Part 6

Modern classification of the pocket gophers begins with Baird in 1858. The important classifications are summarized in Table 1; a few that do not depart essentially from those listed have been omitted owing to limited space for the tabular arrangement, but are discussed in the following account.

Baird probably was strongly influenced by the arrangement proposed by Waterhouse in 1848, but was opposed to separating geomyids from heteromyids as was done by Brandt. Baird was convinced of the close relationship of the geomyids and heteromyids, and referred both groups to one family, the Saccomyidae, as Waterhouse had done earlier. In order to recognize the morphological specializations he used two subfamilies, Geomyinae and the Saccomyinae. In the 20 years that followed, some authors followed Brandt and others followed Baird.

Gill, in 1872 (p. 71), proposed a classification essentially like Baird's of 1858, but Gill raised Baird's subfamilies to the rank of family (see Table 1). In referring all pocket gophers to the Geomyidae, Gill used that name as a family term for the first time. Also he established the superfamily Saccomyoidea to include his two families, Geomyidae and Saccomyidae; therefore, the Saccomyoidea was equivalent to the group Saccomyina of Waterhouse (1848) and the Saccomyidae of Baird (1858). Coues (1877), in his classic monograph of the Geomyidae followed the arrangement proposed by Gill in treating the pocket gophers as a family. Alston in 1876 proposed another classification based on Baird (1858), with two subfamilies, the Geomyinae and the Heteromyinae, united together in the family Geomyidae; thus, he recognized that the genus Saccomys Frédéric Cuvier, 1823, was a synonym of Heteromys Desmarest, 1817, as had been pointed out by Gray (1868:201) and Peters (1874:356). Coues (1877:487-490) acknowledged the invalidity of the genus Saccomys, but refused to give up the name in supergeneric classification. Winge, first in 1887 and subsequently in 1924, classified the geomyids and heteromyids together in the family Saccomyidae as did Baird in 1858, and like Coues, Winge too ignored the synonymy of Saccomys with Heteromys and insisted on retaining the technical terms Saccomyidae and Saccomyini.

Up to the time of Merriam's classic revision of the Recent Geomyidae in 1895 all the known species of living pocket gophers were referred to two genera, Geomys and Thomomys. Merriam described much new material, especially from México and Central America, and proposed seven new genera (see Table 1). His complete and detailed study of the dentitions and osteology of the skull remains today as the definitive work on this subject, and is the point where most studies of the Geomyidae must begin. His treatment of the Recent genera survived for 52 years without change until Hooper (1946:397) arranged Platygeomys as a synonym of Cratogeomys. However, Merriam's genera have been recognized in all subsequent classifications except for the current review (see Table 1).

Cope described the first known fossil geomyids in 1878, and published an excellent review of the two genera, Pleurolicus and Entoptycus, in 1884 (pp. 855-870, pl. 64, figs. 1-9). Both genera were recovered from the John Day Miocene deposits of Oregon. Cope did not propose a new systematic arrangement of these geomyids, but referred them to the family Saccomyidae and mentioned that the Saccomyidae was equivalent to the family Geomyidae of Alston. Winge, in 1887, followed Cope in referring Pleurolicus and Entoptycus to the Saccomyidae along with the living genera Thomomys and Geomys. Miller and Gidley (1918), in their synopsis of the supergeneric groups of rodents, proposed a new subfamily, Entoptychinae, to include the divergent Miocene pocket gophers. Miller and Gidley also revived the old subfamily Geomyinae of Baird (1858), but restricted its application to the modern pocket gophers and their immediate ancestors. In 1936, A. E. Wood revised the taxa of the subfamily Entoptychinae, and described the first Miocene genus, Dikkomys, of the Geomyinae. He followed the supergeneric classification of Miller and Gidley (1918).

The recent classifications of Simpson (1945) and Wood (1955) have combined the classifications of Merriam (1895) and Wood (1936). Wood (1955) brought up to date the list of genera, including those that were described after the publication of Simpson's classification (1945). In Table 1, the list of genera is principally from Simpson (1945) but generic names used by Wood (1955) are included. This is the currently accepted classification.

The new classification proposed in this paper (see Table 1) includes three tribes proposed as vertical units; they are intended to stress the phyletic trends in the known evolutionary sequences by placing immediate ancestors together with their descendants.

Pliogeomys is placed in the same tribe (Geomyini) as Zygogeomys, Geomys, Orthogeomys, and Pappogeomys. That tribe includes the most specialized Geomyinae. Zygogeomys, Geomys, Orthogeomys, and Pappogeomys are lineages resulting from a Pleistocene radiation in which all the lineages diverged from a common Pliocene ancestor. The radiation of the Geomyini was well under way by the close of the late Pliocene. Although Pliogeomys may not be the actual ancestor, it closely resembles the primitive morphotype.

