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Quaternary International 217 (2010) 30–36

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Quaternary International

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Covert biases, circularity, and apomorphies: A critical look at theNorth American Quaternary Herpetofaunal Stability Hypothesis

Christopher J. Bell a,*, Jacques A. Gauthier b, Gabe S. Bever c

a Department of Geological Sciences, 1 University Station – C1100, The University of Texas at Austin, Austin, TX 78712, USAb Department of Geology and Geophysics, Kline Geology Lab, Yale University, PO Box 208109, New Haven, CT 06520, USAc Division of Paleontology, American Museum of Natural History, New York, NY 10027, USA

a r t i c l e i n f o

Article history:Available online 18 August 2009

* Corresponding author. Tel.: þ1 512 471 7301; fax:E-mail addresses: cjbell@mail.utexas.edu (C.J. Bel

(J.A. Gauthier), gbever@amnh.org (G.S. Bever).

1040-6182/$ – see front matter � 2009 Elsevier Ltd adoi:10.1016/j.quaint.2009.08.009

a b s t r a c t

The North American Pleistocene Herpetofaunal Stability Hypothesis was established in the 1980s basedon summary reviews of primary descriptive paleoherpetological literature. The hypothesis posits thatNorth American herpetofaunas essentially were taxonomically and geographically stable throughout theQuaternary. Modern biogeographic distribution of species served as an important criterion for primaryidentification of the majority of fossil herpetofaunal specimens, rendering the Herpetofaunal StabilityHypothesis circular. Apomorphy-based identifications are an important and well-established alternativeto the traditional approach, but are used only rarely within the Quaternary paleontology community.Preliminary applications of the apomorphy-based approach reveal that species-level resolution often isnot possible for herpetofaunal remains. Lack of species-level resolution may be a consequence of poordocumentation of evolutionary morphology for most herpetofaunal species lineages. Development ofadequate skeletal collections and the search for apomorphies in the anatomical systems typicallypreserved in Quaternary deposits will yield meaningful insights into evolutionary morphology, anenhanced appreciation of Quaternary faunal dynamics, and a greater concordance with modern neon-tological approaches. Maintenance of traditional methods of identification will yield a false perception ofclarity, erroneous interpretations of Quaternary faunal dynamics, and may perpetuate misleading data inQuaternary paleontology as well as other fields that rely on those data.

� 2009 Elsevier Ltd and INQUA. All rights reserved.

‘‘Perhaps the sentiments contained in the following pages, arenot yet sufficiently fashionable to procure them general favor;a long habit of not thinking a thing wrong, gives it a superficialappearance of being right, and raises at first a formidable outcryin defence of custom. But the tumult soon subsides. Time makesmore converts than reason.’’Thomas Paine, 1776

1. Introduction

One of the most interesting and important contributions of theQuaternary fossil record is that it serves as a bridge that linksunderstanding of the modern biota with deep-time paleontologicalperspectives. As a ‘shallow’ deep-time record, Quaternary biotasinclude fossilized remains of extant species as well as extinct species,

þ1 512 471 9425.l), jacques.gauthier@yale.edu

nd INQUA. All rights reserved.

all encapsulated within a sedimentary system that permits unpar-alleled chronological control. Quaternary deposits are common,widely distributed, and richly fossiliferous. In recent decades, theyhelped scientists to craft a multidisciplinary perspective on globalchange issues over the last two million years. Studies of faunaldynamics through the Quaternary are particularly important tomany biologists seeking insights, using the last major global warm-ing events, into the impact of warming on biotic communities(Barnosky et al., 2003; Barnosky, 2009). Those of us studyingQuaternary biotas now play an increasingly important role in dia-logues about the nature of modern global change issues and theirprobable and possible impacts. The insights we gain from thematerial and theoretical components of our work now take onrenewed importance as a global community decides how, orwhether, to approach the challenges predicted for coming centuriesby modern climate models and an increasingly diverse assemblage ofscientists (e.g., IPCC, 2007).

From the earliest studies of Quaternary mammals in NorthAmerica it was clear that the Pleistocene fossil record would providean exciting and dynamic perspective on the past biotas of the

C.J. Bell et al. / Quaternary International 217 (2010) 30–36 31

continent. Initially, Quaternary paleontologists centered theirattention on large-bodied and charismatic extinct forms includingground sloths, saber-toothed cats, proboscideans, giant beavers,horses, and camels. Following the development of efficient tech-niques for the recovery of small-bodied vertebrate remains (e.g.,screen-washing; Hibbard, 1949), Quaternary paleontologists wereable to address new questions about faunal dynamics that includeda broader spectrum of the total biota, including small mammals, fish,amphibians, turtles, crocodilians, and squamates (early worksreviewed by Hibbard et al., 1965; see also Kurten and Anderson,1980; FAUNMAP Working Group, 1994; Holman, 1995).

