Five million years of pocket gopher history in the Meade ... Meade... · 868 JOURNAL OF VERTEBRATE...

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Five million years of pocket gopher history in theMeade Basin of southwestern Kansas and northwesternOklahomaRobert A. Martin a , Pablo Peláez-Campomanes b , James G. Honey c , Federica Marcolini d &William A. Akersten ea Department of Biological Sciences, Murray State University, Murray, Kentucky, 42071,U.S.A.b Department of Paleobiology, National Museum of Natural History, C.S.I.C., Jose GuttierezAbascal 2, Madrid, 28006, Spainc Geology Section, University of Colorado, Boulder, Colorado, 80309-0315, U.S.A.d Department of Geological Sciences, University of Roma Tre, Largo S. L. Murialdo 1, 00146,Roma, Italye Department of Biological Sciences, Idaho State University, Stop 8007, Pocatello, ID, 83209Published online: 11 Jul 2011.

To cite this article: Robert A. Martin , Pablo Pelez-Campomanes , James G. Honey , Federica Marcolini & William A. Akersten(2011) Five million years of pocket gopher history in the Meade Basin of southwestern Kansas and northwestern Oklahoma,Journal of Vertebrate Paleontology, 31:4, 866-884, DOI: 10.1080/02724634.2011.576729

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Journal of Vertebrate Paleontology 31(4):866–884, July 2011© 2011 by the Society of Vertebrate Paleontology

ARTICLE

FIVE MILLION YEARS OF POCKET GOPHER HISTORY IN THE MEADE BASIN OFSOUTHWESTERN KANSAS AND NORTHWESTERN OKLAHOMA

ROBERT A. MARTIN,*,1 PABLO PELAEZ-CAMPOMANES,2 JAMES G. HONEY,3 FEDERICA MARCOLINI,4

and WILLIAM A. AKERSTEN5

1Department of Biological Sciences, Murray State University, Murray, Kentucky 42071, U.S.A., [email protected];2Department of Paleobiology, National Museum of Natural History, C.S.I.C., Jose Guttierez Abascal 2, Madrid 28006, Spain,

[email protected];3Geology Section, University of Colorado, Boulder, Colorado 80309-0315, U.S.A., [email protected];

4Department of Geological Sciences, University of Roma Tre, Largo S. L. Murialdo 1, 00146 Roma, Italy,[email protected];

5Department of Biological Sciences, Idaho State University, Stop 8007, Pocatello, ID 83209, [email protected]

ABSTRACT—The Meade Basin record of pocket gophers extends from the latest Miocene (Buis Ranch local fauna) to mod-ern time. A primitive species with hypsodont but rooted cheek teeth, Pliogeomys buisi, characterizes the late Hemphillian.Another species of Pliogeomys, P. louderbachi, is described here as a new species from the early Blancan. It is intermediatein dental and mandibular morphology between Pliogeomys and Geomys. The Geomys minor (= G. smithi) lineage displays astepped dwarfing trend prior to its extinction, whereas the G. jacobi lineage demonstrates an overall pattern of stasis in size.G. jacobi is replaced by G. quinni at the end of the Pliocene, within which there is a significant directional size increase. A latePliocene immigrant, the new species G. floralindae, appears in the Sanders assemblage. It is briefly replaced by an indetermi-nate small species in the Nash 72 local fauna. G. tobinensis is found in the Cudahy local fauna. Morphologically modern G.bursarius appears in the Meade Basin during the early Rancholabrean. A small, primitive species, Geomys adamsi, appearsonly during the early Pliocene at Fox Canyon, and transient Thomomys appear at various times during the Pleistocene, bothapparently during cold intervals. A phylogenetic analysis suggests two clades, one uniting Pliogeomys russelli and Geomysadamsi and another including the remaining Meade Basin geomyines. Enhanced species turnover and the last push for mod-ern mandibular morphology is most pronounced in sediments younger than about 2.6 million years ago, corresponding to thefirst global cooling heralding the Pleistocene.

INTRODUCTION

The testing of macroevolutionary and macroecological theoryrequires at least a dense fossil record in a limited geographic re-gion extending over millions of years. Preferably, such recordsshould also be available in other, nearby regions, for comparison.The Meade Basin, a depositioal trough running approximately48 km from central Meade County, Kansas, through northwest-ern Oklahoma, preserves a record extending from the latestMiocene (latest Hemphillian North American Land MammalAge [NALMA]) through modern time. The late C. W. Hibbardand his students (Hibbard, 1938, 1941. 1950, 1953, 1956, 1964;Paulson, 1961; Woodburne, 1961; Schultz, 1969; Zakrzewski,1975) established a geological and biostratigraphic frameworkthat was later modified and expanded by the Meade BasinRodent Project (MBRP; Martin et al., 2000, 2008), the latter de-signed specifically to create a detailed database with which to ex-amine current hypotheses of mammalian community and mor-phological change. The most recent biostratigraphic hypothesisfor Meade Basin fossil assemblages based on this ongoing re-search is presented in Figure 1. Our work has focused on the ro-dents because they tend to be relatively stenotopic and occur inlarge enough numbers to provide meaningful statistical testing.

Flynn et al. (2007) presented a preliminary taxonomic sum-mary and hypothesis of pocket gopher species replacements inthe Meade Basin. The purpose of this study is to provide the sup-

*Corresponding author.

porting data for that summary, modify it with new information,and add to the growing body of evolutionary information derivedfrom the MBRP. Prior to our work, six extinct gopher species hadbeen described from the Meade Basin: Pliogeomys buisi from theBuis Ranch and Saw Rock Canyon l.f.s (Hibbard, 1954, 1964),Geomys tobinensis from the Tobin and other Cudahy-age lo-cal faunas (l.f.) (Hibbard, 1944; Paulson, 1961), G. jacobi fromthe Rexroad locality 3 l.f. (Hibbard, 1967), G. quinni from DeerPark (Hibbard, 1956), and G. smithi and G. adamsi from the FoxCanyon assemblage (Hibbard, 1967). Hibbard (1967), followingGidley (1922), separated the Geomys species into the subgeneraNerterogeomys and Geomys based on the placement of the men-tal foramen relative to the anterior convergence of the mandibu-lar masseteric crests. In Nerterogeomys the foramen is found un-derneath the ventral crest, whereas in Geomys it is anterior andslightly dorsal to the anterior edge of the crest bundle. Akersten(1973a) considered species with the Nerterogeomys mandibularmorphology and a relatively cuspate occlusal surface of the dp4to represent a separate tribe of primitive geomyines related to,but distinct from, Geomys. Flynn et al. (2007) synonymized G.smithi under G. minor and G. jacobi under G. quinni. They alsoquestioned the taxonomic status of G. tobinensis and suggestedthat the position of the mental foramen might have only lim-ited taxonomic value. Flynn et al. (2007) also referred the ge-omyid material from Saw Rock Canyon to Geomys, whereasAkersten (1973a), in an unpublished dissertation, consideredthe Saw Rock Canyon material to represent a new species ofPliogeomys.

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MARTIN ET AL.—MIOCENE TO RECENT POCKET GOPHERS FROM KANSAS AND OKLAHOMA 867

FIGURE 1. Stratigraphic relationships of Meade Basin local faunas. Abbreviations: Br, Brunhes; C, Chron; C., Canyon; CC, calcium carbonate layer;Coch, Cochiti; Crk, Creek; Ga, Gauss; Gi, Gilbert; Gr., gravel; Huck R, Huckleberry Ridge; Jar, Jaramillo; Ma, million years ago; Mam, Mammoth;MPTS, magnetic polarity time scale; Mt, Matuyama; N and n, normal polarity; Nun, Nunivak; Old, Olduvai; R and r, reversed polarity; Rap, Raptor;Reun, Reunion; Rex, Rexroad; RZ, rodent zones of Martin (2003); Sud, Sidufjall; Thv, Thvera; Tol, Toledo.

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868 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 31, NO. 4, 2011

The alpha taxonomy of modern Great Plains Geomys has be-come rather complicated in the past 30 years, mostly due to theapplication of new molecular techniques and the recognition ofa number of possible sibling species related to G. bursarius, in-cluding G. arenarius, G. knoxjonesi, G. texensis, and G. lutescens(reviewed by Sudman et al., 2006). Six additional species were in-cluded by Jolley et al. (2000) and Sudman et al. (2006) in the per-sonatus, breviceps, and pinetis groups, for a total of 11 species inthe genus. However, not all investigators agree with this conclu-sion, and in the absence of breeding experiments it is difficult toassess the extent of genetic and behavioral isolation among thevarious populations. For example, hybrids of Geomys lutescensand G. bursarius are known (Heaney, 1985). The cranial morpho-logical evaluations that sometimes accompany the genetic studies(e.g., Heaney, 1985) are rarely applicable to fossil material, whereonly dental and mandibular characters are consistently available.For the same reason, new fossil Geomys species described on anoccasional skull (such as G. garbanii White and Downs, 1961)are also impossible to evaluate because crania are almost non-existent in other fossil Geomys samples. Coupled with this sce-nario is the observation that, except for uncommon unworn spec-imens and rare dp4s, the dentition of Geomys is very simple, andeven the most complex permanent teeth, the P4 and p4, offer fewcharacters and states for comparison. A few additional featuresare noted on the mandible. Indeed, the subgenus Nerterogeomys(Gidley, 1922) was named on the basis of a mandibular charac-ter. In a sense, these comments represent an apologia for our in-ability to discriminate among the modern genetic species to anygreat extent. When we find mandibles and dentitions that are in-separable from extant Geomys bursarius from Meade County, werefer them to that species, recognizing that they may be from G.lutescens or, possibly, other modern species, assuming that thesibling species of the bursarius group are valid. What we can sayis that mandibular and dental characters mostly remain consistentwithin the lineages (species) we recognize. Despite our inabilityto recognize modern genetically determined sibling species, therecord of geomyids in the Basin is so dense and sensible that ahistory of both replacements and microevolution emerges thatis unparalleled for mammalian taxa during the later Neogene ofNorth America. In this paper we document at least the occasionalpresence of three genera and 10 species. We establish a long-termstepped dwarfing trend in G. minor, size stasis in G. jacobi, and ashift towards gigantism in G. quinni near the end of the Pliocene.We document the immigration of a potential ancestor for G. bur-sarius, here described as the new species G. floralindae, towardsthe end of the Blancan. Based on newly discovered material inthe field and in collections from Saw Rock Canyon, we describea new species of Pliogeomys, P. louderbachi. It is intermediatebetween Pliogeomys buisi and Geomys in dental hypsodonty andfeatures of the p4. We will also argue that the occasional presenceof diminutive species of both Geomys and Thomomys likely rep-resents periods of depressed winter and summer temperatures,and a major period of instability in gopher history at the closeof the Blancan is probably correlated with the onset of globalcooling.

