Hyoliths are Palaeozoic lophophorates

Analysis of exceptionally preserved fossils of the Cambrian hyolith Haplophrentis leads to a proposed evolutionary relationship with Lophophorata, the group containing brachiopods and phoronids, on the basis of a newly described tentacular feeding apparatus. Shell-like hyoliths from the Burgess Shales Hyoliths were shelly fossils that were common throughout the Palaeozoic. They look like drinking horns with a lid (the operculum), and even had two curved protruberances (called helens) that the drinking horn could stand on, making a tripod with the horn-shaped shell. Because hyoliths are so distinctive, their affinities are difficult to determine, although they are thought to belong to a group of invertebrates known as Lophotrochozoa—annelids, molluscs and lophophore-bearing animals such as brachiopods. Joseph Moysiuk et al . report that a cache of hyoliths from the famous Cambrian Burgess Shales of Canada has remarkable soft-tissue preservation, revealing that they are indeed lophophorates, probably akin to brachiopods, and perhaps taking in the tommotiids, another extinct and enigmatic group of what palaeontologists refer to as 'small shelly fossils'. 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 1 , 2 . 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) 3 , 4 , 5 . Since their first description over 175 years ago, hyoliths have most often been regarded as incertae sedis 4 , 6 , related to molluscs 7 , 8 or assigned to their own phylum 1 , 2 . 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 3 , 9 , 10 , 11 , 12 . 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 (brachiop