Mammalian paleofaunas of central Texas from the late Wisconsinan glacial period to the latest Holocene.
The Balcones fault zone that delimits central Texas to the east and south approximates an undulating line where the ranges of eastern and western groups of modern species of vertebrates and plants meet, overlap, interdigitate, and otherwise interact (Gehlbach, 1991; Fig. 1). The result is a region that harbors a greater diversity of species than occurs only a county or two east or west of this transition zone. The nature (in terms of species composition) and geographic location of this transition zone has varied significantly over time. Such changes are recorded in numerous fossil deposits, particularly on the Edwards Plateau, where animal and plant materials accumulate and fossilize in limestone caves, sinkholes, and fissures.
The purpose of this paper is to survey the primary published information on mammalian paleofaunas of central Texas, which represent approximately the past 30,000 years, with an additional objective of reconstructing past climates and habitats. The interval under study spans from the end of the terminal ice age (Wisconsinan) of the Pleistocene epoch through the ensuing Holocene. The Wisconsinan-Holocene boundary is generally dated at about 12,000 to 10,000 years before present (B.P.) according to Kurten and Anderson (1980:39). This paper is not the first (nor should it be the last) such treatment of the past mammalian faunas of central Texas. Ernest Lundelius has effectively addressed the climatic implications of late Pleistocene and Holocene faunas of the region in several papers (1967, 1974, for example). But because additional sites have been located and studied and more studies have been published since these earlier reports, continuing synthesis is appropriate.
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APPROACH TO THE PROBLEM
Twenty-six paleontological and archaeological sites within, and adjacent to, central Texas were selected for inclusion in this study (Fig. 1, Table 1). Virtually all information included in this paper was obtained from the scientific literature. The time frame covered by these sites ranges from middle late Wisconsinan (Moore Pit, 50,000 to 25,000 B.P.) to historic times (Walton site, about 220 to 160 B.P.). Generally, the proportion of the complete fauna represented as fossils in a deposit increases with decreasing age of the deposit; hence, older Wisconsinan deposits contain fewer species than do the latest Wisconsinan deposits. Contrary to this trend, however, the still younger Holocene deposits, most of which are archaeological deposits, tend to have a poor representation of nonhuman vertebrate remains. One explanation is that humans, unlike raptors, mammalian carnivores, or physical processes that often were the primary depositional agents at certain other sites, probably did not collect and bring into their dwellings the broad spectrum of local vertebrates. A further explanation is that nonhuman vertebrates, particularly the microfauna, frequently are viewed as insignificant (and, therefore, often ignored, discarded, or only casually noted) by persons excavating archaeological sites. Faunal lists for many such Holocene sites thus are undoubtedly incomplete.
Partial faunal listings for 23 selected sites are presented in Table 2. Species included are those indicated in the literature as being sensitive to (and, therefore, indicative of) certain environmental factors such as temperature or precipitation (Harris, 1985:9-13; Lundelius, 1967). Each taxon is assigned to one of four categories on the basis of its modern status: 1) species that are now extinct; 2) species present at sites located within the modern distribution of that species; 3) extralimital taxa--that is, extant species that do not now occur in the vicinity of the site; and 4) those extant species with modern distributions that are inadequately known and thus do not allow designation as extralimital. Absence of one of the presence symbols in Table 2 does not necessarily mean that the species was not a part of the fauna; rather, it is not (yet) known from that deposit, or material in a genus has not been identified to a particular species included in the table.
EVALUATION OF SELECTED FAUNAL COMPONENTS
This section involves evaluation of individual species or groups of species, their chronological distribution in central Texas, and environmental and other habitat features suggested by their presence.
Extant Extralimital Species
Evidence for environmental change is perhaps best demonstrated by extant species that no longer occur in the vicinity of fossil sites. A primary assumption operating here is that ecological requirements of such species are similar now to what they were during earlier depositional times.
Sorex cinereus, the masked shrew, is known from one central Texas deposit, Cave Without a Name, dating near the Wisconsinan-Holocene boundary. This species does not now occur in Texas. Its nearest occurrence is in the mountains of northern New Mexico, with the majority of its range occupying the middle and northern latitudes of the United States and northward into Canada (Lundelius, 1967:300, fig. 4). This shrew inhabits various habitats including grassy fields, marshes, and woodlands (Hamilton and Whitaker, 1979:32). The distributional pattern suggests that cool, moist conditions are requisite for this species.
The short-tailed shrew, Blarina brevicauda, ranges through most of the eastern and central United States (Hamilton and Whitaker, 1979:50). Its Texas distribution lies to the east of central Texas in a variety of habitats including wooded areas and the meadows and openings of these woodlands (Davis, 1974:38). A critical habitat feature for this species seems to be soft, damp soils that allow easy burrowing (Davis, 1974:38). It is known from eight central Texas faunas (Table 2) dated from about 20,000 B.P. to historic times. Its latest occurrence is at the Walton site (220 to 160 B.P.), the easternmost of the central Texas fossil sites from which it is known, and the site nearest its modern range. Recent taxonomic revision of the species (George et al., 1982), however, led to reassignment of modern Texas material to two other species, B. carolinensis in eastern Texas and B. hylophaga, known from Aransas County on the Gulf Coast Plain (Schmidly, 1983:49). The fossil material needs to be re-examined in light of these recent taxonomic changes.