MATERIALS AND METHODS

Specimen Sources—Sequencing of the Meade Basin fossil lo-calities was determined by careful field mapping, radioisotopicand paleomagnetic dating, and biostratigraphy (Martin et al.,2000, 2002, 2003, 2008; Honey et al., 2005), the details of whichwill not be repeated here except for a brief discussion of therelationship of the Vasquez, Newt, and Wiens localities and lo-cal faunas. Fossil assemblages from Vasquez and Newt were ex-cavated from localities that bear those names in East of Alien

Canyon, Meade County (Honey et al., 2005). Newt is isolatedfrom Vasquez by a small ravine (ca. 10 m), and it is likely thetwo were connected during the time of deposition. Two levelsare recognized at Newt, a lower flour sand (NLS in Appendix 3)and an upper sandy silt. Together, they represent a thickness of<0.25 m. Measurements from Vasquez and Newt are presentedseparately in Appendices 2 and 3, but were combined for statis-tical testing. The Wiens locality is found in Alien Canyon about14 km west of Vasquez/Newt, and like the former localities, liesbetween CC1 and CC2, two important regional calcium carbon-ate marker beds (Honey et al., 2005; Fig. 1). As there is <2.0 mof sediment between these layers, it is likely that Vasquez, Newt,and Wiens were contemporaneous. The Wiens (WNS) sample ofG. minor is presented in Appendix 2. A single large incisor of G.cf. jacobi was found at Wiens, but no p4s were recovered fromthe large gopher.

Figure 1 provides a summary of our current understandingof the Meade Basin sequence, including all relevant data ex-cept some biostratigraphic considerations explained elsewhere(Martin 2003; Pelaez-Campomanes and Martin, 2005; Marcoliniand Martin, 2008; Martin et al., 2008). Geomys samples were ex-cavated from single quarries ≤1.0 m2; most were taken from con-siderably smaller volumes. In this study, the name of a quarryis synonymous with the name of the fossil assemblage, or ‘localfauna (l.f.)’ recovered from it. A very primitive brachydont ge-omyoid is present in the late Miocene High Banks l.f. (Fig. 1),but we are currently uncertain of its identification and the posi-tion of this locality in time and we will not consider it further inthis study.

In order to properly assess changes in size and morphologyof the Geomys jacobi and G. quinni lineages, we also incorpo-rated measurements from topotype specimens collected by C.W. Hibbard from the type locality of Geomys quinni (McGrew,1949) at Booth Draw, a tributary of Sand Draw in Brown County,Nebraska. This material is considered to have been depositedduring the late Blancan (Skinner and Hibbard, 1972), and mayrepresent a time in the Kansas sequence between Rexroad 2Aand Borchers.

Specimens, Measurements, and Taxonomic Philosophy—Thep4 and P4 possess sufficient dental characters and variation to beuseful in evolutionary studies of both size and shape. The m1–m3and M1–M2 are simple pegs. The M3 can have some taxonomicutility among modern genera (Russell, 1968), but was not stud-ied in the fossil material because of minimal variability. For thisstudy, we took measurements only on p4. Four measurementswere taken on the occlusal surface of this tooth: (1) Wp4= great-est width across the posterolophid (Fig. 2); (2) Lp4 = greatestlength of p4; (3) WA = width across the anterolophid; and (4)CL = width of dentine isthmus connecting the anterolophid andposterolophid. As the anterior tooth in the mandible is highlycorrelated in rodents with body size (Martin, 1990), we considerLp4 to be a rough indicator of body size in adult Geomys. Thelinea sinuosa on p4 refers to the enamel-dentine junction (EDJ).Enamel-free areas, called dentine tracts, extend upwards fromthe EDJ in most gopher lineages. A labial dentine tract can beseen on the advanced p4 of the extant G. bursarius in Figure 2B,and the linea sinuosa is labeled in Figure 2D. When used with atooth and not a measurement, L = left, R = right.

Figure 2 compares the anterior mandibular morphology andp4 morphology in geomyid rodents. As explained by Akersten(1973a), the evolution of muscle scars (crests) on the lateral sur-face of the geomyid mandible reflects expansion of the masseterbundles from the presumed ancestral morphology in heteromyidrodents. This is consistent with the evolution of a group that usesits incisors for digging as well as masticating food. In heteromyidsand Pliogeomys, dorsal and ventral crests fuse anteriorly andcontinue as an anterior extension known as the ‘heteromyid

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FIGURE 2. Mandibular anatomy in Plio-geomys (A) and Geomys (B) and occlusalmorphology of Geomys p4 (C); lingual view ofPliogeomys p4 (D). Scale bars equal 1 mm.

projection’ (Fig. 2A). The dorsal crest in these forms is named the‘medial masseteric crest’ in heteromyids and the ‘geomyine ridge’in geomyids if another, more dorsal, crest is present, termed the‘dorsal masseteric crest.’ The ventral crest is named the ‘ven-tral masseteric crest’ in all forms. In reality, there is a continuumin the expansion of the dorsal masseteric crest from Pliogeomysthrough modern geomyids. As the masseter muscles expand inbulk, a depression develops above the heteromyid projection fortheir insertion. This development can be seen in Pliogeomys as asmall, shallow depression. In later geomyids, this depression ex-pands dorsally and anteriorly, and engulfs the area of the old het-eromyid projection. A new, relatively short but robust crest (thedorsal masseteric crest) develops dorsal to the old heteromyidprojection and merges with the dorsal edge of the ascending ra-mus. The old medial masseteric crest is retained as the geomyineridge, a thin crest that runs ventral and parallel to the edge ofthe ascending ramus. In Geomys minor and the new species G.floralindae, the area of the old heteromyid projection is still fairlydistinct, although the masseter muscles have begun to expand anda geomyine ridge is present. Even in modern Geomys, the area ofthe heteromyid projection remains discernible, though it is en-gulfed entirely by the crests produced by the enlarged massetermuscles (Fig. 2B). As we shall see, the expansion of the massetersand concomitant production of a dorsal crest occurred indepen-dently in at least two branches of early geomyines.

The basitemporal fossa, a character named by Russell (1968)for the depression on the medial side of the coronoid processnext to m3 (and sometimes m2–m3) for insertion of the tem-poralis muscle on the mandible, is considered here equivalentto Wilkins’s (1984) retromolar fossa. The form of this fossahas taxonomic value, but there are no quantitative data in theliterature charting changes in depth, shape, or overall size, pre-sumably because it is so irregular and difficult to measure. Thecomplete periphery of the insertion area includes, in additionto the pit below the alveoli of m2–m3, a shallow depressionthat runs up part of the medial side of the ramus. As a result,description of the temporalis insertion area has been qualitative,such as Hibbard’s (1967) observation, in the diagnosis of Geomysjacobi (1967:123; 124, fig. 4), that this fossa was very shallow

compared to that of G. quinni. We have confirmed Hibbard’sobservation, and agree that it differentiates G. jacobi from G.quinni. However, we are not satisfied with the lack of datafor this important feature and have begun to consider variousquantification options. Until those studies are completed, we willfollow traditional practice and use qualitative descriptions of thebasitemporal fossa to discriminate between species.

Measurements were taken to the nearest 0.01 mm with an AOfilar micrometer eyepiece on a Spencer stereomicroscope cali-brated with an AO 2.0 mm slide. Raw measurements were con-verted by multiplication with the appropriate correction factor.Statistical analyses were done with Microsoft Excel and Pear-son Education StatCrunch. Because so few statistically signifi-cant differences were found between the G. minor samples forthe three measurements following a significant ANOVA and ex-amination by the Tukey HSD test, instead of reporting the 95%confidence limits for all possible pair-wise comparisons, we repre-sented the results instead as a simple significance matrix, showingonly where significance was uncovered. Raw computations areavailable on request. Ninety-five percent confidence intervals ofTukey’s HSD were presented for the G. jacobi and G. quinni lin-eages. Some graphs were constructed with the R statistical pro-gramming environment. An alpha level of 0.05 was assumed forall comparisons.

