Phylogenetic Taphonomy: a Statistical and Phylogenetic
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Drumheller and Brochu | 1 1 PHYLOGENETIC TAPHONOMY: A STATISTICAL AND PHYLOGENETIC 2 APPROACH FOR EXPLORING TAPHONOMIC PATTERNS IN THE FOSSIL 3 RECORD USING CROCODYLIANS 4 STEPHANIE K. DRUMHELLER1, CHRISTOPHER A. BROCHU2 5 1. Department of Earth and Planetary Sciences, The University of Tennessee, Knoxville, 6 Tennessee, 37996, U.S.A. 7 2. Department of Earth and Environmental Sciences, The University of Iowa, Iowa City, Iowa, 8 52242, U.S.A. 9 email: [email protected] 10 RRH: CROCODYLIAN BITE MARKS IN PHYLOGENETIC CONTEXT 11 LRH: DRUMHELLER AND BROCHU Drumheller and Brochu | 2 12 ABSTRACT 13 Actualistic observations form the basis of many taphonomic studies in paleontology. 14However, surveys limited by environment or taxon may not be applicable far beyond the bounds 15of the initial observations. Even when multiple studies exploring the potential variety within a 16taphonomic process exist, quantitative methods for comparing these datasets in order to identify 17larger scale patterns have been understudied. This research uses modern bite marks collected 18from 21 of the 23 generally recognized species of extant Crocodylia to explore statistical and 19phylogenetic methods of synthesizing taphonomic datasets. Bite marks were identified, and 20specimens were then coded for presence or absence of different mark morphotypes. Attempts to 21find statistical correlation between trace types, marking animal vital statistics, and sample 22collection protocol were unsuccessful. Mapping bite mark character states on a eusuchian 23phylogeny successfully predicted the presence of known diagnostic, bisected marks in extinct 24taxa. Predictions for clades that may have created multiple subscores, striated marks, and 25extensive crushing were also generated. Inclusion of fossil bite marks which have been positively 26associated with extinct species allow this method to be projected beyond the crown group. The 27results of this study indicate that phylogenies can and should be further explored for use as 28predictive tools in a taphonomic framework. 29 INTRODUCTION 30 Bite marks represent direct evidence of diet, feeding behavior (e.g., Davidson and 31Soloman 1990; Forrest 2003; Fuentes 2003), and even inter- or intraspecific fighting (e.g., 32Buffetaut 1983; Williamson 1996; Avilla et al. 2004; Katsura 2004) in the fossil record. These 33bone surface modifications exist at an intersection of ichnology and taphonomy, in which tooth 34shape and structure, jaw morphology and mechanics, and behavior act in concert to create Drumheller and Brochu | 3 35patterns of modification, accumulation, and destruction of remains. Patterns of modern bite 36marks are often observed in order to identify novel traces that could be used for identification 37and interpretation of similar structures in the fossil record (e.g., Njau and Blumenschine 2006; 38Milan et al. 2010; Westaway et al. 2011; Baquedano et al. 2012; Drumheller and Brochu 2014). 39However, sampling of modern bite marks has been uneven across and within clades, and many 40studies focus on a small number of modern groups, sometimes even a single species, to make 41predictions about diverse, extinct clades or morphologies. Drawing such broad conclusions based 42on analyses with strongly restricted study parameters may well be masking patterns of natural 43variation (Lyman, 1994; Haglund and Sorg, 1997). 44 For example, domestic dogs (Canis familiaris) and wolves (e.g., Canis lupus, Canis 45rufus) have an extensive bite mark literature (e.g., Binford 1981; Haynes 1982; Haglund 1997a) 46reflecting their common interaction with forensic sites, importance to studies of early 47domestication, and habit of caching and heavily modifying bones. Among mammals, hyenas 48(e.g., Haynes 1983; Cruz-Uribe 1991; Marean and Spencer 1991), great cats (e.g., Brain 1981; 49Haynes 1983; Domínguez-Rodrigo 1999), bears (e.g., Haynes 1982; Domínguez-Rodrigo and 50Piqueras 2003), and rodents (e.g., Brain 1981; Haglund 1997b; Klippel and Synstelien 2007) 51have also received a substantial amount of attention from bite mark researchers. At the other end 52of the spectrum, bite marks from only one squamate, the Komodo dragon (Varanus 53komodoensis), have been systematically studied, and this dataset stands as the sole modern 54datapoint for comparison with a diversity of ziphodont predators (D’Amore and Blumenschine 552009). 56 When multiple surveys within a clade do exist, it is unclear how the results should be 57synthesized, introducing a large degree of uncertainty into interpretations of ancient bite marks Drumheller and Brochu | 4 58based on comparisons to modern ones. Moreover, differing collection protocols and research 59methodologies can hinder statistical comparisons between these datasets. Researchers frequently 60fall back on simply comparing and contrasting their results (Haynes 1983; Drumheller and 61Brochu 2014). However, if a uniform collection protocol could be applied across multiple 62groups, more rigorous ways to explore patterns across the sampled taxa would become available. 