Peromyscus nasutus, the rock mouse, currently occurs in Texas only in the higher elevations of the Franklin Mountains, El Paso County, from which its range extends into Mexico, New Mexico, and Colorado. Habitat features include Texas madrone-oak and pinyon pine-juniper associations in rocky situations (Davis, 1974:210-211; Schmidly, 1977:111). During the late Wisconsinan, this species occurred in the vicinity of Friesenhahn Cave, Bexar County (Lundelius, 1960:38). The climate was likely cooler and moister when the rock mouse occurred in central Texas.
Several species of microtine rodents (Microtus, Pitymys, Synaptomys, and Ondatra) are members of a variety of central Texas fossil and historical sites spanning nearly the entire 30,000-year range considered here. These species generally do not now occur in the counties from which fossil specimens are known. The pine vole (Pitymys pinetorum) is a species of the eastern United States with spotty occurrence in northeastern (Bowie, Marion, Nacogdoches, Wood counties--Davis 1974:224; Panola County--Parmalee, 1954) and central Texas (Gillespie, Kerr counties--Davis, 1974:224; Hill County--Cleveland et al., 1984:326). The fossil record of Pitymys in central Texas includes the mid-Wisconsinan Moore Pit site and the approximately 200-year-old Walton site, both in or beyond the northeastern part of central Texas. Synaptomys cooperi, the southern bog lemming, ranges through the northeastern United States into the Midwest and central Great Plains as far south as southern Kansas and northeastern Arkansas (Hall, 1981:832). The central Texas fossil records of Synaptomys are restricted to the Edwards Plateau (Cave Without a Name, Longhorn Cavern, and Miller's Cave). Prairie voles (Microtus ochrogaster) and meadow voles (M. pennsylvanicus) are known from seven central Texas deposits ranging in age from about 20,000 to 1000 B.P. The only modern records of M. ochrogaster in Texas are in Hardin County in southeastern Texas (Davis, 1974:223) and Hansford and Lipscomb counties in the extreme northern Panhandle (Manning and Jones, 1988; Choate and Killebrew, 1991); M. pennsylvanicus does not now occur in Texas (Hall, 1981:794). Meadow voles disappeared from central Texas earlier than did prairie voles. Ondatra zibethicus, the muskrat, does not now occur in central Texas despite fossil specimens from Moore Pit and Miller's Cave. Its modern distribution in Texas includes the extreme northern and eastern parts of the state, and along the Pecos and Rio Grande drainages in the west, where marshy habitats are available (Davis, 1974:227). All of the above-listed microtine species now inhabit regions with moister or cooler (or both) conditions than now prevail in central Texas.
The ermine, Mustela erminea, is a small weasel now occupying coniferous forest of Canada and northern regions of the United States; the modern localities nearest Texas are in the mountains of northern New Mexico (Hall, 1981:990; Hamilton and Whitaker, 1979:286). The ermine specimen from Cave Without a Name suggests cooler conditions than now in the southwestern part of central Texas at the end of the Wisconsinan.
The end of the Wisconsinan coincided with the extinction of many vertebrate species, particularly among larger species of mammals (megafauna), throughout their geographic ranges. Megafaunal species lost at the end of the Pleistocene include many known from central Texas fossil deposits. A few of these species, peccaries (Mylohyus, Platygonus) and proboscideans, are indicated in Table 2. Species not listed in Table 2 that met their demise about this time (Lundelius, 1967:290) include short-faced bears (Arctodus), lions (Panthera atrox), sabertooth cats (Smilodon, Dinobastis), camels (Camelops), llamas (Tanupolama), pronghorns (Capromeryx), musk ox (Ovibos), bison (Bison), tapirs (Tapirus), and horses (Equus). Causes implicated in this rash of extinctions coincident with the close of the Pleistocene include humans, as predators who advanced through North America about this time, and change of climate from cool, moist conditions to warmer, drier conditions (Martin and Klein, 1984).
Study of proboscidean and peccary distributions can be useful in reconstructing broad features of habitats in the vicinity of fossil deposits. For example, the peccaries Mylohyus and Platygonus had different habitat affinities as evidenced by dental morphology (Webb, 1974:13). Mylohyus, with low-crowned teeth having rounded cusps, was probably a forest-dwelling animal that fed on relatively soft vegetation. Platygonus possessed hypsodont teeth with sharper crests, a likely adaptation for grazing the abrasive grasses that characterized savannas and grasslands. Friesenhahn Cave is the only central Texas site yet to yield Mylohyus material, thereby suggesting that woodlands occurred extensively in the Friesenhahn vicinity (Bexar County) during the late Wisconsinan. Presence of Platygonus in the Moore Pit, Laubach Cave, Cave Without a Name, and Levi deposits indicates the existence of grasslands or savannas in Dallas, Williamson, Kendall, and Travis counties at various times from the mid-Wisconsinan until the mid-Holocene.