A phylogenetic analysis of pocket gopher dental and mandibu-lar features was constructed with MacClade 4.06 (Maddisonand Maddison, 2001). All character state sequences exceptthose representing premolar size (PMS) were determined to beirreversible, primarily because no reversals were observed inany lineages. Fossil history of character change from a detailedchronology is as close as we can get to primary evidence of suchinformation. All characters except p4 reentrant fold shape (RS)were unweighted; RS was weighted higher (3) to discriminateearly evolution of some advanced features in Geomys adamsi.The characters and data matrix are included as Appendix 1.

Specimens for this study are catalogued in the University ofMichigan, Museum of Paleontology (UMMP), Fort Hays Stern-berg Museum (FHSM), and the Oklahoma Museum of NaturalHistory (OMNH) collections. Modern specimens were studied

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FIGURE 3. Holotype, right mandible, UMMP 31992, of Geomys flo-ralindae from the Sanders l.f., Meade County, Kansas. Scale bar equals 1mm.

and drawn at the University of Michigan, Museum of Zoology(UMMZ).

The taxonomy of phyletic series is complicated and has beendiscussed in previous accounts (Martin, 1993, 1996). In this study,no matter how much change is observed, a lineage is consid-ered a single species; it is not arbitrarily broken into constituentspecies. Previously, Martin (1993, 1996) developed the conceptof a chronomorph, a replacement for the lineage segment con-cept of Krishtalka and Stucky (1985). Both concepts designatedextinct samples as informal name-bearers for lineage segmentsor populations that displayed particular combinations of charac-ters. In certain cases, such as in the rapid dwarfing of Sigmodonminor at the close of the Pliocene (Pelaez-Campomanes andMartin, 2005), this approach has some value, at least as a heuristicdevice and biostratigraphic marker. However, as the fossil recordof a lineage becomes dense and is characterized more by a chron-ocline, as we see, for example, with Geomys minor, the utility ofdesignating multiple chronomorphs becomes questionable. Con-sequently, we have abandoned the geomyine chronomorphs pre-sented by Flynn et al. (2007).

SYSTEMATIC PALEONTOLOGY

Family GEOMYIDAE Bonaparte, 1845Genus GEOMYS Rafinesque, 1817

GEOMYS FLORALINDAE, sp. nov.(Figs. 3, 4B)

Geomys sp. Hibbard, 1941:206.Geomys (Parageomys) tobinensis Hibbard, 1956:183.

Holotype—UMMP 31992 part R mandible with p4–m3.Paratypes—UMMP 31989, edentulous part L mandible;

UMMP 31990, part L mandible with p4–m3; UMMMP 31991 partR mandible with p4–m3; UMMP 31992, part L mandible withp4–m2; UMMP 31995, box with bulk jaws, maxilla, skull pieces.

Locality and Horizon—Sanders l.f., Loc. 2, Meade County,Kansas, late Pliocene (late Blancan); ca. 2.57–3.04 Ma, probablywithin Chron C2an1n of the Gauss interval.

Referred Specimens—Borchers l.f., Meade County, Kansas,late Pliocene (late Blancan); ca. 2.06 Ma. UMMP 35771, box ofjaws.

Diagnosis—G. floralindae is a medium-sized species, approxi-mately the size of early Blancan Geomys minor and small extantG. bursarius, significantly larger than late Blancan G. minor andG. adamsi (Fig. 5, Table 1, Appendix 2). The basitemporal fossais approximately as deep as in G. minor but not as deep as in G.bursarius. The masseteric crest pattern is intermediate betweenearlier species that have a distinct heteromyid projection and a

FIGURE 4. (A) Partial left mandible, UMMP 56274, of Pliogeomysbuisi from the Buis Ranch l.f., Oklahoma. Note absence of dorsal masse-teric crest, presence of heteromyid projection, position of mental foramenbeneath ventral masseteric crest, and well-developed geomyine ridge. (B)Partial left mandible, UMMP 35771 (bulk mandibles), of Geomys flo-ralindae from the Borchers l.f., Meade County, Kansas. Note modestdevelopment of dorsal masseteric crest, remnant of heteromyid projec-tion, and more anterodorsal position of mental foramen. (C) Aspect ofleft mandible of extant Geomys bursarius (labeled G. lutescens), UMMZ108832, from Meade County, Kansas. Note well-developed dorsal mas-seteric crest, remnant of heteromyid projection engulfed by massetericcrests, and more anterodorsal position of mental foramen relative to po-sition of geomyine ridge. Scale bar equals 1 mm.

geomyine ridge (e.g., Pliogeomys buisi) and modern G. bursarius,in which the projection is absent and the dorsal masseteric crest islarge and rounded anteriorly (Figs. 3, 4). The mental foramen isalso intermediate in position; more anterior than in G. minor andless anterior and more closely associated with the ventral masse-teric crest than in G. bursarius.

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FIGURE 5. Box and whisker plots showing changes in measurements(described in text) over time for Geomys minor (Fox Canyon throughRexroad 2A) and G. floralindae (Sanders and Borchers). Abbreviations:

Etymology—floralindae; beautiful flower in Spanish. Named inhonor of the late Linda L. Martin, who graciously hosted a num-ber of Meade Basin field parties and was instrumental in discov-ering new fossil localities in the Basin.

Description and Comparisons

Mandible—The mandible is at least partly preserved in four in-dividuals. It can be duplicated in size by small modern G. bursar-ius from Meade County, Kansas, and by G. minor from the FoxCanyon l.f. because (1) the mandible in modern Geomys variesso much in size with age; and (2) there are so few mandibles fromMeade Basin localities for comparison, we decided to use onlydental dimensions for size evaluations among the samples (seebelow). However, the mandible of G. floralindae is intermediatebetween G. minor and G. bursarius in some qualitative features.The dorsal masseteric crest is better developed than in G. minorand the area of the heteromyid projection is therefore less dis-tinct (Figs. 3, 4). The mental foramen is more anterior than in G.minor, but not as far anterodorsal as in G. bursarius.

Dental Features—The teeth are ever-growing. Enamel in G.floralindae is formed as in Geomys, with no enamel plates onthe posterior border of P4 or the anterior borders of the lowermolars. The form of p4 is conservative within Geomys, com-posed of a rounded to somewhat oval anterolophid connectedto an anteroposteriorly compressed oblong posterolophid by anenamel and dentine isthmus. In Pliogeomys and the diminutiveG. adamsi, reentrant folds are V- rather than U-shaped (Hibbard,1956), as they are also in juvenile p4s of more advanced species.In G. floralindae, as in all Geomys, the reentrants are filledwith cementum. Cementum is sparse in Pliogeomys and usuallyabsent in Thomomys. Three of four (75%) reentrant folds areV-shaped in the sample from Sanders, but only 1 of 13 (7.7%)are V-shaped from Borchers. Given that the reentrants of Ge-omys are V-shaped in juveniles, plus the small sample size fromSanders, we currently view the high percentage of V-shaped reen-trants from the Sanders l.f. to be an artifact of sampling.

The p4 is larger in G. floralindae than in late Blancan G. minorbut about the same size as in the early Blancan G. minor from FoxCanyon (Appendix 2). Table 1 shows that Student’s t-tests com-paring G. floralindae with G. minor are significant for the threemeasurements representing tooth size.

Genus PLIOGEOMYS Hibbard, 1954PLIOGEOMYS LOUDERBACHI, sp. nov.

(Figs. 6, 7)

Pliogeomys sp. Hibbard, 1964:117.

Holotype—UMMP 41401, partial Lp4.Locality and Horizon—Dipoides locality (= University of

Kansas [KU] Rexroad Loc. 6), source of most of the Saw RockCanyon microfauna (Hibbard, 1954). Saw Rock Canyon. SE 1/4,NE 1/4 Sec. 36, T34S, R31W, Seward County, Kansas. EarlyBlancan.

Referred Specimens—UMMP 60474, left mandible withp4–m2. From unnamed quarry 1/8 mi. W of KU Loc. 6 on eastbank of Saw Rock Canyon, Seward County, Kansas. FHSM17456, Lp4, from Fallen Angel B; site lies directly beneath Bishop

← BOR, Borchers; DPB, Deer Park B; FC, Fox Canyon; HOR, Hornet;KAN, modern Kansas; R2A, Rexroad Loc. 2A; R3, Rexroad 3; RIP, Rip-ley B; SAN, Sanders; VAS, Vasquez/Newt; XIT, XIT 1B; WFP, WendellFox Pasture. Box represents first to third quartiles, vertical line in boxequals median, whisker represents minimum and maximum values, andsmall circles indicate outliers.

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TABLE 1. Student’s t-test comparing combined p4 data for Geomys minor and Geomys floralindae.

Difference 95% confidence limits

P (2-tailed) Mean Std. error Lower Upper

WA 0.001 −0.1107 0.03399 −0.17792 −0.04355CL 0.106 0.0138 0.00848 −0.00299 0.03066Wp4 0.000 −0.2186 0.03593 −0.29081 −0.14643Lp4 0.000 −0.1943 0.05174 −0.29656 −0.09205

Measurements are described in the text.

FIGURE 6. Mandible and dentition of Pli-ogeomys louderbachi, new species. (A) Rightmandible (labial view), UMMP 60474, fromunnamed site in Saw Rock Canyon (seetext for details). (B) UMMP 60474, lingualview. (C) Occlusal view of holotype Lp4,UMMP 41401 from the Dipoides locality (KURexroad Loc. 6), Saw Rock Canyon. Note U-shaped reentrant folds with cement. (D andE) Labial and lingual views of UMMP 41401.Cross-hatched area indicates debris (matrix).Stippled area indicates cement. Scale barsequal 1 mm.

