Evolution and Classification of the Pocket Gophers of the Subfamily Geomyinae Part 9

_Orthogeomys matagalpae_ (J. A. Allen, 1910). Bull. Amer. Mus. Nat.

Hist., 28:97, April 30. Type from Pena Blanca, Matagalpa, Nicaragua.

Genus =Pappogeomys= Merriam

1895. _Pappogeomys_ Merriam, N. Amer. Fauna, 8:145, January 31.

1895. _Cratogeomys_ Merriam, N. Amer. Fauna, 8:150, January 31.

Type: _Geomys merriami_ Thomas.

1895. _Platygeomys_ Merriam, N. Amer. Fauna, 8:162, January 31.

Type: _Geomys gymnurus_ Merriam; Hooper, Jour. Mamm., 27:397, November 25, 1946.

_Type._--_Geomys bulleri_ Thomas, 1892, from near Talpa, west slope Sierra de Mascota, 8500 ft. (actually about 5000 ft.), Jalisco.

_Chronologic range._--Late Pliocene, from deposits of early Blancan age (Benson local fauna, Arizona) to the Recent. However in the Pleistocene, only late Pleistocene records are known, and _Pappogeomys_ has not been found in early (late Blancan) or middle (Irvingtonian) Pleistocene local faunas. Presumably the genus was restricted to Mexico during the Pleistocene until post-Wisconsin time.

_Description and discussion._--The size ranges from as little as in the smaller kinds of _Th.o.m.omys_ to the maximum attained in the subfamily and matched elsewhere perhaps in only a few of the larger subspecies of _Orthogeomys grandis_. Depending on the species and subgenus, the form of the skull varies from generalized to specialized. The generalized skulls are short and not especially narrow; the zygomatic arches are spread laterally so far that the breadth across them exceeds the breadth across the mastoid processes.

The most specialized skulls are platycephalic and the breadth across the mastoid processes equals or exceeds the breadth across the zygomatic arches (even so, the zygomatic arches are still relatively widespread). In correlation with the great breadth of the posterior part of the cranium, the rami of the mandibles diverge widely posteriolaterally and the angular processes are remarkably elongated.

The rostrum is moderately broad in most species, but not nearly so broad and heavy as in _Orthogeomys_.

The single deep, median sulcus on the outer surface of the upper incisor is slightly displaced to the inner side of the tooth. The posterior surface of P4 lacks enamel (small vestige found on lingual end of posterior wall in only two adult individuals--UA 3260 and KU 100442, of the subgenus _Pappogeomys_); the other three plates are fully developed as usual. The p4 is provided with four fully developed enamel plates, in the pattern characteristic of the tribe Geomyini. In the p4 of the late Pliocene species (_P. bensoni_) the re-entrant angles are open (obtuse), a trait that is evidently primitive in the Geomyini.

All three lower molars are single, compressed, elliptical columns with enamel on only the posterior surfaces. M1 and M2 are also elliptical in cross-section and decidedly anteroposteriorly compressed, like the lower molars. Nevertheless, the enamel pattern is variable; enamel plates may be retained completely across both the anterior and posterior walls of M1 and M2 or only the anterior plate may be retained without reduction and the posterior plate may be reduced so that only a vestige is retained on the lingual fourth of the tooth or the posterior plate may be completely lost.

M3 tends to remain at least incompletely bilophodont by reason of retaining a permanent l.a.b.i.al re-entrant fold in most species (with exceptions in _Pappogeomys bulleri_ and some old adults of _P.

castanops_). Primitively the occlusal surface of M3 is subtriangular (subgenus _Pappogeomys_), but in the _castanops_ species-group of the advanced subgenus _Cratogeomys_, the posterior loph usually is reduced and the occlusal surface is quadriform or obcordate. Curiously, the trend towards reduction of the posterior loph is reversed in one subspecies (_P. merriami fulvescens_) and, the loph has elongated into a p.r.o.nounced heel in some specimens, resembling the condition in _Orthogeomys_. The entire range of variation occurs in _P. m.

fulvescens_. The subtriangular pattern is retained in the most specialized species of _Cratogeomys_ where that pattern is a.s.sociated with extreme platycephaly in the _gymnurus_ species-group. In most species the posterior loph supports two lateral plates, the outer one always bordering the l.a.b.i.al re-entrant fold. In _Pappogeomys bulleri_ and in the _castanops_ species-group, the outer re-entrant fold of M3 tends to be obsolete, and the tooth becomes quadriform or suborbiculate in some individuals and loses the bilophodont pattern that characterizes other species. The lingual enamel plate is displaced to the posterior surface of the tooth, and one or both plates may disappear with advancing age. Consequently, only the anterior enamel plate remains in some adults, and const.i.tutes the maximum degree of reduction of enamel on M3 in the Geomyinae. In many adults of _Pappogeomys bulleri_, the enamel investment of the posterior loph is complete and the two lateral plates are connected, without interruption around the posterior apex of the tooth, evidently representing the retention of a primitive character of the ancestral lineage.

