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Masticatory Jaw Movement of Exaeretodon Argentinus (Therapsida: Cynodontia) Inferred from Its Dental Microwear

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Masticatory Jaw Movement of Exaeretodon Argentinus (Therapsida: Cynodontia) Inferred from Its Dental Microwear RESEARCH ARTICLE Masticatory jaw movement of Exaeretodon argentinus (Therapsida: Cynodontia) inferred from its dental microwear Tai Kubo1*, Eisuke Yamada1,2, Mugino O. Kubo3 1 The University Museum, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, Japan, 2 Department of Evolutionary Studies of Biosystems, School of Advanced Sciences, SOKENDAI (The Graduate University for Advanced Studies), Shonan Village, Hayama, Kanagawa, Japan, 3 Department of Natural Environmental Studies, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba, Japan a1111111111 * [email protected] a1111111111 a1111111111 a1111111111 Abstract a1111111111 Dental microwear of four postcanine teeth of Exaeretodon argentinus was analyzed using both two dimensional (2D) and three dimensional (3D) methods to infer their masticatory jaw movements. Results of both methods were congruent, showing that linear microwear features (scratches) were well aligned and mostly directed to the antero-posterior direction OPEN ACCESS in all four teeth examined. These findings support the palinal masticatory jaw movement, Citation: Kubo T, Yamada E, Kubo MO (2017) Masticatory jaw movement of Exaeretodon which was inferred in previous studies based on the observation of gross morphology of argentinus (Therapsida: Cynodontia) inferred from wear facets. In contrast, the lack of detection of lateral scratches confirmed the absence of its dental microwear. PLoS ONE 12(11): e0188023. the lateral jaw movement that was also proposed by a previous study. Considering previous https://doi.org/10.1371/journal.pone.0188023 microwear studies on cynodonts, palinal jaw movements observed in Exaeretodon evolved Editor: Alistair Robert Evans, Monash University, within cynognathian cynodonts from the fully orthal jaw movement of its basal member. AUSTRALIA Although there are currently only three studies of dental microwear of non-mammalian cyno- Received: February 9, 2017 donts including the present study, microwear analysis is a useful tool for the reconstruction Accepted: October 30, 2017 of masticatory jaw movement and its future application to various cynodonts will shed light Published: November 29, 2017 on the evolutionary process of jaw movement towards the mammalian condition in more detail. Copyright: © 2017 Kubo et al. 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. Introduction Data Availability Statement: All relevant data are within the paper. Evolution of postcanine occlusal pattern and masticatory jaw movement toward the mamma- lian condition among cynodonts has been studied intensively as one of the historical topics in Funding: TK was funded by JSPS KAKENHI Grant Number 17K14411 (URL:http://www.jsps.go.jp/ vertebrate paleontology [1±3]. The mammalian condition, which enabled precise occlusion of english/.) and Fujiwara Natural History Foundation upper and lower postcanines and jaw movement only on one side, increased their bite force (URL:http://fujiwara-nh.or.jp/), when this study was and consequently contributed to their success [1]. Further, masticatory jaw movements of conducted. cynodonts had been considered to have some phylogenetic significance; for example, palinal Competing interests: The authors have declared (i.e., backward) movement of the mandible of traversodontids and tritylodontids was consid- that no competing interests exist. ered evidence of their ancestor-descendant relationship [2]. However, due to the recent PLOS ONE | https://doi.org/10.1371/journal.pone.0188023 November 29, 2017 1 / 11 Jaw movement of Exaeretodon argentinus advances in understanding of the phylogeny of cynodonts [4], it is now considered that various jaw movement patterns of cynodonts were evolved as an adaptation for various diets. Palinal jaw movements were perhaps adaptations for an herbivorous diet, which evolved indepen- dently among traversodontids and tritylodontids, and also within mammaliaforms among har- amiyidans and multituberculates [3]. Occlusal patterns and jaw movements of non-mammalian cynodonts and early mammals have been largely inferred from the gross morphology of teeth by examining how cusps, ridges and basins of antagonistic teeth occluded with each other during mastication [2,3,5,6]. Based on these simple observation, jaw movements of Exaeretodon, a derived traversodontid that was found mostly from Late Triassic strata of Argentina and Brazil [7], was studied by Crompton [2] and Goñi and Goin [6]. Both studies assumed bilateral movement of mandible, in other words, there is no mobility at the