As in the case of the peccaries, various genera of proboscideans also demonstrate different ecological affinities and preferences (Lundelius, 1967:297); both the dentition and habitat preferences of mastodons (Mammut sp.) approximated those of the forest-dwelling Mylohyus. Mastodon cheekteeth have rows of elevated, rounded cusps, whereas mammoth (Mammuthus sp.) cheekteeth have relatively flat occlusal surfaces with numerous sharp, transversely oriented enamel ridges--an adaptation for grazing on abrasive vegetation. Mastodons were the predominant proboscidean of the Texas coastal plains, although mammoths, too, ranged widely over coastal areas; conversely, mammoths were more common than mastodons in central Texas (Fox et al., 1992; Slaughter et al., 1962). The two genera co-occurred at two (Moore Pit, Friesenhahn Cave) of the six central Texas sites bearing proboscidean material. The evidence from peccaries and proboscideans suggests that both woodlands and grasslands-savannas existed simultaneously through much of central Texas. These two broad habitat types occurred perhaps as an interdigitating mosaic with woodlands most likely occupying stream corridors, canyons and similar situations, and with the savannas and prairies being more common on the drier uplands.
Extant Resident Species
Also presented in Table 2 are several species found in central Texas paleontological and archaeological sites that currently reside in or near the vicinty of those deposits. Study of these species allows evaluation of somewhat subtler changes in geographic distribution than can be shown by the extinct or entirely extralimital taxa.
Cynomys ludovicianus, the black-tailed prairie dog, still ranges through much of the western part of central Texas (Davis, 1974:155). Cynomys was a member of two Edwards Plateau paleofaunas (Friesenhahn Cave, Cave Without a Name) located within the modern range of the species. Yet, the late Wisconsinan material from Laubach Cave (Williamson County) demonstrates that this range has retracted westward. Even historically, however, the distribution of Cynomys in central Texas was scattered.
Pocket gophers (genus Geomys) have been identified from at least nine central Texas deposits located within the current range of the species. The presence of Geomys bursarius in three deposits (Rattlesnake Cave, Schulze Cave, Felton Cave) in the southwestern part of the region indicates that the range of this species extended farther to the southwest during early Holocene times than now. Semken (1961:300, 305) speculated that the range reduction in the Holocene corresponded to a loss of the deeper soils required by Geomys, due primarily to increased soil erosion related to drier climate and overgrazing by domesticated livestock. Fossils of Botta's pocket gopher (Thomomys bottae) are known from at least four sites in central Texas (Klein Cave, Cueva Quebrada, Schulze Cave, Fowlkes Cave) that are within or near the modern range of the species. The record of T. bottae from Klein Cave (=Hall's Cave--R. S. Toomey, personal communication) is at about the eastern edge of the modern range. Dalquest and Kilpatrick (1973:6) posited that T. bottae has advanced eastward in modern times to reoccupy its former more extensive range. Though this scenario may well be so, further study of fossil material and additional field work to accurately ascertain the modern distribution of T. bottae is necessary. In only the past decade, several central Texas counties have been added to the current distribution (Hollander et al., 1987; Wilkins, 1990, 1991) of this gopher. These recent reports of range extension probably do not reflect recent expansion of range, but rather discovery of some of the surely many isolated (and presumably relictual) pockets of these fossorial rodents in central Texas.
The northern pygmy mouse (Baiomys taylori) has expanded its range considerably in Texas over the past century (Pitts and Smolen, 1989; Choate et al., 1990). About 1905, Baiomys was known in Texas only in parts of the coastal plain of the Gulf of Mexico and the brush country of the eastern area of southern Texas (Bailey, 1905:102). By 1957, it ranged into Dallas County in north-central Texas (Hunsaker et al., 1959:447). Records from Cottle County document invasion of the Texas Panhandle by 1972 (Diersing and Diersing, 1979:707). Seemingly, this advance from southern Texas to the east, north, and west still continues, largely along grassland corridors opened by agriculture and along highway and other rights-of-way (Schmidly, 1983:187-189). Of the four central Texas sites yielding this species (Table 2), only the Barton Springs Road site (Travis County) lies within the current distribution of the species (Davis 1974:198). Baiomys occurred farther west in Sutton (Felton Cave), Val Verde (Cueva Quebrada), and Kinney (Rattlesnake Cave) counties during the interval 14,300 to 7800 B.P. (Table 2) when the climate was probably cooler and wetter than at present. Collection of several pygmy mice near El Dorado, Schleicher County, in 1986 (Hollander et al., 1987:7) brings the current distribution close to the Felton Cave fossil locality. Similarly, in 1983 and 1984, Baiomys taylori has been taken near Uvalde and Utopia in Uvalde County, bringing the current range nearer the Rattlesnake Cave locality in Kinney County (Pitts and Smolen, 1989). Further study undoubtedly will reveal a highly dynamic biogeographic history for B. taylori. Already, two phases are evident: an eastward or southeastward withdrawal from the western Edwards Plateau, followed by advance to the north, west, and northwest.
