisting of the same parts as the preceding tooth.

The molars may be distinguished by the presence of a tiny median cusp on the rear of the tooth, behind the crescent, which, when the tooth is worn, makes a median spur to the rear. In both the premolars and molars, the teeth are characterized by their plumpness, and the isolation and lowness of the cusps.

The two halves of the lower jaw are completely fused at the symphysis. The horizontal ramus is thick, but low dorso-ventrally, giving the appearance of a slender jaw. The posterior angle is prolonged backward and bent inward. The fossa for the masseter muscle, while large, is but faintly outlined. The ascending ramus hardly rises above the level of the teeth, except as the slender coronoid projects to a good height above the articular condyle and curves backward over it.

The dorsal vertebrae have short, wide, and somewhat depressed centra (in this individual the epiphyses are free, though this is the only indication of youth). The lower rib facets are small, that on the posterior margin of the centrum being a mere streak, while the one on the anterior margin is narrow. The upper rib facet is a rounded convex surface on the end of a short stout transverse process. The prezygapophyses are convex surfaces, wide transversely, but narrow in the antero-posterior direction, while the postzygapophyses are correspondingly narrow concave facets under the rear of the spines. The spines are thin and high, and the neural canal is nearly circular in section.

The lumbar vertebrae have laterally compressed, deep centra, with very long transverse processes, shorting spines, and zygapophyses of the subcylindrical interlocking type. In all their features the vertebrae resemble those of Theosodon, being nearly as highly specialized and in the same manner.

The femur is short and very stocky. The rounded head is carried on a short neck, and does not rise nearly as high as the greater trochanter, the sulcus for the round ligament being a broad, deep notch on the posterior margin. The greater trochanter is rugose, heavy, and high, but not incurved at the top. The lesser trochanter is a small, thin ridge well below the head. The third trochanter is a large, thin process, projecting almost directly backward, though curved inward at the end, and is situated well below the middle of the bone. The shaft of the femur is flattened above, but thick, and changes in the lower part to subcylindrical. The condyles are small, subequal in size, and widely separated, while the rotular trochlea is relatively wide and shallow.

The tibia is about three-fourths the length of the femur, very stocky and heavily built. On the proximal end, the convex external condyle is much narrower antero-posteriorly than the larger and slightly concave internal condyle. The low spine is bifid. A cnemidial crest extends to the middle of the bone. On the distal end, the broad and shallow external articular facet is separated from the narrow and deeper internal facet by a low intercondylar ridge. The fibula is fused to the tibia at the upper end, but is free below, being approximated to the tibia along a rugose surface nearly an inch long. This bone is rather slender and strongly bowed outward. Distally, there is a large facet for the outside of the astragulus, the back part of which rests on the calcaneum. This is peculiarly developed so that the articulation represents what is two separate facets, the one for the outside of the astragulus the other for the calcaneum. Here, however, they are blended.

While in general the tarsus is similar to that of Theosodon, there are some marked contrasts. The astragulus has an asymetrical trochlea with a shallow groove, the external condyle being higher and narrower than the internal. The head is depressed in the dorso-plantar plane, is carried on a moderately long neck, and has a broad convex facet for the navicular on which alone it articulates. On the plantar side, the ectal facet is broadly oval and slightly concave, differing from that of Theosodon in having no sulcus dividing it into lobes. The broad sustentacular facet is slightly convex, and widely separated from the ectal. On the external side the astragulus carries an expanded articular facet for the inner side of the fibula, which, instead of being vertical, is expanded below, making an oblique face which is continuous with the fibular facet on the calcaneum. In this feature Protheosodon is, as far as the feet are known, unique.

The calcaneum is a long bone with a club-shaped expansion of the upper end. The fibular facet is small, being continuous, as above described, with that on the outer side of the astragulus. On the face toward the astragulus, the ectal facet is broadly convex (not divided as in Theosodon), while the sustentacular facet is slightly concave. The distal end is occupied by the large concave facet for the cuboid.

The navicular is of moderate height, with a prominent hook behind. On the upper surface is only the broad, deep facet for the astragulus; while the lower surface is divided into facets for the three cuneiforms, and the external distal margin is beveled to make a narrow facet for the cuboid. This navicular differs from that of Theosodon in that the facet for the ectocuneiform is not cut step-like into its external face.

