Pliocene Amphibians and Reptiles from Clark County, Nevada.
CHRISTOPHER J. BELL 
Abstract.--Pliocene herpetofaunas are uncommon in the published literature. Here we report on a small Pliocene (Blancan Land Mammal Age) herpetofauna from the White Narrows local fauna (Clark Co., Nevada) dating approximately 4.9 to 4.3 Ma. At least two species of Bufo, a phrynosomatid lizard, Xantusia, and a colubrine snake were recovered from sediments deposited in a basin that held a small lake. Climate was probably subhumid, but a cooling and aridification trend was occurring. The vegetation may have been a mosaic of oak-juniper (?) woodland and a riparian community. The White Narrows herpetofauna is depauperate in comparison to the present community which is probably due to sampling bias and not a climatic factor.
In contrast with the mammalian faunas, there are relatively few described amphibian and reptile faunas from the late Tertiary of what is now the Intermountain Region of western North America. This paucity of localities, and the consequent gaps in our understanding of herpetofaunal biogeography and biodiversity in this region renders any new information noteworthy. In this paper we describe the amphibian and reptile components of a small, relatively diverse vertebrate assemblage (White Narrows local fauna) recovered from a locality in Clark County, Nevada, and discuss their biogeographic significance in relation to the changing physiographic setting.
The fossils reported here were recovered from San Bernardino County Museum locality 2.12.16, which was discovered and collected as a result of paleontological mitigation efforts conducted in the late 1980s by the museum along the right-of-way for the Kern River Gas Transmission Line. The locality is situated at 520 m elevation, approximately 2 km south of the Muddy River in Moapa Valley, about 8 km upstream from the confluence with Meadow Valley Wash at Glendale, Clark County, Nevada. The San Bernardino County Museum Regional Paleontologic Locality Inventory places the locality (referred to as Moapa Bluffs) at the SW 1/4 of the NE 1/4 of the SW 1/4 of the SW 1/4 of Section 7, T15S, R66E (Mount Diablo Base and Meridian), Moapa West quadrangle, on US Bureau of Land Management holdings. Abbreviations for museums include: MCZ, Museum of Comparative Zoology, Harvard University; SBCM, San Bernardino County Museum; UCMP, University of California Museum of Paleontology.
Overview, Stratigraphic Setting, and Chronology
The establishment of ancient drainage systems in the region of the Muddy River in Moapa Valley in southeastern Nevada were governed by major structural features that developed during the Miocene. These events and their consequences were discussed by Longwell (1936), Lucchitta (1972, 1979, 1990), Dicke (1985), Duebendorfer and Wallin (1991), Schmidt et al. (1996), and Williams (1996). During the Miocene, extensional stress, subsidence, and normal faulting set up the development of the basins and ranges of Nevada and relative uplift of the western margins of the Colorado Plateau. Slow subsidence and basin filling occurred from approximately 24 to 13 Ma. From about 13 to 10 Ma, normal faulting became prevalent, resulting in the rise of mountain ranges and subsidence of basins. In some of these basins, including the one in which the fossils discussed herein were deposited, large lakes and lake systems formed. The ancestral Colorado River began flowing into and through the Muddy Creek basin at approximately 5.5 M a and ultimately breached the lake basin, establishing external drainage.
Late Tertiary alluvial deposits in the vicinity of Moapa Valley and nearby Meadow Valley received the attention of geologists and paleontologists for many decades. Early studies placed all the sediments into a single unit, the Muddy Creek Formation. Although published reports of fossils from the Muddy Creek Formation exist (Lucchitta 1979; Taylor and Smith 1981; Taylor 1983; Reynolds and Lindsay 1999), remains of fossil vertebrates are relatively uncommon. Vertebrate fossils from the Muddy Creek Formation were first studied by Stock (1921) who assigned the sediments a Mio-Pliocene age based on vertebrate taxa present within the unit.
