SEXUAL DIMORPHISM IN THE CRETACEOUS-PALEOCENE BAENID TURTLE NEURANKYLUS

Fossil turtles are often studied as though they were static objects rather than the remains of living, growing individuals. This is particularly true of archaic groups with no living descendants. Sexual dimorphism has largely been either lumped in with more random variations or, in some cases, the two genders were recognized as distinct species. Baenid turtles are a family of cryptodire turtles from Cretaceous-Eocene strata of western North America, and include Paleocene-Eocene Neurankylus, particularly well known from the San Juan Basin of New Mexico and the Grand Staircase-Escalante National Monument of Utah. Several large specimens of Neurankylus baueri excavated in the Kaiparowits Formation of Utah were described as two distinct taxa, N. hutchisoni and N. utahensis. This was in part based on the assumption that the smaller Neurankylus known from the San Juan Basin were ontogenetically mature. We suggest that rather than being mature and not fusing their shells at maturity, as in other baenids, the San Juan Basin specimens were still immature and thus capable of further growth. These larger specimens in Utah show the same pair of one elongate and one round shell morphotype seen in the San Juan Basin specimens. It is more parsimonious to consider them one species with two morphotypes than two individual lineages that happen to always occur together in the Kaiparowits, Fruitland, Kirtland and Nacimiento formations, independently. The most common form of dimorphism present in extant turtles with two morphotypes is sexual dimorphism, most commonly with the male being smaller. With this will often come differences in shell shape, such as that seen in Graptemys, in which females are more elongate as well as larger. Another common dimorphism seen in some modern river turtles is that midline spines present in both genders as juveniles will be more reduced in one gender. For example, many adult female Graptemys almost completely lack these midline spines but they remain prominent in males. A great degree of variation in the development of the midline spines is seen in Neurankylus and may have a similar cause. Thus, we consider N. hutchisoni and N. utahensis to be based on two mature sexual dimorphs of N. baueri.