Evolution and Classification of the Pocket Gophers of the Subfamily Geomyinae Part 6

Genus =Pliogeomys= Hibbard

1954. _Pliogeomys_ Hibbard, Michigan Acad. Sci., Arts and Letters, 39:353.

_Genotype._--_Pliogeomys buisi_ Hibbard, 1954, from Buis Ranch local fauna (middle Pliocene), Beaver County, Oklahoma.

_Chronologic range._--Latest Middle Pliocene, known only from the highest part of the Hemphillian mammalian fauna (Buis Ranch local fauna, Oklahoma). Professor Hibbard informs me (personal communication) that he found the type, a right ramus, lying on the surface near the base of the fossil beds. The isolated teeth of small geomyids from the Saw Rock Canyon local fauna (see Hibbard, 1953:392) may also be referable to this genus. The Saw Rock Canyon local fauna may also be middle Pliocene in age but is considered to be from the later part of the late Pliocene, and, therefore, somewhat younger than the Buis Ranch local fauna (Hibbard, _op. cit._:342).

_Description and discussion._--The size of members of this small genus of the Geomyinae is about the same as in smaller adults of _Geomys bursarius_. According to Hibbard (_op. cit._:353), the holotype is smaller than specimens from the Rexroad local fauna referred to _Geomys quinni_ and larger than specimens referred to _Zygogeomys_ cf.

_minor_. The cheek teeth are rooted, and the crowns are as high as those of living geomyids. The upper incisor is bisulcate, and the inner groove is fine and indistinct in places.

Of the molariform dent.i.tion only the lower premolar and first two lower molars are known. The enamel investment of p4 is complete, and would not be subject to interruption at any stage of wear; the two prisms are joined at their mid-points, and the isthmus of dentine is relatively broad (as in _Pliosaccomys_) when compared with modern pocket gophers of this tribe. Also, the re-entrant folds, rather than having parallel sides, diverge broadly to the sides. The divergence is especially noticeable in the l.a.b.i.al fold. The lower deciduous premolar would have formed essentially the same enamel pattern with wear as observed in _Nerterogeomys_ [= _Zygogeomys_] cf. _minor_ (see Hibbard, 1954:fig. 5, A and B) and _Pliosaccomys dubius_ (see Wilson, 1936; pl.

1, fig. 1). Each molar is a single column in the final stages of wear; pre-final stages are unknown. Anterior and posterior enamel plates are present on m1 and m2 (m3 has not been recovered). The dentine tracts of m1 are exposed over a relatively wide surface; therefore, the enamel plates are distinctly separated. The tracts of dentine of m2 are much narrower than in m1 and the enamel plates are barely separated at the anterolateral margin of the tooth. Possibly the enamel band of m2 was continuous in an earlier stage of wear.

The mandible is stout and its general construction not unlike that in modern geomyines. The capsule at the base of the angular process that receives the terminal end of the lower incisor is well developed. The base of the angular processes is preserved, and suggests that the process was short and decidedly smaller than in living examples of the tribe. The ma.s.seteric ridge is distinct but weakly developed, and not at all ma.s.sive as in living pocket gophers. The mental foramen is immediately anterior, and slightly ventral, to the anterior extension of the crest. The basitemporal fossa is absent as such, but its position is marked by a slight depression.

_Specimens examined._--Two rami; nos. 29147 (holotype) and 33446; several isolated teeth 30194 and 30195, including an upper incisor and a dp4 (deciduous lower premolar), all from Univ. Michigan Mus. Paleo.

_Referred species._--One.

*_Pliogeomys buisi_ Hibbard, 1954. Papers Michigan Acad. Sci., Arts, and Letters, 39:353. Type from Buis local fauna, latest middle Pliocene, Beaver County, Oklahoma.

Genus =Zygogeomys= Merriam

1895. _Zygogeomys_ Merriam, N. Amer. Fauna, 8:195, January 31.

1942. _Nerterogeomys_ Gazin, Proc. U. S. Nat. Mus., 92:507 (type, _Geomys persimilis_ Hay, 1927).

_Type._--_Zygogeomys trichopus_ Merriam, 1895, from Nahuatzen, Michoacan.

_Chronologic range._--Late Pliocene (Benson and Curtis Ranch local faunas, Arizona, and ?Rexroad Formation, Kansas) to Recent.

