Quantitative Analysis of Conodont Tooth Wear and Damage As a Test of Ecological and Functional Hypotheses
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Quantitative analysis of conodont tooth wear and damage as a test of ecological and functional hypotheses Mark A. Purnell and David Jones This pdf file is licensed for distribution in the form of electronic reprints and by way of personal or institutional websites authorized by the author(s). A copyright license has been purchased to make this possible. Paleobiology, 38(4), 2012, pp. 605–626 Quantitative analysis of conodont tooth wear and damage as a test of ecological and functional hypotheses Mark A. Purnell and David Jones Abstract.—Analysis of dental wear and damage is becoming an increasingly important tool in unraveling the trophic ecology of a wide range of vertebrates, and when applied to fossils it can provide evidence of both diet and feeding kinematics that is independent of morphological analysis. Conodonts have the best fossil record among vertebrates and their skeletal elements are known to exhibit surface wear and damage generated in vivo as a consequence of their function as teeth. We report the results of the first systematic survey and analysis of the frequency and extent of this wear and damage in conodonts (based on P1 elements from a range of Carboniferous genera). This has revealed that wear and damage are remarkably common, present in all conodont elements sampled. Multivariate analysis reveals that patterns of wear and damage differ significantly among different conodont taxa, and exploratory ANOVA and linear discriminant analyses show that wear and damage differ according to the position of taxa in an onshore-offshore gradient, and whether they are likely to have had a benthic or pelagic mode of life. The incidence of denticle tip spalling in particular is higher in more-offshore environments and in taxa likely to have had a pelagic mode of life. Aspects of the data also reflect the occlusal kinematics of the elements, providing a means of testing hypotheses of element function. Our results have wide-ranging implications for unlocking the fossil record of conodonts, by, for example, furnishing direct evidence of the diet-mediated processes that may have driven observed patterns of evolutionary change, and reducing the confounding effects of depth segregation when using conodonts in isotope-based paleotemperature studies. Mark A. Purnell* and David Jones.** University of Leicester, Department of Geology, Leicester LE1 7RH, United Kingdom. E-mail: [email protected], [email protected]. *Corresponding author. **Present address: Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, United Kingdom Accepted: 11 April 2012 Introduction in large part upon understanding their ecolo- Conodonts are an important clade of early gy. Although conodonts were nektonic, pelag- vertebrates. The toothlike skeletal elements of ic taxa, particularly those occupying the their oropharyngeal feeding apparatus have a oceanic realm, are preferable for biostratigra- superb fossil record (Foote and Sepkoski 1999; phy because they are more likely to have wide Purnell and Donoghue 2005), and conodonts distributions and are less likely to be con- are consequently the paramount group for trolled by local environmental heterogeneity. biostratigraphic correlation and relative dat- Ecological factors are frequent drivers of ing through most of the Paleozoic and Triassic. evolutionary change. Interpretation of shifts The quality of their fossil record also makes in conodont oxygen isotope values in terms of them an invaluable resource for studying sea-surface temperature and/or sea-level evolutionary pattern and process. Additional- changes must exclude alternative explanations ly, conodonts are becoming increasingly im- that differences reflect variation in the depth, portant tools for paleotemperature studies and hence temperature, at which conodonts based on stable isotope analysis (e.g., Wenzel lived. et al. 2000; Joachimski and Buggisch 2002; Nonetheless, many fundamental aspects of Joachimski et al. 2006; Bassett et al. 2007; conodont ecology are poorly understood. Buggisch et al. 2008; Zigaite et al. 2010; Chen Facies distributions and patterns of taxon co- et al. 2011). occurrence have been documented and re- The biostratigraphic, evolutionary, and pa- viewed (e.g., Driese et al. 1984; Merrill and leoclimatological utility of conodonts depends von Bitter 1984; Pohler and Barnes 1990; Ó 2012 The Paleontological Society. All rights reserved. 