Palaeontologia Electronica palaeo-electronica.org Oriented microwear on a tooth of Edestus minor (Chondrichthyes, Eugeneodontiformes): Implications for dental function Wayne M. Itano ABSTRACT The symphyseal tooth whorls of the Carboniferous chondrichthyan Edestus con- sist of files of teeth having sharply-pointed, serrated crowns, joined at their bases. A single tooth whorl was present in each jaw. How these tooth whorls functioned is unclear, since their convex curvature allows only a few of the most lingual crowns of opposing tooth whorls to occlude. Rather than working in opposition, like scissors, the more labial teeth might have been used to cut and disable prey with a vertical motion of the anterior part of the body. Provided the scratches observed on the surface of Edes- tus teeth can be inferred to have been generated in the process of feeding, their orien- tation might be used to distinguish whether the teeth were used mainly in occlusion, to cut prey trapped between the jaws, or mainly to cut prey situated outside the oral cav- ity. Edestus minor teeth having unusually good surface preservation were examined for microwear. The teeth are from the Strawn Group (Desmoinesian, Middle Pennsylva- nian) of San Saba County, Texas, USA. The best-preserved crown surfaces display scratches 50 to 500 micrometers long. The scratches are oriented predominantly transversely to the basal-apical axis. This observation appears to support the vertical slashing hypothesis. However, the possibility that interaction with the substrate contrib- uted to the observed wear cannot be discounted. Wayne M. Itano. Museum of Natural History, University of Colorado, 1995 Dartmouth Ave., Boulder, Colorado 80305, USA. [email protected] Keywords: Pennsylvanian; Texas; Chondrichthyes; Edestidae; sharks; functional morphology; dental microwear Submission: 3 November 2018. Acceptance: 26 May 2019. INTRODUCTION is a chondrichthyan in the order Eugeneodon- tiformes Zangerl, 1981, that possessed symphy- Edestus sensu lato (including Lestrodus seal tooth whorls of similar sizes in both jaws. It is Obruchev, 1953, and Edestodus Obruchev, 1953) known from Pennsylvanian deposits ranging in age Itano, Wayne M. 2019. Oriented microwear on a tooth of Edestus minor (Chondrichthyes, Eugeneodontiformes): Implications for dental function. Palaeontologia Electronica 22.2.39A 1-16. https://doi.org/10.26879/831 palaeo-electronica.org/content/2019/2603-edestus-minor-microwear Copyright: July 2019 Society of Vertebrate Paleontology. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. creativecommons.org/licenses/by/4.0/creativecommons.org/licenses/by-nc-sa/4.0/ ITANO: EDESTUS MINOR MICROWEAR from Bashkirian to early Kasimovian and geo- part of the body by using the pectoral fins like graphically from North America, Britain, and Russia wings (Itano, 2014, p. 215-216). At present, there (Itano et al., 2012). is no postcranial fossil evidence of Edestus to Edestus sensu lato may be unique among either support or refute either of these two possibil- chondrichthyans in possessing two large, symphy- ities. seal tooth whorls of similar sizes, with blade-like, Macrowear sustained during life has been sharply-pointed, serrated crowns. Teeth are shed reported on a tooth of E. minor from the Smithwick from the outer, labial end and replaced at the inner, Shale (Pennsylvanian, late Atokan = Moscovian) of lingual end, as in a normal chondrichthyan tooth San Saba County, Texas, USA (Itano, 2015). The file. This unusual dentition is best known from the apex of the crown is truncated, and the surface of holotype of Edestus mirus Hay, 1912, which shows the remaining part is smooth and convex, as if both tooth whorls of a single individual (Figure 1). worn by repeated contact with an abrasive surface, The convex curvature of the tooth whorls prevents such as the skin of a large fish covered with scales all but the two or three lingualmost crowns of each or denticles. The fact that the abraded surface is whorl from occluding with the corresponding roughly perpendicular to the basal-apical axis of crowns of the opposing whorl. Itano (2014) pro- the crown appears to support the slashing-mode posed that the tooth whorls were used in an uncon- hypothesis over the scissors-mode hypothesis. ventional way, not to cut prey caught between Similar apical wear has recently been reported on opposing tooth whorls (scissors-mode), but to two crowns of a partial tooth whorl of E. heinrichi slash prey situated outside the oral cavity, with a from a shale bed overlying the Herrin (No. 6) Coal vertical motion of the whole head (Figure 2). (Pennsylvanian, late Desmoinesian = late Mosco- Given that possession of a flexible neck, as in vian) of Randolph County, Illinois, USA (Itano, the Devonian tetrapod-like fish Tiktaalik (Daeschler 2018). et al., 2006), is unknown in any chondrichthyan, Observations of dental microwear have long the vertical motion might have been carried out by been used to elucidate dental function