With the emergence of widespread radiocarbon dating, andreconstructions of the history of glacial advances and retreats, thelate Pleistocene became a model for studying the impacts of climatechange over relatively short durations of time. The attraction ofdetailed chronological control over the last approximately 50 000years, the widespread occurrence of late Pleistocene and earlyHolocene fossiliferous deposits, and the appearance of humans onthe continent within that interval made that epochal transition anespecially important one for understanding faunal dynamics. In thelatter part of the 20th century, considerable attention was paid tothe biogeographic and taxonomic status of the North AmericanQuaternary amphibian and non-avian reptile faunas (collectively,the herpetofauna). The pattern reported is one of general stasis, orstability, both in terms of taxonomic composition and geographicdistribution of species. This pattern stands in stark contrast to thoseof other tetrapod vertebrates. Both the mammalian and avianfaunas suffered extinctions of large-bodied species during theQuaternary (Martin and Klein, 1984; Barnosky et al., 2004), and thefossil record shows that geographic distributions of these groups,and especially of the mammals, expanded and contracted in mostgeographic regions of the continent throughout the Quaternary(FAUNMAP Working Group, 1994, 1996).

The Herpetofaunal Stability Hypothesis is well-accepted amongQuaternary paleontologists and, in the context of modern concernsabout global amphibian decline (e.g., Alford and Richards, 1999;Lannoo, 2005) and possible parallel decline in non-avian reptiles(Gibbons et al., 2000), may be used to highlight the complexitiesassociated with making predictive statements about future changebased on the Quaternary fossil record. As our epigraph suggests,however, widespread and long-standing acceptance of an idea isnot an argument for its cogency. In this paper, we review theHerpetofaunal Stability Hypothesis, offer a new interpretation ofthe data that underlie that hypothesis, conclude that the hypothesisis ultimately based on circular reasoning, and propose alternativemethodological and philosophical approaches to working withQuaternary fossil herpetofaunas. Those alternative approachespose new challenges, but avoid some of the biases that stronglyshape our current perspective on this important aspect of Quater-nary faunal dynamics.

2. The Herpetofaunal Stability Hypothesis

The Herpetofaunal Stability Hypothesis encompasses bothtaxonomic and geographic stability, and suggests that throughoutthe Quaternary the herpetofauna experienced relatively fewextinctions, no speciation, and few geographic range adjustmentsacross most of North America north of Mexico. The hypothesis hasits roots in basic observations derived from some of the earlieststudies of fossil herpetofaunas in North American Quaternarydeposits. The introductory paragraph of the first major summary ofthe North American Quaternary non-avian reptile record encap-sulated the basis of the stability hypothesis by noting that ‘‘{i}fthere is any one thing that characterizes the reptiles of the entire

last half of the Cenozoic, it is their stability’’ (Auffenberg and Mil-stead, 1965:557).

Tentative exploration of regional stability in eastern NorthAmerica was first presented by Fay (1984a,b, 1986a,b), and latePleistocene through Holocene stability across Appalachia wasformally proposed in 1988 (Fay, 1988). A continental-scale analysisof purportedly extinct and extralimital forms was completed byBrewer (1985). He emphasized the pattern of relative taxonomic andgeographic stability for most lizard, crocodilian, and turtle groups,with notable exceptions among the tortoises. Following Brewer’swork, additional regional works were completed or expanded(e.g., LaDuke, 1991; Holman, 1992, 1999a; Mead and Bell, 1994), andsynthetic studies were completed that explored the herpetofaunalrecords across North America north of Mexico (Holman, 1991, 1995).In some of those later works, the stability hypothesis was explicitlyexpanded beyond the late Pleistocene to encompass the w1.8million years of the Quaternary (e.g., Holman, 1991, 1995, 1999a). Itspossible extension to European faunas was first suggested byHolman (1989), but basic physiographic differences between thecontinents force significant modifications (Holman, 1999b).

