Evolution and Classification of the Pocket Gophers of the Subfamily Geomyinae Part 12

A. _Pliogeomys buisi_, No. 29157 (UMMP), holotype, Buis Ranch (Upper Middle Pliocene), Beaver Co., Oklahoma. Right lower, p4-m2 (m3 unknown).

B and C. _Zygogeomys trichopus trichopus_, adult female, No. 51971 (FMNH), Mt. Tancitaro, 10,500 ft., Michoacan. Left upper (B), P4-M3; right lower (C), p4-m3.

D and E. Subgenus _Orthogeomys_. _Orthogeomys grandis guerrerensis_, adult female, No. 39807 (KU), 1/2 mi. E La Mira, 300 ft., Michoacan. Left upper (D), P4-M3; right lower (E), p4-m3.

F and G. Subgenus _Heterogeomys_. _Orthogeomys hispidus hispidus_, adult female, No. 23975 (KU), 4 km. W Tlapacoyan, 700 ft., Veracruz. Left upper (F), P4-M3; right lower (G), p4-m3.

H and I. Subgenus _Macrogeomys_. _Orthogeomys heterodus cartagoensis_, adult female, No. 60664 (KU), Rancho Redando, Volcan Lroza, Prov. San Jose, Costa Rica. Left upper (H), P4-M3; right lower (I), p4-m3.

The subcordate form is characterized by p.r.o.nounced anteroposterior compression, and retention of a distinct l.a.b.i.al re-entrant fold. The posterior loph apparently has been rotated in such a way that what was previously its posterior border now lies on the outer margin of the tooth; therefore, the axis of the posterior loph is strongly oblique in relation to the anteroposterior bearing of the maxillary tooth-row, and the median enamel plate also has been rotated and so lies transversely across the posterior wall of the tooth. Owing to the rotation of the posterior loph, the apex of the obcordate tooth is at its lingual side. The subcordate type is ill.u.s.trated by Merriam (_loc.

cit._) in Figures 27 (3 and 4), 28 (a and b), 34 (3 and 4), and 35 (5, 6, and 7). The suborbicular and quadriform types are less specialized than the two described above. Both are characterized by reduction, often obliteration, of the bicolumnar pattern of the subtriangular ancestral form, especially marked by the decrease in depth of the lateral re-entrant folds and the decrease in length of the posterior projection of the posterior loph. With these changes, the tooth becomes essentially monocolumnar, its occlusal surface oval in outline in one and squarish in shape in the other. Occlusal views of the suborbicular form are presented by Merriam (_loc. cit._) in Figure 33 (1, 5, 6, 7, 11, and 12) and the quadriform tooth is depicted in Figure 29. Grooved upper incisors are characteristic of the living Geomyini, but variation occurs in the number of grooves, and, if only one groove is present, its position on the anterior face of the tooth varies. Except for the previously mentioned (p. 480) abnormal tooth having three grooves, incisors with no more than two grooves are found in these pocket gophers, and this number of grooves is taken to be primitive. Loss of one or the other of the two grooves of the bisulcate pattern, therefore, is regarded as specialization. However, complete loss of both grooves never occurs in the Geomyini. Each of the four major lineages is characterized by one of the three patterns of grooving, and the particular groove-pattern is remarkably stable in each group.

Shape of skull varies from dolichocephalic to platycephalic. The morphology of each has been described in foregoing accounts. The dolichocephalic skull is highly specialized for planing, a grinding action of the teeth; whereas, the platycephalic skull is highly specialized for shearing, a slicing action of the teeth. Of course, concomitant specializations of the dent.i.tion, as described above, are closely a.s.sociated with both specialized trends in the skull. Most kinds of living Geomyini have generalized skulls that show no tendency toward either of the specialized conditions.

Increase in size of body and skull is seen in most Pleistocene lineages of the Geomyini. Judging from the smallness of the skull in late Pliocene species, representing the base of three of these lineages, the ancestral species of the living a.s.semblage were no larger than the living species of the subgenus _Pappogeomys_ or the smaller subspecies of _Geomys bursarius_. The recorded range of variation in condylobasal length is 36.1 to 45.5 in _Pappogeomys bulleri_, including both adult males and females. Probably the skulls of the ancestral species were not significantly larger. Maximum dimensions of males in living species are 74.5 (subgenus _Cratogeomys_) and 75.0 (subgenus _Orthogeomys_). These are more than twice the minima observed in _Pappogeomys bulleri_.

Zygogeomys

This is the least specialized and most primitive of the four lineages, has a generalized type of skull, two grooves on the anterior face of each upper incisor, an enamel plate on the posterior wall of P4, open or divergent lateral re-entrant angles on the premolars, and a bicolumnar and elongated M3. All of these features are primitive and essentially as in the ancestral morphotype. No other modern genus retains so much of the primitive structure. Phyletic trends in _Zygogeomys_ are not well doc.u.mented in the fossil record; and only a few fossils are known and they are fragmentary as discussed before.

