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

Abstract

ABSTRACTIsolated 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.