TABLE 1.--History of the classification of the Superfamily Geomyoidea

===============+==============+==================+================ Baird 1858 | Gill 1872 | Winge 1887 | Merriam 1895 | Coues 1877 | and 1924 | Ellerman 1940 ---------------+--------------+------------------+---------------- Family | Family | Family | Family Saccomyidae | Geomyidae | Saccomyidae | Geomyidae ---------------+--------------+------------------+---------------- Subfamily | | "Group" | Geomyinae | | Geomyini | -- -- -- -- -- +-- -- -- -- --+-- -- -- -- -- -- +-- -- -- -- -- - | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Thomomys | Thomomys | Thomomys | Thomomys | | | | | | | | | | | | | | | | | | Zygogeomys | | | | | | Geomys | Geomys | Geomys | Geomys | | | | | | Orthogeomys | | | Heterogeomys | | | Macrogeomys | | | | | | Pappogeomys | | | Cratogeomys | | | Platygeomys -- -- -- -- -- +-- -- -- -- --+-- -- -- -- -- -- +-- -- -- -- -- - | | | | | | | | | | | *Pleurolicus | | | | | | | | | *Entoptychus | ---------------+--------------+------------------+---------------- | | | | | | | | | | | "Group" | | | Gymnoptychine** | | | Gymnoptychus | ---------------+--------------+------------------+---------------- Subfamily | Family | "Group" | Saccomyinae | Saccomyidae | Saccomyini | -------------+-------------+------------------+----------------

=====================+=====================+=================== Wood 1935 | Simpson 1945 | Names used in Wood 1936 | Wood 1955 | present paper ---------------------+---------------------+------------------- Family | Family | Family Geomyidae | Geomyidae | Geomyidae ---------------------+---------------------+------------------- Subfamily | Subfamily | Subfamily Geomyinae | Geomyinae | Geomyinae -- -- -- -- -- -- -- +-- -- -- -- -- -- -- +-- -- -- -- -- -- - | | Tribe | | Dikkomyini | | *Dikkomys | *Dikkomys | *Dikkomys | *Pliosaccomys | *Pliosaccomys | | | | Tribe | | Thomomyini | | *Pleisothomomys | *Pleisothomomys | } Thomomys | Thomomys | } Thomomys | | | | Tribe | | Geomyini | | | *Pliogeomys | *Pliogeomys Zygogeomys | Zygogeomys | } | *Nerterogeomys | } Zygogeomys | | Geomys | Geomys | } | *Parageomys | } Geomys Orthogeomys | Orthogeomys | } Heterogeomys | Heterogeomys | } Orthogeomys Macrogeomys | Macrogeomys | } | | Pappogeomys | Pappogeomys | } Cratogeomys | Cratogeomys | } Pappogeomys Platygeomys | Platygeomys | } -- -- -- -- -- -- -- +-- -- -- -- -- -- -- +-- -- -- -- -- -- - Subfamily | Subfamily | Subfamily Entoptychinae | Entoptychinae | Entoptychinae | | *Pleurolicus | *Pleurolicus | *Pleurolicus *Gregorymys | *Gregorymys | *Gregorymys *Grangerimus | *Grangerimus | *Grangerimus *Entoptychus | *Entoptychus | *Entoptychus ---------------------+---------------------+------------------- | Geomyidae | Geomyidae | incertae sedis | incertae sedis | | | | *Gidleumys | *Diplolophus | *Diplolophus | *Griphomys | *Griphomys ---------------------+---------------------+------------------- Family | Family | Family Heteromyidae | Heteromyidae | Heteromyidae ---------------------+---------------------+-------------------

* Denotes extinct genera.

** Winge included in his family Saccomyidae the "group" Gymnoptychine and the contained genus Gymnoptychus Cope, 1873, which genus currently is placed in the family Eomyidae. The type of Gymnoptychus Cope, 1873, is synonymous with Ischyromys Leidy, 1856, and the valid name for the genus is Adjidaumo Hay, 1899.

Pliosaccomys, on the other hand, represents the terminal stages of a long trend that began with the Dikkomys-like Geomyinae of the early Miocene. In this lineage, the rate of evolution in the dentition and the skull was slow; therefore, the differences between early Miocene (Dikkomys) and middle Pliocene (Pliosaccomys) are not great and the two are united into the tribe Dikkomyini. The Dikkomyini is the ancestral geomyinen trunk from which the modern groups have diverged.