The most expansive treatment and discussion of the hypothesiswas that by Holman (1995). In his view, extinction and geographicrange adjustments were the two major categories of change thatwere important for vertebrate populations in the Quaternary. Hereviewed and interpreted the herpetofaunal fossil record in bothcontexts. Most of the purportedly extinct species still accepted atthe time of his work were rejected after study of increased samplesizes of comparative specimens of the extant species (one frog, twoturtles, one snake), or were of doubtful validity and requiredadditional study for evaluation (two salamanders, two anurans, andtwo turtles). Only two frogs, seven turtles, three snakes, and onelizard were considered by Holman (1995) to represent (at thattime) unquestioned extinct species. Of those, both frogs were laterconsidered nomina dubia by Sanchiz (1998), one of the snakes wassynonymized with an extant form by Parmley (1994) in a paperHolman was not able to include in his book, and many of thepurportedly diagnostic features of the lizard (‘Eumeces’ carri)previously were suggested by Estes (1983) to be the result of post-mortem erosion. Thus, the extinct Quaternary herpetofauna ofNorth America north of Mexico consists of seven turtles and twosnakes. Six of those turtles are referred to Geochelone, but thetaxonomy and systematics of those fossils are in dire need ofrevision (Preston, 1979; Holman, 1995). The snake species may beconsidered as extinct only because of a lack of clear and detailedunderstanding of patterns of evolutionary morphology amongNorth American snakes (see below), and their possible synonymywith extant forms was noted by Holman (1999b). In any event,there are certainly few extinct herpetofaunal species recognizedfrom Quaternary deposits in North America.

Holman recognized eight herpetofaunal regions in North Amer-ica, and characterized them in terms of overall pattern of geographicrange adjustment (Holman, 1995). Stability was the general condi-tion for all except the Great Lakes region. One or several extralimitalspecies were reported from the Southeastern, Ozark, NorthernPlains, Southern Plains, and Rocky Mountain-Great Basin regions; noextralimital species were recognized from the Appalachian andPacific Coast regions (Holman, 1995). In the Great Lakes region theoverall pattern was one of extirpation followed by recolonization ofglaciated areas as the ice retreated (Holman, 1992, 1995).

All of the authors evaluating and promoting the stabilityhypothesis included extensive discussion of climatic and environ-mental factors that may have influenced faunal dynamics. Manymade specific attempts to explain why herpetofaunal stability wasthe norm when basic expectations are that ectothermic organismsshould be more sensitive to climatic and environmental

C.J. Bell et al. / Quaternary International 217 (2010) 30–3632

perturbations. Our concern, however, is not with testing the varioushypotheses put forward to explain stability, nor with specific chal-lenges to the hypothesis based upon reanalysis of paleodistributiondata of particular species (e.g., Hutchison et al., 1999). We are con-cerned with the methodological and philosophical approaches thatunderlie the primary identifications of herpetofaunal fossils; in otherwords, with the material foundation of the Herpetofaunal StabilityHypothesis itself.

3. Biases that shape the stability hypothesis

In all branches of science, it is useful and informative to pausenow and then and reflect upon the assumptions and biases thatunderlie our work, and to assess how those assumptions mightshape our results and the conclusions we draw from them. To thatend, we reviewed the primary literature in which the NorthAmerican Quaternary herpetofauna was documented anddescribed. We identified several consistent patterns that reflectsubtle and usually unacknowledged biases in the methodologicalapproaches taken to identify those fossils. These can be categorizedas geographic biases, identification biases, and temporal biases.

In many early descriptive works, herpetofaunal fossils werecompared only with the immediately local extant biota in thevicinity of the fossil deposit itself. Identifications were then basedupon resemblance of the fossil to a particular local taxon. In otherpapers, including those of recent authors, identifications are biasedby restriction of the pool of species with which comparisons aremade, but across broader geographic space than the immediatevicinity of the fossil deposit. This is most commonly manifested byrestriction of comparison to modern species found within certaingeopolitically defined areas (e.g., states) or to well-defined phys-iographic provinces (e.g., the Great Basin; the Colorado Plateau).Identifications are based upon resemblance to one of the species inthe comparative pool. Because geography is directly shaping thecomparative sample under both of these approaches, taxonomicand geographic stability is predetermined, and any secondary claimsof emergent biogeographic patterns are necessarily based oncircular reasoning (i.e., identifications based on biogeographycannot then be used to support biogeographic conclusions). This istrue regardless of whether or not the specimens in question actu-ally do represent the species to which they are attributed; one canreach the right conclusion for the wrong reason.

The importance of modern geographic data is expressed in othersubtle ways, as well. In a review of Holman’s book on the fossilrecord of North American snakes (Holman, 2000), the seeminglyincongruous appearance of 45 color plates of extant snakes ina volume that otherwise provided few novel illustrations of fossilsnakes was noted by Albright (2001). The appearance of thoseplates is, however, completely consistent with the operationalpractices used by Holman to identify Quaternary snakes. Themorphologic resemblance of extant forms and the geographicdistribution of those forms were essential tools for Holman’sapproach to identification of fossils.