Recent evaluation of a discrete sedimentary package in the upper portion of the Muddy Creek Formation resulted in a proposal for the recognition of a new formation, tentatively named the White Narrows (Schmidt et al. 1996). This currently informal formation (no description outside of the 1996 Open-File report has yet appeared; see Salvador 1994:23) was described as a white calcic claystone and clayey limestone, with two distinct subfacies: 1) green claystone, and 2) channel fill deposition in degradational channels. A fossiliferous marl bed, 20 cm thick and about 2 m above the base of the formation, produced the mammals described by Reynolds and Lindsay (1999) and the amphibian and reptilian remains presented here (SBCM 2.12.16; see Schmidt et al. 1996). It is this package of vertebrate remains that compose the White Narrows local fauna.
No radiometric dates are available for the White Narrows formation, and age estimates provided by Schmidt et al. (1996) were based on preliminary identifications of the rodents SBCM 2.12.16. Unfortunately, the mammalian biochronology does not provide unambiguous evidence upon which an age can be based. A recent review of the mammalian fauna recovered from the White Narrows local fauna was provided by Reynolds and Lindsay (1999) who suggested that this depositional unit represents a transition from the late Miocene/earliest Pliocene Hemphillian Land Mammal Age to the Pliocene Blancan Land Mammal Age. The small mammals they reported include the murid rodents Paronychomys cf. P. lemredfieldi., Copemys cf. C. vasquezi, Peromyscus valensis, Repomys cf. R. gustleyi, Calomys (Bensonomys) cf. C. coffeyi, Calomys (Bensonomys) cf. C. arizonae, Neotoma (Paraneotoma) cf. N. vaughani, and several heteromyid taxa, including Dipodomys gidleyi, Oregonomys cf. O. sargenti, and undetermined species of Perognathus and Prodipod omys (Reynolds and Lindsay 1999).
The lack of confident species identifications for most of the rodent taxa in the fauna renders biochronological age estimates difficult. Most of the genera can be found in Hemphillian and Blancan deposits and definitive species identifications are required for refined biochronological age estimates. Of the nine taxa listed by Reynolds and Lindsay (1999) as being "restricted temporally" to either the Hemphillian or Blancan land mammal ages, only two (Peromyscus valensis and Dipodomys gidleyi) are definitively identified to species. Peromysucs valensis is currently known only from the Hemphillian and D. gidleyi only from the Blancan. In addition, the shrew genus Sorex is represented in the White Narrows local fauna (possibly by two species; see table 3, p. 473, in Reynolds and Lindsay 1999), and is elsewhere known only from Blancan and younger faunas (Repenning 1967; McKenna and Bell 1997). The locality is either of latest Hemphillian age, dating between approximately 6.5 and 4.9 Ma, or is earliest Blancan, da ting between approximately 4.9 and 4.3 Ma. The overriding compliment of taxa seems to imply a Blancan age for the fauna, and is therefore the assumed age for this report (this is in agreement with the conclusions of Reynolds and Lindsay 1999).
All specimens are housed in the Section of Geological Sciences at the San Bernardino County Museum and are catalogued under collection number SBCM L-2515. Each specimen has a separate catalog number. Terminology for anurans follows Tihen (1962) and Sanchiz (1998). Iguanian familial designation follows Frost and Etheridge (1989).
Class AMPHIBIA Linnaeus, 1758
Subclass Lissamphibia Haeckel, 1866
Order Anura Rafinesque, 1815
Family Bufonidae Gray, 1825
Genus Bufo Laurenti, 1768
Bufo boreas/canorus/exsul group
Material.--Left ilia 570-571, 2720-2729; right ilia 572, 2730-2734.
Identification.--The dorsal protuberance is relatively high (varies from high to medium-low on the fossils), but the anterior and posterior edges of the protuberance are even with the top of the dorsal prominence (the protuberance does not stand out from the prominence). A distinct, projecting protuberance occurs on the living B. cognatus, B. microscaphus, B. ret iformis, and B. woodhousei and on the extinct B. hibbardi and B. pliocompactilis (Wilson 1968; Taylor 1 941A) implying that our specimens do not belong to any of these species. The extinct B. suspectus has a prominence with the posterior slope steeper than the anterior slope; it was assumed by Tihen (1962) to belong to the B. valliceps species group and by Estes and Tihen (1964) to belong to the B. americanus species group. The specimens listed above have a dorsal prominence similar in size to that described for B. suspectus, but do not exhibit the steeper posterior slope. The range of individual variation in this character needs to be thoroughly ex amined for all North American representatives of Bufo.