The m3 of _P. bensoni_ from the late Pliocene is distinguished by minute lateral inflections suggesting the primitive biprismatic pattern. Also the posterior enamel plates of m1 and m2 are remarkably long, extending around the ends of the tooth. The a.s.sociated upper incisor was unisulcate as in the modern species, and the basitemporal fossa of the mandible is well developed and deep.

The lower jaw is stout and relatively short. The ma.s.seteric ridge is well developed and has an especially thick crest. The basitemporal fossa is deep. In most living species, the pelage is soft and dense, but in one species, _Pappogeomys fumosus_, the hairs are coa.r.s.e and hispid somewhat as in _Orthogeomys_.

Key to the Subgenera of _Pappogeomys_

A Enamel plates completely developed across posterior walls of M1 and M2, except in one species (_P. alcorni_) having enamel restricted to lingual fourth in M1; sagittal crest lacking owing to impressions of temporal muscles remaining separated (even in old adults); zygomata slender, and without platelike expansion at lateral angle. Subgenus _Pappogeomys_ p. 534

A' Enamel lacking on posterior walls of M1 and M2; p.r.o.nounced sagittal crest developed in adults of both s.e.xes by union of temporal impressions at middorsal line; zygomata stout and wide, with lateral angle expanded into broad plate.

Subgenus _Cratogeomys_ p. 535

Subgenus =Pappogeomys= Merriam

1895. _Pappogeomys_ Merriam, N. Amer. Fauna, 8:145, January 31.

_Type._--_Geomys bulleri_ Thomas, 1892, from near Talpa, west slope Sierra de Mascota, 8500 ft. (actually about 5000 ft.), Jalisco.

_Chronologic range._--Late Pliocene (Benson local fauna, Arizona) to Recent, but no specimens known from Pleistocene.

_Description._--Small, approximately same size as small subspecies of _Th.o.m.omys umbrinus_ but forefeet larger and claws longer; skull of generalized shape, broad, relatively short, smoothly rounded, not especially compressed dorso-ventrally; zygomatic breadth great but not exceeding mastoid breadth; zygomata relatively slender for geomyid and lacking platelike expansions at lateral angles; rostrum relatively narrow; sagittal crest lacking, owing to impressions of temporal muscles remaining separated; angular process of mandible not especially elongated; enamel plates extending completely across posterior wall of M1 and M2, except in one species, _P. alcorni_, where posterior plate of M1 remains only on lingual fourth of posterior wall (remainder of plate lacking); with wear, plates sometimes exceptionally thin completely across posterior face of M2 and especially M1 in a few individuals of _P. bulleri_ much as Paulson (1961:138-139) describes in extinct _Geomys tobinensis_; one or both plates rarely disappear in final stages of attrition in old individuals resulting in same dental pattern found in _Cratogeomys_; M1 and M2 retaining enamel plate on anterior wall throughout life; M3 usually subtriangular in cross-section but sometimes suborbiculate or ovoid, crown slightly bilophodont owing to shallowness of l.a.b.i.al re-entrant angle in modern species; posterior loph of M3 not especially elongated and crown not significantly longer than wide; both lateral enamel plates of M3 usually well developed and approximately equal in length, occasionally plates reduced in length and rarely one or both plates are lost with wear in old individuals; patch of whitish or buffy hairs surrounding nose of most individuals.

The primitive character of the lower dent.i.tion, as described in the species account above, suggest that _Cratogeomys_ [= _Pappogeomys_]

_bensoni_ Gidley should be referred to the subgenus _Pappogeomys_ rather than _Cratogeomys_. Only the upper dent.i.tion would make positive identification possible; however, reference to the subgenus _Pappogeomys_ seems to be the best arrangement at this time.

_Referred species._--Three (one extinct):

*_Pappogeomys bensoni_ (Gidley), 1922. U. S. Geol. Surv. Prof.

Papers, 131:123. Type from Benson local fauna (late Pliocene), Cochise County, Arizona.

_Pappogeomys alcorni_ Russell, 1957. Univ. Kansas Publ. Mus. Nat.

Hist., 9(11):359. Type from 4 mi. W Mazamitla, Jalisco.