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FIGURE 7. Lp4 of Pliogeomys louderbachi from Fallen Angel B,Meade County, Kansas, FHSM 17456. A, labial view; B, lingual view; C,occlusal view. D, Lp4 of Pliogeomys buisi from Buckshot Arroyo, BuisRanch, Beaver County, Oklahoma, OMNH 56757, occlusal view. Notethe V-shaped reentrant folds and the complete enamel band on the pos-terolophid; this tooth is from a much younger animal than the specimenfrom Fallen Angel B. E, Lp4 of Geomys minor from Rexroad Loc. 2A,Meade County, Kansas, FHSM 14835, occlusal view. Note the U-shapedreentrant folds filled with copious cement. Scale bar equals 1.0 mm.

gravel near top of small canyon about 0.25 km south of Saw RockCanyon, Seward County, Kansas. Early Blancan.

Diagnosis—An advanced species of Pliogeomys with hyp-sodont cheek teeth that tend to form roots late in development.The adult p4 is rooted and displays U-shaped reentrant foldsfilled with cementum. In P. buisi the p4 is relatively brachydontand reentrant folds on adult p4s are distinctly V-shaped. Differsfrom P. carranzai (Lindsay and Jacobs, 1985) in having a deepmedial as well as lateral groove on upper incisor. P. louderbachiis much larger than P. parvus (Zakrzewski, 1969) and P. russelli(Korth, 1995) and the latter have p4s lacking cementum in thereentrant folds.

Etymology—Named for Max Louderbach, deceased, who gen-erously allowed us access to the Saw Rock Canyon area on hisranch and who once kindly dragged our jeep out of the sand withhis pickup.

Description of Holotype and Referred Specimens—The holo-type is a adult partial left p4; the posterior half of the pos-

TABLE 2. Measurements of p4 in Pliogeomys louderbachi and P. buisi.

Specimen number WA Lp4 Wp4 CL

P. louderbachiUMMP 60474 1.69 2.46 2.20 0.28UMMP 41401 (holotype) 1.64 — — 0.06FHSM 17456 1.64 — — 0.07Mean 1.66 — — 0.14P. buisiOMNH 56757 1.66 2.42 2.25 0.21UMMP 30195A 1.49 2.19 2.07 0.23UMMP 30185B 1.67 2.58 2.17 0.12UMMP 56274 1.91 2.02 2.31 0.11Mean 1.68 2.30 2.20 0.17

Measurements are described in the text.

terolophid is missing (Fig. 6). Roots are well developed. In oc-clusal view, the p4 exhibits two dentine tracts that break theenamel surface; the lingual tract is wider than the labial tract. Thereentrant folds are U-shaped and partly filled with cementum,though not to the extent as in adult Geomys. With the exceptionof the sparse cementum, the occlusal morphology can be dupli-cated among a number of Geomys species. An adult Lp4, UMMP17456, from Fallen Angel B is shown in Figure 7. Cementum isslightly developed in the reentrant folds and may have been moredeveloped, as some was likely eroded away subsequent to deposi-tion. The occlusal pattern, although not displaying the U-shapedfolds as in the holotype, is not as distinctly V-shaped as is typicalfor P. buisi. This morphology is interpreted as part of the vari-ation in a large sample of P. louderbachi. The lingual aspect ofthe anterolophid also differs from the holotype in possessing twodentine tracts that would have fused into one large tract in latewear. Measurements of the holotype and referred specimens ofP. louderbachi are compared with those of P. buisi in Table 2.Both species are medium-sized and of similar dimensions.

Molars and incisors are represented in the UMMP and FHSMcollections from Saw Rock Canyon and Fallen Angel B but willnot be described in detail (see Akersten, 1973a). All molars arehypsodont and lack roots but most, probably adult, display con-strictions at their bases, indicating the possibility of late rootdevelopment. Enamel plates are present on both anterior andposterior faces of lower molars in juveniles, but the posteriorplate is lost with wear. The upper incisors of P. louderbachi, likethose of P. buisi, are bisulcate as in Geomys.

The referred mandible includes the p4–m2. The lingual sideof the mandible was broken and, after cleaning off matrix, thebase of p4 was visible. The p4 is very hypsodont and occlusalfeatures indicate the animal was likely a juvenile. Two smallroots are clearly observable at the base (Fig. 6). The m1 lacksroots, but has a distinct constriction at its base. The m2 is in themandible but the base is obscured by debris and glue. It does notappear to have roots. In occlusal view, the reentrants on p4 areV-shaped and, at the surface, no cementum is observable. How-ever, cementum is present <0.25 mm below the mastication sur-face and would obviously fill the reentrants with further wear(age). The reentrants widen beneath the occlusal surface andwould likely have displayed a U-shaped form. In P. buisi, the V-shaped form is retained to the tooth base. Dentine tracts are high(Fig. 6).

The mandible is composed of many fragments and the deter-mination of labial morphology is difficult. Nevertheless, in over-all size and form, it can be duplicated by mandibles from the typelocality of Pliogeomys buisi. There is a hole in the area wherethe dorsal masseteric crest would have converged with the ven-tral crest, but based on the angle of the geomyine ridge as it de-scends from the condyle, it is likely that a heteromyid projectionwas present (Fig. 6A). The mental foramen lies directly under

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the ventral masseteric crest (or heteromyid ridge, if present), asit does in P. buisi.

Relationships—The p4 and mandibular morphology of P.louderbachi indicate a species of Pliogeomys similar in many waysto P. buisi but also displaying features of the p4 found only inGeomys. The adult p4s of P. buisi from the type locality, BuisRanch, all are relatively brachydont, with V-shaped reentrantslacking cementum (Fig. 7). The p4s of P. louderbachi tend tobe more hypsodont, with V- to U-shaped reentrants with ce-mentum. This combination of features was also reported in Pli-ogeomys carranzai from the late Hemphillian Yepomera local-ity, Chihuahua, Mexico (Lindsay and Jacobs, 1985) but, as theynoted, the medial groove in the bisulcate upper incisor fragmentsof P. carranzai is much shallower than in any fossil and modernGeomys. Roots were not preserved on any of the P. carranzaicheek teeth, but a constriction at the base of the type juvenilep4 suggests that it would, like P. louderbachi, have formed rootslate in development. The upper incisors are bisulcate in an an-cestral geomyine Progeomys sulcatus (Dalquest, 1983) from theHemphillian Coffee Ranch l.f. of Texas, and both grooves areshallow. Pliogeomys buisi from Buis Ranch is likely older thanP. carranzai from Yepomera (Bell et al., 2004), lacks cemen-tum in the reentrants of p4 in adults, and has distinctly bisulcateupper incisors. The evidence suggests that P. carranzai is moreclosely related to extant Mexican geomyines that have lost themedial groove on the upper incisor, such as Orthogeomys. Thishypothesis further supports the concept, noted above regardingthe relationships of G. adamsi, that the Geomyinae as presentlyconstituted may be polyphyletic. This possibility was recognizedmany years ago by Akersten (1973a), and a thorough revision offossil and modern geomyids would be most helpful in the nearfuture.

As noted above, the one mandible of P. louderbachi we haveseen is damaged in the area of the convergence of the masse-teric scars, but seems to have had a heteromyid projection. If thisstructure is reduced or absent in other mandibles of P. louder-bachi from localities in and around Saw Rock Canyon beneaththe Bishop gravel (Fig. 1), then the evidence would be consid-erably more persuasive that this species was likely ancestral toGreat Plains Geomys.

PHYLOGENETIC RELATIONSHIPS OF MEADE BASINGEOMYS

Progeomys sulcatus from the Coffee Ranch of Texas(Dalquest, 1983) was chosen as the outgroup for a cladistic anal-ysis of the Meade Basin gophers (Fig. 8). Characters and theirstates are given in Appendix 1. Hibbard (1967) named the largerof two gopher species from Fox Canyon (Hibbard’s locality UM-K1–47) G. smithi, based on its smaller size relative to G. mi-nor from Rexroad Loc. 3, in conjunction with a less-developedbasitemporal fossa as compared with modern G. bursarius. Themental foramen is located beneath the masseteric crest bundle,which tends, in G. minor, to come to a more acute anterior an-gle than in the subgenus Geomys, in which it is more rounded(Figs. 3, 4). The geomyine ridge is also very well developed.With the many new samples we have collected in recent years,we can see a stepped chronocline (as opposed to monotonicgradual change) in size in G. minor. A significant size de-crease, as reflected by Lp4, occurs from Fox Canyon throughRipley (Fig. 5, Table 3), after which size appears to remainin stasis until a final dwarfing in G. minor from Rexroad2A, recorded in Table 3 as a statistically significant compari-son with the sample from Ripley B. Pieces of mandibles fromRexroad Loc. 3, XIT 1B, and Fox Canyon show no signif-icant qualitative differences; in all three samples the men-tal foramen lies below the ventral masseteric crest. The p4sfrom these localities are identical in morphology, with U-

FIGURE 8. Box and whisker plots showing changes in measurementsfor Geomys jacobi (Vasquez through Rexroad Loc. 3) and G. quinni(Deer Park B through Borchers). Abbreviations: BOR, Borchers; HOR,Hornet; VAS, Vasquez/Newt; WFP, Wendell Fox Pasture; DPB, DeerPark B; R2A, Rexroad Loc. 2A; R3, Rexroad Loc. 3; SDR, Sand Draw.