The ventral acetabulur expansion is not flared out and flattened as on B. punctatus, or anteriorly rounded as on B. debilis (young and adult specimens). Specimen 571 is not similar in size or in robustness to B. holmani (Parmiey 1992), nor does it have the medially-directed crest on the dorsal surface near the middle of the shaft as is characteristic of B. hibbardi (Taylor 1941A). The prominence and morphology of the protuberance on 571 is identical to living B. boreas specimens with an sv1 of approximately 30 mm. Because the ilia listed above exhibit characters most similar to those found in the Bufo boreas species group, we refer our specimens to the B. boreas/canorus/exsul group. As indicated above, the White Narrows adult specimens are always small, sv1 approximately 26 to 40 mm, and in this character resemble more the living toads B. exsul and B. canorus and not the larger B. boreas. Today Bufo exsul and B. canorus do not live near the Moapa Valley region; B. boreas is known from the Muddy River drainag e (Stebbins 1985; Hovingh 1997).
Remarks.--Specimen 571 (Figure lA) is the most complete of the ilia from White Narrows local fauna and is used here as representative of all the specimens referred to this group. The ilium is complete except for the extreme anterior end of the shaft. The overall size of most specimens is that of a toad with a svl (snoutvent length) from 26 to 30 mm (2720 is equal in size to modern toads of svl approximately 40 mm).
Bufo suspectus Tihen, 1962
Material.-Left ilia 2716-2717; right ilia 2718-2719.
Identification.--The low dorsal prominence of the ilium, with the posterior slope steeper than the anterior, and the position of the dorsal protuberance somewhat posterior to the midpoint of the base of the prominence are characteristic for the species. The four ilia listed above all show these features (Figure 1B).
Remarks.--Bufo suspectus is a small species of toad originally described from the Blancan Fox Canyon Locality in Kansas (Tihen 1962). Additional material (consisting of a frontoparietal and two ilia) was subsequently recovered from the Miocene Valentine Formation of Nebraska, but was named a distinct species (B. valentinensis) on the basis of the supposed geographic and temporal improbability that a species from the Pliocene of Kansas could be the same as a Miocene species of Nebraska (Estes and Tihen 1964). Subsequent reports of "B. valentinensis" (e.g., Holman 1970, 1973; Chantell 1971; Parmley 1992) continued this practice, but Sanchiz (1998) recently synonymized the two species under B. suspectus, which we endorse.
Material.--Angulosplenial 563, 2735-2737; right ilium 573; humerus 564-. 565, 2738-2743.
Identification.--These specimens were not preserved well enough to permit a more refined identification.
Remarks.-All specimens are from adults of small species of Bufo that appear similar in size to living species with svl of approximately 25 to 45 mm.
Class REPTILIA Laurenti, 1768
Order Squamata Oppel, 1811
Family Phrynosomatidae Fitzinger, 1843
Material.--Right maxilla fragment 602; right dentary fragment 603.
Identification.--The fragmentary nature of this material prevents identification beyond the family level. We compared the specimens with modem skeletal material of Callisaurus, Cophosaurus, Holbrookia, Petrosaurus, Phrynosoma, Sceloporus, Uma, Urosaurus, and Uta. We were unable to detect any derived characters that would permit definitive allocation to any of these taxa, but in their overall size and morphology, the specimens most closely resemble modern specimens of various species in the genus Sceloporus.
