A Silurian pseudocolonial pterobranch

Abstract

Pterobranchs, a major group of the phylum Hemichordata, first appear in the fossil record during the Cambrian,¹ and there are more than 600 fossil genera dominated by the mainly planktic graptolites of the Paleozoic, which are widely used as zone fossils for correlating sedimentary rock sequences.² Pterobranchs are rare today; they are sessile marine forms represented by Rhabdopleura, which is considered the only living graptolite, and Cephalodiscus. Unlike their sister taxon, the colonial graptolites, cephalodiscids are pseudocolonial.^3,4 Here, we describe a problematic fossil from the Silurian (Pridoli) Bertie Group of Ontario (420 mya), a sequence of near-shore sediments well known for its remarkably preserved diversity of eurypterids (sea scorpions).⁵ The fossil, Rotaciurca superbus, a new genus and species, was familiarly known as Ezekiel's Wheel,⁵ with reference to the unusual circular arrangement of the tubes that compose it. The structure and arrangement of the tubes identify Rotaciurca as a pterobranch, and phylogenetic analysis groups it with the cephalodiscids. We place it in a new family Rotaciurcidae to distinguish it from Cephalodiscidae. A large structure associated with the tubes is interpreted as a float, which would distinguish Rotaciurca as the only known planktic cephalodiscid-thus cephalodiscids, like the graptolites, invaded the water column. This mode of life reflects the rarity of pseudocolonial macroinvertebrates in planktic ocean communities, a role occupied by the tunicates (Chordata) known as salps today. Our estimates of divergence times, the first using relaxed total-evidence clocks, date the origins of both hemichordates and pterobranchs to the earliest Cambrian (Fortunian).

Keywords: Bertie Group; Paleozoic plankton; Silurian; cephalodiscid; graptolite; hemichordate; phylogeny; problematica; pterobranch.