The genus is represented in the late Pliocene (_Z. minor_), middle Pleistocene (_Z. persimilis_), and Recent (_Z. trichopus_). The living species is a relict population in the mountains of Central Mexico.

Judging from the known material, the phyletic trends in the genus have been increase in size, reduction of enamel on the posterior face of P4 (occurring only in the living species) where a short enamel plate is retained on the lingual side of the tooth (see Fig. 7B), loss of the outer fourth of the enamel blade on the posterior wall of M1 and M2 (also occurring only in the living species), development of a more p.r.o.nounced heel on the M3 by progressive elongation of the posterior loph, reduction in size of the jugal and its displacement ventrally, which allows the maxillary and squamosal bones to meet along the dorsal border of the zygomatic arch. The last specialization is seen in at least one taxon of _Orthogeomys_ (_Orthogeomys cherriei costaricensis_). In my opinion, too much weight has been given to this feature in past cla.s.sifications. Reduction of enamel in the upper dent.i.tion evidently occurred in the late Pleistocene, since the posterior plates on the upper cheek teeth were complete in specimens from the middle Pleistocene (_Z. persimilis_).

Geomys

_Geomys_, slightly more specialized than _Zygogeomys_, must also be regarded as one of the most primitive of the living genera. Primitive features that have been retained are the generalized type of skull, the bisulcate pattern of grooves on the upper incisor, and the retention of enamel plates on both the anterior and posterior walls of M1 and M2 (see Fig. 9A). All of these primitive features are shared with _Zygogeomys_. In addition, three other trends, or specializations, in evolution characterize the phyletic development of _Geomys_. One major trend is toward loss of the enamel plate from the posterior wall of P4. No trace of enamel remains on the posterior wall of this tooth in late Pleistocene or Recent species of _Geomys_, and at least one of the earlier species (_quinni_) was also characterized by loss of this enamel plate. Secondly, M3 retains only a vestige of the primitive bicolumnar pattern after the initial stages of wear. In most Recent specimens, especially of the species _G. bursarius_, the lateral re-entrant fold and the heel of M3 are small, and the re-entrant inflection is hardly evident. The lateral fold is more frequently well-developed in Irvingtonian species than in living species (White and Downs, 1961:13), ill.u.s.trating progressive loss of the bicolumnar pattern in Pleistocene evolution. A third trend involves the modification of the lateral folds of the premolars.

Primitively the angles of these folds are broadly open or divergently V-shaped, and some of the earliest species of _Geomys_, for example _G. quinni_, have retained this feature throughout life. Nevertheless, the main trend is toward progressive compression of the folds resulting in their walls being more nearly perpendicular, and parallel, to the long axis of the tooth. Obtuse re-entrant angles persist in premolars of young individuals of Irvingtonian species, but the adults are characterized by well-compressed folds, as in Recent species.

Remains of _Geomys_ are abundant, especially from Pleistocene deposits of the Great Plains, but in most instances specific a.s.signment is difficult or impossible since only isolated teeth or fragments of skulls have been preserved. Estimates of phyletic relations.h.i.+ps of the known species of _Geomys_ are depicted in Figure 8; those estimates are useful in discussing the phyletic development of the genus. One of the earliest known species, _Geomys quinni_, ranges from Upper Pliocene to the later stages of the Lower Pleistocene (Aftonian interglacial deposits). The dent.i.tion of _G. quinni_ is essentially the same as in the living species except that open lateral re-entrant angles are retained in the premolars. _Geomys paenebursarius_, also of the early Pleistocene, is a smaller species and seems to be more directly in the line of evolution of the modern species. As yet unnamed smaller species of _Geomys_ from the Rexroad fauna (late Pliocene) and Saunders fauna (latest Aftonian) may also be on the main line of evolution. Surprisingly, _Geomys tobinensis_ and _Geomys garbanii_ of later Irvingtonian provincial age are less specialized than either _Geomys quinni_ or _Geomys paenebursarius_. It is likely that _G. tobinensis_ and the unnamed species from the Dixon are closer to the main line of descent than _G. paenebursarius_ suggesting that the direct ancestral lineage of the living species of _Geomys_ was more conservative and less specialized than _Geomys paenebursarius_ of the Lower Pleistocene. _Geomys quinni_ and _G. paenebursarius_ seem to have acquired specialized dental features in the early Pleistocene.

_Geomys quinni_ was successful on the Great Plains, and persisted into the late Blancan. The main line may be represented in the early Pleistocene by _Geomys paenebursarius_ from the Hanc.o.c.k formation of the Texas Trans-Pecos. The structure of _G. paenebursarius_ indicates that it is in or close to the main line of descent, and probably evolved from one of the more primitive late Pliocene species of _Geomys_ from the Rexroad fauna.