The difficulties in identifying isolated skeletal elements of Sceloporus species were discussed by Etheridge (1964) and Larsen and Tanner (1974). The fossils were compared with modem skeletal material from S. clarkii, S. graciosus, S. jarrovii, S. magister, S. occidentalis, S. olivaceous, S. orcutti, S. poinsettii, S. scalaris, S. undulatus, and S. virgatus. Our comparisons failed to reveal any diagnostic features of anterior maxillae or dentaries in extant species, but the fossils are different in some respects from certain species. The teeth are not the relatively heavy, transversely compressed trilobate teeth typical of the extinct S. robustus (Twente 1952). The fossils are larger and more robust than modern specimens of S. graciosus, S. scalaris, and most S. virgatus, S. undulatus, and S. occidentalis. The fossil specimens differ from the living S. clarkii and S. orcutti in that they lack the more pointed, slightly recurved anterior teeth often seen in these two living species. The fossil teeth do not sh ow any trend toward being trilobate and wide, as in modern S. magister, however, the robust nature of the anterior portion of the fossil dentary, and the appearance of the dental shelf and meckel's groove are most similar to that species.
Remarks.--The maxilla is a fragmented (3.4 mm long) anterior portion with two blunt conical teeth (Figure 1C). The anterior dentary fragment (3.5 mm long) contains four blunt, conical teeth (Figure 1D).
Family Xantusiidae Baird, 1858
Xantusia Baird, 1858
Material.--L maxilla 591; R spleniodentary 595; R spleniodentary fragment 592, 594, 2744; L spleniodentaries 599-600; L spleniodentary fragments 598, 601.
Identification.--Specimen 595 (Figure lE) is described here and is representative of the Xantusia material from the White Narrows. Specimen 595 is a right spleniodentary with three complete teeth preserved at mid-dentary. The teeth are unicuspate, are of a uniform width at the base and gently taper at the crown. The anterior 10% of the spleniodentary is missing. The dental gutter is well developed.
These fossil specimens are readily identifiable as xantusiids. The White Narrows focal fauna specimens are small, of relatively slight build, and have closely spaced, simple teeth that lack lateral cusps. The splenial appears to be fused with the dentary in all specimens, and does not extend posteriorly beyond the apex of the coronoid in specimens where this character can be evaluated. A pronounced dental gutter is present in all of the spleniodentaries, but there is a varying degree of development in this feature. In all specimens, however, the dental gutter is markedly more developed than in any modern specimens of Xantusia examined (Figure lE).
The small size, slight build, and simple, conical, unstriated teeth of the fossils distinguish them from Klauberina riversiana and from the Paleogene genus Palaeoxantusia. The dentary teeth of Lepidophyma have lingually-directed cusps that superficially resemble, in dorsal view, a mammalian tribosphenic tooth, a feature distinctly lacking in the White Narrows specimens. The limited availability of skeletal material of Cricosaura typica makes adequate comparisons impossible at this time, but MCZ 10454 has much more delicately constructed jaw elements than any of the specimens from the White Narrows local fauna. The spleniodentary of X. henshawi is longer, more gracile, and has more teeth on average than do the fossil specimens. The fossil specimens are most similar to Xantusia vigilis, but are more robust and are slightly longer than the corresponding bones of that species.
A detailed description of an extinct form of Xantusia from the Anza-Borrego Desert in California was provided by Norell (1989). The preserved material shows characteristic plesiomorphic features for the genus, but lacks apomorphies. The plesiomorphic nature of the fossils recovered from Anza-Borrego led Norell to describe Xantusia [downsi.sup.*] as a new metaspecies (see Gauthier et al. 1988 for explanation of the use of "*" for metaspecies). The White Narrows Xantusia specimens are similar to those described by Norell in that they are all characterized by small size, the plesiomorphic presence of a dental gutter, and the synapomorphic reduction of tricuspid teeth. Morphologically, the specimens cannot be distinguished from X. [downsi.sup.*], or from numerous other (unpublished) plesiomorphic xantusiids ranging in age from Barstovian (middle Miocene) to early Irvingtonian (early Pleistocene; CJB, pers. obs.). Given the uncertain relationships among plesiomorphic xantusiids, we choose not to identify our mate rial as X. [downsi.sup.*] and simply refer the specimens to Xantusia.