The nine-banded armadillo (Dasypus novemcinctus) is another recent invader from the south. The species, which now ranges over most of eastern and central Texas and northward into the Great Plains of Oklahoma, Kansas, and Nebraska (Hall, 1981:284), is reported from only one Pleistocene deposit in the United States--the Slaton fauna from the Texas Panhandle (Dalquest, 1967:4). However, the report of D. novemcinctus from Slaton probably is erroneous; the specimen is more likely of a young individual of the larger extinct species, Dasypus bellus (Kurten and Anderson, 1980:131). Patton (1963:22) suggested the presence of D. novemcinctus in Texas by about 3000 B.P. in the Miller's Cave deposit. However, Lundelius (1967:313) speculated that the Miller Cave D. novemcinctus material is a modern intrusive. Hence, the earliest undisputed records of D. novemcinctus in Texas are from the mid-19th century (Audubon and Bachman, 1854). Humphrey (1974) detailed the northward advance of the nine-banded armadillo over the past century in a report based on questionnaires sent to wildlife biologists working in the modern range of the species. He determined that these armadillos occupy regions having 380 or more millimeters of precipation annually and having no more than nine days per year where the maximum temperature does not reach or exceed 0[degrees]C (32[degrees]F).
During the late Wisconsinan-Holocene interval, the short-tailed grasshopper mouse (Onychomys leucogaster) ranged through much of central Texas. Only the southwestern part of the region now falls within the overall range of this species (Davis, 1974:192). Central Texas deposits that include O. leucogaster, but which are east of this modern distribution of the species, are Longhorn Cavern (late Wisconsinan, Burnet County) and Kyle site (1389 to 674 B.P., Hill County). Apparently the range has retreated westward substantially within only the last millennium.
The distribution of lagomorph material (jackrabbits, Lepus; cottontails, Sylvilagus) among central Texas deposits is seemingly so ubiquitous in time and space as to be uninformative of changing environmental conditions. This interpretation has prevailed largely due to the difficulty of distinguishing between the several species of Sylvilagus that occur in Texas. The value of being able to discriminate among cottontail species is that each indicates somewhat different environmental conditions. The swamp rabbit, S. aquaticus, is adapted to semiaquatic habitats that are now found in eastern Texas; the Texas range of the swamp rabbit extends westward to the Balcones Escarpment (Davis, 1974:243), which approximately defines the eastern limits of central Texas as here defined. Audubon's cottontail, S. audubonii, has a distribution (western Texas eastward to eastern portions of central Texas) that is essentially complementary to that of the swamp rabbit. S. audubonii occupies semiarid to arid habitats including grasslands, creosote brushlands, and cactus deserts (Davis, 1974:241). The eastern cottontail (S. floridanus) occupies virtually the entire state of Texas, save extreme western portions; it occurs in a wider variety of habitats than either of the above cottontail species (Davis, 1974:239-240).
Hulbert (1984) utilized multivariate statistical techinques (discriminant function analysis) to develop means of identifying the Sylvilagus species with a high degree of confidence. He applied this identification scheme to lagomorph material from 18 paleontological and archaeological deposits in central Texas. Hulbert's identifications have been incorporated into Table 2. S. aquaticus presently occurs in the vicinity of only one of the sites considered here (Friesenhahn Cave in Bexar County), near the eastern edge of the Balcones Escarpment. Its only other past records in central Texas are Kincaid Shelter (Uvalde County, late Rancholabrean) and Centipede Cave (Val Verde County, 5000 B.P.), both far west of the modern range. For such a distribution to have existed, moister conditions must have prevailed from at least about 12,000 to 5000 B.P. All the records (Table 2) of both Sylvilagus audubonii and S. floridanus fall within (or extremely close to) the modern ranges of these species. Records of jackrabbits (Lepus) also are widespread in time and space; all paleontological and archaeological records lie within the current range of Lepus californicus.
FAUNAL CHANGES AS INDICATORS OF CLIMATIC CHANGE
From the preceding analysis, it is apparent that past and present mammalian faunas in central Texas differ in species composition. Some species were lost to extinction, whereas others retreated northward, eastward, or westward; yet others made their initial entries into the region. These faunal changes seem largely to be responses to changes in distributions of major floral environments in concert with shifting climate. Some 15,000 B.P., a cooler moister climate suitable for eastern coniferous forest prevailed in east-central Texas (Bryant and Holloway, 1985), and likely this Arcto-Tertiary forest dominated central Texas as well (Gehlbach, 1991). With climatic warming and drying, this eastern flora waned during the ensuing 5000 years, being replaced west of the Balcones Escarpement by evergreen woodland that presently characterizes the Edwards Plateau. Also about 10,000 B.P., the tallgrass prairie, that now separates eastern deciduous forest from western evergreen woodland, emerged. Coincident with these floral changeswere adjustments in distributions of vertebrates associated with these floras. Further consideration of the distributional histories of various mammalian species generally corroborates the shifts in floras and climate as summarized by Gehlbach (1991).