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TABLE 3. Tukey’s HSD significance matrix for G. minor samples.

R2A R3A WFP HOR VAS WNS RIP

RIP X — O — — O — — — — — — — — — — — — — — —XIT X � O X — O X — — — — — — — — X — — — — —FC X � O X — O X — — X — — X — — X — — X — —

Only statistically different samples are shown: X, Lp4; filled square, WA; circle, Wp4. Locality abbreviations: R2A, Rexroad Loc. 2A; R3A, RexroadLoc. 3A; WFP, Wendell Fox Pasture; HOR, Hornet; VAS, Vasquez; RIP, Ripley B; XIT, XIT 1B; FC, Fox Canyon. Measurements are described inthe text.

shaped reentrants in adults filled with cementum. Therefore,we synonymize Geomys smithi (Hibbard, 1967) under G. minor(Gidley, 1922). Similarity in size of the earliest G. minor samplesto those of G. floralindae is important and will be considered fur-ther below.

In his description of Geomys jacobi from Rexroad Loc. 3 andWendell Fox Pasture, Hibbard (1967) noted its resemblance to G.quinni from the late Blancan Sand Draw l.f. of Nebraska. Indeed,Hibbard (1967) also referred the large gopher from the DeerPark A l.f. to G. quinni, after he had originally referred it to G. to-binensis (Hibbard, 1956). Geomys quinni was distinguished fromother Geomys by McGrew (1949) on its large size and featuresof the rostrum. G. jacobi was considered distinct from G. quinniby Hibbard (1967) based on somewhat smaller size and a rela-tively shallow basitemporal fossa. Appendix 3 and Figure 8 showthat the Deer Park A and Rexroad 2A Geomys referable to G.quinni on morphological grounds (see below) are within the sizerange of G. jacobi. Mandibles from Rexroad Loc. 3 (G. jacobi)and Deer Park A (G. quinni) are similar in that the dorsal masse-teric crest is large and rounded. The basitemporal fossa is shallowin the Rexroad Loc. 3 specimens, whereas it is deep in the onewell-preserved mandible from Deer Park A (UMMP 33675). Infour mandibles of G. quinni we examined from the type locality,Sand Draw, the basitemporal fossa was relatively deep. In bothG. jacobi and G. quinni the dorsal masseteric bundle scars areso heavily developed that the geomyine ridge is mostly obscured.The mental foramen is as in G. floralindae, intermediate betweenG. minor and G. bursarius. It appears that G. jacobi (shallow ba-sitemporal fossa) was ancestral to G. quinni (deep basitemporalfossa), but we doubt that the basitemporal fossa deepened grad-ually. It seems more likely that it deepened in a relatively briefburst that also shifted the mental foramen slightly more forward.Certainly, this could happen in a single species lineage, and there-fore represent a case of punctuated gradualism (Malmgren andBerggren, 1984), or stairstep evolution, but as we currently haveno detailed evidence for this kind of shift in the Meade Basin,we will tentatively retain Hibbard’s (1964) Geomys jacobi for thepre-Deer Park A large gopher and assume that G. quinni re-placed G. jacobi in the basin at some point near the end of thePliocene. Extreme development of the dorsal masseteric scar inG. jacobi and G. quinni separates these species from both G. flo-ralindae and G. bursarius. It is for this reason we propose that

G. bursarius arose from a species more like G. floralindae thana species in the G. jacobi/G. quinni group, in which both dwarf-ing and a morphological reversal (reappearance of a distinct ge-omyine ridge) would be required.

Although the ANOVAs for WA, Wp4, and Lp4 were signif-icant for G. jacobi samples, there was no significant differencebetween the earliest (Vasquez/Newt) and latest (Rexroad Loc.3) samples, as shown in Table 4, a summary of the results fromthe Tukey’s HSD post hoc test. The most critical variable, Lp4,our proxy here for body size, was significant only for the Hor-net versus the Rexroad Loc. 3 sample. The box plots for G. ja-cobi in Figure 8 are suggestive of a slight size increase, but thestatistical results indicate stasis. Table 5 provides the results ofTukey’s HSD test following significant ANOVAs for G. quinnisamples. The Borchers Lp4 sample is not significantly differentfrom that of Sand Draw, whereas both are significantly largerthan the Rexroad 2A and Deer Park B samples. In WA and Wp4there are various combinations of overlap, with the Borchers andSand Draw samples significantly larger than samples from theother localities. With regard to body size, as indicated by Lp4, theanimals from Borchers and Sand Draw were likely most similar insize and generally larger than those from Rexroad 2A and DeerPark B. The potential of character displacement in the MeadeBasin gophers will be examined in a separate study.

Geomys adamsi from Fox Canyon is unique in the MeadeBasin, as a diminutive species in which the p4s have V-shapedreentrants filled with cementum. Unlike small G. minor, the mas-seteric scar bundle more closely resembles that in G. quinni,with the geomyine ridge sometimes obscured by a robust dorsalmasseteric scar, although the holotype mandible, UMMP 43880,shows a lightly developed geomyine ridge (Hibbard, 1967; Fig. 9).The mental foramen, concomitantly, appears slightly more ante-rior to the masseteric bundle than in G. minor. Table 6 showsthat G, adamsi is significantly smaller for all measurements thanG. minor except for CL, the width of the isthmus connecting theanterolophid and posterolophid on p4. G. adamsi has also beenreported from the early Blancan Pipe Creek Sinkhole l.f. of In-diana (Martin et al., 2003). Hibbard (1967) referred G. adamsito the subgenus Geomys because of the more anterior positionof the mental foramen and rounded form of the masseteric scars.However, based on the V-shaped reentrant folds, another sce-nario seems more likely and will be explored below in combina-

TABLE 4. Ninety-five percent confidence intervals for Tukey’s HSD test for Geomys jacobi p4 samples.

95% confidence intervals

WA Wp4 Lp4

WFP × R3A 0.089044 0.341937 0.085086 0.429359 −0.062249 0.521779WFP × HOR −0.257826 0.151159 −0.392303 0.182303 −0.830492 0.036491WFP × VAS −0.198122 0.281456 −0.223570 0.280704 −0.644574 0.490736R3A × HOR −0.460910 −0.076736 −0.634395 −0.090050 −1.033959 −0.219570R3A × VAS −.4031247 0.055477 −0.475976 0.006531 −0.854517 0.240987HOR × VAS −.1850131 0.375013 −0.2058404 0.460840 −0.314567 0.954567

Locality abbreviations: as in Table 3. Measurements are described in the text. Statistically significant differences between sample means are indicatedin bold text.

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TABLE 5. Ninety-five percent confidence intervals for Tukey’s HSD test for Geomys quinni p4 samples.

95% confidence intervals

WA Wp4 Lp4

BOR × SDR −0.496248 −0.183418 −0.506783 −0.058550 −0.639736 0.049736BOR × DPB −0.600370 −0.273630 −0.696081 −0.227918 −1.058150 −0.354578BOR × R2A −0.717808 −0.236859 −0.936690 −0.363393 −1.461320 −0.508680SDR × DPB −0.253582 0.059248 −0.403449 0.044783 −0.747443 −0.075284SDR × R2A −0.373304 0.098305 −0.645946 −0.887201 −1.154841 −0.225159DPB × R2A −0.280808 0.200141 −0.474692 0.098691 −0.748730 0.1914575

Abbreviations: BOR, Borchers; SDR, Sand Draw; DPB, Deer Park B; R2A, Rexroad Loc. 2A. Measurements are described in the text. Statisticallysignificant differences between sample means are indicated in bold text.

tion with the overall pattern of change in geomyine dental andmandibular morphology.

As is the case with early cricetids and arvicolids we have sam-pled from the Meade Basin, the earliest Geomys species demon-strate an almost random combination of primitive and advancedfeatures. Geomys adamsi evolved diminutive size, a hyper-robustdorsal masseteric crest, and a slightly elevated mental foramen,but retains V-shaped reentrant folds, albeit now filled with ce-mentum. Geomys minor (= G. smithi) retains the geomyineridge, has minimal development of the dorsal masseteric crestand a ventral mental foramen, but develops U-shaped reentrantswith cementum. The giant G. quinni developed U-shaped reen-trants on p4, but also followed the diminutive G. adamsi in evolv-ing an expanded dorsal masseteric crest. Geomys floralindae ismedium-sized, has a well-developed geomyine ridge, but the dor-sal masseteric scar is better developed and the mental foramenis more anterodorsal than in G. minor. G. floralindae stands inboth a temporal and morphological position as a potential an-cestor for modern Geomys if we assume a speciation event thatfurther separated the mental foramen from the masseteric scar

FIGURE 9. A, Partial right mandible, UMMP 28269, of Geomys adamsifrom the Fox Canyon l.f., Meade County. Note lack of geomyine ridgeand well-developed dorsal masseteric crest continuous with dorsal edgeof ascending ramus in this otherwise primitive species. B, Partial leftmandible, UMMP 28258, of Geomys minor from the Fox Canyon l.f. ofMeade County. Note well-developed geomyine ridge, absence of distinctdorsal masseteric crest (lightly developed in other specimens), and po-sition of mental foramen ventral to ventral masseteric crest. Scale barsequal 1 mm.

bundle and slightly increased mandibular and dental size and ro-bustness. However, we know from G. adamsi and G. quinni thatthese tendencies can be derived independently. Indeed, Korth(1995) described a small Hemphillian species of Pliogeomys, P.russelli, from the ZX Bar l.f. of Nebraska that may very wellhave been ancestral to G. adamsi. Like P. buisi, P. russelli re-tains rooted cheek teeth, and the p4 has low dentine tracts andlacks cementum in its V-shaped reentrants. However, P. russelliis very small, only slightly larger than the diminutive P. parvus(Zakrzewski, 1969) from the Hagerman beds of Idaho. Never-theless, the heteromyid projection on the mandible is absent, thegeomyine ridge is reduced, and the dorsal masseteric crest is ex-panded anteriorly and dorsally, as it is in G. adamsi. These ob-servations suggest that Geomys with ever-growing cheek teethmay have descended from more than one Pliogeomys ances-tor, and the genus may therefore be polyphyletic. A cladogramrepresenting this possibility, based on the character matrix inAppendix 1, is presented in Figure 10. In order for this to bethe most parsimonious cladogram, the character RS had to beweighted strongly for G. adamsi, recognizing the importance ofan early-appearing species of small size with ever-growing andV-shaped, rather than U-shaped, reentrants on p4 filled withcementum.

