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Full text of "A new Argyrohippus from the Deseado beds of Patagonia"

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. 

No. 344 161 



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. 



650. •:> 



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A New Argyrohippus from Patagonia 



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163 



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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