Evolution and Classification of the Pocket Gophers of the Subfamily Geomyinae Part 3

_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 cla.s.sification 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 -- -- -- -- -- +-- -- -- -- --+-- -- -- -- -- -- +-- -- -- -- -- - _Th.o.m.omys_ _Th.o.m.omys_ _Th.o.m.omys_ _Th.o.m.omys_ _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 Th.o.m.omyini *_Pleisoth.o.m.omys_ *_Pleisoth.o.m.omys_ } _Th.o.m.omys_ _Th.o.m.omys_ } _Th.o.m.omys_ 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 dent.i.tion 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 _Th.o.m.omys_ is unknown but probably resembled _Pliosaccomys_, with which it may have been a contemporary. _Th.o.m.omys_ is the least specialized of the modern Geomyinae, and, consequently, shows the most resemblance to the ancestral tribe. The specializations of _Th.o.m.omys_, however, clearly preclude its reference to the tribe Dikkomyini; therefore, it is set apart in the monotypic tribe Th.o.m.omyini. 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, _Th.o.m.omys_ is set apart in cla.s.sification 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 relations.h.i.+ps. 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 dent.i.tion, 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 relations.h.i.+p 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 a.s.semblage. 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 cla.s.sed in the genus _Platygeomys_. Even so, in my opinion, the differences are insufficient to warrant even subgeneric recognition.

CLa.s.sIFICATION

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, ma.s.sive, angular, relatively broad, and flattened; distinctly murine form, but having zygoma.s.seteric structure of advanced sciuromorphs, including small infraorbital ca.n.a.l (that transmits no part of ma.s.seter muscle) and well-developed, broad zygomatic plate; zygomata ma.s.sive and widely flaring, jugals stout; rostrum robust, relatively broad and deep, and without evidence of transverse ca.n.a.l (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 ca.n.a.l 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); pos...o...b..tal process lacking, except for rudimentary k.n.o.blike 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 f.l.a.n.g.elike, 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 s.e.xt.i.tuberculate 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

Subfamily ENTOPTYCHINAE Miller and Gidley, 1918

Anterior face of upper incisor usually smooth, sometimes bearing faint groove in center or near medial margin of tooth, at least in _Gregorymys_; cheek teeth hypsodont, medium to high crowned, and rooted in all but _Entoptychus_ (has rootless, ever-growing teeth); cheek teeth identical in form, premolars resembling molars and lower cheek teeth mirror images of upper teeth; crowns biprismatic, having two columns joined at edge of protomeres (for description of term, see discussion of primitive morphotype on page 537) and with persistent lateral fissure between them; lateral re-entrant fold deep, penetrating at least half width of crown, from external side in upper teeth and internal side in lower teeth (in specialized genus _Entoptychus_ lophs, upon additional wear, join also at edge of parameres, thus uniting columns at both ends and thereby enclosing interior part of lateral fissure as a transverse fossette in center of tooth); enamel investment of prisms usually complete, including inflection bordering re-entrant folds, occlusal pattern becoming interrupted with wear only in _Entoptychus_, where enamel disappears first from sides of crowns (following union of anterior and posterior columns at both sides) and later, in final stages of attrition, from anterior wall of lower molars and posterior wall of upper molars.

Maxillary bone without p.r.o.nounced vertical depth in part supporting cheek teeth, its inferior border only slightly lower than inferior border of premaxillary and alveolar lips of molariform teeth consequently approximately level with, or slightly below, alveolar lip of upper incisor; squamosal without lateral expansion, therefore, meatal tube of auditory bulla separated from zygomatic process of squamosal by deep, well-developed postglenoid notch; angular part of mandible below alveolar level of mandibular cheek teeth; angular process only slightly reflected laterally; coronoid process low, tip only slightly above condyle.

For information concerning the structure and relations.h.i.+ps of the known genera, and for accounts of species, see Wood (1936). A list of the named genera in order of specialization is as follows:

*_Pleurolicus_ Cope, 1878. Proc. Amer. Phil. Soc., 18:66.

*_Gregorymys_ Wood, 1936. Amer. Mus. Novit., 866:9.

*_Grangerimus_ Wood, 1936. Amer. Mus. Novit., 866:13.

*_Entoptychus_ Cope, 1878. Proc. Amer. Phil. Soc., 18:64.

Five new species have been described since Wood's (1936) revision.

They are: _Pleurolicus clasoni_ MacDonald (1963:180); _Gregorymys kayi_ Wood (1950:335); _Gregorymys montanensis_ Hibbard and Keenmon (1950:198); _Grangerimus dakotensis_ MacDonald (1963:182); _Grangerimus sellardsi_ Hibbard and Wilson (1950:623).

