Variation and Implications of Intra-Dentition Hunter-Schreger Band Pattern in Fossil Hyaenids and Canids (Carnivora, Mammalia)

Cenozoic mammalian carnivores evolved stereotypical morphologies that can be correlated to adaptations for specific dietary capabilities (Werdelin, 1996; Van Valkenburgh, 2007). The diagnosis of such ecomorphologies is based on a suite of morphological characters of the teeth and skeleton, using a presumed relationship between form and function (Van Valkenburgh, 1985, 1988; Lauder, 1995). Among the morphological indicators of functional capability in extinct carnivorans, a series of studies on the enamel microstructure of carnivoran teeth have shown a well-established link between enamel prism decussation patterns and diet in modern carnivorans, and their equivalents in the fossil record (Tomes, 1906; Stefen, 1994, 1997; Rensberger and Wang, 2005; Rensberger and Stefen, 2006; Ferretti, 2007). In this study, new data are presented on evolutionary patterns revealed by examination of enamel microstructure using optical light, highlighting aspects of intra-dentition variation of enamel microstructure in extinct hyaenid and canid carnivorans. A general survey of enamel microstructure in carnivorans by Stefen (1994, 1997) demonstrated the widespread occurrence of structural modification and a gradient of complexity in the arrangement and decussation of enamel prisms. Layers (or bands) of prisms that decussate in opposing orientations are called Hunter-Schreger bands (HSBs). The three-dimensional organization of enamel prisms that occur in carnivorans can be categorized into three major types of HSBs: undulating, acute-angled, and zig-zag (Stefen, 1997, 1999). These types were defined by Stefen (1997) based on the approximate angles of the enamel bands, measured as the angle between the boundaries of a given enamel prism fold: >140◦ for undulating, 140◦–70◦ for acuteangled, and 70◦–50◦ (or smaller) for zig-zag HSBs. Each type represents a grade along which the teeth possessing them are adapted for consumption of hard foods; undulating HSB is the plesiomorphic condition, whereas extensive zig-zag HSB is found in specialized bone-cracking ecomorphologies (Stefen, 1997). There is a close association between zig-zag HSBs in premolars and bone-consumption in modern spotted hyaenas (Stefen and Rensberger, 2002; Rensberger and Stefen, 2006). Convergently evolved zig-zag HSBs are observed in extinct borophagine canids (Stefen, 1999; Rensberger and Wang, 2005). Specialized HSB patterns have also been observed in other mammal groups such as creodonts, condylarths, and entelodonts (Stefen, 1997). Previous studies of HSB adaptation in fossil hyaenids and canids, and indeed for most other carnivorans, focused on only a few tooth positions and reported broad trends of specialization across lineages (Stefen, 1994, 1999; Ferretti, 2007). Others reported on variation of HSB orientations within a single tooth (Rensberger and Wang, 2005; Rensberger and Stefen, 2006). Even though this latter variability of intra-tooth HSB pattern has been recognized by many workers, the variability of HSB patterns across different tooth positions within the dentition of a given taxon has not been dealt with in more detail. Whereas broad trends of HSB changes through the evolutionary history of entire lineages can contribute evidence regarding patterns of changing diets and paleoecology, the pattern by which different teeth in the dentition evolved specialized HSB patterns can further reveal potential selective drivers in the evolving functionality of the heterodont carnivoran dentition. Focusing on such dentition-level patterns, which have been previously recognized but not emphasized (Stefen, 1999; Stefen and Rensberger, 1999), this report describes the intra-dentition variation of HSB enamel microstructure within two carnivoran lineages. Enamel microstructure patterns of fossil hyaenids and canids were studied to demonstrate the utility of examining entire dentitions in revealing trends and timing of structural adaptations that complement more generalized surveys of higher-level lineages and groups. Institutional Abbreviations—AMNH, American Museum of Natural History, New York, U.S.A. ; BM, Natural History Museum, London, U.K.; F:AM, Frick Collection, American Museum of Natural History; HMV, Hezheng Paleozoological Museum, Gansu, China; IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; LACM, Natural History Museum of Los Angeles County, Los Angeles, California, U.S.A. ; LACMCIT, California Institute of Technology collection, housed in the Natural History Museum of Los Angeles County; PMU, Evolutionsmuseet, University of Uppsala, Sweden; PPHM, Panhandle-Plains Historical Museum, Canyon, Texas, U.S.A. ; UCMP, University of California Museum of Paleontology, Berkeley, California, U.S.A. ; USNM, National Museum of Natural History (Smithsonian), Washington, D.C., U.S.A.