and diet in (1) flexing of the whole body between a U and extant and extinct animals. The first application of inverted-U shape or (2) vertical motion of the front FIGURE 2. Reconstruction of the head of Edestus, illus- trating the hypothesized, whole-head, vertical slashing motion. The curvature of the tooth whorls is based on FIGURE 1. USNM V7255, holotype of Edestus mirus the holotype of Lestrodus (originally Edestus) newtoni. (scale in cm). Copyright © Smithsonian Institution, all Modified with permission from copyrighted artwork by G. rights reserved. Used with permission. Raham, www.biostration.com. 2 PALAEO-ELECTRONICA.ORG dental microwear in paleontology seems to have been by Simpson (1926), who used scratch direc- tions to infer chewing movements in Mesozoic mul- tituberculate mammals. More recently, 2D, feature- based dental microwear observations have been used to infer diet in mammals, as in distinguishing browsers from grazers, using a scanning electron microscope (SEM) (Walker et al., 1978) or a low- magnification binocular light microscope (Solounias and Semprebon, 2002). 2D microwear analysis has been applied to fishes in a few cases (Purnell et al., 2006; Purnell et al., 2007). A 2D microwear analysis relies on human identification of features such as scratches and pits and is sub- ject to strong intra- and inter-observer errors (Mihl- bachler and Beatty, 2012). A 3D dental microwear texture analysis uses a confocal profilometer and scale-sensitive fractal analysis to measure surface textures without requiring the counting of individual features (Ungar et al., 2003; Scott et al., 2006). A 3D dental microwear analysis is subject to smaller intra- and inter-observer errors (DeSantis et al., FIGURE 3. Schematic diagram showing the orientation 2013; Arman et al., 2016) but requires specialized of feeding-related scratches (red) on a tooth of Edestus for tooth whorls used in opposition (1) or in vertical equipment. The method has been applied to stud- slashing mode (2). Serrated crowns are blue, lingually ies of dental microwear in mammals (e.g., DeSan- extended bases are brown. tis, 2016) and fishes (Purnell et al., 2012; Purnell and Darras, 2016), including, very recently, sharks (McLennan, 2018). that those outer teeth carry a heavy cost in terms of In the present study, a form of 2D microwear weight and water resistance, so it is not obvious analysis was used. The orientation of microwear why they would be retained. features on the crown surfaces of an Edestus tooth Other chondrichthyans, both extant and might help to distinguish between the two afore- extinct, have had non-occlusal tooth-like struc- mentioned alternative hypotheses (scissors vs. tures, but attached to their rostra. These chon- slashing). Scratches oriented mainly parallel to the drichthyans comprise the extant Pristidae (sawfish) basal-apical axis would support the hypothesis that and Pristiophoridae (sawsharks) and the extinct the whorls were used in opposition, like scissors Sclerorhynchidae. Analogies in function between (Figure 3.1), while scratches oriented mainly per- Edestus tooth whorls and the toothed rostrum of pendicular to that axis would support the vertical- the extant sawfish Pristis were proposed soon after slashing hypothesis (Figure 3.2). This presupposes the discovery of Edestus (Hitchcock, 1856; Leidy, that the observed wear is indeed feeding-related, 1857). The rostrum of Pristis microdon is used both which is not known a priori. There is also a possibil- to sense and to capture prey, by stunning or impal- ity that scratches could have been generated ing them or by pinning them to the substrate through interaction with the substrate. In addition, (Wueringer et al., 2012). Observations of wear acquired postmortem needs to be distin- microwear on rostral teeth of the sawshark Pristio- guished from wear acquired in vivo. phorus cirratus suggest that it also uses its rostrum The two hypothesized functions of the teeth to capture prey, though not necessarily to impale need not be mutually exclusive. Possibly teeth at them (Nevatte et al., 2017). In that work, numbers an early stage of ontogeny might have been used of scratches were counted, but the directional dis- in occlusion, while they were still within the oral tributions of the scratches were assessed only cavity, and then used in a slashing, non-occlusal qualitatively. The extinct sclerorhynchid manner at a later stage of ontogeny. However, the Schizorhiza stroemeri has rostral teeth that are tri- possibility that the innermost teeth were used to cut angular, with sharp cutting edges, so that its ros- prey, while the outer two-thirds to three-quarters of trum might have been used as a slashing weapon, the teeth had no function runs into the objection in a similar manner to the function that is hypothe- 3 ITANO: EDESTUS MINOR MICROWEAR sized here for the Edestus tooth whorls (Kirkland Edestus teeth in museum collections originate from and Aguillón-Martinez, 2002; Smith et al., 2015).