The stability hypothesis is, then, not a revelation or conclusiondrawn from analysis of the data themselves, but is instead a pre-determined outcome of a methodological approach. That it has notbeen widely recognized as such is attributable to the fact that thegeographic biases mentioned above are not explicitly stated norjustified in primary publications; they simply result from standardoperational practices that have been in place for decades.

Some biases in the identification of fossils, and their possibleimpact on the stability hypothesis, were discussed by Holman (1995).He pointed out that the majority of identifications of fossil herpeto-faunal specimens are based on certain skeletal elements(e.g., vertebrae for salamanders and snakes, the ilium for anurans,

tooth-bearing elements for non-snake squamates). The unstatedcorollary to this is that many other skeletal elements remain unrec-ognized, or at least unstudied. Those other elements do exist incollections, and in some cases are equally or more diagnostic ofparticular species than the ‘standard’ elements (e.g., vertebralelements of the lizard Anniella; Bell et al., 1995). Holman also notedthat cryptic species (those which are reproductively isolated, and inwhich genetic differentiation may be extreme, but morphologicaldifferentiation is subtle or absent) can be recognized today,undoubtedly occurred in the Pleistocene, but necessarily remainunrecognized in the fossil record. In addition, some extant speciesthat can be readily distinguished on the basis of external morphologyshare common osteological features, and it is sometimes difficult orimpossible to distinguish some species based on individual skeletalelements, including those that are the ‘standard’ elements for iden-tifying taxonomic groups (e.g., snake vertebrae; Holman, 1995).

In a particularly revealing remark, Holman (1995:208) notedanother prevalent bias in Quaternary systematics, specifically ‘‘theingrained idea among many paleoherpetologists that all Pleisto-cene herpetological material, other than certain large tortoisefossils, must belong to modern species.’’ That observation suggeststhe importance of species-level resolution for Quaternary paleon-tology generally.

Species-level taxonomic resolution from the fossil record isa laudatory goal for paleontologists, but is simply not alwayspossible to achieve. For many of the basic questions that Quater-nary paleontologists seek to answer (e.g., ‘How do species respondto global warming?’), species-level resolution is necessary toaddress the question. We suspect that ultimately it was the limi-tations of the morphological data commonly available in the fossilrecord, either perceived or real, combined with the perceived needto achieve species-level resolution, that drove Quaternary paleo-herpetologists to utilize geographic data to help them refineidentifications of fossil specimens.

It is particularly interesting that Holman (1995) mentioned theimpossibility of distinguishing among some species based oncertain elements. When the body of primary Quaternary paleo-herpetological literature is considered as a whole, it is unusual tofind such open acknowledgement of limitations in the identifica-tion of fossil specimens. In our perusal of the primary literature, wefound a nearly ubiquitous presence of euphemistic language,possibly utilized to avoid or sidestep the unpleasant and undesiredconsequences of open acknowledgement that species-level reso-lution is not always attainable. In common phrasing such as ‘thesespecies are difficult to distinguish’, the ‘difficult’ often is a euphe-mism for ‘impossible,’ yet species-level identifications often stillwere made. Other common phrases that appear in the primaryliterature are framed along the lines of ‘the fossil compares favor-ably with {taxon name}’ or ‘the fossil is indistinguishable from thesame element in {taxon name} inhabiting the area today’. Thesestatements are sometimes all that was provided in the way ofa justification for identification of a fossil element to a particulartaxon. The widespread use of the ‘cf.’ convention was later used asan additional means to express uncertainty in the identification ofa fossil element. In itself, such an appellation to the taxon name isan appropriate way to express uncertainty, but authors have a dis-turbing tendency to disregard the ‘cf.’ when making summary orconcluding statements in the discussion or conclusion sections oftheir papers. In extreme cases, the ‘cf.’ appellation was dropped infaunal lists and tables of taxa, and was included only in thesystematic paleontology section of the paper, or was dropped insubsequent review papers (Murray, 2008).

Many paleontologists also uncritically accept previously pub-lished statements about the relative utility of certain morphologicalfeatures for identifying species or higher taxa. Many of those

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published characteristics were originally applied only in local ornarrow geographic contexts (e.g., this character works to distin-guish a species currently found in a particular region from otherspecies in that region). However, these characters often fail toachieve such fine-scale taxonomic resolution when they areconsidered in a broader (global or regional) context that includes allrelated species across the geographic range of an entire lineage. Insuch broader contexts, many characters actually are found to applyto much more inclusive taxa.