_Description and discussion._--The size is small to medium for the subfamily Geomyinae. This genus is distinguished princ.i.p.ally by the retention of primitive features. In the living species, the skull is generalized, rather than specialized toward either extreme dolichocephaly or platycephaly. The angular process is short, barely exceeding the lateral extensions of the mastoid process of the squamosal. The rostrum is remarkably narrow in relation to its length.

The jugal is reduced and displaced ventrally, causing the maxillary arm of the zygomata to articulate with the squamosal arm of the zygomata along the dorsal border of the zygomatic arch (a feature observed also in _Orthogeomys cherriei costaricensis_).

The upper incisor, recovered in material from the late Pliocene and middle Pleistocene, is bisulcate as in the genus _Geomys_ and the primitive genus _Pliogeomys_. The enamel plate across the posterior wall of P4 is either complete (late Pliocene to late Pleistocene) or restricted to the lingual half of the tooth (always restricted in living species). The Pliocene specimens of the Rexroad local fauna referred to _Nerterogeomys_ cf. _minor_ by Hibbard (1950:138-139) are exceptional. In these specimens the length and position of the posterior enamel plate is variable; however, all but one specimen had persistant enamel. Evidently, in approximately 43 per cent of the specimens, a complete enamel blade was present (see Paulson, 1961:139), and in the others (except the one without any enamel) the plate was restricted to a small area of the ventral surface, usually on the lingual side of the loph. Hibbard suggested that the decrease in size of the plate, and its restriction to the lingual side, may be a function of age. Hence, most adults would be characterized by the reduced posterior plate on the upper premolar. Although age may be the important factor, intragroup variation cannot be ruled out. It is of interest to note that in all specimens from the Benson (type series of _P. minor_) and Curtis Ranch local faunas, the former of late Pliocene age and the latter of middle Pleistocene age, the enamel plates are complete on the posterior face of the upper premolar. As mentioned before, the specimens from Kansas may actually represent the transitional stages of the early evolution of _Geomys_ in which the posterior plate of P4 is entirely lost. The enamel pattern of p4 is like that in other members of the tribe (excepting the genus _Pliogeomys_). The re-entrant angles of P4 and p4 are widely open (obtuse) in the examples recovered from late Pliocene and middle Pleistocene deposits, representing retention of a trait that is primitive in the Geomyini (see account of phylogeny).

M1 and M2 are elliptical in cross-section and each has an enamel plate on both the anterior and posterior surface. In the living species (_Z.

trichopus_), the posterior enamel plate fails to reach the l.a.b.i.al margin of the tooth and is restricted to the lingual two-thirds of the posterior surface; however, the enamel plates are complete in the late Pliocene species (_Z. minor_) and the middle Pleistocene species (_Z.

persimilis_), being only slightly separated from the anterior plate by narrow tracts of dentine on the ends of the tooth. M3 is partly biprismatic in the living species, the two incompletely divided lophs being separated by a distinct outer sulcus. The posterior loph is elongated and forms a conspicuous heel paralleling the evolution of this trait in the genus _Orthogeomys_; therefore, the crown is longer than wide. The posterior part of the tooth is protected by two lateral enamel plates; of the two, the lingual plate is especially long and extends to the end of the heel. M3 has not been recovered in the Pliocene species, but in the middle Pleistocene species (_Z.

persimilis_) M3 is subtriangular, no longer than wide, and the lateral inflections are weakly developed. The trend towards elongation of M3 evidently occurred in late Pleistocene evolution of the genus. All three of the inferior molars are elliptical, and only the posterior enamel plate is present (as in all other genera of the tribe except _Pliogeomys_).

The ma.s.seteric ridge of the mandible is well developed. In the late Pliocene species _Z. persimilis_ and _Z. minor_ the mental foramen is directly beneath the anterior extension of the ma.s.seteric ridge, but in the living species, _Z. trichopus_, the foramen lies well anterior to the ridge. The basitemporal fossa in the living species is well developed and deep; in the Pliocene species it is usually distinct but shallow (late Pliocene specimens of _Z. minor_).

_Referred species._--Three (two extinct and one living; the last has two subspecies):

*_Zygogeomys minor_ (Gidley), 1922. U. S. Geol. Surv. Prof. Paper, 131:123, December 26. Type from Benson local fauna (late Pliocene), Cochise County, Arizona; also known from the Rexroad local fauna, Meade County, Kansas.

*_Zygogeomys persimilis_ Hay, 1927. Carnegie Inst. Was.h.i.+ngton Publ., 136. Originally described by Gidley, 1922 (U. S. Geol.

