Part 2

#Heterogeomys onerosus# new species

Holotype.--Los Angeles County Museum (C.I.T.) No. 2384, an incomplete left ramus, bearing incisor and p4; the alveolus of m1-m3 is present (Fig. 1a). Paratypes: Two isolated and unnumbered right upper incisors, one isolated premolar, and five additional rami, Nos. 2385, 2386, 2388, and two with no number.

Horizon and type locality.--Upper Pleistocene, Cueva de San Josecito, province of Aramberri, near the town of Aramberri, Nuevo León, México; California Institute of Technology, Vertebrate Paleontology Locality 192.

Description of Holotype.--Differs from any known living species of Heterogeomys, by the significantly heavier and deeper ramus (see Table 1 and Fig. 1). The holotype is compared with the largest adult male of Heterogeomys hispidus (H. torridus is smaller than hispidus) available to me in Table 1. Relative to the length of the ramus (measured from the anterior mental foramen to the posterior margin of the capsule that surrounds the root of the lower incisor), the depth of the ramus anterior to the molariform tooth-row is 33.0 per cent in H. onerosus compared with 27.3 per cent in H. hispidus. If the fossil ramus is that of a female (females are significantly smaller than males in Heterogeomys) then the differences would be greater than recorded.

TABLE 1. DEPTH OF MANDIBULAR RAMUS

A: Least depth in front of premolar (See A to A' on Fig. 1c) B: Depth of ramus opposite re-entrant angle of p4 (B to B' on Fig. 1c) C: Depth from a point in front of capsule for incisor (See C to C' on Fig. 1c)

=====================================+======+======+====== | A | B | C -------------------------------------+------+------+------ H. onerosus holotype | 11.0 | 17.4 | 11.7 -------------------------------------+------+------+------ H. h. hispidus ad., 23979 KU| 9.1 | 15.2 | 10.5 -------------------------------------+------+------+------

The angle between the anterior border of the coronoid process and the dorsal border of the ramus of the mandible is more acute, and the posteroventral margin of the ramus is more nearly straight, in onerosus than in hispidus. The molariform tooth-row in onerosus is only slightly longer (13.9 in contrast to 13.5) than in hispidus and torridus. The ventral border of the massenteric ridge is weakly developed in onerosus and hardly discernable whereas in the living species of Heterogeomys the massenteric ridge is strongly developed posteriorly forming a noticeable prominence.

Description of Paratypes.--The fossils are referable to the genus Heterogeomys on the basis of the short lateral angular processes of the lower jaw and on the basis of the associated upper incisors, which have a single distinct sulcus that lies toward the inner margin of each tooth. The isolated lower premolar that is referred to the new species is as large as that of the holotype and has the enamel pattern of Heterogeomys.

FIG. 1a. Heterogeomys onerosus, lateral view of left lower jaw of holotype.

FIG. 1b. Heterogeomys onerosus, front view of right, upper incisor.

FIG. 1c. Heterogeomys hispidus, lateral view of left lower jaw, No. 23979, adult, from 3 km. E San Andrés Tuxtla, Veracruz.]

One jaw fragment, L.A.C.M. (C.I.T.) No. 2368, is smaller than the others and probably is from a young individual. Two others L.A.C.M. (C.I.T.) No. 2384 and one unnumbered, are smaller than the holotype, and possibly are the remains of females; however, they have the same characteristic shape as the holotype. Nevertheless, the two rami mentioned above are significantly larger than in adult males of modern Heterogeomys and are especially larger than in females. Another jaw fragment, L.A.C.M. (C.I.T.) No. 2385, is seemingly as large as, or perhaps larger than, the holotype, although the posterior part of the ramus behind the alveolus of m2 is missing. An additional unnumbered ramus is of somewhat lighter construction than the holotype, but is important since it bears not only the incisor and p4 but also the first two lower molars. The only other material referable to Heterogeomys onerosus is a fragmentary and isolated lower molar tooth that has a single posterior enamel blade, a feature characteristic of a number of Recent genera of pocket gophers, and some limb bones which are slightly larger than corresponding elements in Recent species of Heterogeomys.

Remarks.--Pocket gophers do not inhabit caves; therefore gophers were brought into the cavern probably by birds of prey, the remains of which were common in the deposits (Miller, 1943:152-156), or conceivably by carnivorous mammals. Since most of the raptorial predators that would prey on pocket gophers do not have a wide hunting territory, it is likely that the gophers were taken within a short distance of the cave. The presence of the genus Heterogeomys in the deposits strongly suggests a tropical situation in the vicinity of the cave when these gophers were taken, because the distribution of this genus today is entirely within the Tropical Life-zone.

Since the presumably early time when tropical conditions, or more nearly tropical conditions, prevailed at San Josecito Cave, climatic shifts account for a humid boreal environment there and its associated fauna. Findley (lit. cit.:635-636) reports from San Josecito the remains of the boreal shrew Sorex cinereus that today occurs no nearer than 800 miles to the northward in the mountains of north-central New Mexico. As he points out, that species requires hydric communities of cool climates, and in the Wisconsin Glacial age such climates probably prevailed in the high mountainous region where San Josecito is located. Since the time when a more mesic boreal environment occurred at San Josecito, climatic shifts have favored more xeric conditions as are found in the vicinity of the cave today. The more arid environments would support the occurrence of Cratogeomys and Thomomys; however the ecological affinities of the fragment here referred to Cratogeomys sp. are unknown.

The more nearly tropical environment there could have occurred either during a Wisconsin interglacial period or during the Sangamon Interglacial age. Heterogeomys onerosus perhaps lived near the cave during an interglacial period; since then it became extinct or evolved into the Recent species Heterogeomys hispidus. Heterogeomys has not previously been recorded from Pleistocene or earlier deposits.

LITERATURE CITED

BAKER, R. H.

1953. The pocket gophers (genus Thomomys) of Coahuila, México. Univ. Kansas Publ., Mus. Nat. Hist., 5:499-514, 1 fig. in text, June 1.

CUSHING, J. E., JR.

1945. Quaternary rodents and lagomorphs of San Josecito Cave, Nuevo León, México. Jour. Mamm., 26:182-185, July 19.

FINDLEY, J. S.

1953. Pleistocene Soricidae from San Josecito Cave, Nuevo León, México. Univ. Kansas Publ., Mus. Nat. Hist., 5:633-639, December 1.

HANDLEY, C. O., JR.

1955. A new Pleistocene bat (Corynorhinus) from México. Jour. Washington Acad. Sci., 45:48-49, March 14.

HOOPER, E. T.

1952. A systematic review of the harvest mice (genus Reithrodontomys) of Latin America. Miscl. Publ. Mus. Zool., Univ. Michigan, 77:1-255, 9 pls., 24 figs., 12 maps, January 16.

JAKWAY, G. E.