[Ill.u.s.tration: FIG. 8. Tentative arrangement of species of the genus _Geomys_, depicting phylogenetic trends and probable relations.h.i.+ps within the genus.]

Isolated teeth, to which the name _Geomys bisulcatus_ probably applies, from Illinoian deposits on the Great Plains, show that the dent.i.tion characteristic of the living _Geomys_ had been developed by that time. Actually, the Illinoian material is too fragmentary to show clearly its taxonomic or phyletic affinities with the species of the later Pleistocene. Even so, the two main stocks of living _Geomys_, _G. bursarius_ and _G. pinetis_, had certainly been differentiated by Sangamon time. The other living species evidently evolved from one or the other of these two stocks in a period of isolation from the main population, probably in either the Wisconsin or post-Wisconsin. For example, _Geomys arenarius_ clearly differentiated from populations of _Geomys bursarius_ that were isolated by the eastward retreat of the main population from the southwestern United States as that region became more arid in the post-Wisconsin.

In review, it seems that the Recent species, represented basically by _bursarius_ and _pinetis_, evolved from Illinoian species (_Geomys bisulcatus?_), which descended in turn from the more primitive species of the early Pleistocene, possibly _Geomys paenebursarius_ or possibly from descendants of the Saunders species. Actually the Saunders species may prove to be _Geomys paenebursarius_. At any rate, three trends that took place during the Pleistocene stage of evolution, in the direction of the modern species, were an increase in size, progressive loss of the posterior enamel plate on P4, and a decrease in the vertical depth of the enamel cap as a result of which the dentine is reached in the initial phases of attrition on the tooth of a juvenile. _Geomys garbanii_, occurring at the periphery of the range of the genus, is regarded as a sterile offshoot of the primitive _tobinensis_-line of evolution.

Orthogeomys

This is one of the more specialized genera of the Geomyini. Save for one record in the late Pleistocene (_Orthogeomys onerosus_), there is no fossil history of the genus upon which to reconstruct its phylogeny; therefore, its phyletic development must be estimated by comparing it and the primitive morphotype of the tribe. Results of that comparison suggest that _Orthogeomys_ has closer affinities with _Zygogeomys_ than with any of the other genera, and that _Orthogeomys_ may have originated in an early dichotomy of primitive _Zygogeomys_ stock instead of descending from the ancestral stock of the tribe.

Except for the unisulcate incisors and the longer posterior loph on the third upper molars, the teeth of the two genera do not differ significantly. As in _Zygogeomys_, the enamel blade on the posterior wall of P4 has been reduced to a short plate restricted to the lingual third of the tooth (see Fig. 7F and H). In _Orthogeomys_, the trend in reduction of enamel is carried to its extreme only in the subgenus _Orthogeomys_, where this plate has been completely lost in most taxa (see Fig. 7D). The most significant trends in _Orthogeomys_, and the princ.i.p.al basis for recognizing the genus, are the dolichocephalic specializations of the skull, as described elsewhere, and the adaptive traits that have equipped the genus for living in tropical environments. The dolichocephalic features are more sharply defined in the subgenera _Orthogeomys_ and _Macrogeomys_, and are less developed in the subgenus _Heterogeomys_. Aside from the general dolichocephalic specializations, trends in _Orthogeomys_ include: Increase in size; loss of the median one of the two grooves on the anterior face of the upper incisor in the ancestral stock; increase in the anteroposterior length of each of the cheek teeth, as well as the aforementioned elongation of the posterior loph of M3; compression of the lateral angles of the premolars; and the remarkable increase in the size of the rostrum.

Pappogeomys

The genus _Pappogeomys_, as it is conceived of in this study, is comprised of two subgenera; one, _Pappogeomys_, is generalized and primitive, and the other, _Cratogeomys_, is specialized, and includes the most highly specialized of the modern pocket gophers. The subgenus _Pappogeomys_ is regarded as the ancestral lineage, and the subgenus _Cratogeomys_ is regarded as an early offshoot, probably in the early Pleistocene, that became progressively more specialized in the course of its subsequent evolution. In the same period of time, the subgenus _Pappogeomys_ changed little. It is known only from late Pliocene fragments and from the living species. The ancestral morphotype is preserved in _Pappogeomys_. Primitive characters are: (1) Small size; (2) skull generalized and smoothly rounded; (3) temporal ridges separate (not uniting into a sagittal crest); (4) enamel plates retained on both anterior and posterior walls of M1 and M2; (5) M3 bilophate, its posterior loph short. Basic specializations are few and include loss of the inner groove from the anterior face of the upper incisor; anteroposterior compression of the lateral re-entrant folds of the premolars; and loss of enamel from the posterior wall of P4.