The subgenus Nerterogeomys was originally diagnosed by thepresence of the mandibular mental foramen under the ventralmasseteric crest (Gidley, 1922), and Akersten (1973a) suggestedthat this feature, plus a relatively cuspate dp4, was sufficient torecognize a clade of geomyids distinct from those species withthe mental foramen more forward on the mandible. This issue re-mains a point of debate among the authors. This ventral positionof the mental foramen is also found in Pliogeomys buisi and P.louderbachi, and therefore may be a plesiomorphic feature, if weassume that the P. buisi mandibular morphology was ancestral tothe extant Geomyinae. Alternatively, P. buisi, P. louderbachi, G.minor, and G. floralindae could represent a branch unrelated tothe Geomyinae and another ancestor remains to be discovered.Based on the characters noted in Appendix 1, the most parsimo-nious cladogram (Fig. 10) was not represented by one in whichspecies were separated by the ventral placement of the mentalforamen.

TABLE 6. Student’s-t test comparing p4 data for Fox Canyon G.adamsi and G. minor (combined). Measurements described in text.

95% confidencelimits

p (2-tailed)Mean

DifferenceDifferenceStd. Error Lower Upper

WA 0.000 0.3339 0.04083 0.24917 0.41854CL 0.122 −0.0316 0.01966 −0.07233 0.00919Wp4 0.000 0.3029 0.05798 0.18228 0.42344Lp4 0.000 0.4800 0.08240 0.30911 0.65089

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FIGURE 10. Cladogram of extinct and extant geomyines from the Central Great Plains. Outgroup is Progeomys sulcatus. Abbreviations: P., Plio-geomys; G., Geomys; Pr., Progeomys. Some important character changes are noted.

Geomys tobinensis was named by Hibbard (1944) for a singlejuvenile Geomys mandible with p4–m2 from the middle Pleis-tocene Tobin l.f. of central Kansas. The defining character in thediagnosis was a continuous enamel band around the anterolophidof p4. However, this feature is occasionally encountered in ju-venile Geomys of all species (Martin et al., 2003); in fact, Hi-bbard (1944:736) recognized that the holotype of G. tobinensiswas from a juvenile. Two additional adult p4s from the Tobinl.f. in the University of Kansas collections (KU 6718) lack theenamel band. However, Paulson (1961) emended the diagnosis,referring a number of specimens from the Cudahy l.f. to G. to-binensis. Paulson’s diagnosis included one mandibular and threeskull and maxillary characters. Although not included in the di-agnosis, Paulson (1961) made the additional observation that athin enamel band may be present on the posterior face of P4and the anterior face of the lower molars in approximately 5% ofthe Cudahy specimens. This band may extend to the base of thetooth and is not, therefore, a juvenile feature. He also pointed outthat some Geomys minor specimens from Fox Canyon displaythis condition. In further discussion, Paulson (1961:138) clearlydistinguished the Cudahy specimens from the Geomys materialfrom Sanders, which we have named here the new species G. flo-ralindae. Paulson noted that the mental foramen was more ven-

tral in position in the Sanders mandibles and the basitemporalfossa was shallower. From our observations, the dorsal massetericscar also seems better developed in G. tobinensis. We agree withPaulson (1961) that there are also some minor differences be-tween the Cudahy and modern Geomys mandibles, and the oc-casional full enamel plate on some Cudahy p4s and P4s is a char-acter we have not observed in adult modern Geomys. We cannotbe certain of the relationships or phylogenetic position of G. tobi-nensis, although on the cladogram in Figure 10 it falls out close toboth G. floralindae and G. bursarius. Despite an earlier conclu-sion to the contrary (Flynn et al., 2007), we will here restrict theappearance of modern Geomys bursarius to post-Cudahy Kansasl.fs. (post Lava Creek B ash at 0.64 Ma) and tentatively accept theintegrity of G. tobinensis. Geomys bursarius, then, becomes a lat-est Irvingtonian or early Rancholabrean immigrant to the centralGreat Plains.

The fossil history of Florida Geomys is also instructivein understanding geomyid character evolution. Wilkins (1984)proposed a phyletic series from the Blancan Geomys propinetisthrough the extant G. pinetis, the latter of which first appearsin its modern form in the early Irvingtonian Leisey l.f. (Morganand White, 1995). Wilkins (1984) documented changes in boththe dentition and the mandible, including loss of an enamel plate

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on the posterior surface of P4, increased depth of the basitem-poral fossa, and a shift from an elongated, bilophate M3 to onethat was subtriangular. The reentrants on p4 are V-shaped in G.propinetis and U-shaped in G. pinetis. However, Ruez (2001) pro-vided an alternative interpretation, arguing that the earlier formwas referable to Orthogeomys, and that G. propinetis was re-placed in Florida by G. pinetis during the Irvingtonian. In par-ticular, Ruez (2001) pointed out that the greater proportion ofupper incisors of G. propinetis from Inglis 1A (89%) possessedonly a single groove, as in Orthogeomys.

The oldest known geomyine that can be directly ancestral toGeomys is Progeomys sulcatus from the Coffee Ranch l.f. ofTexas (Dalquest, 1983). An ash dated by Izett (1975) to 6.6 Maon fission tracks lies directly above the quarry. The holotype p4is rooted and the linea sinuosa is a wavy line with no distinct den-tine tracts. Two upper incisor fragments attributed to P. sulcatuscontain two shallow grooves in the same position as in modernGeomys incisors. Upper incisors of P. buisi, likely from youngerHemphillian sediments in northern Oklahoma (Hibbard, 1954),exhibit two deep grooves as in Geomys. These observations cer-tainly suggest that the ancestral Geomys possessed bisulcate up-per incisors, and for now we follow Ruez (2001) and consider Ge-omys pinetis an immigrant that replaced Orthogeomys propinetisat the beginning of the Irvingtonian.

THE MEADE BASIN GEOMYID REPLACEMENTCHRONOLOGY AND ITS POSSIBLE CORRELATION

WITH CLIMATIC EVENTS

Geomys minor and G. adamsi are found together in the FoxCanyon l.f., approximately 4.3 Ma (Fig. 11). The Fox Canyon l.f.was recovered from a sandy lens at the top of the Bishop gravel(Honey et al., 2005). This locality has also produced copious re-mains of the ancient arvicolid Pliophenacomys finneyi, but lackscotton rats, which are often the most common rodents in Blancansites from the Meade Basin. Following Pelaez-Campomanes andMartin (2005), we interpret this combination to indicate a coldinterval, with especially cold winters that limit the northern dis-tribution of cotton rats. Martin (1986) and Pelaez-Campomanesand Martin (2005) have previously documented the climatic con-ditions during modern times that restrict the northward dis-persal of Sigmodon hispidus in this area. Geomys adamsi hasalso been recorded from the Indiana Pipe Creek Sinkhole l.f.(Martin Goodwin and Farlow, 2002), and its presence at FoxCanyon, considered as a northern disjunct, is consistent with theclimatic hypothesis.

From Fox Canyon time onwards, G. minor demonstratesa dwarfing trend (Fig. 5) until its final appearance in thePaloma l.f. about 2.8 Ma. Geomys jacobi enters the basin inthe Vasquez/Newt l.fs. between the CC1-CC2 geological markers(Fig. 1) about 3.6 Ma and remains until Deer Park time, about 2.9Ma, when it is replaced by G. quinni. Although there are no p4s,a single large gopher incisor from Wiens suggests that G. jacobimight have been present at that locality as well. Large gophershave not yet been recovered from Paloma or Sanders. G. floralin-dae first appears in the basin in the Sanders assemblage about 2.6Ma, during the period when deep-sea foraminiferan oxygen iso-tope ratios record the initiation of ice rafting in the north AtlanticOcean. It replaces both the G. minor and the larger G. jacobi andG. quinni lineages. G. quinni reappears at 2.06 Ma as a giant formin the Borchers l.f., sympatric with G. floralindae. At the sametime, Sigmodon minor undergoes a dwarfing bout, and is recog-nized in the Borchers l.f. as the chronomorph S. m./minor. Sub-sequent to Borchers (e.g., Short Haul, Aries A, Nash 72) fromabout 2.0–1.70 Ma, we see the shift to a small mammal fauna ofmore north temperate nature, including the first Microtus and thelemming Mictomys kansasensis (Martin et al., 2008). During thistime we also observe the brief return of a small gopher the sizeof G. minor in the Nash 72 l.f., followed by the appearance of

presumably G. tobinensis at Rick Forester, perhaps 1.4 Ma. G.tobinensis persists until sometime after the Lava Creek B ash at0.64 Ma, when it is replaced by modern G. bursarius of approxi-mately the same size (Fig. 11). Thomomys cf talpoides appears atvarious points during the middle and late Pleistocene, apparentlyalso during cold intervals, as cotton rats are absent from localitieswith Thomomys.