Hulbert (1984:203-206) reconstructed the climatic history of the last 9000 years for the southwestern part of central Texas on the basis of lagomorphs from archaeological sites in Val Verde County. He relied more on the proportions of the rabbit community comprised of particular species than on mere presence-absence data such as appears in Table 2. Hulbert's climatic inferences for the southwestern area of central Texas follow: 9000 to 5000 B.P., more mesic than today (annual rainfall greater than 46 centimeters); 5000 to 3000 B.P., more xeric than today (annual rainfall less than 31 centimeters); 3000 to 400 B.P., similar to modern conditions (annual rainfall 31 to 41 centimeters). Xeric conditions were indicated by an assemblage predominated by Sylvilagus audubonii, whereas a greater presence of S. floridanus or S. aquaticus, or both (even if S. audubonii still made up the majority) suggested moister conditions. Although Hulbert (1984:197) did not directly address temperature regime, he noted that effective moisture results from an interaction of precipitation and temperature. He thereby intimated that warmer temperatures correspond to lower precipitation and that lower temperatures correspond to greater precipitation. Support for Hulbert's scenario derives from the presence of Baiomys taylori at Cueva Quebrada (Val Verde County), suggesting more mesic conditions during the depositional interval of about 14,300 to 12,280 B.P. (Lundelius, 1984:466).
Latest known dates at which nine species of now-extralimital small mammals were present in central Texas are given in Table 3. These latest appearances extend over a period of at least 10,000 years, indicating that loss of species from central Texas was not a single synchronous event, but that climatic change was gradual. Further, these latest dates of occurrence offer an indirect way of evaluating Hulbert's climatic reconstruction for the southwestern part of central Texas. As moist conditions prevailed until about 5000 B.P., then mesic-adapted species from Table 3 should have remained in central Texas, especially the northeastern portions thereof, until at least that time. And, as the moisture gradient in Texas increased from west to east, dates of loss of mesic taxa would be expected to be later for sites lying farther east. Perusal of Table 3 reveals that the boreal species Sorex cinereus, Mustela erminea, and Synaptomys cooperi vanished about 10,000 B.P. Others lost during this interval include Peromyscus nasutus, now a species of higher elevations, and Ondatra zibethicus, a species with a distribution that seems more limited by the need for adequate moisture than by temperature. Hangers-on, perhaps lingering in small refugia of suitable habitat in central Texas until drier, warmer conditions much like those of the present day, included Onychomys leucogaster, Pitymys pinetorum, and Blarina brevicauda. Modern central Texas is probably too arid or warm for Pitymys and Blarina, but may well be objectionable to Onychomys because conditions are too moist or cool.
The difficulty of identifying fragmentary specimens to species level required that all Microtus material be combined (Table 3). Yet, it may be hazardous to consider all Microtus species together because no single set of environmental conditions suits all species possibly found in central Texas deposits. The northern-adapted Microtus pennsylvanicus, known in central Texas only from Cave Without a Name, likely left the region about 10,000 B.P. with other boreal mammals. At least one population of M. ochrogaster survived in southeastern Texas (Hardin County) into the present century, and M. mexicanus still occurs in montane western Texas (Guadalupe Mountains). The latter two species might be expected in central Texas fossil sites of younger age than those containing remains of M. pennsylvanicus.
Inclusion of three additional Texas sites (Sims Bayou, Ben Franklin, Fowlkes Cave) permits brief development of the broader climatic perspective in regions bounding central Texas. The Sims Bayou fauna (Harris County, 50,000 to 25,000 B.P.) was deposited before the time of maximum extent of the Wisconsinan glacier (Slaughter and McClure, 1965:404). This time interval represents an interstadial when climate was warmer than during the ensuing Wisconsinan glaciation, perhaps quite similar to present day climate. Virtually all mammalian species present in the fauna reflect this warmer climate. A savanna-grassland setting is suggested by the presence of grazers (Equus, Bison, Breameryx, Mammuthus) and of various rodents that inhabit modern prairie settings--Cynomys ludovicianus, Spermophilus sp. (ground squirrels), Sigmodon hispidus (hispid cotton rat), Oryzomys palustris (marsh rice rat), Microtus cf. ochrogaster ludovicianus. No markedly boreal species occurred in this fauna; indeed, the climatic affinity is more neotropical as demonstrated by Sigmodon hispidus and extinct xenarthrans, Holmesina septentrionalis (extinct armadillo) and Boreostracon (glyptodon).