Surv. Prof. Papers, 131:123, December 26) as _Geomys parvidens_ which was preoccupied by _G. parvidens_ Brown, 1908. Type from Curtis Ranch local fauna (middle Pleistocene), Cochise County, Arizona.

_Zygogeomys trichopus trichopus_ Merriam, 1895. N. Amer. Fauna, 8:196, January 31. Type from Nahuatzen, Michoacan.

_Zygogeomys trichopus tarascensis_ Goldman, 1938. Proc. Biol. Soc.

Was.h.i.+ngton, 51:211, December 23. Type from 6 mi. SE Patzcuaro, 8,000 ft., Michoacan.

Genus =Geomys= Rafinesque

1817. _Geomys_ Rafinesque, Amer. Monthly Mag., 2(1):45, November.

1817. _Diplostoma_ Rafinesque, Amer. Monthly Mag., 2(1):44-45, November. Included species: _Diplostoma fusca_ Rafinesque [= _Mus bursarius_ Shaw] and _Diplostoma alba_ Rafinesque [= _Mus bursarius_ Shaw] from the Missouri River region.

1820. _Saccophorus_ Kuhl, Beitr. Zool. und Vergl. Anat., pp. 65, 66.

Type: _Mus bursarius_ Shaw, from upper Mississippi Valley.

1823. _Pseudostoma_ Say, Long's Expd. Rocky Mts., I, pp. 406. Type: _Pseudostoma bursaria_ [= _Mus bursarius_ Shaw], from upper Mississippi Valley.

1825. _Ascomys_ Lichtenstein, Abh. K. Akad. Wiss. Berlin (1822), p. 20., fig. 2. Type: _Ascomys canadensis_ Lichtenstein [= _Mus bursarius_ Say], probably from upper Mississippi Valley.

1944. _Parageomys_ Hibbard, Bull. Geol. Soc. Amer., 55:735, June.

Type: _Parageomys tobinensis_ Hibbard, from Pleistocene, Cudahy (Tobin) local fauna, Russell Co., Kansas.

_Type._--_Geomys pinetis_ Rafinesque, 1817, restricted to Screven County, Georgia, in region of the pines.

_Chronologic range._--Late Pliocene faunas of Blancan age (Rexroad, Kansas, and Sand Draw, Nebraska, local faunas) to Recent. Reported from numerous Pleistocene deposits of all stratigraphic levels, especially from the Great Plains, where common today.

_Description and discussion._--Pocket gophers of this genus are medium-sized geomyids; none is so small as the average-sized _Th.o.m.omys_. The skull is generalized and lacks the dolichocephalic and platycephalic specializations seen in the genera _Orthogeomys_ and _Pappogeomys_, respectively. _Geomys_ closely resembles _Zygogeomys_, but retains fewer of the primitive characters of the ancestral stock.

At the same time, _Geomys_ has several specializations. Even so, a considerable amount of parallelism is evident in the phyletic trends of the two genera.

The upper incisor of _Geomys_ is bisulcate as in _Pliogeomys_ and _Zygogeomys_; the deeper grove is medial and the shallower grove lies near the inner border of the tooth. The premolar, above and below, is bicolumnar; and two columns are joined at their mid-points (deep re-entrant angles separate the columns at the sides). A permanent enamel plate protects the anterior face of the anterior loph, and enamel bands outline each of the re-entrant folds. In p4 a complete enamel plate covers the posterior surface of the posterior loph. All of the enamel bands are interrupted by tracts of dentine, except in the initial stages of wear of the occlusal surface of the newly erupted tooth. For a short time in living _Geomys_, the enamel bands are continuous as observed in juveniles of _Geomys bursarius major_ (KU 5628, 8531, and 41540). But, the enamel cap is thin and the dentine tracts, which are high on the sides of the tooth, are soon revealed by a minimum of wear on the crown. Therefore, the adult, or final, pattern characterized by interrupted enamel plates emerges early in life and remains throughout the life of the individual.

Evidence from fossil _Geomys_, especially from specimens from early and late Pleistocene deposits, suggests that the final adult pattern appears later, ontogenetically, than in Recent specimens. Some of the fossil premolars in initial stages of wear have continuous and uninterrupted bands of enamel. _Geomys quinni_ of the late Pliocene and early Pleistocene has the interrupted pattern seen in late Pleistocene and Recent _Geomys_. Also, in late Pliocene and early Pleistocene species, the re-entrant folds diverge laterally and form "open" angles. In later taxa (middle Pleistocene to Recent) the folds are compressed and parallel-sided, and the "open" folds are found only in the early stages of wear.