TEMPO AND MODE IN GOPHER DENTAL ANDMANDIBULAR EVOLUTION

What constitutes significant morphological evolution in pocketgopher morphology? In order to determine if morphologicalchange in the Meade Basin gophers corresponds to the hypoth-esis of punctuated equilibrium (Eldredge and Gould, 1972), wemust be able to answer this question. Pocket gopher mandiblesand dentitions appear much the same. They may be giant or smalland have, in extinct species, rooted or rootless (ever-growing)cheek teeth, cementum or no cementum in the reentrant foldsof p4, V- versus U-shaped reentrants on p4, a mental foramenplaced under a well-developed heteromyid projection, or no pro-jection at all. The basitemporal fossa may be shallow or deep.Some modern taxa retain enamel on the posterior wall of P4 (e.g.,Zygogeomys) whereas others do not (Geomys). The M3 in a fewextant species may be elongate and have a slightly more complexform (e.g., Orthogeomys and Pappogeomys). The upper incisormay be smooth or faintly unisulcate (Thomomys), unisulcate (Or-thogeomys), or bisulcate (Geomys). Akersten (1973b) noted con-siderable variation in extant geomyine incisor grooves, and bothgain and loss of grooves may have happened multiple times ingopher lineages. We see changes in many of these features in thefossil record, but how do we determine which, if any, of theseshifts represent significant breaks? As can be seen by examiningtheir diversity, modern geomyids are a very successful clade, withat least six modern genera and numerous species, ranging fromnorthern South America through much of the United States. Thatis, speciation events have occurred on numerous occasions. AllThomomys species retain V-shaped reentrant folds on p4 with nocementum. Is the appearance of U-shaped folds with cementuma major change? Is it equivalent to the absence of roots? Sizechanges in the G. minor lineage, but seems stable in other lin-eages, unless G. quinni is the endpoint in the G. jacobi lineage(in which case we would subsume G. jacobi under G. quinni). Asshown by Martin (1993), size changes dramatically affect the ecol-ogy of a given species in numerous ways, modifying metabolicrate, home range, litter size, and life span. Although some authorsargue that size changes should be ignored in determinations ofevolutionary significance, anything that so clearly affects the nat-ural history of an organism can hardly be dismissed so cavalierly.Because the characters noted above are all there is to distinguishthe dentitions and mandibles of geomyids, then they must be thestuff we need to consider when making determinations of evolu-tionary tempo and mode. So, how do these characters change?

Size changes in a directional but stepwise manner in G. mi-nor (Fig. 5). The shift to gigantism in G. quinni is accompaniedby dwarfing in the Sigmodon minor lineage in the Meade Basin(Martin, 1986; Pelaez-Campomanes and Martin, 2005). Perhapsboth are responding by natural selection to climatic or habi-tat changes at the close of the Pliocene. We do not know thetempo of change in the basitemporal fossa, and we do not knowfor certain if it changes gradually or at speciation events, al-though we have speculated that it deepened with the origin of G.quinni. We know it deepened in the origin of G. bursarius froman ancestral species such as G. floralindae, in which it is some-what deeper than in its presumed ancestor, G. minor. Reentrantfold shape on p4 seems to be stable in a given species, thoughthere is some variation in G. floralindae that we have tentativelyconcluded represents sampling bias rather than an evolutionarychange. Roots are present (Pliogeomys) or absent (all Geomys).

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FIGURE 11. Summary of geomyid replacement in the Meade Basin of southwestern Kansas. Abbreviations: Ma, million years ago; NLM, NorthAmerican Land Mammal Age; PAL, Magnetic polarity time scale. Other abbreviations as in Figure 1. Pair of diagonal lines represents a period ofabout 0.70 million years with no mammalian record.

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Likewise, form of the mandibular masseteric crests appears tobe stable within a species. Cementum is lightly developed in thereentrant folds of Pliogeomys buisi p4s, though it is usually ab-sent near the crown in teeth with light wear. Phyletic size changeis seen in Geomys, as it was in Ogmodontomys and Sigmodon(Pelaez-Campomanes and Martin, 2005; Marcolini and Martin,2008), but it may be that the most pronounced changes, such asloss of anatomical roots, extreme shift in position of the mentalforamen, gain of cementum on p4, loss of the heteromyid projec-tion associated with expansion of the masseteric musculature, anddeepening of the basitemporal fossa, occur more rapidly, duringspeciation events (here defined as branching events; we wouldinterpret punctuated gradualism as stepped phyletic evolution ina single lineage). The mandibular anatomy of Geomys bursar-ius (including its sibling species, such as G. lutescens) is differentfrom most Meade Basin fossil Geomys species and is easily rec-ognizable. In none of the extinct species do we see the combi-nation of an anterodorsal mental foramen coupled with envelop-ment of the heteromyid projection and an expanded and robustdorsal masseteric crest, though that morphology is approached inG. adamsi, G. jacobi, and G. quinni. This certainly suggests thatthe modern Plains pocket gopher mandibular morphology final-ized in a single speciation event in the middle or late Pleistocene,quite possibly from G. floralindae. These considerations suggestthat the majority of morphological changes in the dentition andmandible of Meade Basin Geomys took place rapidly, during spe-ciation events.

CONCLUSIONS

The dense record of geomyids in the Meade Basin of south-western Kansas contributes to our deepening knowledge of thehistory of small mammal community structure on the CentralGreat Plains and the possible forces determining its dynam-ics. Three genera and at least 10 species of geomyines haveexisted in the Meade Basin over the last 5 million years. Onespecies, Geomys minor, remained for almost 1.5 million years andanother, G. jacobi, was present for most of 1.0 million years.Rapid turnover of species subsequent to about 2.6 Ma is likelydue to climatic change at the close of the Pliocene. The immigra-tion of diminutive geomyids such as G. adamsi and Thomomys cf.talpoides probably indicates cold periods. Indeed, the disappear-ance of Pliogeomys louderbachi in the Meade Basin may be theresult of an environmental change leading to a geomyid turnoverduring Fox Canyon time. Wholly modern gopher mandibularmorphology does not appear until subsequent to the Lava CreekB ash (post-Cudahy, 0.64 Ma), perhaps resulting from a finalspeciation event from a potential ancestor, the newly describedGeomys floralindae. A phylogenetic analysis of Great Plains ge-omyines recognizes two groups, one including Pliogeomys russelliand Geomys adamsi, and another with Pliogeomys buisi, P. loud-erbachi, and various Geomys species.

Directional size microevolution is documented in G. minor andG. quinni. Numerous speciation events have occurred within theGeomyinae without significant character change, if by that weimply changes that alter body form in some important and rec-ognizably unique way. Nevertheless, certain morphologies, suchas presence or absence of anatomical roots, presence or absenceof cementum on p4, or mandibular masseteric crest morphology,appear to be stable within lineages, and may have changed onlyat speciation events.

ACKOWLEDGMENTS

We appreciate the loan of material by G. Gunnell and P. Gin-gerich at the Museum of Paleontology, University of Michigan,and by N. Czaplewski, University of Oklahoma. L. D. Martinkindly allowed study of specimens at the Museum of NaturalHistory, University of Kansas. We thank C. Mecklin, Depart-ment of Mathematics and Statistics at Murray State University,for help with statistical analysis. This research was funded by

grants from the National Geographic Society (5963-97, 6547-99)and the National Science Foundation (EAR 0207582) and theSpanish Ministerio de Ciencia e Innovacion (projects CGL2004-02094/BTE and CGL2008-04200/BTE). F. Marcolini was sup-ported by a Marie Curie Fellowship (MC-OIF-CT-2005-008522).

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Submitted May 5, 2010; accepted March 22, 2011.Handling editor: Anjali Goswami.

APPENDIX 1. Characters and data matrix for phylogeneticanalysis of Central Great Plains late Neogene geomyids.Progeomys sulcatus was the outgroup.

Characters and States

(1) PMS: Premolar size: (0) small; (1) medium; (2) large.Unordered.

(2) HP: Heteromyid projection: (0) well developed; (1) re-duced; (2) absent. Irreversible.

(3) CMC: Anterior conjunction of masseteric crests: (0) acute;(1) somewhat rounded; (2) rounded. Irreversible.

(4) PMF: Position of mental foramen: (0) under ventral masse-teric crest; (1) at least slightly anterior to ventral massetericcrest in >50% of sample; (2) usually higher than conjunc-tion of masseteric crests. Irreversible.

(5) BF: Basitemporal fossa: (0) shallow; (1) moderately deep;(2) very deep. Irreversible.

(6) DM: Dorsal masseteric scar: (0) absent or poorly developed;(1) moderately developed; (2) well developed; (3) hyper-developed; occasionally obscuring geomyine ridge.

