Evolution and Classification of the Pocket Gophers of the Subfamily Geomyinae Part 1

Evolution and Cla.s.sification of the Pocket Gophers of the Subfamily Geomyinae.

by Robert J. Russell.

INTRODUCTION

When C. Hart Merriam wrote his monograph of the subfamily Geomyinae in 1895, he had no opportunity to examine fossil specimens. No doubt his phylogenetic conclusions and cla.s.sification would have been greatly influenced had he enjoyed that opportunity because study of fossil geomyids reveals the historic sequence of phyletic development, and this sequence provides a firm basis for distinguis.h.i.+ng specialized from primitive characters. The history of the Geomyinae has been characterized by the evolution of specializations. These evolutionary trends begin, as we presently know them, with a generalized ancestral stock in the early Miocene. The direction, degree, and rate of change, beginning with the primitive morphotype of the subfamily, has not been the same in the various lineages. The cla.s.sification within the subfamily is based upon the phyletic interpretations of available data and the relations.h.i.+ps they disclose. In turn, a new, and I hope more realistic, phylogeny and cla.s.sification is offered.

MATERIALS AND ACKNOWLEDGMENTS

Recent specimens were studied of all the known genera, subgenera and 29 of the 36 living species. Most of the species not studied are monotypic and have restricted geographic ranges. They are: _Geomys colonus_, _G. fontanelus_, and _G. c.u.mberlandius_, _Orthogeomys cuniculus_ and _O. pygacanthus_ of the subgenus _Orthogeomys_, and _O. dariensis_ and _O. matagalpae_ of the subgenus _Macrogeomys_.

Examination of these modern species would not radically change the estimation of the degree of phyletic development of the genera and subgenera involved. All of the major polytypic and widespread species were studied.

Specimens of the extinct genera _Dikkomys_, _Pliosaccomys_, _Pliogeomys_, _Nerterogeomys_, and _Parageomys_ also were studied, as were examples of the extinct species _Geomys quinni_, _Geomys tobinensis_, and _Orthogeomys onerosus_. Considerable fossil material of living species, especially of the genera _Geomys_ and _Pappogeomys_, was used.

Inasmuch as the present account concerns mainly structural changes in the subfamily Geomyinae at the level of subgenera and above, and the temporal sequence of those changes, no attempt is made in the present account to revise taxonomy below the level of subgenera. Considerable modification of the cla.s.sification below that level (for species and subspecies) is to be expected in _Orthogeomys_ and Pleistocene taxa of _Geomys_ when available specimens are studied.

I thank Prof. Robert W. Wilson for his a.s.sistance in securing fossil geomyids for study, and those in charge of the paleontological collections at the California Inst.i.tute of Technology, Prof. Bryan Patterson, formerly of the Field Museum of Natural History, and Prof.

Claude W. Hibbard of the University of Michigan, Museum of Zoology.

For their kindness in lending Recent species, I thank Mr. Hobart M.

Van Duesen of the American Museum of Natural History, Dr. David H.

Johnson of the U. S. National Museum, and Dr. Oliver P. Pearson of the California Museum of Vertebrate Zoology, the late Colin C. Sanborn of the Field Museum of Natural History, and Profs. Emmet T. Hooper and William H. Burt of the University of Michigan Museum of Zoology.

I am especially grateful to Prof. E. Raymond Hall for his guidance and helpful criticisms with the ma.n.u.script. For a.s.sistance with paleontological problems, I thank Drs. Robert W. Wilson and William A. Clemens. Several persons have offered helpful suggestions and encouragement in the course of my study. For a.s.sistance of various sorts I especially thank Drs. J. Knox Jones, Jr., Rollin H. Baker, A. Byron Leonard, Sydney Anderson, James S. Findley, Robert L.

Packard, and Robert G. Anderson. Advice concerning the drawings of the dent.i.tions was generously given by Mr. Victor Hogg, and the drawings were done by Mrs. Lorna Cordonnier under his direction and by Mr.

Thomas H. Swearingen. For a.s.sistance with secretarial tasks I thank Valerie Stallings, Violet Gourd, Ann Machin, Toni Ward, Sheila Miller, and my wife, Danna Russell.