Family Colubridae Oppel 1811
Subfamily Colubrinae Oppel 1811
Material.--Trunk vertebra 618.
Identification.--The specimen conforms to the diagnosis provided by Holman (1979) for colubrine trunk vertebrae. It lacks the hypapophyses characteristic of anterior trunk vertebrae (and those of natricines) and the subcentral paramedian lymphatic fossae characteristic of posterior trunk vertebrae (LaDuke 1991). In overall morphology and proportions, it most closely resembles trunk vertebrae of the 'racers' Coluber and Masticophis. Auffenberg (1963) provided measurements to distinguish mid-trunk vertebrae of Coluber and Masticophis in the eastern U.S., but LaDuke (1991) noted geographical variation in Auffenberg's characters. Due to the somewhat poor preservation of this specimen, generic determination is not attempted. Numerous highly fragmented specimens of snake vertebrae were also recovered but are not identifiable.
Discussion and Conclusions
As demonstrated above, the overriding compliment of mammalian taxa seems to imply a Blancan age for the fauna. It appears that the White Narrows local fauna is early Blancan in age ([sim]4.9[sim]4.3 Ma), but probably post-dates the early Blancan Panaca Formation from the nearby Meadow Valley (which dates to about 4.95 Ma; Lindsay et al. 1999; Mou 1999; Reynolds and Lindsay 1999) and predates the well-known Blancan faunas (e.g., Hagerman) of the Glenns Ferry Formation, southern Idaho (see review in Mead et al. 1998).
The termination of the Hemphillian Land Mammal Age in the earliest Pliocene is marked by the extinction of most of the typical mammalian species of the late Hemphillian, including taxa with long Miocene histories. The late Hemphillian mammalian faunas suggest that the process of change was taking place at different rates in different geographic regions of North America (Tedford et al. 1987). The accelerating pace of mammalian immigration from Hemphillian into Pleistocene Lime reflects the availability of dispersal routes into North America and, possibly most importantly, an increase in environmental instability.
The herpetofauna of North America became essentially modern at the familial level during the Miocene (Holman 1995). Many modern families and genera of anurans make their first appearance in North America during the Miocene, including Bufo, Rana, Pelobatidae, and Hylidae, Many families of lizards occur, including Iguanidae (sensu lato), Scincidae, Teiidae, Anguidne, Helodermatidae, Xantusiidae, and Rhineuridae. Snakes are represented by five families, including Aniliidae, Boidae, Colubridae, Elapidae, and Viperidae (Holman 1995). These families (except Aniliidae) persist in North America into the Pliocene and through to the Recent. Although Pliocene herpetofaunal records are rare in North America, those available indicate a basically modern fauna (Estes and Baez 1985; Parmley and Holman 1995; Holman 1995).
The majority of the published herpetofaunas of Neogene age from Nevada are late Pleistocene (Rancholabrean) or Holocene in age (Banta 1966; Mend and Bell 1994). At present, our knowledge of late Tertiary herpetofaunas from Nevada is extremely limited. The White Narrows local fauna and deposits located farther north at Panaca, Nevada (Lindsay et al. 1999; Mou 1996, 1997, 1999) appear to be the only sites of this age being studied in the region today. Scaphiopus alexanderi was reported from sediments in the Esmeralda Formation of Miocene age (Zweifel 1956) and an undescribed turtle from the same formation was listed by MacDonald and Pelletier (1958). Miopelodytes gilmorei, a pelodytid anuran, was described from the middle Miocene of Elko County, Nevada (Taylor 1941B). From localities farther north and west of the White Narrows local fauna, Rana johnsoni and R. plax were described from sediments that appear to be of Clarendonian to Hemphillian age (upper Miocene; La Rivers 1953, 1966), and an undescribed Claren donian turtle is included in the Chalk Spring Locality fauna of Elko County (MacDonald and Pelletier 1958). An unidentified snake was recorded from the late Miocene Thousand Creek fauna by MacDonald and Pelletier (1958). A single specimen of snake previously identified as Coluber/Masticophis was recovered from Hemphillian sediments in the Truckee Formation of west-central Nevada (Ruben 1971).