The posterior enamel plate of P4 disappears in the final stages of wear as the interrupted enamel pattern is formed. In the late Pleistocene and Recent _Geomys_, the loss of the posterior plate occurs early in life, usually in the first phases of wear on the occlusal surface of the newly erupted tooth, but in fossils of _Geomys_ of corresponding ontogenetic age from the early and middle Pleistocene, the posterior plate is retained in some individuals until a later phase of wear, thereby delaying the appearance of the final pattern. Indeed, in five or fewer per cent of the individuals (see Paulson, 1961:138-139; and White and Downs, 1961:18) a vestige of enamel is retained throughout life or at least until late in adulthood. In _Geomys tobinensis_, for example, a thin, but transversely complete, plate of enamel occurs all the way down to the base of the loph (Paulson, _loc. cit._) and would persist throughout life. In _Geomys garbanii_, a vestige on the lingual side of the posterior surface of a fully adult specimen was noted by White and Downs (_loc. cit._). Vestiges of the posterior plate occur less frequently in living geomyids. Paulson (_loc. cit._) found a posterior plate in one of 75 specimens of _Geomys bursarius dutcheri_. A young (suture present between exoccipitals and supraoccipital) female of _Geomys pinetis austrinus_ (KU 23358) has a vestige of the posterior plate on the lingual side of the tooth as White and Downs (_loc.

cit._) observed in a specimen of _Geomys garbanii_. The enamel, I suspect, tends to be thicker on the lingual than on the l.a.b.i.al side of the loph and extends farther down the lingual surface in some individuals; therefore, wear on the occlusal surface erodes it down to the dentine more rapidly on the l.a.b.i.al than on the lingual side. The tendency of enamel to be retained is a primitive feature.

A lower molar of _Geomys_ is a single elliptical column, and enamel is restricted to the posterior surface as in _Zygogeomys_, _Orthogeomys_, and _Pappogeomys_. Paulson (_loc. cit._) found a thin enamel plate on the anterior surfaces of the lower molars in about five per cent of the individuals of _Geomys tobinensis_ from the Cudahy local fauna (middle Pleistocene, deposits of the late Kansan glaciation). An anterior plate is unknown in other members of the tribe Geomyini, except in the primitive genus _Pliogeomys_ of the middle Pliocene.

Occurrence of the plate in _Geomys tobinensis_ is an atavistic trait.

Primitive dental patterns occur occasionally in geomyids, as pointed out above, but the frequency of occurrence in _G. tobinensis_ is higher than would be expected.

M1 and M2, like the lower molars, are elliptical in cross-section.

Complete enamel plates on the anterior and posterior surfaces are separated by tracts of dentine on the sides of each tooth. M3 is usually suborbicular (sometimes subtriangular) in cross-section. The tooth is not especially elongated posteriorly and usually has no definite heel; therefore, it is not significantly longer than wide.

Living species of _Geomys_ rarely have a well defined outer re-entrant fold on M3; less than 10 per cent of the individuals (and usually only one side in each individual in which it occurs) have it, although a shallow inconspicuous groove occurs more frequently. The biprismatic molar characteristic of the ancestral morphotype is less often found in _Geomys_ than in any other living member of the tribe Geomyini. The outer re-entrant fold and biprismatic pattern are more often present in the extinct species _Geomys garbanii_ of the Middle Pleistocene than in other species. Less than 24 per cent of the third upper molars in _Geomys garbanii_ lack a tract of the re-entrant fold and more than 38 per cent have a well developed outer fold (see White and Downs, 1961:13, 18). The bicolumnar pattern, although incomplete, would be clearly evident in those teeth having a well marked re-entrant fold; the pattern occurs less frequently in those teeth with no fold or only a slight one. M3 of geomyids is not usually recovered and, therefore, the occlusal pattern of M3 is unknown in most extinct kinds of _Geomys_. In Recent _Geomys_ the fold is more common in the eastern _pinetis_ species-group than in the western _bursarius_ species-group.

The ma.s.seteric ridge on the outer side of the mandible is well developed in all species of the genus. The position of the mental foramen relative to the anterior part of the ridge varies with individuals and according to species. The basitemporal fossa is always present, but is shallower in the late Pliocene and Pleistocene species than in Recent species. The angular process is short.

_Referred species._--The twelve species, five of which are extinct, are as follows:

_quinni_ species-group