All species are finite in time and space, but recovering the actualtemporal duration of lineages is difficult. The temporal duration ofmany species often is assessed based on their reported occurrencein the fossil record. Temporal bias in the identification of fossilsoften takes the form of assuming temporal restrictions on durationof species. For example, the toad Bufo valentinensis Estes and Tihen(1964) is morphologically identical to Bufo suspectus Tihen (1962),but was named as a distinct species because of the perceivedimprobability that a Pliocene toad known from Kansas would befound in Miocene deposits in Nebraska (Estes and Tihen, 1964): itwas subsequently synonymized, but not until 34 years later(Sanchiz, 1998). For Quaternary remains, temporal bias is man-ifested primarily by the conception that Quaternary specimensshould be referable to extant species (Holman,1995), and that fossilsfrom a particular geologic horizon are most likely to representremains of species previously reported from that same horizon.

These geographic, identification, and temporal biases are opera-tional biases that are not explicitly stated as such in the literature.They are, however, real and important. They permit elucidation ofa central bias in the identification of Quaternary herpetofaunal (andother biotic) remains, namely, that the primary operational methodfor identifying those remains is based on gross morphologicalresemblance. Further, the pool of species against which resemblanceis judged is derived from consideration of known spatiotemporaldistributions of species, and especially of the extant biota.

4. Resemblance versus apomorphy

All paleontologists must approach their work with the founda-tional idea that we can use skeletal morphology to identify fossils.We certainly do not dispute this central claim; indeed, we all arestrong proponents of the value and relevance of fossils for eluci-dating vertebrate history. The ascendancy of molecular biology inrecent decades did little to diminish the value and importance offossils in systematic analyses of lineages (e.g., Gauthier et al., 1988;Smith and Turner, 2005) or for interpreting the material record ofevolution.

Discovering the relationship between morphological changeand speciation remains an important, if elusive, goal for manyevolutionary biologists. Ironically, despite the fact that Darwin(1859) emphasized the importance of variation as a potentialdriving force for evolutionary change, our understanding ofevolutionary morphology for the majority of species is far fromcomprehensive. Most authors reporting on skeletal morphology ofthese groups failed to investigate and report data pertaining tomajor sources of variation, including sexual dimorphism, ontoge-netic transformations in morphology, epigenetic polymorphisms,ecophenotypic plasticity, and teratologies. In the absence of suchdata, only preliminary characterizations of morphological ‘space’for species can be made, and that poses special challenges for usingmorphology to reconstruct reliable phylogenetic hypotheses(e.g., Joyce and Bell, 2004), to interpret data from the fossil record,and to understand the evolutionary history of the developmentalsystems responsible for post-natal morphologies (Bever, 2009).As an example, the varying opinions regarding the validity of thepurportedly extinct skink ‘Eumeces’ carri are based in part on

ignorance of the range of morphological conditions in extant NorthAmerican skinks (Estes, 1983). Morphological variants derived fromthose sources of variation can, when found expressed in fossils, bemistaken as representing different species. This does not appear tobe a problem for interpretations of Quaternary fossils; instead,paleontologists working in the latter part of the Cenozoic oftenassume that modern morphology of a species (at whatever level ofdetail it is currently understood) characterizes that lineage for itsentire evolutionary history. This essentially denies that mosaicevolution can take place, and robs us of the ability to gain newinsights from the fossil record.

Over the last few decades a major shift in the focus of paleon-tology began. That shift involved a departure from traditionalapproaches of studying transitional species, and a renewedemphasis on the study of evolutionary transformations in themorphological characteristics of species. This is one of the impor-tant aspects of cladistic methodology, and serves as the foundationfor recognizing a distinction between plesiomorphic characteristics(evolutionarily ‘primitive’ features inherited from deep ancestry)on one hand and apomorphic characteristics (evolutionarilyderived features) on the other. The determination of plesiomorphicor apomorphic condition of a feature (its evolutionary polarity) isnot self-evident from the feature itself, but is interpreted in thecontext of the relationship of all known features and their distri-bution across an evolutionary tree (e.g., Rieppel, 1994).