The two other peripherally located faunas considered here are from late Wisconsinan deposits in northeastern Texas (Ben Franklin, Delta County--Slaughter and Hoover, 1963) and in Trans-Pecos Texas (Fowlkes Cave, Culberson County--Dalquest and Stangl, 1984). As expected from the influence of the waning glacial climate, a number of boreal species occur in these sites: Ben Franklin includes Sorex cinereus, Microtus pennsylvanicus, and Synaptomys cooperi. The numerous boreal species at Fowlkes Cave include Sorex vagrans (vagrant shrew), Sorex palustris (water shrew), Eutamias cinereicollis (gray-collared chipmunk), and Marmota flaviventris (yellow-bellied marmot). Yet, southern or desertadapted species also occurred at these sites; Sigmodon hispidus and Dasypus bellus were components of the Ben Franklin fauna. The warmdry adated forms at Fowlikes Cave included Notiosorex crawfordi (desert shrew), Myotis velifer (cave myotis, a bat), Sylvilagus audubonii, Lepus californicus, Spermophilus spilosoma (spotted ground squirrel), Cynomys ludovicianus, Cratogeomys castanops (yellow-faced pocket gopher), Peromyscus eremicus (cactus mouse), Onychomys leucogaster, O. torridus (southern grasshopper mouse), and Neotoma albigula (white-throated woodrat).
That few of these northern and southern taxa coexist today suggests that climatic conditions prevalent in late Wisconsinan times do not now exist anywhere within the ranges of these species. The overlaps that existed in environmental tolerances of the northern and southern species facilitated geographic overlap in the past. Slaughter and Hoover (1963:144) addressed this enigma: "The paradox is resolved if one remembers that it is the blistering summers, not the lack of bitter winters, that restrict northern types, and that it is the presence of prolonged extreme winter temperatures, not the lack of hot summers, that limit the range of southern species."
Such equable climates, wherein both summer and winter extremes are moderated, likely prevailed in central Texas during the late Wisconsinan to early Holocene interval as suggested by presence of both northern and southern species in many "disharmonious" (Semken, 1974) or "intermingled" (Graham, 1975) central Texas faunas (Table 2). These disharmonious faunas contained more species than modern faunas. Lundelius (1989) attributed greater species richness to existence of patchier environments supporting a larger number of niches than at present.
Few of the nonmammalian vertebrate faunas from central Texas sites have been studied in detail. However, those that have offer further support for climatic interpretations derived from evaluation of mammalian paleofaunas. The species composition of the Rancholabrean Schulze Cave local fauna allows a somewhat detailed reconstruction of climate and habitat prevailing in Edwards County at the end of the Wisconsinan glaciation, some 15,000 to 12,000 B.P. (Parmley, 1986). The herpetofauna suggests an ecosystem of grassland or woodland, possibly savanna. Strictly aquatic forms, such as Rana and Acris (genera of frogs), Chrysemys (pond slider turtles), and Nerodia (water snakes), were not present in the deposit, indicating thereby that aquatic habitats in the vicinity were seasonal or temporary. Species present in the fauna that require at least temporarily moist conditions included Ambystoma (salamanders) and Thamnophis (garter and ribbon snakes). The only extant member of this herpetofauna that does not now occur in Edwards County is the rat snake (Elaphe obsoleta), the modern range of which includes central and eastern parts of central Texas (Dixon, 1987:244). Interestingly, E. obsoleta inhabits woodlands at the western edge of this modern range; Parmley (1986:8) speculated that earlier access of this rat snake into central Texas was via woodlands that formerly characterized parts of the region. The existence of a more equable climate (cool summers and mild winters) is indicated not only by the mammalian fauna (Dalquest et al., 1969:273), but also by the two species of turtles present. Presence of the giant land tortoise (Geochelone) suggests winters without freezing temperatures although this tortoise probably could have survived a few nights with light frost if ensuing daytime temperatures reached at least 60[degrees]F (=16[degrees]C--Parmley, 1986:9). The box turtle (Terrepene ornata) in this deposit possessed a somewhat thicker shell than modern counterparts, perhaps as an adaptation to cooler summer temperatures (Parmley, 1986:9).
One of the fascinating aspects of Wisconsinan and Holocene faunas of central Texas pertains to their species composition. Living together in some of the faunas were species that do not now occur together anywhere. The presence of both northern and southern species in an area argues for climates more moderate than at present: in these equable climates, summers were not as hot and winters were not as cold as at present. During the late Wisconsinan and into the early Holocene, equable climates apparently prevailed widely over Texas from the Trans-Pecos through central Texas and into northeastern part of the state as attested to by many disharmonious faunas.
Hulbert (1984) presented a somewhat more refined accounting of climatic change for the southwestern part of central Texas since 9000 B.P. From then until 5000 B.P., climate was moister and cooler than at present. Drier, warmer conditions than at present prevailed for the next 2000 years. Conditions similar to those of today obtained by around 3000 B.P. Geographic distribution of lagomorphs, and presumably of many other environmentally sensitive vertebrate species, fluctuated there and presumably over all of central Texas.
