title: Hyoliths are Palaeozoic lophophorates.
abstract: Hyoliths are abundant and globally distributed 'shelly' fossils that appear early in the Cambrian period and can be found throughout the 280 million year span of Palaeozoic strata. The ecological and evolutionary importance of this group has remained unresolved, largely because of their poorly constrained soft anatomy and idiosyncratic scleritome, which comprises an operculum, a conical shell and, in some taxa, a pair of lateral spines (helens). Since their first description over 175 years ago, hyoliths have most often been regarded as incertae sedis, related to molluscs or assigned to their own phylum. Here we examine over 1,500 specimens of the mid-Cambrian hyolith Haplophrentis from the Burgess Shale and Spence Shale Lagerstätten. We reconstruct Haplophrentis as a semi-sessile, epibenthic suspension feeder that could use its helens to elevate its tubular body above the sea floor. Exceptionally preserved soft tissues include an extendable, gullwing-shaped, tentacle-bearing organ surrounding a central mouth, which we interpret as a lophophore, and a U-shaped digestive tract ending in a dorsolateral anus. Together with opposing bilateral sclerites and a deep ventral visceral cavity, these features indicate an affinity with the lophophorates (brachiopods, phoronids and tommotiids), substantially increasing the morphological disparity of this prominent group.
authors.affiliation: Departments of Ecology and Evolutionary Biology and Earth Sciences, University of Toronto, 25 Willcocks Street, Toronto, Ontario M5S 3B2, Canada.
authors.forename: Joseph
authors.initials: J
authors.lastname: Moysiuk
authors.affiliation: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK.|Department of Earth Sciences, Mountjoy Site, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Martin R
authors.initials: MR
authors.lastname: Smith
authors.affiliation: Departments of Ecology and Evolutionary Biology and Earth Sciences, University of Toronto, 25 Willcocks Street, Toronto, Ontario M5S 3B2, Canada.|Department of Natural History, Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada.
authors.forename: Jean-Bernard
authors.initials: JB
authors.lastname: Caron
mesh_terms: D060105:Animal Shells / Q000033:anatomy & histology; D000818:Animals; D002170:Canada; D005580:Fossils; D007448:Invertebrates / Q000033:anatomy & histology* / Q000145:classification* / Q000502:physiology; D008954:Models, Biological; D010802:Phylogeny