(7) RS: Reentrant shape: (0) V-shaped; (1) U-shaped. Irre-versible.

(8) C: Cement in reentrant folds: (0) absent; (1) sparse; (2) co-pious. Irreversible.

(9) PE: Premolar/molar enamel: (0) still easily discernible onposterior half of tooth after juvenile condition; (1) occasion-ally absent on posterior half of tooth after light wear; (2) ab-sent on posterior half of tooth in >95% of specimens afterlight wear. Irreversible.

(10) RP4: Roots on p4: (0) present; (1) absent. Irreversible.(11) HP4: Height p4: (0) brachydont; (1) moderately developed;

(2) hypsodont. Irreversible.(12) LS: Linea sinuosa: (0) undeveloped; (1) moderately de-

veloped; dentine tracts breaking through occlusal sur-face after moderate wear; (2) well developed; dentinetracts breaking through occlusal surface in early wear.Irreversible.

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.Character Matrix

Species PMS HP CMC PMF BF DM RS C PE RP4 HP4 LSP. sulcatus 0 ? ? ? ? ? 0 0 0 0 0 0P. russelli 0 2 1 0 ? ? 0 0 0 0 1 1P. buisi 1 0 0 0 0 0 0 1 0 0 2 1P. louderbachi 1 0 0 ? ? 0 1 1 0 0 2 1G. minor 0 2 0 0 0 0 1 2 1 1 2 2G. jacobi 2 2 2 0 0 3 1 2 1 1 2 2G. quinni 2 2 2 0 1 3 1 2 2 1 2 2G. floralindae 1 2 1 1 1 1 1 2 2 1 2 2G. tobinensis 1 2 1 1 2 3 1 2 1 1 2 2G. bursarius 1 2 2 2 2 2 1 2 2 1 2 2G. adamsi 0 2 1 1 0 3 0 2 2 1 2 2

APPENDIX 2. Descriptive statistics for small and middle-size Geomys from the Meade Basin. Abbreviations: CUD, Cudahy; RF, Rick Forester;NAS, Nash 72; BOR, Borchers; SAN, Sanders; R2A, Rexroad Loc. 2A; DP, Deer Park B; BEN, Bender; R3A, Rexroad Loc. 3A; WFP, Wendell FoxPasture; HOR, Hornet; VAS, Vazquez; NLS, Newt lower sand; WNS, Wiens; RIP B, Ripley B; XIT, XIT 1B; FC, Fox Canyon; s, standard deviation;Min, smallest value; Max, largest value; N, number of specimens. Measurements are described in the text.

Lp4 Wp4 WA CL

Modern Mean 2.81 2.17 1.61 0.03G. bursarius s 0.07 0.05 0.04 0.01(Meade Co.) Min 2.46 1.85 1.40 0.00

Max 3.08 2.46 1.85 0.07N 11 11 11 11

CUD Mean 2.68 2.13 1.60 0.09G. tobinensis s 0.05 0.05 0.03 0.01

Min 2.21 1.72 1.32 0.04Max 2.92 2.46 1.85 0.24N 15 15 15 15

RF Mean 2.64 2.11 1.50 0.06G. tobinensis s 0.02 0.11 0.05 0.01

Min 2.62 2.03 1.46 0.06Max 2.65 2.18 1.53 0.07N 2 2 2 2

NAS Mean 2.11 1.63 1.37 0.08Geomys sp. s 0.11 0.20 0.12 0.03

Min 2.00 1.37 1.29 0.04Max 2.21 1.85 1.51 0.12N 4 4 3 4

BOR Mean 2.41 1.97 1.43 0.09G. floralindae s 0.24 0.16 0.16 0.03

Min 1.99 1.67 1.16 0.04Max 2.88 2.39 1.76 0.14N 20 20 20 20

SAN Mean 2.40 1.91 1.38 0.10G. floralindae s 0.17 0.20 0.14 0.04

Min 2.10 1.56 1.19 0.04Max 2.67 2.26 1.67 0.15N 10 10 10 10

R2A Mean 1.95 1.51 1.17 0.06G. minor s 0.16 0.10 0.08 0.06

Min 1.67 1.30 1.06 0.00Max 2.26 1.70 1.29 0.25N 15 17 16 22

DP Mean 1.88 1.46 1.17 0.09G. minor s — 0.04 0.01 0.05

Min — 1.41 1.16 0.06Max — 1.51 1.17 0.17N 1 4 2 4

R3A Mean 2.14 1.54 1.20 0.06G. minor s 0.20 0.14 0.12 0.02

Min 1.81 1.23 1.02 0.02Max 2.43 1.78 1.47 0.09N 14 14 14 14

WFP Mean 2.04 1.67 1.21 0.06G. minor s 0.08 0.06 0.05 0.02

Min 1.97 1.59 1.16 0.03Max 2.16 1.75 1.27 0.07N 5 5 4 5

HOR Mean 2.17 1.71 1.23 0.06G. minor s 0.08 0.14 0.17 0.02

Min 2.07 1.52 1.06 0.03Max 2.25 1.89 1.40 0.09N 4 5 3 5

(Continued on next page)

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APPENDIX 2. (Continued)

Lp4 Wp4 WA CL

VAS Mean 2.00 1.65 1.20 0.07G. minor s 0.21 0.14 0.03 0.03

Min 1.85 1.51 1.18 0.05Max 2.14 1.85 1.22 0.11N 2 4 2 4

NLS Mean 2.14 1.74 1.23 0.02G. minor s 0.05 0.16 0.08 0.02

Min 2.10 1.63 1.15 0.00Max 2.17 1.85 1.31 0.04N 2 2 3 3

WNS Mean 1.95 1.77 1.29 0.05G. minor s 0.09 0.10 0.15 0.02

Min 1.85 1.71 1.18 0.03Max 2.03 1.88 1.39 0.07N 3 3 2 3

RIP B Mean 2.29 1.79 1.29 0.12G. minor s 0.18 0.15 0.11 0.08

Min 1.99 1.52 1.01 0.05Max 2.66 2.18 1.55 0.55N 35 45 38 44

XIT Mean 2.36 1.85 1.47 0.15G. minor s 0.23 0.18 0.20 0.07

Min 1.97 1.57 1.12 0.02Max 2.92 2.23 1.97 0.35N 20 22 21 22

FC Mean 2.50 1.89 1.41 0.12G. minor s 0.17 0.17 0.10 0.03

Min 2.09 1.54 1.23 0.07Max 2.72 2.13 1.62 0.19N 13 13 13 13

FC Mean 2.06 1.61 1.10 0.16G. adamsi s 0.17 0.10 0.07 0.04

Min 1.85 1.48 1.01 0.10Max 2.31 1.75 1.21 0.24N 10 10 10 10

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APPENDIX 3. Descriptive statistics for large Geomys from the Meade Basin and Sand Draw, Nebraska. Abbreviations: BOR, Borchers; SD, SandDraw; R2A, Rexroad Loc. 2A; DP, Deer Park B; R3A, Rexroad Loc. 3A; WFP, Wendell Fox Pasture; HOR, Hornet; VAS, Vazquez; NLS, Newtlower sand; s, standard deviation; Min, smallest value; Max, largest value; N, number of specimens. Measurements are described in the text.

Lp4 Wp4 WA CL

BOR Mean 3.83 2.82 2.26 0.17G. quinni s 0.38 0.26 0.13 0.04

Min 3.19 2.46 2.05 0.11Max 4.38 3.30 2.50 0.25N 10 10 10 10

SD Mean 3.54 2.53 1.92 0.14G. quinni s 0.23 0.18 0.14 0.09

Min 3.26 2.29 1.72 0.07Max 3.94 2.85 2.18 0.41N 12 12 12 12

R2A Mean 2.85 2.17 1.79 0.10G. quinni s 0.30 0.20 0.21 0.05

Min 2.46 1.97 1.59 0.04Max 3.17 2.46 2.01 0.16N 4 5 3 7

DP Mean 3.12 2.35 1.83 0.11G. quinni s 0.28 0.12 0.11 0.06

Min 2.71 2.14 1.61 0.06Max 3.78 2.49 1.98 0.22N 11 10 10 9

R3A Mean 3.36 2.56 1.92 0.11G. jacobi s 0.31 0.18 0.11 0.04

Min 3.00 2.21 1.75 0.03Max 4.04 2.98 2.11 0.19N 17 18 17 18

WFP Mean 3.11 2.29 1.70 0.11G. jacobi s 0.14 0.08 0.09 0.02

Min 2.90 2.09 1.57 0.07Max 3.42 2.37 1.81 0.14N 10 10 9 10

HOR Mean 2.72 2.18 1.65 0.08G. jacobi s 0.23 0.19 0.17 0.01

Min 2.53 1.97 1.46 0.08Max 3.00 2.33 1.80 0.09N 4 3 3 3

VAS Mean 3.04 2.34 1.75 0.18G. jacobi s 0.49 0.18 0.15 0.08

Min 2.69 2.21 1.64 0.12Max 3.38 2.46 1.85 0.23N 2 2 2 2

NLS Mean – 2.28 – 0.075G. jacobi s – 0.25 – 0.05

Min – 2.1 – 0.04Max – 2.46 – 0.11N 0 2 0 2

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Five million years of pocket gopher history in the Meade ... Meade... · 868 JOURNAL OF VERTEBRATE...

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Transcript of Five million years of pocket gopher history in the Meade ... Meade... · 868 JOURNAL OF VERTEBRATE...