TAXONOMIC CHARACTERS

Morphological features of the fossils and their stratigraphic provenience provide the information upon which phylogenetic interpretations are based. Although the most critical sequences of the fossil record are lacking, and although the existing fossils are mostly fragmentary and therefore seldom furnish ideally suitable data for the interpretations that have been made, phylogenetic conclusions drawn from fossil materials are superior to those drawn on other bases. The especially relevant characters are those disclosing primary trends in the evolution of the modern a.s.semblages. The higher systematic categories recognized in the following account are based primarily upon such characters.

The most important characters found are in the teeth, although several structural changes in the lower jaw, especially those a.s.sociated with the insertion of cranial musculature, are almost as important.

_Prismatic Character of Molars_

In primitive geomyines the molar consisted of two columns united at their mid-points and forming a figure 8 or H-pattern (see Fig. 4B).

Both l.a.b.i.al and lingual re-entrant folds were formed between the two columns. The primitive pattern is retained in the premolars of all known Geomyinae. Therefore, in the earliest (Miocene) members of the subfamily, the pattern of the molars was essentially like that of the premolars.

In Pliocene Geomyinae the two columns of the molars tend to merge into one. This is evident on the worn occlusal surface of the teeth; the lateral re-entrant folds are shallow vertically and progressively recede laterally until only a slight inflection remains. In the final stages of attrition, the inflection disappears and the tooth is a simple elliptical column. In the Pleistocene the monoprismatic pattern appears at earlier stages of wear owing to the decrease in depth of the re-entrant folds, and in Geomyinae of Recent time the initial stages of wear on the enamel cap of infants erase the last vestiges of two columns in the molar teeth.

The general trend in evolution, therefore, has been from a bicolumnar to a monocolumnar pattern. The particular patterns of wear characterizing each genus are described in detail beyond.

The third upper molar has evolved less rapidly than the first and second and in one of the modern lineages (tribe Geomyini) tends to retain at least a vestige of the primitive bicolumnar pattern in the final stage of wear. Therefore, the loss of any trace of the bicolumnar pattern in M3 is considered to be a much specialized condition. Unfortunately, the fossil record of the third upper molar is less complete than that for the first molar and second molar; the tooth drops out of its alveolus more often than does any one of the other molariform teeth and is seldom recovered.

_Character of Enamel Patterns_

In the primitive genera the enamel pattern is bilophate and the enamel loop (see Fig. 4B) is continuous on the occlusal surface of a worn molar. Concomitant with the union of the double columns, the bilophodont pattern is reduced to a single loph, but the enamel still completely encircles the dentine.

In the molars of modern geomyines, the enamel loop is not continuous but is interrupted on the sides of the crown by vertical tracts of dentine that are exposed at the occlusal surface of the tooth during early stages of wear. Therefore, a continuous enamel band is to be found only in a juvenal individual whose teeth have been subjected to only slight attrition on the enamel cap. In molars lacking enamel on the l.a.b.i.al and lingual sides, anterior and posterior enamel plates, or blades, are found on each molar. The premolar also has an enamel plate on the anterior surface and another on the posterior surface, and in addition both re-entrant angles are protected by a V-shaped investment of enamel. One or the other of the various plates can be reduced or lost accounting for the several distinctive tooth-patterns of the modern geomyines. If loss occurs, it usually is the anterior plate in the lower dent.i.tion and the posterior plate in the upper dent.i.tion, including the upper premolar. When reduction of the posterior plate of the upper cheek teeth occurs, enamel is first lost from the l.a.b.i.al side of the tooth, thus leaving only a short vestigial plate on the lingual end of the crown.

_Grooving of Incisors_

The incisors are smooth with no trace of a groove in the ancestral lineage. In the specialized a.s.semblage (tribe Geomyini) p.r.o.nounced grooves are always developed on the anterior face of the upper incisor. The pattern of grooving is constant in each species and thus provides characters of taxonomic worth for grouping species into genera. The only inconstancy noted was an incisor of _Geomys_ from the Tobin local fauna of the middle Pleistocene which has three grooves rather than the normal two (No. 6718 KU). The extra groove is an obvious abnormality, and the tooth was a.s.sociated with others of the same species from the same quarry that were normally grooved.

Grooves on the lower incisors are unknown. The functional significance of grooving has been debated on numerous occasions in the literature.

