Disarticulated centra recovered from the Campanian Dinosaur Park Formation (Cretaceous) are used to describe hiodontid age, growth, and longevity. Previous studies of Cretaceous centrum morphology have shown only one morphotype. However, growth characteristics can be used to test the one-taxon hypothesis. Age and growth characteristics indicate that Cretaceous hiodontids were shorter-lived (i.e., 7 years) and significantly smaller than extant Hiodon. The fossil centra also exhibit two significantly different growth patterns. Initially it was predicted that two factors could explain the presence of the two growth patterns: (1) sexual size dimorphism, and (2) variation in centrum size from multiple species. Sexual size dimorphism did not produce the two growth patterns because (1) H. alosoides maximum total length is significantly correlated with temperature, and (2) the paleotemperature in the depositional environment fluctuated approximately 6 °C mean annual temperature resulting in continuous growth variation. Concerning the second factor, the coefficient of variation (CV) observed in the Cretaceous centrum size data (0.23 CV) is 22% greater than that of the two extant species of Hiodon combined (0.18 CV) suggesting that at least two species of hiodontids were present in the Dinosaur Park Formation. The distribution of sizes of the smaller Cretaceous centra is not different from one extant species but is significantly different from two species suggesting that three taxa were not present.

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