All three features have been perpetuated in the advanced subgenus _Cratogeomys_, suggesting that they were already developed in the early evolution of the subgenus _Pappogeomys_ before _Cratogeomys_ diverged. Agreement with _Geomys_ is demonstrated by the lack of enamel on the posterior wall of P4 (see Fig. 9) and by retention of the posterior enamel plate on M1 and M2. In _Pappogeomys (Pappogeomys) alcorni_ the enamel from the posterior face of M1 has been lost from all but the lingual fourth or so of the posterior wall (Fig. 9E).

Reduction of enamel in M1 provides an example of parallelism with the more advanced subgenus _Cratogeomys_, discussed below.

There is no record as yet of the early evolution of the subgenus _Cratogeomys_. The features that characterize the subgenus were already well developed in the first known fossils which are from Wisconsin deposits of the late Pleistocene. _Cratogeomys_ is not a h.o.m.ogenous a.s.semblage; instead it is composed of two groups of living species, the generalized _castanops_ group and the specialized _gymnurus_ group. The _castanops_ group may be survivors of the ancestral lineage that diverged in two different stages in the phyletic development of the main line. Even so, the _castanops_ group has acquired its peculiar specializations. Indeed, _P. merriami_ of the _castanops_ group differs from the hypothetical stem more than does _P. castanops_. Judging from the structure of the living species of the subgenus _Cratogeomys_ and from the primitive subgenus _Pappogeomys_, the subgenus _Cratogeomys_ featured five major trends: (1) Increase in size; (2) formation of sagittal crest by union of the temporal impressions; (3) increase in rugosity and angularity of the skull; (4) progressive development of platycephalic specializations, including the elongation of the angular process of the mandible; (5) complete loss of enamel plates from the posterior wall of M1 and M2.

Each trend is thought to be adaptive.

[Ill.u.s.tration: FIG. 9. Molariform dent.i.tions of the Tribe Geomyini.

Drawings ill.u.s.trating enamel patterns characteristic of _Geomys_ and _Pappogeomys_ (including the subgenera _Pappogeomys_ and _Cratogeomys_). 5.

A and B. _Geomys bursarius bursarius_, adult female, No. 46275 (KU), Elk River, Sherborne Co., Minnesota. Left upper (A), P4-M3; right lower (B), p4-m3.

C and D. Subgenus _Pappogeomys_. _Pappogeomys bulleri albinasus_, adult female, No. 31002 (KU), W side La Venta, 13 mi. W and 4 mi. N Guadalajara, Jalisco. Left upper (C), P4-M3; right lower (D), p4-m3.

E and F. Subgenus _Pappogeomys_. _Pappogeomys alcorni_, adult female, No. 31051 (KU), holotype, 4 mi. W Mazamitla, 6600 ft., Jalisco. Left upper (E), P4-M3; right lower (F), p4-m3.

G and H. Subgenus _Cratogeomys_. _Pappogeomys gymnurus tellus_, adult female, No. 31051 (KU), 1 mi. NE Tala, 4400 ft., Jalisco. Left upper (G), P4-M3; right lower (H), p4-m3.

Loss of enamel is a trend common to all living genera of the tribe Geomyini, but the greatest loss has occurred in _Cratogeomys_. It has lost the plates on the posterior walls of M1 and M2 (Fig. 9G). If the lateral plates of M3 are considered as one functional plate and the lateral plates on either side of P4 together as two transverse plates, then, the transverse cutting blades in _Cratogeomys_ number seven in the upper and seven in the lower cheek teeth compared with 10 in the upper and seven in the lower in the primitive morphotype. Indeed, in some species of the subgenus, one or both of the lateral plates on M3 is also lost, usually in old age, resulting in even greater reduction of enamel. Loss of enamel from the posterior walls of the upper molars may be a.s.sociated with changes in the mechanics of mastication from anteroposterior planing to anterotransverse shearing, as discussed elsewhere. Merriam (1895:95-96) argues convincingly that the posterior cutting blades of the upper molars would hinder efficient shearing action of the teeth; hence, selection would favor their reduction and eventual loss. Changes in the shape of the skull also seem to be correlated with the s.h.i.+ft from a planing to a shearing type of mastication. More efficient shearing action, which depends upon lateral movement of the jaw, can be developed if the functional muscles insert farther laterally than is possible in the generalized type of skull. Therefore, platycephalic specializations involved lateral expansion of the braincase and mandible. p.r.o.nounced lateral expansion has been developed only in the _gymnurus_ group of species, suggesting that the dental specializations evolved earlier in the evolution of the subgenus than did the platycephalic specializations of the skull, and that the _castanops_ group separated from the _gymnurus_ group before the common ancestor had developed the more extreme trends in platycephaly. It is interesting to note that the subtriangular M3 (Fig. 9G) postulated for the ancestral morphotype and that characterizes the subgenus _Pappogeomys_ is retained also in the _gymnurus_ group.