Abstract

Reinterpretation of two enigmatic Canadian edrioasteroids, Thresherodiscus ramosus Foerste, 1914 from the Upper Ordovician Verulam Formation of Ontario and Lispidecodus plinthotus Kesling, 1967 from the Lower Mississippian Banff Formation of Alberta, shows that previous accounts of these taxa were largely erroneous. Unlike other isorophid edrioasteroids, Thresherodiscus has branching ambulacra, an unusual hydro-gonopore, differentiated interambulacral plates bearing respiratory structures, and ambulacra that extend onto the peripheral rim. Except for the turreted thecal shape, Lispidecodus is a rather typical discocystinid edrioasteroid. Phylogenetic analysis weakly supports the placement of Thresherodiscus within Agelacrinitidae and strongly places Lispidecodus within Discocystinae.