Subfamily GEOMYINAE Baird, 1858

Anterior face of upper incisor primitively smooth, grooves consistently developed only in one modern lineage (Geomyini); cheek teeth hypsodont, primitively rooted and having crown of medium height (late Oligocene to middle Pliocene), being higher crowned, rootless and ever-growing in modern lineages (late Pliocene to Recent); primitively crowns of cheek teeth biprismatic, having two columns joined at mid-points by narrow isthmus and entire crown sheathed in continuous band of enamel; premolars retaining primitive biprismatic form, anterior and posterior columns never uniting at edge of protomeres or parameres, and with both lateral re-entrant folds persistent throughout life; primitive biprismatic pattern becoming decidedly modified in molars (except in M3), having two prisms progressively uniting into one column by reduction and loss of lateral inflections, primitive biprismatic patterns being retained only in early stages of ontogeny; third upper molars retaining, at least partially, primitive bicolumnar pattern (except in Th.o.m.omyini), with relatively broad isthmus and horizontally shallow re-entrant folds, lingual fold sometimes wanting; enamel pattern becoming discontinuous (late Pliocene to Recent) owing to loss of enamel from sides of each column; remaining enamel restricted to anterior and posterior plates, or cutting blades, and enamel bordering lateral inflections in premolars (considering both sides together, these plates const.i.tute essentially two transverse cutting blades); enamel pattern of M3 varying, depending on taxon; with specialization, anterior plates of lower molars and posterior plates of upper premolar and molars may be reduced or lost; except in primitive species (early Miocene), no enamel fossettes retained in adult dent.i.tions.

Maxillary bone having p.r.o.nounced vertical depth in part supporting cheek teeth, inferior border arching downward well below inferior border of premaxillary; consequently, alveolar lips of molariform teeth decidedly below level of alveolar lip of upper incisor; squamosal with marked lateral expansion at expense of postglenoid notch; notch compressed and reduced between meatal tube of auditory bulla and zygomatic process of squamosal; angular part of mandible mostly above alveolar level of mandibular cheek teeth; angular process reflected laterally at right angles to axis of ramus and developed into heavy k.n.o.blike projection; coronoid process well developed, tip decidedly higher than condyle; fossorial specializations remarkably well developed in advanced lineages, degree of specialization of primitive Miocene species unknown but probably only semi-fossorial as in Entoptychinae.

Key to the Tribes of the Geomyinae

A Enamel investment complete and uninterrupted, even in final (adult) stages of wear; cheek teeth rooted, with crowns of medium height; third lower molar biprismatic, the two columns separated by inner and outer re-entrant folds as in lower premolar.

Tribe Dikkomyini p. 515

A' Enamel investment incomplete and discontinuous, reduced, at least in final (adult) stages of wear, to interrupted enamel plates; cheek teeth rootless and ever-growing (except in extinct genus _Pliogeomys_), crowns of maximum height; third lower molar monoprismatic, without trace of inner and outer re-entrant folds as in first and second lower molars.

B Upper incisors smooth, occasionally with a fine indistinct groove near inner margin of tooth; form of third upper molar same as M1 and M2, monoprismatic, anteroposteriorly compressed, and having transverse enamel plates on both anterior and posterior faces, and without suggestion of either l.a.b.i.al or lingual re-entrant folds; basitemporal fossa absent (except for a shallow depression in one Recent species, _T. townsendii_); forefoot small and narrow with claws not elongated for digging.

Tribe Th.o.m.omyini p. 518

B' Upper incisors grooved, bearing either one or two sulci; form of third upper molar distinctly different from M1 and M2, fully or partially biprismatic (with a few exceptions discussed beyond), without marked anteroposterior compression (either subtriangular, elongated, suborbicular or quadriform in cross-section, but not elliptical as in M1 and M2), and having typical transverse anterior plate and two lateral plates (varying in their development, depending on taxa), but no posterior plate, and with lateral re-entrant folds usually developed, especially l.a.b.i.al inflection (although sometimes minute in a few species, as described beyond); basitemporal fossa well-developed, although occasionally shallow or absent (primitive species of _Zygogeomys_); forefoot large and broad, with elongated claws for digging.

Tribe Geomyini p. 521

Tribe DIKKOMYINI, new tribe

_Genotype._--_Dikkomys_ Wood, 1936.

_Chronologic and geographic range._--Early to Middle Pliocene (early Arikareean to mid-Hemphillian) in western United States. Known from Miocene fossil sites in Montana, South Dakota, and Nebraska and Pliocene sites in South Dakota, Oregon, Nevada, and southern California. For precise localities see accounts of _Dikkomys_ and _Pliosaccomys_ beyond.

_Diagnosis._--Small Geomyinae; lacking specializations of more advanced tribes; upper incisors smooth, at least in _Pliosaccomys_; molariform teeth always rooted and having crowns of medium height; enamel investment of cheek teeth complete and uninterrupted in all stages of wear; crowns of molars primitively biprismatic, having two columns united at mid-points, thus forming narrow isthmus separating lateral re-entrant folds as in premolars, and, with wear, also uniting secondarily at protomeres (with exception of third lower molars), consequently, isolating remnant of that inflection as shallow fossette (columns uniting first at protomeres in _Pliosaccomys_); anterior and posterior columns of first and second molars, both above and below, becoming progressively united into one column in advanced Dikkomyini (early and middle Pliocene), but m3 (M3 unknown) retaining primitive biprismatic pattern, with columns joined at centers but never at protomeres (for details of dent.i.tion see generic accounts); mandible stout, its angle mostly above mandibular tooth-row; ma.s.seteric ridge low; basitemporal fossa barely discernable in some fragments of _Pliosaccomys_; postcranial skeleton unknown.