Apomorphies for many herpetofaunal lineages are now known orhypothesized in the context of particular phylogenetic hypotheses(e.g., Estes et al., 1988; Larson and Dimmick, 1993; Lee, 2005a,b).Although not all phylogenetic hypotheses are congruent, even forhigher-order relationships (e.g., Estes et al., 1988; Townsend et al.,2004), these can be used as a framework for the identification ofQuaternary fossil specimens. Most hypothesized apomorphies,however, apply to more inclusive taxonomic groups, are based onarticulated skulls or postcranial skeletons, and may be difficult toassess in the disarticulated and dissociated material commonlyfound in North American Cenozoic deposits. Nonetheless, carefulexamination of even fragmentary fossil material can result inrecognition of apomorphies established on articulated material(e.g., Smith, 2006). More importantly, investigation of disarticulatedskeletal elements within a phylogenetic context can lead to thediscovery of new apomorphies that are relevant not only for theidentification of fossils, but for enhancing robusticity of phylogeneticanalyses. This was elegantly demonstrated recently by Bhullar(2008) in his exhaustive analysis of articulated and disarticulatedskeletal elements of fossil and extant xenosaurid lizards.

Mark Norell was a pioneer in the use of apomorphies to identifyQuaternary fossils. His work centered on analysis of the lizards fromthe Pliocene and Pleistocene sediments in the Anza-Borrego Desert insouthern California (Norell, 1989). He was able successfully diagnoseone extant species (the desert iguana, Dipsosaurus dorsalis), and usedapomorphies to establish one new genus and three new species ofiguanians. He proposed characters that would permit species-levelidentification of the horned lizards (Phrynosoma), but was not able toachieve low-level resolution (genus or species) for other phrynoso-matid lizard fossils. Genus-level resolution was obtained for Xantu-siidae and Scincidae, but higher-order resolution only for Anguidaeand Teiidae (Norell, 1989). Norell’s ground-breaking approach wasnot adopted by other Quaternary paleoherpetologists until the 1990swhen apomorphies were used to diagnose Pliocene and Pleistocenefossils of the legless lizard Anniella from California (Bell et al., 1995).Subsequent efforts applied this method to the identification ofQuaternary snakes (Bell and Mead, 1996; Czaplewski et al., 1999;Head et al., 2006; Head and Bell, 2007) and other lizards (Mead andBell, 2001 [in part]; Mead et al., 2008), crocodilians (Brochu, 2000,2007; Mead et al., 2002), anurans (Bever, 2005), and of entire

C.J. Bell et al. / Quaternary International 217 (2010) 30–3634

Pleistocene herpetofaunas (Bell et al., 2004; Kennedy and Bhullar,2008). The results of these applications paralleled those of Norell(1989); in most, but not all, cases, species-level resolution was notachieved using apomorphies, and in some cases generic-level reso-lution was unattainable. In all cases, stability was demonstrated athigher taxonomic levels; for example, natricine snakes were presentat high elevations in central Colorado during the middle Pleistocene,just as they are today (Bell et al., 2004).

There is no question that an apomorphy-based approach to theidentification of fossils is different from, and to some degreeinconsistent with, traditional approaches. In many cases it isimpossible to replicate traditional identifications when an apo-morphy-based approach is adopted. This was discussed to someextent for snakes by Head (2002) and Bell et al. (2004), and wasclearly demonstrated by Good (1988) and Bever (2005). Whenusing apomorphies to reassess Mesozoic and Tertiary fossilspreviously assigned to Gerrhonotinae, a group of North Americananguid lizards, only three of 166 specimens showed apomorphicevidence supporting assignation to Gerrhonotinae (Good, 1988).A reanalysis of all purportedly diagnostic features of the ilium inNorth American toads of the genus Bufo by Bever (2005), resulted inthe surprising conclusion that there were no diagnostic charactersor character combinations that would permit species-level identi-fication of extant species. In that case the failure at the species levelof an apomorphy-based approach was particularly striking becausethe ilium was long accepted as the most diagnostic and usefulskeletal element for identification of anuran species.

It is equally clear that in some cases finer-scale resolution isreadily available; for example, 32 unambiguous morphologicalsynapomorphies were identified for Phrynosoma by Reeder andWiens (1996), and species-level resolution is possible for someisolated skeletal elements within that clade (Norell, 1989). In othercases, species-level resolution remains elusive. This may bea result of higher-order ambiguity. For example, there are noknown apomorphies on trunk vertebrae of snakes that permitreliable separation of all natricine snakes from all elapid snakes(Bell et al., 2004). In these cases, it may be beneficial to utilizegeographic data for refinement, but that utilization should beexplicitly stated; for example, North American natricines can bedistinguished from North American elapids (Bell et al., 2004). Inmany cases, a less refined taxonomic resolution relative to thatachieved using standard methods results from inadequateunderstanding of morphology of the relevant groups, or froma paucity of data regarding apomorphic conditions in relevantskeletal elements of those groups. In at least two instances,authors used apomorphy approaches for particular groups withina biota, but not for the biota as a whole; this was the case forsnakes within Papago Springs Cave (Czaplewski et al., 1999) andsome of the squamates from the White Narrows fauna (Mead andBell, 2001). Clarity in presentation of data and communication ofmethods is key.

