L I E) RARY

OF THE

U N I VER.SITY

or ILLINOIS

REMO

GEOLOGICAL SERIES

OF

FIELD MUSEUM OF NATURAL HISTORY

Volume VI Chicago, May 15, 1935 No. 12

A NEW ARGYROHIPPUS FROM THE DESEADO BEDS OF PATAGONIA

By Bryan Patterson Assistant in Paleontology

Results of the First Marshall Field Paleontological Expedition TO Argentina and Bolivia, 1922-24

The notohippid genus Argyrohippus has hitherto been known from two species, A. jrater cuius and A. boulei, both of which occur in the Colhu^-Huapi beds. Simpson (1932, p. 12) has recently designated A. fraterculus as the genotype. The present communica- tion describes a third species from the eariier Deseado beds. The excellent specimen of A. fraterculus collected by the Scarritt Pata- gonian Expedition has greatly facilitated the description and com- parison of the new form. I am indebted to Mr. Elmer S. Riggs for the privilege of describing the species. The drawings are by Mr. Carl F. Gronemann, Staff Illustrator, Field Museum.

The premolars of the holotype were described and figured on page 97 of this volume under the name of Argyrohippus sp.

Argyrohippus praecox sp. nov.

Holotype.— FM. No. P13834. Incomplete palate with RP^-M* and alveolus of P^ and the root of the canine, LM^-^; horizontal ramus of left mandible with Pi-Ms and alveoli of anterior teeth. Collected by George F. Sternberg, 1924.

Paratypes.—FM. No. P13486. Palatal fragment with RM^-^ F.M. No. P13475. Fragment of left mandible with M i-^. Collected by John B. Abbott, 1924. F.M. No. P14697. Fragment of left mandible with P4-M5. Collected by George F. Sternberg, 1924.

Horizon and locality. — Deseado beds, La Flecha, province of Santa Cruz, Argentina.

Diagnosis. — Teeth more brachyodont than those of A. frater- culus; small upper canine present and in series with the premolars; P^ relatively large; infraorbital foramen smaller than in A. frater- culus and well removed (15 mm.) from anterior border of orbit.

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Kaiurs] Kf tt rr Survey

162 Field Museum of Natural History— Geology, Vol. VI

Discussion. — As shown by the American Museum specimen of A. fraterculus, the most characteristic features of the genus are the cup-Hke postero-internal cingula^ and the absence of anterior cingula on P--*, and the thick covering of cement. The new species possesses these characters^ and hence may be referred with confidence to Argyrohippus. The characters given in the diagnosis readily distinguish A. praecox from A. fraterculus.^ The teeth of the Deseado species are about one-fourth shorter than those of the genotype. The large infraorbital foramen in A. frater- culus is only 7 mm. from the anterior rim of the orbit. The upper canines in the Colhu^-Huapi species, according to Ameghino (1902, p. 82), are situated, when present, in the middle of the diastema between I- and P-; in A. praecox the upper canine is in series with the cheek teeth. The differences in the structure of P^ between A. praecox and A. fraterculus have been discussed on pages 96 and 97 of this volume. They may be of specific value but in the absence of a series of specimens I am not prepared to admit them as such. In the holotype of A. praecox there are four mental foramina, one in front of P2, two beneath P2 and one beneath the trigonid of P?. In a specimen of A. fraterculus, F.M. No. P13587, there are three, one beneath the diastema, one beneath P2 and one beneath M 1 ; on the right side of this specimen there are two small additional foramina one above and one below the large foramen beneath the diastema. Mental foramina, however, tend to be variable and a wider range of specimens is necessary before admitting these differ- ences as specific characters. Pr may have been present in A. praecox and the upper and lower dentitions may have been in series without diastemata but these points cannot be determined from the available material.

' Ameghino did not mention this character in his description of the genus (1902, pp. 81-83). Presumably his material was too much worn to show it.

2 The cement is considerably abraded on the specimens of A. praecox but enough remains to show its former extent. A vestige of an anterior cingulum is present on P^ of the holotype.

^ The second Colhue-Huapl species, A. boulei, was distinguished by Ameghino (1902, p. 83) from A. fraterculus on the basis of larger size, simpler enamel folds in the lower cheek teeth, more procumbent symphysis and more proclivous incisors. The difference in size between the two is about 15 per cent, not necessarily in itself a specific character. The slight differences in the enamel folds can hardly be considered as more than individual, possibly even age, variations. The reported differences in the mandibular symphysis constitute the only positive claim of A. boulei to specific distinction. Unfortunately, Ameghino has given no figures so that it is impossible to ascertain how much variation exists in this region between the types of the two species. The differences may well be due to pressure after burial. A. boulei may be doubtfully retained as valid, pending further information.

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A New Argyrohippus from Patagonia

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164 Field Museum of Natural History — Geology, Vol. VI

A. praecox and A. fraterculus compare very closely. The former, as shown by the differences mentioned above, is the more primitive of the two species, a fact that is in accord with its geological horizon. It is possible that the Deseado species was directly ancestral to the genotype.

The paratype specimens of A. praeeox are somewhat older than the holotype and satisfactorily show the age characters of the teeth.

