Ontogenetic heterodonty in Reticulodus synergus (Chondrichthyes, Hybodontiformes) from the Upper Triassic of the southwestern U.S.A., with a redescription of the genus

ABSTRACT

Isolated teeth from the extinct hybodontoid Reticulodus synergus Murry and Kirby are known from Upper Triassic strata of Revueltian age in Arizona, Utah, and New Mexico, U.S.A. Here, we provide evidence for ontogenetic heterodonty in Reticulodus based on a reappraisal of the type and newly discovered material from the Upper Triassic Bull Canyon Formation in east-central New Mexico. We reexamined the ∼4000 specimens of the type series and measured representative teeth (n = 88) for mesiodistal length, crown width, and crown height. Reticulodus exhibits moderate monognathic heterodonty. We then assigned almost all teeth to one of three morphotypes (I–III) defined by ratios of crown width to mesiodistal length, with morphotype I being the most equilateral and morphotype III the most elongate. A fourth morphotype (IV) possesses very low crown width to mesiodistal length ratios and is tentatively considered as posterolateral in position. Adult teeth measure 1–10 mm long, with apical surfaces ornamented with numerous reticulations, crenulations, and pits, and appear subrounded, rectangular, or hexagonal in apical view—these teeth probably enabled a durophagous diet. Juvenile teeth measure approximately 0.7–2 mm in length, exhibit greater variation in crown shape than do adult teeth, and possess a clutching-type dentition that would facilitate the capture and consumption of softer-bodied organisms. The dentition thus transitioned from a clutching-type in juveniles to a grinding-type in adults, which would have allowed both adults and juveniles to occupy the same habitat and suppressed intraspecific competition between the two age groups.

ACKNOWLEDGMENTS

We thank L. Martin for providing the sediment that was picked for the shark teeth based on an agreement that he had with the landowner, whom we also thank. Discussions with A. Hungerbühler of the Mesalands Dinosaur Museum clarified the stratigraphy of the site. The MNA, especially J. Gillette, loaned the thousands of specimens of Reticulodus we reexamined here. P. Weaver of the NCSM accessioned and cataloged specimens we recovered from the ‘Clam Site.’ This paper is based on a senior thesis by J.T.V. and was supported by the Appalachian State University Office of Student Research and Department of Geology with both Undergraduate Research Assistantships and Travel Grants. A University Research Council Grant to A.B.H. provided screenwashing equipment. J. Fischer and D. K. Zelenitsky examined this paper in its preliminary stages and improved it with their suggestions. We also thank H. Avrahami, A. L. Hendrix, D. K. Hoffman, N. Brand, S. K. Jenks, and other members of ‘Finding Fossils on Fridays,’ without whom many of the fossils we recovered would have gone undiscovered and unidentified. The manuscript was handled by editor Dr. Charlie Underwood and improved by comments from reviewers Drs. Gilles Cuny and Jan Fischer.