TABLE 1. Selected paleontological and archaeological sites in and around central Texas, listed in approximately chronological order. Approximate ages, expressed as years before present (B.P.), taken from the literature, primarily from Lundelius (1967, 1984), Kurten and Anderson (1980), Lundelius et al. (1983), Dalquest and Stangl (1984), Redder (1985), Story (1985), and Winkler (1990). Map number Site County Age (B.P.) 1 Moore Pit Dallas 50,000-25,000 2 Waco Mammoth Site McLennan 28,000 3 Sims Bayou Harris [greater than or equal to] 23,000 4 Laubach Cave Williamson 23,200-13,900 5 Friesenhahn Cave Bexar 20,000-9,200 6 Cueva Quebrada Val Verde 14,300-12,280 7 Ben Franklin Delta 11,135-9,550 8 Cave Without A Name Kendall 10,900 9 Bonfire Shelter Val Verde 10,230-9,920 10 Clamp Cave San Saba late Rancholabrean (1) 11 Kincaid Shelter Uvalde late Rancholabrean (1) 12 Fowlkes Cave Culberson latest Pleistocene 13 Longhorn Cavern Burnet late Wisconsinan 14 Miller's Cave Llano late Wisconsinan 15 Rattlesnake Cave Kinney post-late Wisconsinan 16 Klein (=Hall's) Cave Kerr 13,050-modern 17 Horn Shelter McLennan 10,310-8,400 18 Levi Shelter Travis 10,000-6,750 19 Schulze Cave Edwards 9,680-9,310 20 Wilson-Leonard Site Williamson 9,000-<2,000 21 Felton Cave Sutton 7,800 22 Wunderlich Site Comal 5,405-4,170 23 Centipede Cave Val Verde 5,000 24 Barton Springs Road Travis 3,450-1,015 25 Kyle Site Hill 1,389-674 26 Walton Site McLennan 220-160 (1) Another way of subdividing geological time is on the basis of chronological distribution of species. Vertebrate paleontologists recognize numerous biostratigraphic time units that are termed North American Land Mammal Ages (NALMA). The Rancholabrean NALMA includes the latter part of the Pleistocene epoch, which encompasses the entire Wisconsinan glacial period. Hence, "late Rancholabrean" is essentially equivalent to late Wisconsinan. TABLE 2. Occurrence of selected, environmentally sensitive, mammalian species at 22 paleontological and archaeological sites in central Texas. Presence of a species in a deposit is denoted by symbols: open circle (0) indicates extinct; plus (+) indicates site is within current distribution of species; x indicates extralimital (species extant but not now present in vicinity of site); ? indicates extant species with a modern distribution not well enough understood to assign it to other categories. Sites are listed chronologically. See Table 1 for approximate ages. Sites Moore Waco Mammoth Laubach Friesenhahn Species Pit Site Cave Cave Sorex cinereus Masked shrew Blarina brevicauda x Short-tailed shrew Cynomys ludovicianus x + Black-tailed prairie dog Geomys bursarius + + + Plains pocket gopher Thomomys bottae Botta's pocket gopher Peromyscus nasutus x Rock mouse Baiomys taylori Northern pygmy mouse Onychomys leucogaster Short-tailed grasshopper mouse Microtus ochrogaster Prairie vole Microtus pennsylvanicus Meadow vole Microtus sp. x Voles Pitymys pinetorum x Pine vole Ondatra zibethicus x Muskrat Synaptomys cooperi Southern bog lemming Mustela erminea Ermine Mammut americanum 0 0 American mastodon Mammuthus sp. 0 0 0 0 Mammoths Lepus californicus + + Black-tailed jackrabbit Lepus sp. Jackrabbits Sylvilagus floridanus + Eastern cottontail Sylvilagus audubonii + Audubon's cottontail Sylvilagus aquaticus + Swamp rabbit Sylvilagus sp. + Cottontails Mylohyus nasutus 0 Long-nosed peccary Platygonus sp. 0 0 Flat-headed peccary Sites Cave Cueva Without A Bonfire Clamp Kincaid Species Quebrada Name Shelter Cave Shelter Sorex cinereus x Masked shrew Blarina brevicauda x Short-tailed shrew Cynomys ludovicianus + Black-tailed prairie dog Geomys bursarius Plains pocket gopher Thomomys bottae Botta's pocket gopher Peromyscus nasutus Rock mouse Baiomys taylori x Northern pygmy mouse Onychomys leucogaster Short-tailed grasshopper mouse Microtus ochrogaster Prairie vole Microtus pennsylvanicus x Meadow vole Microtus sp. x Voles Pitymys pinetorum Pine vole Ondatra zibethicus Muskrat Synaptomys cooperi x Southern bog lemming Mustela erminea x Ermine Mammut americanum 0 American mastodon Mammuthus sp. 0 Mammoths Lepus californicus + + + + Black-tailed jackrabbit Lepus sp. + Jackrabbits Sylvilagus floridanus + + Eastern cottontail Sylvilagus audubonii + + Audubon's cottontail Sylvilagus aquaticus x Swamp rabbit Sylvilagus sp. + + + + + Cottontails Mylohyus nasutus Long-nosed peccary Platygonus sp. 0 Flat-headed peccary Sites Klein Longhorn Miller's Rattlesnake (=Hall's) Species Cavern