5. Discussion and conclusion

The North American Herpetofaunal Stability Hypothesis ulti-mately is based upon circular reasoning: stability is the onlyconclusion that can be drawn based on the way fossils specimenswere compared and identified. Stability at higher taxonomic levels(i.e., above the species) is demonstrable, but not necessarily infor-mative for the questions Quaternary paleontologists typically ask,many of which revolve around response of species lineages toclimatic or environmental perturbations.

Historical and most current methodological approaches to theidentification of Quaternary herpetofaunal remains relied almostentirely on a gross overall resemblance criterion. Geographic biases

of various scales were used to restrict the pool of extant specieswith which comparisons of fossils were made, and an appreciationof evolutionary morphology played a minimal role in the process ofidentification.

The identifications of these relatively young fossils may beaccurate, and in many cases are likely to be so. But it is imperativethat we separate our opinions, however well-justified they might be,from the scientific methods we use to test them. Maintenance of thestatus quo approach yields a false perception of clarity in ourunderstanding of the herpetofaunal fossil record throughout theQuaternary and results in an inadequate or erroneous understandingof faunal dynamics; the North American herpetofauna may havebeen taxonomically and geographically stable throughout theQuaternary, but stability has not been demonstrated at the specieslevel, and we simply do not know. The nearly consistent ability toachieve species-level resolution using skeletal resemblance withina context of a geographically and/or stratigraphically limitedcomparative sample also gives a false perception of clarity regardingour understanding of skeletal morphology. The ability to achievespecies-level resolution using that methodology provides nothing toenhance our understanding of skeletal variation in the search fornew, meaningful characters. In contrast, the lack of species-levelresolution that often results from a more global, apomorphy-basedapproach can only result in one of two conclusions – both of whichare evolutionary meaningful and serve as an impetus to conduct newstudies in comparative evolutionary morphology. One, the skeletalmodule in question did not undergo any skeletal transformationswithin the examined time frame (evolutionary stasis), or two, welack the comparative data that permit recognition of charactertransformations (apomorphies) that are present (Bever et al., 2008).

An apomorphy-based approach to understanding and interpret-ing the extant biota is now nearly ubiquitous. The wider communityof neontologists appreciates the potential importance of Quaternaryfaunal data, but most members of that community have onlyminimal understanding of the general biases of the fossil record andthe operational approaches used to identify Quaternary fossils. Asa result, paleontological data may be interpreted in ways that areinconsistent with the methodological and philosophical contextsunder which they were initially derived. The result is a perpetuationof misleading data into other disciplines, yielding a potentially falsesense of security in the foundations of biological science.

Apomorphy-based approaches to the identification of Quater-nary herpetofaunal fossils are possible, are ultimately based onappreciation of the evolutionary history of lineages and ofmorphology, and are independent of geographic and temporalbiases that strongly influence the status quo. Those approachesgenerally (but not universally) fail to yield species-level resolution.This is not a limitation of the method as much as it is a result oflimitation in the current state of knowledge of apomorphies withinherpetofaunal clades.

There certainly are challenges associated with the adoption ofan apomorphy-based approach to identification of Quaternaryremains. First, the vast majority of Quaternary fossils consist ofdisarticulated and dissociated elements, and evaluation of isolatedelements for apomorphies is now in an inchoate state of develop-ment. The few studies completed thus far clearly indicate thatundiscovered and unrecognized apomorphies exist in relevantanatomical systems, and can be derived from the study of isolatedskeletal elements. This ultimately is a benefit not only to Quater-nary paleontologists, but also to systematists seeking a morethorough and comprehensive data set to elucidate evolutionaryrelationships.