Fig. 35. Argyrohippus praeeox sp. nov. x \. a, Holotype, F.M. No. P13334. RC-M*. 6, Para- type, P.M. No. P13486. RMi^-». c, Paratype, F.M. No. P14697. LPj-M,. d, Paratype, F.M. No. P13475. LMt-3.

As in many notoungulates, the grinding surfaces of the molars increase in transverse diameter and the details of the crown pattern become obscured by wear. There is a fold in the posterior face of the enamel of the protolophs of M^-^ of F.M. No. P13486. As shown by a photograph supplied by Professor W. B. Scott, a similar fold is present on one of Ameghino's specimens of A. fraterculus. It is impossible to state at present whether this feature is an age character or an individual variation.

Ameghino considered that Argyrohippus was descended from Morphippus and ancestral to Notohippus. The latter suggestion may eventually be proved correct but Notohippus is too little known at present for any definite statement to be made. The former suggestion was unlikely on morphological grounds and the discovery

A New Argyrohippus from Patagonia 165

of A. praecox as a contemporary of Morphippus in the Deseado greatly increases its improbability.

Measurements. — The antero-posterior measurements have been taken from the mid-points of the anterior and posterior faces of the teeth. The transverse measurements of the lower cheek teeth have been taken across the entoconids, except for P?, which lacks this cusp. Transverse measurements of the upper cheek teeth are not given for the sufficient reason that this diameter in most notoun- gulates with high-crowned teeth varies so much with age as to be useless.

Field Mus. Field Mus. Field Mus. Field Mus. No. P13334 No. P13486 No. P13475 No. P14697

Canine,* antero-posterior diameter 4

Canine, transverse diameter. . ." 3.25

Length, F^-M^ 90

Length, Pi-* 45

F^,^ a.p 5.25

P^, a.p 9.5

P^, a.p 12

P*, a.p 12

Length, M^-^ 46 48.5

MJ-, a.p 16 15

M2,a.p 17.5 17

M^, a.p 17 17.5

Length, P^-M^ 90.5

Length, P^-* 34.5

P5,a.p 10

Pj, tr.. 5.75

P3, a.p 12

P3, tr 7.25

P^, a.p 12.75 13.25

Pi, tr 7.75 7.75

Length, Mj-^ 54.5

Ml, a.p 15.5 15.25

Mi, tr 9.25 9 9

Mt, a.p 17.5 17 17.5

M J, tr 9 10.25 9.25

M5, a.p 21.75 23.5

M3, tr 7.75 9

Depth of ramus beneath alveolus be- tween P:f and M 1 , external 36

' Measured from the root. - Measured from the alveolus.

THE AGE OF THE LA FLECHA DEPOSIT

When the collection from this locality was prepared it was believed that the beds were a redeposit containing both Deseado and Colhu^-Huapi fossils, and on page 15 of this volume a specimen was stated to have come from the latter horizon. The belief that specimens of Colhu^-Huapi age were included was based on the specimen described above, which was at that time referred to A.

166 Field Museum of Natural History — Geology, Vol. VI

fraterculus, and on a series of leontiniids with a dental formula of l>o4>l> which were tentatively assigned to Colpodon. Direct com- parison of the Argyrohippus with the American Museum specimen of A. fraterculus has now revealed that the La Flecha form represents a distinct, more primitive species. The identification of the leontiniids was made on the authority of Loomis, who gave (1914, pp. 108-109) the dental formula of Colpodon as l,or4,f and of the Deseado Ancylocoelus as f >o>l>l. In his first account of Ancylocoelus, Ameghino (1895, p. 650) held that Pi was lacking. In his second report (1897, p. 475), however, he stated that the tooth which was lacking above and below and formerly believed by him to be the Pi might be the canine. The number of teeth of Ancylocoelus, according to Ameghino, is therefore the same as in Colpodon. During the summer of 1934 I was able to study some casts of Ameghino's specimens of Ancylocoelus in the British Museum (Natural History) . The casts are identical with the leontiniids from La Flecha in Field Museum. These specimens show that the tooth that is lacking is the canine, thereby making the dental formula of Ancylocoelus identical with that of Colpodon. The entire fauna from La Flecha is therefore of Deseado age. The specimen described and figured by me on pages 15 to 17 of this volume under the name of Colpodon sp. is in reality Ancylocoelus frequens Ameghino.

REFERENCES

Ameghino, F.

1895. Premiere contribution a la connaissance de la faune mammalogique des couches a Pyrotherium. Bol. Inst. Geog. Arg., 15, pp. 603-660.

1897. Mammiferes cr6taces de I'Argentine (Deuxieme contribution a la con- naissance de la faune mammalogique des couches a Pyrotherium). Ibid., 18, pp. 406-521.

1902. Premiere contribution a la connaissance de la faune mammalogique des couches a Colpodon. Bol. Acad. Nac. Cienc. Cordoba, 17, pp. 71-132.

Loomis, F. B.

1914. The Deseado Formation of Patagonia. Amherst, 1914, pp. 1-232

Simpson, G. G.

1932. New or Little-known Ungulates from the Pyrotherium and Colpodo Beds of Patagonia. Amer. Mus. Nov., 576, pp. 1-13,

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