Grooves appear in a number of only distantly related rodents and in lagomorphs. The grooving occurs always in small herbivorous mammals, and in some way may be related to feeding habits.

The grooves provide a serrated cutting edge on the occlusal edge of the upper incisor. In the genus _Geomys_, for example, the two incisors, including the slight s.p.a.ce between them, present a total of five serrations, which may facilitate cutting and piercing tuberous and fibrous roots upon which _Geomys_ feeds. Also the sulci would perform the same function as the longitudinal groove on the side of a bayonet, and would aid the animal in extracting its upper incisors from coa.r.s.e, fibrous material. In gathering food, the gopher sinks its upper incisors into a root, and then, with the upper incisors firmly anch.o.r.ed, slices off small chunks by means of the lower incisors.

Therefore, in pocket gophers, grooving may be an adaptation for feeding on fibrous or woody material. Finally, grooves increase the enamel surface of the incisor without additional broadening of the tooth itself. There could be a selective advantage for sulcation if the extra enamel and the serrate pattern strengthen the incisors, which are under heavy stress while penetrating or prying off pieces of coa.r.s.e material. Few broken incisors of pocket gophers are found.

_Ma.s.seteric Ridge and Fossa_

This ridge and fossa are on the lateral surface of the ramus. The crest on the ridge begins at the base of the angular process and terminates slightly anterior to the plane of the lower premolar. The ma.s.seteric fossa receives the insertion of the rostral or superficial division of the ma.s.seter muscle. The mental foramen lies immediately anterior, or anteroventral, to the fossa.

In the ancestral lineage, the ridge is distinct but relatively low; the ma.s.seteric fossa is shallow and is a poorly developed area for attachment of the superficial ma.s.seter muscle. In modern Geomyinae the ridge is ma.s.sive and forms a high crest, especially anteriorly, and the ma.s.seteric fossa is a deep, prominent cup along the dorsal side of the crest. The elaboration of the crest and fossa evidently is a.s.sociated with an increase in size of the superficial ma.s.seter muscle, which enlarges and provides increased power for the propalinal type of mastication. A high crest has evolved independently in both modern lineages, Th.o.m.omyini and Geomyini.

_Basitemporal Fossa_

The name basitemporal fossa is suggested here to denote the deep pit that lies between the lingual base of the coronoid process and the third lower molar. The basitemporal fossa receives the insertion of the temporal muscle. The fossa, which until now has not been named, is a unique feature in advanced Geomyinae, being unknown in either primitive Geomyinae or in other rodents.

The temporal is one of several muscles holding the occlusal surface of the lower molariform dent.i.tion firmly against the upper cheek teeth during mastication. In primitive geomyines that masticate food by a planing action, the temporal muscle also moves the mandible posteriorly and food is ground between the enamel plates when the lower jaw is retracted as well as when it is moved forward.

The basitemporal fossa appears in late Pliocene geomyines and increases the attachment surface of the temporal muscles that powers the planing action important in utilizing woody and fibrous foods. The basitemporal fossa developed in only one of the modern lineages (tribe Geomyini), the same lineage in which grooved incisors evolved.

Both features probably are adaptations for feeding on coa.r.s.e food.

The fossa is not greatly developed in either the ancestral tribe Dikkomyini or the modern tribe Th.o.m.omyini, although in some specimens a slight depression marks the site of the basitemporal fossa.

[Ill.u.s.tration: FIG. 1. Types of skulls in the subfamily Geomyinae. 1.

A. and B. Generalized type of skull. _Geomys bursarius lutescens_, adult, male, No. 77955 KU, 10 mi. N Springview, Keya Paha Co., Nebraska.

A. Dorsal view of skull.

B. Ventral view of lower jaw.

C. and D. Dolichocephalic type of skull. _Orthogeomys_ (_Orthogeomys_) _grandis guerrerensis_, adult, female, No. 39807 KU, 1/2 mi. E La Mira, 300 ft., Michoacan, Mexico.

C. Dorsal view of skull.

D. Ventral view of lower jaw.

E. and F. Platycephalic type of skull. _Pappogeomys_ (_Cratogeomys_) _gymnurus tellus_, adult, female, No. 33454 KU, 3 mi. W Tala, 4300 ft., Jalisco, Mexico.

E. Dorsal view of skull.

F. Ventral view of lower jaw.

_Specializations of Skull_