Rotaciurca

Rotaciurca superbus is a Silurian cephalodiscid pterobranch, known from the Bertie Group of Ontario.

Description
Rotaciurca is a pseudocolonial (containing multiple clustered individuals which are not connected) pterobranch, unique for being a cephalodiscid rather than a graptolite. The cluster consists of a ring of radiating tubes, likely containing the living individuals, attached to a conical structure interpreted as a float, giving the original fossil the nickname of "Ezekiel's wheel". The whole organism is roughly 4 cm long, seemingly with two distinct rows of tubes. Its pterobranch affinity is confirmed by lineations within the tubes, which are traces of fuselli, and the irregular arrangement of these group it with the cephalodiscids. Rotaciurca has a number of similarities to Yuknessia, another enigmatic fossil recently described as a pterobranch, including rigid and similarly sized tubes, although those of Yuknessia are much finer and have more tightly packed fuselli.

Ecology
Rotaciurca has been interpreted as a planktonic filter-feeder, with the conical float providing buoyancy like that of a Portuguese man o' war. It is unique in this aspect, as all other known cephalodiscids are benthic and sessile, while the only other planktonic pterobranchs are graptolites, which shows the water column was colonised several times by hemichordates.

Distribution
Rotaciurca is known from ten specimens, with the holotype nearly complete. Unusually for a planktonic animal, it has only been found in one locality, that being the Bertie Group, however since it was likely soft-bodied, the exceptional preservation of this site may have been necessary for Rotaciurca to fossilise.

Etymology
Rotaciurca is named in honour of Samuel J. Ciurca, Jr., who donated thousands of eurypterid fossils to the Yale Peabody Museum, alongside the Latin word rota, meaning "wheel". Its specific name, superbus, from the Latin word meaning "beautiful", acknowledges that Ciurca labelled the holotype fossil "the most beautiful fossil ever found".