Unusually preservedMetaconularia manni(Roy, 1935) from the Silurian of Iowa, and the systematics of the genus

Abstract

The Welton Member of the Scotch Grove Formation at Shaffton Quarry, near Camanche, Iowa, is about 427 Ma old and contains numerous metaconulariid specimens, many of which are preserved in apex-downward orientation. Some of these show an unusual, splayed, “Maltese cross” configuration. Apex-downward configurations suggest rapid burial, consistent with the soft part preservation known in other taxa from this locality. The abundance ofMetaconulariaat Shaffton Quarry, and of topotype specimens ofM. aspersa, the generic type species, permits evaluation of the degree of individual and intracollectional variation in peridermal ornament. Variation within and among individuals precludes reliance on ornamental differences in species differentiation in most cases. In view of these results we assign all Shaffton specimens toMetaconularia manni(Roy, 1935), and reviseMetaconulariabased on its type material from Europe and other material from Europe and North America. An exploratory phylogenetic analysis highlights aspects of character distribution within the genus, but the small number of characters states and possible taphonomic influences upon them limit confidence in the clade topology. Subgroups within the genus are characterized by larger, discoidal papillae, and by strong transverse corrugation and sinuous rows of smaller papillae. The genus itself comprises those conulariids with an external ornamentation of simple, round, small papillae, paired internal septae along the midlines, and a thin periderm that was to some degree pliable during life.