The Pliocene ancestor of Thomomys is unknown but probably resembled Pliosaccomys, with which it may have been a contemporary. Thomomys is the least specialized of the modern Geomyinae, and, consequently, shows the most resemblance to the ancestral tribe. The specializations of Thomomys, however, clearly preclude its reference to the tribe Dikkomyini; therefore, it is set apart in the monotypic tribe Thomomyini. That tribe has not undergone an adaptive radiation comparable to that of the tribe Geomyini or that of the Entoptychinae in the early Miocene. Here, for the first time, Thomomys is set apart in classification from the other living pocket gophers.

Merriam's genera Orthogeomys, Heterogeomys, and Macrogeomys are closely related. Each of these taxa is retained as a subgenus of a single genus, Orthogeomys. Some species of Macrogeomys seem to be more closely allied to the subgenus Orthogeomys and others to the subgenus Heterogeomys. A revision of the genus is needed; it might show that the currently recognized subgenera are artificial, and that a different arrangement of the species would more clearly express their evolutionary relationships. The subgenus Heterogeomys seems to be the most nearly uniform of the subgenera, and it is the least specialized. Radiation within the genus may have begun relatively recently, but the many special adaptations for tropical environments suggest that the genus has been in the Neotropical Zone a long time. Therefore, discovery of an early dichotomy from the common ancestral stock of the tribe would come as no surprise.

Nerterogeomys Gazin here is arranged as a junior synonym of Zygogeomys. Both are less specialized than any of the other Geomyini, except Pliogeomys. The single living species (Zygogeomys tricopus) is obviously a relic. Its range is small. The two subspecies differ only in minor features. The living species does have a few unique characteristics, only to be expected in the surviving species of a long phyletic lineage. Some of these are specializations. Otherwise, Zygogeomys and Nerterogeomys are closely related and the latter is best placed as a synonym of the former. Both are admittedly closely related to Geomys. Zygogeomys and Geomys share several characters, particularly primitive ones; there is considerable parallelism, especially marked in Irvingtonian species of Geomys. Nevertheless, Geomys is more specialized, particularly in the dentition, and it has developed some Pappogeomys-like specializations. Zygogeomys has retained more of the primitive characters of the tribe. A strong case could be made for recognizing only one genus, Geomys, containing Zygogeomys as one of two subgenera. Nevertheless, the characters separating Zygogeomys and Geomys are of considerable importance and I consider the two kinds to be distinct genera.

The species of Geomys, both living and extinct, form a distinct and well-marked group. The genus is less primitive in most respects than Zygogeomys and Orthogeomys and it is less specialized than Pappogeomys, excluding the ancestral stock (subgenus Pappogeomys). Some specimens of species of Irvingtonian age (Geomys tobinensis and Geomys garbanii, especially the former) retain primitive enamel plates as does Zygogeomys; but this is true of only a small percentage of the individuals. Also the adult dental pattern developed somewhat later in ontogeny in these middle Pleistocene species of Geomys than in either Recent or late Pliocene and early Pleistocene representatives (Geomys paenebursarius, Geomys quinni) of the genus. Whether these features represent a stage in the evolution of the late Pleistocene and Recent species or a terminal stage in members of a sterile and primitive branch of the main line of evolution of Geomys is uncertain. At present I favor the latter explanation, and view G. paenebursarius and G. quinni as early progressive species that evolved dental specializations that were maintained in the main line of phylogeny.

Hibbard proposed the generic name Parageomys (1944:55), but later regarded it as a subgenus of Geomys (1956:182) that includes those species retaining continuous enamel bands until relatively late in ontogeny; no other differences have been noted. When the early phylogeny of Geomys is better understood, Parageomys may serve as a subgeneric taxon in which the primitive species of Geomys can be grouped, but as of now Parageomys is arranged as a synonym of Geomys.

Pappogeomys and Cratogeomys also form a natural group. Their close relationship is best reflected in formal taxonomy by including them in the same genus. Their dissimilarities are of the sort that separate a primitive ancestral lineage from a divergent and progressively more specialized assemblage. The fossil record is inadequate, and I can only speculate that Cratogeomys diverged from primitive Pappogeomys-stock in the earlier Pleistocene, at least before the end of the Irvingtonian. Cratogeomys probably originated on the Mexican Plateau and probably underwent its subsequent evolution there. The living species of the subgenus Pappogeomys are evidently relics of the ancestral stock of the genus. Hooper (1946:397), I think correctly, considered Platygeomys as congeneric with Cratogeomys, although the highest degree of specialization of the genus is attained in those species formerly classed in the genus Platygeomys. Even so, in my opinion, the differences are insufficient to warrant even subgeneric recognition.