mandibular symphysis and occlusion occurred simulta- neously in postcanine tooth rows of both mandibular rami. Both studies also assumed after the occlusion of upper and lower postcanines by orthal movement, palinal movement occurred as a masticatory jaw movement, which pulled the mandible backward against the upper jaw [2,6]. Goñi and Goin [6], but not Crompton[2], assumed after these orthal and palinal jaw move- ments, transverse movement occurred, where lower postcanines move medially against upper postcanines. Microwear analysis examines microscopic scars (e.g. pits and scratches) on tooth enamel surfaces that were produced during mastication. Historically, the study of dental microwear began for the purpose of reconstructing jaw movement, because the directions of scratches are likely to reflect the relative direction of jaw movement [8,9]. This inference was confirmed through experimental study on extant mammals [10]. It is now widely accepted that the direc- tion of scratches at occlusal surfaces reflects the relative movement of antagonistic teeth and direction of jaw movement can be reconstructed by integrating it with other information such as tooth morphology and tooth position in relation to the skull. Along with diet, jaw move- ment was often considered as the main information that can be revealed by dental microwear analysis [11]. This method was widely used in reconstructing the jaw movements of Mesozoic tetrapods [12±16], however, tooth microwear of non-mammalian cynodonts has been exam- ined only in two studies so far [15,17]. Grine [17] examined microwear of a basal cynognathian cynodont, Diademodon, and found pitted tooth surface texture that indicate direct pounding between upper and lower teeth due to the wholly orthal jaw movement. Goswami [15] showed that the microwear of the traversodontid, Dadadon isaloi, and two unnamed traversodontid specimens were dominated by scratches, which were oriented in a bimodal manner. Scratches of both upper and lower teeth predominantly run in a dorso-posterior orientation. Upper teeth display a second dominant orientation of scratches in an antero-posterior orientation, whereas the second dominant orientation of lower teeth is in the dorso-ventral orientation. These scratches were interpreted as the result of orthal and palinal jaw movements. The aim of this study is to analyze dental microwear of Exaeretodon argentines to recon- struct its jaw movement and compare the results with previous reconstructions based on gross morphology of wear facets [2,6]. The direction of the masticatory jaw movement was recon- structed by measuring the orientation of scratches from two dimensional images of tooth sur- faces and also by calculating the direction of surface texture anisotropy obtained from 3D surface data of dental microwear. Materials and methods Molds were taken from 13 teeth of six Exaeretodon argentines specimens stored in the Instituto y Museo de Ciencias Naturales of the Universidad Nacional de San Juan (PVSJ). Specimen PLOS ONE | https://doi.org/10.1371/journal.pone.0188023 November 29, 2017 2 / 11 Jaw movement of Exaeretodon argentinus numbers are PVSJ 74, PVSJ 109, PVSJ 623, PVSJ 702, PVSJ 707, and PVSJ 1091. These Exaere- todon specimens were collected from the Upper Triassic Ischigualasto formation (Carnian- Norian) in San Juan Province, northwestern Argentina [18]. The tooth surfaces of these speci- mens were examined using a stereomicroscope to locate well preserved shiny enamel regions. These surfaces were cleaned using acetone and cotton swabs and molded using Affinis Regular Body polyvinylsiloxane (Coltène Whaledent Inc. AltstaÈtten, Switzerland) that has a medium viscosity and medium consistency (ISO4823, Type2). The obtained dental impressions were scanned using a confocal laser microscope (VK-9700, Keyence Co, Japan) using a violet laser (wavelength 408nm) with 10× and 50× objective lens (numerical apertures 0.30 and 0.55 respectively) to obtain surface images. On each of the XY coordinate points, light intensity of reflected laser, color information and height data (Z position) were obtained at the focus point. Using these data the VK-9700 device generates real color ultra-depth images. By exam- ining images of mold surfaces, preservation of microwear was confirmed for four specimens. Specimen number and anatomical positions of these four specimens are: the posterior edge (hereafter referred as transverse ridge following [2]; see Fig 1B for its position) of the second postcanine tooth of right upper jaw of PVSJ 1091 (Fig 1); the posterior lingual cusp (hereafter referred as internal cusp following [2]; see Fig 1B for its position) of the second postcanine tooth of left upper jaw of PVSJ 707; and lingual side of occlusal surfaces of third and fourth postcanine
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