Paleobiology of the Late Cretaceous sclerorhynchid sawfish,Ischyrhiza mira (Elasmobranchii: Rajiformes), from North America based on new anatomical data

Abstract

We describe seven associated skeletal remains of Ischyrhiza mira, a Late Cretaceous sclerorhynchid sawfish, from the Campanian?lower Maastrichtian of Tennessee and Alabama, U.S.A., to decipher its paleobiology. Ischyrhiza mira had about 16 or 17 functional spines and about the same number of replacement spines on each side of the rostrum in which tall erect spines occupied the anterior one-half to two-third of the rostrum followed posteriorly by smaller spines. Whereas small hat-shaped dermal denticles were distributed on the rostrum, large thorn-like dermal denticles were present on the dorsal side of the body characteristic of sluggish, benthic batoids. We concur with the interpretation that specimens previously identified as rostral spines of Peyeria are actually enlarged thorn-like dermal denticles of a sclerorhynchid. We suggest that the ratio between the rostrum length and total body length of sclerorhynchids was generally about 1:3.27. Our vertebra-based ontogenetic analysis of I. mira gives an age estimate of 12.4 years for a 190-cm-long individual, the size at birth of about 0.5 m, and the maximum possible length for the species of no more than 3 m. Compared to extant pristid sawfishes, I. mira probably became sexually mature much earlier with a slightly faster rate of rostrum development.