Morphology and phylogeny of some Early Silurian "Diplograptid" genera from Cornwallis Island, Arctic Canada

A diverse assemblage of Llandovery (mainly Aeronian), biserial graptoloids has been recovered from the Cape Phillips Formation on Cornwallis Island, Arctic Canada. A study of the astogenetic patterns among these taxa can be used to define phylogenetically meaningful taxa at the generic and suprageneric level. Using previously defined astogenetic patterns, five families can be defined within the Diplograptoidea: (1) Normalograptidae - Pattern H genera including Normalograptus, the root stock of the Monograptoidea; (2) Petalolithidae - Pattern I genera including Petalolithus and Glyptograptus sensu stricto; (3) Retiolitidae-Pattern R, the Silurian ancora-bearing retiolitids; (4) Dimorphograptidae - Pattern J, including the uni-biserial Dimorphograptus and the fully biserial Akidograptus and Parakidograptus; (5) Monograptidae - the uniserial, Pattern M. Of these five families, only the Normalograptidae occur in strata lower than highest Ashgill. Normalograptus — Akidograptus — Dimorphograptus - Atavograptus is suggested as a likely evolutionary sequence for the origin of the monograptids. This is consistent with the observed astogenetic patterns, requires few astogenetic modifications (apomorphies) at each step, and is not in conflict with the currently known stratigraphical ranges of these taxa.