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| Rotaciurca superbus Briggs & Koch, 2023 |
Summary
Pterobranchs, a major group of the phylum Hemichordata, first appear in the fossil record during the Cambrian, 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. 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. 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 Ezekielidae 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).
Derek E.G. Briggs and Nicolás Mongiardino Koch. 2023. A Silurian pseudocolonial pterobranch. Current Biology. In Press, DOI: 10.1016/j.cub.2023.10.024
The last turn of ‘Ezekiel’s Wheel’ honors a Yale-affiliated fossil hunter
Yale paleontologists have identified a “problematic” fossil as an ancient sea creature that lived in the plankton 420 million years ago.