The herpetofauna from the White Narrows is not taxonomically diverse. Anurans
and a xantusid lizard dominate the fauna. The anurans are expected given the reconstruction of lake, riparman, and oak-juniper (?) open woodland communities nearby (Axelrod 1991; Thompson 1991). The anuran taxa recovered are expected in the region, although species identifications are not precise for most specimens. The recovery of bufonid anurans in the Bufo borea/canorus/exsul group might be predicted based on the Recent forms found in the region today (Hovingh 1997), however, this record represents a temporal range extension back to the early Pliocene.
The presence of plesiomorphic Xantusia in the White Narrows local fauna is significant for a number of reasons. These specimens represent the northernmost fossil record of the genus reported to date. Norell (1989) reported similar plesiomorphic xantusiids from late Pliocene and early Pleistocene sediments in Anza-Borrego Desert, San Diego County, California. Norell also mentioned that potentially similar forms were present in the Barstow Formation and in Clarendonian age sediments of the Dove Springs Formation of California. Whistler and Burbank (1992) list the Dove Springs Formation material as Xantusia sp., and this material resembles (in its plesiomorphy) X. [downsi.sup.*]. We confirm the presence of plesiomorphic Xantusia specimens in at least seven Barstovian deposits in California (UCMP localities V5501, V6447, V6449, V6463, V65145, V65147, V6604; also SBCM locality 1.130.37, possibly equivalent to UCMP V65147). Fossil material identified as "Iguanidae or Xantusiidae" from the early Miocene Boron Local Fauna in San Bernardino County, California (Whistler 1984) cannot be definitively referred to either taxon at this time; we examined the material and it is too fragmentary to be informative. The geographic and temporal distribution of these fossils raises interesting questions regarding the propriety of allocating geographically and temporally distinct fossils to the same metaspecies. Resolution of this taxonimic problem, and of the systematic position of the fossils in question is beyond the scope of this paper, but will be addressed at a later date.
It would appear from the related data presented above that the basin holding White Narrows sediments contained a lake, with a chemistry permitting the deposition of marl (Schmidt et al. 1996). Climate was probably still subhumid, but a cooling and aridification trend was occurring. Based on assumed reconstructions from the paleobotanical records, this lake might have been surrounded by a mosaic of oak-juniper woodland, possibly other deciduous trees and shrubs, and a riparian community along the lake margins in stream courses. A subhumid climate around a lake with surrounding riparian and open woodland communities would seem ideal for a much more diverse local herpetofauna than was actually recovered. Certainly the lake environment supported a diverse fauna; according to Reynolds and Lindsay (1999) the sediments contained (in addition to the mammals discussed above) remains of perch and sucker fish, gastropods, and ostracods. The low taxonomic diversity is probably due to sampling bias. Additional stratigrap hic, paleomagnetic, and paleontologic work is required to better understand the herpetofaunal changes that took place during the Pliocene and immediately prior to the glacial/interglacial phases of the Quaternary.
Sue Beard, George Billingsley, Ernie Duebendorfer, Joel Pederson, Dwight Schmidt, and Van S. Williams provided helpful information about the Muddy Creek and White Narrows formations. Eric Scott and Scott Springer helped with locality information for the SBCM material. We thank Jacques Gauthier for insightful discussions about xantusiid lizard evolution. G.E. Swartz provided helpful comments on an earlier version of this report. Figures were prepared by Leigh Anne McConnaughey, for whose patience we are most grateful. We thank Kathy Springer and Betsy Slemmer (SBCM) for assistance with the loan of herpetological remains from the White Narrows local fauna.
(1.) Department of Geology and Quaternary Sciences Program, Northern Arizona University, Flagstaff, Arizona 86011
(2.) Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712
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|Author:||MEAD, JIM I.; BELL, CHRISTOPHER J.|
|Publication:||Bulletin (Southern California Academy of Sciences)|
|Date:||Apr 1, 2001|
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