63 Here, we present the results of an actualistic survey of bite marks collected from 21 64species of extant Crocodylia. Recent studies have expanded our existing knowledge of the bone 65surface modifications this clade can generate, including potentially diagnostic traces (Njau and 66Blumenschine 2006; Milan et al. 2010; Westaway et al. 2011; Baquedano et al. 2012; 67Drumheller and Brochu 2014). The fossil record of bite marks generated by crocodylians and 68their more distant relatives also has garnered increased attention and study (Davidson and 69Soloman 1990; Carpenter and Lindsey 1980; Erickson 1984; Schwimmer 2002, 2010; Forrest 702003; Fuentes 2003; Cisneros 2005; Mikulás et al. 2006; Martin 2013). In addition to serving as 71important predators and taphonomic agents in their own rights (Noto et al. 2012; Boyd et al. 722013), crocodylians are popular proxies for several extinct clades, including a variety of other 73archosaur-line reptiles (Brazaitis and Watanabe 2011; Drumheller et al. 2014). Therefore a better 74understanding of their feeding behaviors and resulting traces has broad potential for application 75to a large number of paleoecological and paleobiological questions. 76 We further explore patterns of feeding trace expression across this collection of 77crocodylian bite marks with associated metadata representing details characterizing each animal 78and collection methodology used during specimen collection. This allows statistical tests of 79significance between presence or absence of mark types and variables such as age, sex, specimen 80type, and collection protocol. The possibility that phylogeny could inform taphonomic patterns is Drumheller and Brochu | 5 81explored by applying a modified version of the extant phylogenetic bracket method (Witmer 821995). This study will serve not only as a survey of bite marks expected among crocodylians and 83their more distant relatives, but also as a case study demonstrating how future datasets 84addressing a variety of taphonomic and behavioral questions might be collected and synthesized. 85 MATERIALS AND METHODS 86 Bite Mark Collection and Preparation 87 Partially butchered cow hind limbs and pig femora were obtained from meat packaging 88plants and transported to the St. Augustine Alligator Farm (SAAF) in St. Augustine, Florida. 89Cow specimens retained significant amounts of flesh at the joints, and included articulated 90femora, tibiae, patellae, and varying numbers of tarsals depending on how much of the foot was 91removed during the butchering processes. Pig femora experienced more initial processing and 92were largely defleshed. 93 Bite marks were collected from 65 individuals, representing juveniles to very large 94adults, from 21 of the 23 generally recognized extant crocodylian species, excluding Gavialis 95gangeticus and Crocodylus palustris (Supplementary Table 1). Bite marks were collected from 96isolated individuals, so that veterinary data kept by SAAF staff could be correlated to specific 97samples (Fig. 1). Smaller animals were presented with isolated pig femora, and larger animals 98were given partially butchered cow hind limbs. Samples usually were collected once voluntarily 99abandoned, although some animals had to be distracted by handlers to prompt sample 100abandonment once active biting ceased. Animals were unrestrained while bite mark samples 101were collected, except when the SAAF staff felt that handler and animal safety was a concern. In 102those cases, often related to the small size relative to bone samples or the aggressive nature of the 103individuals, animals were held near the base of the skull by a handler or, with larger animals, Drumheller and Brochu | 6 104secured by the handler sitting on the back of the animal. Specific feeding behaviors, such as axial 105rolling or violent lateral thrashing, were recorded. Collection protocol and vital statistics (sex, 106mass, age, total length, snout-vent length, cranial length, and whether each animals was born in 107captivity or in the wild, restrained or free during specimen collection, and whether presented 108with cow limbs or pig femora) were recorded for each animal and are presented in 109Supplementary Table 1. 110 After collection, samples were defleshed and degreased via simmering in water with 111Borax®, an enzymatic detergent. After rinsing and manual removal of any remaining soft tissue, 112bones were cleaned again with Luminox® and Dawn®, milder, non-enzymatic detergents. 113Sponges