CLASSIFICATION

Family GEOMYIDAE Gill, 1872

Rodents of the superfamily Geomyoidea specialized for completely fossorial life (early Pliocene to Recent); specialized earlier (late? Oligocene and early Miocene) for semi-fossorial habits; body thickset, fusiform without apparent neck (in modern geomyids); legs short; forelegs especially stout; eyes and ears small (pinna reduced to inconspicuous crest concealed beneath pelage); tail tactile, shorter than head and body; lips closing behind incisors; cheek pouches external, fur-lined; baculum rodlike, arched, having expanded quadriform platelike base; pelage long, soft without underfur, covering body in thick coat (in some species of Orthogeomys scant, harsh or scattered bristles); color varying from pale tints of buffy (almost white) to metallic black.

Skull thick-walled, massive, angular, relatively broad, and flattened; distinctly murine form, but having zygomasseteric structure of advanced sciuromorphs, including small infraorbital canal (that transmits no part of masseter muscle) and well-developed, broad zygomatic plate; zygomata massive and widely flaring, jugals stout; rostrum robust, relatively broad and deep, and without evidence of transverse canal (as in Heteromyidae); anterior projection of nasals only slightly exceeding that of upper incisors; interorbital region usually constricted, narrower than rostrum; anterior opening of infraorbital canal far forward on side of rostrum, about half way between zygomatic plate and upper incisor and just behind premaxillary-maxillary suture, its opening countersunk in oblique sulcus (for protection from muscle contraction); postorbital process lacking, except for rudimentary knoblike projection in subgenus Macrogeomys; palate relatively narrow, its deeply sculptured surface sloping steeply downward posteriorly causing region supporting maxillary tooth-row to be markedly depressed; palatine bone reduced, forming, on two abruptly different levels, posterior margin of hard palate behind tooth-rows; parietals compressed and narrow, and most of cerebral cavity roofed by squamosals (in some species squamosals overlap lateral parts of parietals); tympanic bullae completely inferior in position and fully ossified, external meatus being developed laterally as elongated tube; mastoid not inflated, but broadly exposed at posterolateral margin of the skull; occiput large, its surface usually rugose, and paroccipital processes large and flangelike, at least in advanced groups (early Pliocene to Recent); ramus relatively short and stout, having distinct crest and ridges for muscle attachments; coronoid process well developed, erect; articular condyle prominent; angular process prominent, reflected laterally, and in modern groups lateral extension protruding from posterior border of ramus nearly at right angle; capsule for root of lower incisor, prominent between angular process and articular condyle.

Anterior surface of incisors broad and flat, always smooth on lower teeth, but either smooth or grooved on upper teeth depending on taxon; cheek teeth hypsodont, becoming progressively higher crowned in modern groups, rooted in primitive groups (late? Oligocene to middle Pliocene), rootless and ever-growing in modern groups (late Pliocene to Recent); upper and lower premolars persistently bicolumnar; upper and lower molars bicolumnar only in primitive groups (late? Oligocene and early Miocene), becoming progressively monocolumnar in advanced groups (early Pliocene to Recent), primitive bicolumnar pattern being retained on occlusal surface only in early stages of ontogeny and in third molar throughout life; enamel pattern of occlusal surface of cheek teeth based on sextituberculate prototype (see Wood and Wilson, 1936:388-391), having cusps arranged in two transverse rows of three cusps each, excepting three anterior cusps of premolars that are arranged in trefoil, especially on p4 (sometimes only one or two, rather than three, cusps develop in a particular set, especially in p4), conules absent; protostyle and endostyle in upper teeth and protostylid and hypostylid in lower teeth formed from cingulum; cusps of each row uniting with wear into transverse enamel lophs (or lophids), each tooth having two lophs, one on anterior column, protoloph and protolophid, and one on posterior column, hypoloph and hypolophid, that unite with additional wear forming continuous enamel band; enamel lacking on sides of each column in advanced lineages, thereby restricting enamel to anterior and posterior walls; with extreme reduction, posterior plates of upper teeth and, more commonly, anterior plates of lower molars, missing. Dental formula: 1/1, 0/0, 1/1, 3/3.

Key to the Subfamilies of Geomyidae

A Angular process of ramus mostly below alveolar level of mandibular tooth-row; pattern of premolar like that of molars, consisting of two subequal crests united at one or both margins of tooth; molars persistently bicolumnar; molariform teeth always rooted. Subfamily Entoptychinae p. 513

A´ Angular process of ramus mostly above level of mandibular tooth-row; pattern of permolar unlike that of molars, consisting of two prisms differing in size and united at their mid-points but never at either margin; molars progressively monocolumnar, except for early Miocene forms; molariform teeth rooted only in primitive genera (late? Oligocene to middle Pliocene), and rootless and ever-growing in later genera (late Pliocene to Recent). Subfamily Geomyinae p. 514