A more troublesome challenge is that the search for apomorphiccharacters on isolated elements must be conducted withina comprehensive understanding of evolutionary morphology, one

C.J. Bell et al. / Quaternary International 217 (2010) 30–36 35

that embraces and incorporates the many sources of variation inthe vertebrate skeletal system. Lamentably, adequate skeletalcollections simply do not yet exist to permit such an understanding.Ultimately, it may well be paleontologists who assume the burdenof building those collections, but we do so now in a logisticallychallenging time of increasingly acute conservation concerns.Where the appropriate collections do exist, a concentrated searchfor new apomorphic characters that permit species-level resolutioncan be undertaken. The search for apomorphies will yield clearrecognition of the gaps in our understanding of morphology andwill, therefore, help to guide our efforts in building additionalcollections and interpreting our data in comparative evolutionarycontexts. Special attention must be paid to seeking those charactersin the anatomical systems we typically find preserved in the fossilrecord. When apomorphic characters cannot be discovered, pale-ontologists must accept the limitations of our data (the morpho-logical features preserved on fossils), and identify specimens onlyto the taxonomic level that those data themselves actually permit.

Any method or approach to the identification of fossils comeswith biases or operational constraints. We are not making anargument of exclusivity; an apomorphy-based approach is not theonly method that can be used to identify fossils. However, such anapproach does permit an independent test of the conclusionsdrawn from traditional resemblance approaches that rely ongeographic and stratigraphic ‘parsimony’ to refine taxonomicresolution. We do not completely reject resemblance criteria. Ourunderstanding of apomorphies in relevant anatomical systems formost species is insufficiently mature to permit widespread andimmediate adoption of an apomorphy-based approach to identifi-cation of Quaternary fossils, at least at the species level. Never-theless, because all anatomical features arose at some point in thehistory of Life, the challenge for paleontologists is to discover thegenealogical level at which a particular feature is an apomorphy.Moreover, because species exist in particular times and places, wethink it may be appropriate to use those data to aid identification;but one cannot then use species identified in that way to drawconclusions about temporal and geographic relations withoutcircularity in reasoning. Whatever criteria are used, authors shouldexplicitly state them to enable subsequent generations clearly tosee the biases present in identifications and interpret themaccordingly. Specimen identifications constitute our primary data,and those data are used in secondary and tertiary analyses such asbiostratigraphic correlations, faunal turnover rates, paleoecologicalreconstructions, assessments of spatiotemporal shifts in faunalprovinces, and estimates of divergence times in lineages. Lack ofcare in the presentation of primary data can, therefore, haveunanticipated consequences (Bell and Bever, 2006).

Although hampered by current understanding of evolutionarymorphology and frustrated by the lack of adequate collections, anapomorphy-based approach is demonstrably viable, will yieldfruitful insights into the history of Quaternary biotas, and is free ofthe covert biases that currently hamper clear understanding ofQuaternary biodiversity and faunal dynamics. Such an approach isphilosophically and methodologically consistent with the domi-nant neontological paradigm, and is widely used by vertebratepaleontologists working with older fossil deposits (especially thoseof the Mesozoic). Its adoption by scientists studying Quaternarybiotas would facilitate greater integration of paleontological andneontological data. Although our focal point here is centered on theHerpetofaunal Stability Hypothesis, we recognize that many of ourarguments in favor of an apomorphy-based approach apply morebroadly to all Quaternary biotas.

At a more fundamental level it is important for Quaternarypaleontologists to accept the limitations of the data sets with whichwe work. It is incumbent upon us to seek out and explore the

potential benefits of alternative methods. Exploration of theconsequences of an apomorphy-based approach to identification offossils is underway. One consequence that emerges is that apo-morphy-based approaches typically do not yet yield species-leveltaxonomic resolution. In the short term (until apomorphies arediscovered that will yield species-level resolution), this may requirethat Quaternary paleontologists adjust their questions, or framethem more carefully. This inconvenience is, we suggest, a smallprice to pay for enhanced clarity in our data, and a more robust,repeatable, and testable set of hypotheses derived from theextensive Quaternary fossil record.

Acknowledgments

We benefited from years of both constructive and acrimoniousdiscussions with many colleagues who share our interest in thefossil record. Those who particularly helped us to frame thethoughts expressed in this paper are Anjan Bhullar, Chris Brochu,Julia Clarke, Russ Graham, Jason Head, Marci Hollenshead, ChrisJass, Ernie Lundelius, Jim Mead, Mark Norell, Jen Olori, Tim Rowe,Blaine Schubert, and Krister Smith. We gratefully acknowledge thepatience and encouragement we received from Eric Scott duringpreparation of this manuscript. Logistical support was provided byDennis Trombatore and Mary Poteet. Comments on earlier versionsof this manuscript were provided by Geraldine Swartz, Jim Mead,and an anonymous reviewer. Errors of fact, interpretation, andjudgment are our own, but we appreciate the input and insightsshared by all these colleagues.

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