_Pappogeomys bulleri_ Thomas, 1892. Ann. Mag. Nat. Hist., Ser. 6, vol. 10:196, August. Type from "near Talpa," west slope of Sierra Madre de Mascota, Jalisco.

Subgenus =Cratogeomys= Merriam

1895. _Cratogeomys_ Merriam, N. Amer. Fauna, 8:150, January 31.

1895. _Platygeomys_ Merriam, N. Amer. Fauna, 8:162, January 31.

Type: _Geomys gymnurus_ Merriam, 1892.

_Type._--_Geomys merriami_ Thomas, 1893, from "Southern Mexico,"

probably in Valley of Mexico.

_Chronologic range._--Late Pleistocene, from Wisconsin deposits (San Josecito Cave, Nuevo Leon, Upper Bercerra, Mexico, and Burnet Cave, New Mexico, local faunas) to the Recent.

_Description._--Size medium to large; skull becoming angular and rugose with age, and tending towards platycephaly and dorso-ventral compression; zygomata stout, each bearing platelike expansion at anterolateral angle into which anterior end of jugal becomes morticed; breadth across zygomata great relative to length of skull; rostrum relatively broad; squamosals expanding medially with age eventually growing over lateral parts of parietals, and sometimes also expanding laterally displacing postglenoid notch; sagittal crest well developed in adults of both s.e.xes, but especially high and bladelike in males; lambdoidal crest prominent in all but young animals, having dorsal outline broadly convex posteriorly in most species but strongly sinuous in _gymnurus_-group; enamel plate on posterior wall of P4 absent; enamel plates present only on anterior walls of M1 and M2; M3 variform in occlusal shape (as described in species account), either subtriangular (_gymnurus_-group), quadriform or obcordate (_castanops_-group, with exceptions as noted before); lateral plates of M3 usually present in all species, l.a.b.i.al plate approximately as long as lingual plate in _gymnurus_-group (like that in subgenus _Pappogeomys_) or distinctly shorter in _castanops_-group (l.a.b.i.al plate scarcely extending beyond border of l.a.b.i.al re-entrant fold); one or both lateral plates tending to disappear with wear in _castanops_-group, with lingual plate usually disappearing first; breadth across angular processes clearly more than breadth across zygomatic processes, especially in _gymnurus_-group.

_Remarks._--In the species of the _castanops_-group the skulls can be spoken of as generalized and the least platycephalic of the subgenus.

Indeed, the species of the _castanops_-group are hardly more specialized in this respect than is the subgenus _Pappogeomys_.

In these skulls the breadth across the squamosal processes is less than that across the zygomatic arches, although the two dimensions are almost equal in some examples of _P. merriami_ of the _castanops_-group (where squamosal breadth varies from 85 to 98% of zygomatic breadth). In the species having marked platycephalic skulls (_gymnurus_ species-group) the breadth across the squamosal processes equals or exceeds the breadth across the zygomatic arches (squamosal breadth rarely 97 to 99% of zygomatic breadth), except in _P. zinseri_ and _P. tylorhinus zodius_.

The variable character of the third upper molar as between species suggests that this tooth is presently undergoing active evolution. The structure of this tooth, although differing between taxa, is remarkably stable in other kinds of Geomyini. The most remarkable modification of M3 in _Cratogeomys_ is the obcordate pattern developed in _P. merriami_ of the _castanops_-group. The posterior loph and entire tooth is shortened somewhat resembling in shape that of _Th.o.m.omys_. Moreover, the posterior loph is twisted l.a.b.i.ally; consequently, its posterior surface now forms the l.a.b.i.al border of the weakly defined posterior loph. Owing to the torsion, the lingual enamel plate has been rotated to the posterior surface of the tooth.

Therefore, the tooth is provided with two transverse enamel plates, including the plate on the anterior wall of the tooth. The l.a.b.i.al plate is greatly reduced, its total surface being restricted to the small l.a.b.i.al inflection. The highly specialized obcordate M3 is not found in the most specialized platycephalic skulls characteristic of the _gymnurus_ species-group. Instead the _gymnurus_-group retains the primitive subtriangular pattern without significant modification.

_Referred species._--Seven:

_castanops_ species-group

_Pappogeomys castanops_ (Baird, 1852). Report Stanbury's Exp'd. to Great Salt Lake, p. 313, June. Type from "Prairie road to Bent's Fort," near present town of Las Animas, Colorado.

_Pappogeomys merriami_ (Thomas, 1893). Ann. Mag. Nat. Hist., ser. 6, 12:271, October. Type from "southern Mexico," probably Valley of Mexico (see Merriam, 1895:152).

_gymnurus_ species-group