MORPHOMETRIC ANALYSIS REVEALS A NEW GIANT HELICOPRION SHARK FROM THE PERMIAN PHOSPHORIA FORMATION, SOUTHEAST IDAHO

Helicoprion belonged to a long-lived, but dead-end, lineage of Pennsylvanian—Triassic sharks that evolved an enigmatic whorled dentition. Ten species of Helicoprion are described worldwide on the basis of very few incomplete specimens, and almost no accounting for variation within or among species concepts. Collections at the Idaho Museum of Natural History house more than 15 Helicoprion specimens from the Waterloo and Gay phosphate mines of southeast Idaho, including previously described H. ferrieri and three specimens of very large whorls. This study conducted morphometric analysis on the Phosphoria Helicoprion specimens to assess the variability in tooth and whorl dimensions, and to compare them to published measurements for various species.

Two distinct size groupings of the Phosphoria Helicoprion specimens emerge from the analysis. Both share similar maximum ranges in tooth count (up to 125–150) and number of volutions in the whorl (3.5–4), but differ in maximum tooth size and total whorl diameter. The giant morph grew a whorl up to 45 cm in diameter and its 128^th tooth measures 97.9 mm in height; whereas the small morph, H. ferrieri, grew a whorl up to 25 cm in diameter and its 130^th tooth measures 29.2 mm. In all specimens tooth height, and width as a function of tooth count follow a two-phase growth trend characterized by accelerated growth following the first ~80 teeth, but in the giant morph the accelerated second-phase growth is more than 5 times that of the small H. ferrieri, which accounts for the striking size difference among these specimens. The distinct size range and growth history of the giant Helicoprion whorl likely merits new taxonomic classification, having formed in an animal conservatively estimated to exceed 10 m in body length. Variation among the Phosphoria specimens appears to overlap with some previously described type specimens of Helicoprion, suggesting taxonomic revision may be necessary for several species in the genus.