0094-8373/12/3804-0007/$1.00 606 MARK A. PURNELL AND DAVID JONES Zhang and Barnes 2000), but determining the 2. Do patterns of surface wear and damage range of depths at which conodonts lived, and vary significantly among taxa? whether particular species occupied benthic or 3. Do patterns of surface wear and damage pelagic niches, is generally achieved using differ between conodonts that inhabited simplistic ecological models of spatial distri- different environments? bution and lateral segregation that are known In order to address these questions and test to be unreliable (Klapper and Barrick 1978). the associated null hypotheses, we have Similarly, understanding of conodont ele- developed the first rigorous protocols for ment functional morphology is rudimentary recording, quantifying, and analyzing the and limited to a few taxa (Purnell and von frequency and extent of wear/damage in Bitter 1992; Donoghue and Purnell 1999b), and conodont elements. knowledge of conodont dietary niches and how they differed between taxa is essentially Previous Observations of Surface Wear and non-existent. Consequently, evaluating the Damage in Conodonts selective pressures on conodont element mor- Despite some early observations of possible phology, and testing hypotheses that evolu- surface wear in some conodont elements, a tionary changes were adaptive, is currently strong consensus developed among cono- impossible. donts workers that elements generally do not Analysis of wear and damage in conodont show surface wear, and that it is certainly not elements may provide more robust constraints present at the levels that would be expected if on conodont ecology and element function. In elements had functioned as teeth (Huddle mammals, analysis of how tooth shape is 1934; Hass 1941; Rhodes 1954; Pierce and changed by wear from feeding (mesowear; Langenheim 1970; Nicoll 1987). Two of these Fortelius and Solounias 2000; Kaiser and studies were detailed SEM-based investiga- Solounias 2003), analysis of tooth breakage tions that failed to detect even microscopic (Van Valkenburgh 2009), and analysis of the evidence of wear (Pierce and Langenheim microscopic textures developed on tooth wear 1970; Nicoll 1987). Weddige’s (1990) work facets (microwear; e.g., Walker et al. 1978; documenting ‘‘pathologies’’ of conodont ele- Scott et al. 2005), furnishes direct information ments did include surface wear (his ‘‘abrasio’’ on diet and trophic niche. Previous studies of pathology) but the possible functional signif- dental wear and damage have focused pre- icance of this was not explored because dominantly on primates and ungulate mam- Weddige suspected it was postmortem in mals, but tooth wear studies are starting to be origin. Some of Weddige’s other classes of applied more widely, for example, to test ‘‘pathology’’ we would interpret, as he did, as hypotheses of feeding mechanics in non-avian damage resulting from in vivo toothlike dinosaurs (Williams et al. 2009) and to occlusion between elements. differentiate between fishes (living and fossil) More recent SEM-based work was able to occupying benthic and limnetic trophic niches go further, partly because of clearer under- (Purnell et al. 2006, 2007) and with different standing of P1 element occlusion (Fig. 1) diets (Purnell et al. 2012). Such studies suggest (Purnell 1995; Donoghue and Purnell 1999a). that analysis of dental damage and wear has This allowed interactive surfaces on which the potential to provide similar constraints on functional wear might develop to be differen- conodont ecology and function. Our aim here tiated from non-occlusal surfaces on which is to present a novel approach to interrogating damage was likely to be postmortem in origin the conodont fossil record, and to illustrate its (a key factor in studies of tooth wear in fossils potential, by testing null hypotheses linked to [Teaford 1988]). Thus Purnell (1995) docu- the following questions: mented wear patterns that provide direct evidence that elements functioned as teeth. 1. Do conodont elements commonly exhibit This was confirmed by subsequent analyses of surface wear and damage? surface wear and damage in occlusal pairs of TOOTH WEAR, FUNCTION, AND ECOLOGY OF CONODONTS 607 FIGURE 1. Diagram illustrating arrangement of elements within the conodont oropharyngeal skeleton, with biological orientation terms. Modified from Purnell et al. (2000) and Purnell (1995). P1 elements from articulated skeletons that, of surface wear (Jeppsson 1979; Donoghue because they were articulated, could not have and Purnell 1999a) such that only those been subject to postmortem abrasion (Donog- elements from conodonts that died toward hue and Purnell 1999b). the end of a functional cycle will preserve Donoghue and Purnell (1999a) provided evidence of significant wear. further examples of functional damage in Despite the evidence that conodont ele- elements, preserved on the external surface ments do in fact preserve wear on occlusal (the functional surface at time of death) and as surfaces (i.e., in exactly the places where it internal discontinuities