Fig. 8.Tentative arrangement of species of the genusGeomys, depicting phylogenetic trends and probable relationships within the genus.
Fig. 8.Tentative arrangement of species of the genusGeomys, depicting phylogenetic trends and probable relationships within the genus.
Isolated teeth, to which the nameGeomys bisulcatusprobably applies, from Illinoian deposits on the Great Plains, show that the dentition characteristic of the livingGeomyshad been developed by that time. Actually, the Illinoian material is too fragmentary toshow clearly its taxonomic or phyletic affinities with the species of the later Pleistocene. Even so, the two main stocks of livingGeomys,G. bursariusandG. 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 arenariusclearly differentiated from populations ofGeomys bursariusthat 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 bybursariusandpinetis, evolved from Illinoian species (Geomys bisulcatus?), which descended in turn from the more primitive species of the early Pleistocene, possiblyGeomys paenebursariusor possibly from descendants of the Saunders species. Actually the Saunders species may prove to beGeomys 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 primitivetobinensis-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 thatOrthogeomyshas closer affinities withZygogeomysthan with any of the other genera, and thatOrthogeomysmay have originated in an early dichotomy of primitiveZygogeomysstock 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 inZygogeomys, the enamel blade on the posterior wall of P4 has been reduced to a short plate restricted to the lingual third of the tooth (seeFig. 7F and H). InOrthogeomys, the trend in reduction of enamel is carried to its extreme only in the subgenusOrthogeomys, where this plate has been completelylost in most taxa (seeFig. 7D). The most significant trends inOrthogeomys, and the principal 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 subgeneraOrthogeomysandMacrogeomys, and are less developed in the subgenusHeterogeomys. Aside from the general dolichocephalic specializations, trends inOrthogeomysinclude: 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 genusPappogeomys, 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 subgenusPappogeomysis regarded as the ancestral lineage, and the subgenusCratogeomysis 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 subgenusPappogeomyschanged little. It is known only from late Pliocene fragments and from the living species. The ancestral morphotype is preserved inPappogeomys. 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 subgenusCratogeomys, suggesting that they were already developed in the early evolution of the subgenusPappogeomysbeforeCratogeomysdiverged. Agreement withGeomysis demonstrated by the lack of enamel on the posterior wall of P4 (seeFig. 9) and by retention of the posterior enamel plate on M1 and M2. InPappogeomys (Pappogeomys) alcornithe enamel from the posterior face of M1 has been lost from all but thelingual fourth or so of the posterior wall (Fig. 9E). Reduction of enamel in M1 provides an example of parallelism with the more advanced subgenusCratogeomys, discussed below.
There is no record as yet of the early evolution of the subgenusCratogeomys. The features that characterize the subgenus werealready well developed in the first known fossils which are from Wisconsin deposits of the late Pleistocene.Cratogeomysis not a homogenous assemblage; instead it is composed of two groups of living species, the generalizedcastanopsgroup and the specializedgymnurusgroup. Thecastanopsgroup may be survivors of the ancestral lineage that diverged in two different stages in the phyletic development of the main line. Even so, thecastanopsgroup has acquired its peculiar specializations. Indeed,P. merriamiof thecastanopsgroup differs from the hypothetical stem more than doesP. castanops. Judging from the structure of the living species of the subgenusCratogeomysand from the primitive subgenusPappogeomys, the subgenusCratogeomysfeatured 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.
Fig. 9.Molariform dentitions of the Tribe Geomyini. Drawings illustrating enamel patterns characteristic ofGeomysandPappogeomys(including the subgeneraPappogeomysandCratogeomys). × 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. SubgenusPappogeomys.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. SubgenusPappogeomys.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. SubgenusCratogeomys.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.
Fig. 9.Molariform dentitions of the Tribe Geomyini. Drawings illustrating enamel patterns characteristic ofGeomysandPappogeomys(including the subgeneraPappogeomysandCratogeomys). × 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. SubgenusPappogeomys.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. SubgenusPappogeomys.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. SubgenusCratogeomys.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.
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. SubgenusPappogeomys.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. SubgenusPappogeomys.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. SubgenusCratogeomys.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 inCratogeomys. 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 inCratogeomysnumber 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 posteriorwalls of the upper molars may be associated 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 shift 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. Pronounced lateral expansion has been developed only in thegymnurusgroup 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 thecastanopsgroup separated from thegymnurusgroup 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 subgenusPappogeomysis retained also in thegymnurusgroup.
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Transmitted May 29, 1967.[]
Transcriber's NotesAll obvious typographical errors were corrected. Minor changes were made to standardize the text to match the most prevalent form used.Typographical CorrectionsPageCorrection477cumberlandicus => cumberlandius535breath => breadth
Transcriber's Notes
All obvious typographical errors were corrected. Minor changes were made to standardize the text to match the most prevalent form used.
Typographical Corrections