The endocuneiform is a small scale-like bone with a narrow facet on the navicular, and overlapping the inner side of Mt. II. The mesocuneiform is small, with a flat facet above for the navicular, and a convex one below for Mt. II, which is carried wholly on this bone. The ectocuneiform is far the largest of these three bones, and carries a broad facet above for the navicular, a similar one below for Mt. III, a small facet on the internal side for the mesocuneiform and a second one below that for the side of Mt. II, while externally there are facets for the cuboid and for the side of Mt. IV.

The cuboid is large, the external side being longer than the internal. The upper surface is entirely occupied by the facet for the calcaneum, while the lower face is mostly devoted to the facet for Mt. IV, with a narrow streak on the external margin for the vestige of Mt. V. The internal face carries a boss beveled above by the facet for the navicular, and below by the facet for the ectocuneiform.

All the metatarsals are short and heavy as compared with those of Theosodon. Mt. II is compressed above, but enlarges below into a subcylindrical bone, ending in an extensive articular trochlea for the phalanx, the trochlea carrying a carina which extends into the upper surface of the articular area. Proximally, it is so closely approximated to the adjacent metatarsus that these could have had very little independent movement. On the upper internal surface, there is a roughened area, where the endocuneiform overlaps this bone. Mt. III is slightly heavier than the others. On the distal end, its articular trochlea extends well onto the dorsal surface, as does also the carina. Mt. IV is a trifle shorter than the others and stouter. Mt. V is absent but its former presence is indicated by the beveled facet on the cuboid, and by the small roughened surface on Mt. IV.

The phalanges of the third digit are a trifle heavier than those of the other two digits, but of approximately the same lengths. The ungual phalanges were broad compressed hoofs, without traces of clefts.

Restoration

In order to get a comparison of what is known of this form with Theosodon I have outlined a restoration of the animal as a whole, realizing that some essential parts are lacking, but the general proportions can hardly vary greatly from those given. It appears, first, that this form has an unusually short back. Though the limbs and lower jaw are ⅗ the length of those of Theosodon garrettorum, the vertebrae are ½ as long. I have assumed that the number of vertebrae would prove to be the same as in Theosodon. While the limb bones are ⅗ as long as in the Theosodon, they are relatively half again as heavy and with the processes much more developed. The greatest difference is found in the tarsus which is only ⅓ as long as that of Theosodon, though relatively as heavy, and the foot was carried in a nearly plantigrade position the heel raised but a little from the ground, though the anticular ends of the metatarsals and the phalanges indicate that there was a considerable freedom of movement of the various elements. The form seems to be fairly close to the ancestral types such as Lambdaconus of the Casamayor, the limbs of which, however, are entirely unknown, but I should expect that when found these earlier forms would prove to be approximately plantigrade.

Coniopternium Ameghino

Coniopternium Amegh., 1895 Bol. Inst. Geog. Argen., t. 15, p. 632.
Coniopternium Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 453.

The genus is based on a calcaneum and astragulus of the macrauchenid type, but of unusually large size. The real generic characters are not evident in the description, but the presence of these bones, and of three cervical vertebrae, which we also found, indicating a macrauchenid of about the same size, are evidence that a form larger than the Santa Cruz representatives will turn up in the Deseado beds, for which this name may be reserved. The material is described under the specific name C. andinum.

Adianthidae Ameghino

This family is based primarily on the genus Adianthus of the Santa Cruz to contain some macrauchenid-like forms which, however, are of much smaller size, and differentiated by the narrow character of the teeth and their early tendency to hypsodonty. It seems to be a valid series of dwarf types, which are all scarce and known only by the most fragmentary remains. Two genera are described from the Deseado, Tricoelodus, peculiar in having the posterior lobe of the lower molars somewhat subdivided so that the tooth appears three-lobed; and Proadianthus, known only by premolars which however show an unusual development of the styles on the inner side of the teeth.

Tricoelodus Ameghino

Tricoelodus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 454.

The genus is based primarily on the three-lobed character of the molars, which is a secondary effect of an infolding on the inner side of the posterior lobe. They are rooted, but strongly hypsodont. The margins of the crescents are well developed and the “pillar” is a prominent feature in the posterior crescent.

Tricoelodus bicuspidatus Ameghino

T. tricuspidatus Amegh., loc. cit. above.