The smallest recorded specimen of Xiphactinus audax from the Niobrara Chalk of Kansas and its implications for juvenile ichthyodectid ecology

Abstract

ABSTRACT Juvenile specimens of Xiphactinus audax are rarely preserved in the fossil record and are typically represented by disarticulated cranial elements. Here we report the smallest specimen of Xiphactinus to date, consisting of the left and right dentaries and the right preoperculum. Based on the size of the new specimen and the rarity of small ichthyodectids in the Niobrara Chalk, it is interpreted as a juvenile. We suggest that the early life of Xiphactinus was spent in shallow margins of seaways for protection and to utilize rich food resources, and they were rare in open marine water. Today, juvenile predatory large-bodied fish are rare in open-water sediments, but shallow, nearshore deposits are lacking from the Western Interior Seaway, providing a complicating bias. The teeth of this juvenile X. audax specimen indicate that the diet did not notably change during growth and that even the small specimens would have been piscivorous predators. This is typical of large-bodied predatory fish such as tarpon that utilize estuaries and other shallow environs to mitigate juvenile mortality and do not change their diets much throughout ontogeny. This use of nearshore environments could explain why our Xiphactinus is so small and why such small specimens are rare.