Heckethornia, a new genus of dimeropygid trilobites from the Lower Ordovician (Ibexian; Tulean and Blackhillsian) of the Great Basin, western USA

Abstract

Heckethornia n. gen. is a morphologically striking clade of dimeropygid trilobites known from the Pogonip Group (Ordovician; Ibexian Series, Tulean and Blackhillsian stages) of western Utah and eastern Nevada. It includes seven species based on silicified material. All occur in the Fillmore Formation of western Utah, and four are also known from the Yellow Hill Limestone of eastern Nevada. Diagnostic features of Heckethornia include a highly vaulted exoskeleton with a tall pygidial “wall” made of fused outer pleurae, three pairs of large glabellar spines, two to three pairs of prominent pygidial spines, a single (or paired) large occipital spine(s), and an arc of tubercles on the librigenal field. Cladistic parsimony analysis suggests that the genus comprises two subclades, one including species with a single median occipital spine, and species with paired occipital spines or tubercles in the other. New species are H. smithi , H. hyndeae , H. numani , H. bowiei , H. morrisseyi , and H. ballionae .