Cave Cave Cave Sorex cinereus Masked shrew Blarina brevicauda x x Short-tailed shrew Cynomys ludovicianus Black-tailed prairie dog Geomys bursarius + + x x Plains pocket gopher Thomomys bottae ? Botta's pocket gopher Peromyscus nasutus Rock mouse Baiomys taylori x + Northern pygmy mouse Onychomys leucogaster x + + Short-tailed grasshopper mouse Microtus ochrogaster x x Prairie vole Microtus pennsylvanicus x Meadow vole Microtus sp. x Voles Pitymys pinetorum Pine vole Ondatra zibethicus x Muskrat Synaptomys cooperi x x x Southern bog lemming Mustela erminea Ermine Mammut americanum American mastodon Mammuthus sp. Mammoths Lepus californicus + Black-tailed jackrabbit Lepus sp. + + Jackrabbits Sylvilagus floridanus + + Eastern cottontail Sylvilagus audubonii + + Audubon's cottontail Sylvilagus aquaticus Swamp rabbit Sylvilagus sp. + + + Cottontails Mylohyus nasutus Long-nosed peccary Platygonus sp. 0 Flat-headed peccary Sites Levi Schulze Wilson-Leonard Felton Species Shelter Cave Site Cave Sorex cinereus Masked shrew Blarina brevicauda x x Short-tailed shrew Cynomys ludovicianus Black-tailed prairie dog Geomys bursarius + x x x Plains pocket gopher Thomomys bottae ? Botta's pocket gopher Peromyscus nasutus Rock mouse Baiomys taylori x Northern pygmy mouse Onychomys leucogaster x + Short-tailed grasshopper mouse Microtus ochrogaster Prairie vole Microtus pennsylvanicus Meadow vole Microtus sp. x x x Voles Pitymys pinetorum Pine vole Ondatra zibethicus Muskrat Synaptomys cooperi Southern bog lemming Mustela erminea Ermine Mammut americanum American mastodon Mammuthus sp. Mammoths Lepus californicus + + + Black-tailed jackrabbit Lepus sp. Jackrabbits Sylvilagus floridanus + + + Eastern cottontail Sylvilagus audubonii + Audubon's cottontail Sylvilagus aquaticus Swamp rabbit Sylvilagus sp. + Cottontails Mylohyus nasutus Long-nosed peccary Platygonus sp. 0 Flat-headed peccary Sites Barton Wunderlich Centipede Springs Kyle Walton Species Site Cave Road Site Site Sorex cinereus Masked shrew Blarina brevicauda x x Short-tailed shrew Cynomys ludovicianus Black-tailed prairie dog Geomys bursarius + + + Plains pocket gopher Thomomys bottae Botta's pocket gopher Peromyscus nasutus Rock mouse Baiomys taylori + Northern pygmy mouse Onychomys leucogaster x Short-tailed grasshopper mouse Microtus ochrogaster Prairie vole Microtus pennsylvanicus Meadow vole Microtus sp. x Voles Pitymys pinetorum x Pine vole Ondatra zibethicus Muskrat Synaptomys cooperi Southern bog lemming Mustela erminea Ermine Mammut americanum American mastodon Mammuthus sp. Mammoths Lepus californicus + Black-tailed jackrabbit Lepus sp. Jackrabbits Sylvilagus floridanus + Eastern cottontail Sylvilagus audubonii + Audubon's cottontail Sylvilagus aquaticus x Swamp rabbit Sylvilagus sp. + + + Cottontails Mylohyus nasutus Long-nosed peccary Platygonus sp. Flat-headed peccary TABLE 3. Latest documented occurrences (in years before present, B.P.) of selected small-mammal species in Wisconsinan and Holocene sites in central Texas. Approximate Species date (B.P.) Site County Peromyscus nasutus 20,000-9200 Friesenhahn Cave Bexar Sorex cinereus 10,900 Cave Without A Name Kendall Mustela erminea 10,900 Cave Without A Name Kendall Ondatra zibethicus ca. 10,000 Miller's Cave Llano Synaptomys cooperi ca. 10,000 Miller's Cave Llano Microtus (all species) 3450-1015 Barton Springs Road Travis Onychomys leucogaster 1389-674 Kyle Site Hill Pitymys pinetorum 1389-674 Kyle Site Hill Pitymys pinetorum 600 Mac's Cave Travis Blarina brevicauda 220-160 Walton Site McLennan Literature Species source Peromyscus nasutus Lundelius, 1967 Sorex cinereus Lundelius, 1967 Mustela erminea Lundelius, 1974 Ondatra zibethicus Lundelius, 1974 Synaptomys cooperi Lundelius, 1974 Microtus (all species) Lundelius, 1967 Onychomys leucogaster Lundelius, 1967 Pitymys pinetorum Jelks, 1962 Pitymys pinetorum Lundelius, 1974 Blarina brevicauda Story, 1985
This paper was written during a research sabbatical sponsored by the Baylor University Sabbatical Program. F. R. Gehlbach and E. L. Lundelius, Jr., offered thoughtful criticism of an earlier draft of this paper.
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KENNETH T. WILKINS
Department of Biology and Strecker Museum, Baylor University, Waco, Texas 76798-7388
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|Author:||Wilkins, Kenneth T.|
|Publication:||The Texas Journal of Science|
|Date:||Aug 1, 1992|
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