We met these modest internal molds of the mysterious hyoliths about five years ago. With a dramatic new development in hyolith studies, they are worth seeing again.
These fossils are internal molds (the sediment that filled the shell) of of flattened cones composed of the carbonate mineral aragonite. The aragonite shells dissolved away after burial, leaving the cemented sediment behind. That’s what we see above, in their stark simplicity. (We also see wiggly indentations that are the trace fossil Arachnostega, which is what I collected them for in the first place.) They were found in the Middle Ordovician of Estonia.
Hyalites, though common throughout the Paleozoic, have been difficult to place in a taxonomic category. Because of their easily-dissolved aragonite skeletons, most fossils are like these — simple molds and casts. A few were found with some preserved internal organs, which added to the intrigue. Their flattened conical shells had a hinged lid (operculum) over the open end. Extending from each side in the space between the operculum and cone were two calcareous rods called helens (a name deliberately chosen so as not to evoke a particular function). They were rumored to be deposit-feeders, based on no real evidence, it turns out.
An excellent paper appeared earlier this month showing dramatic evidence of hyolith soft parts in the Cambrian of western Canada (Moysiuk et al., 2017). The authors reconstruct the iconic Cambrian hyolith Haplophrentis “as a semi-sessile, epibenthic suspension feeder that could use its helens to elevate its tubular body above the sea floor”. Their primary evidence is a magnificently preserved lophophore (tentacular filter-feeding apparatus) and a U-shaped digestive tract with a dorsolateral anus. These features not only give the hyoliths a life mode and feeding habit, they place them systematically among the lophophorates, a group that includes brachiopods, phoronids and bryozoans.
Haplophrentis in the Burgess Shale (Middle Cambrian) at the Walcott Quarry, Burgess Pass, British Columbia, Canada.
It’s not often we see such dramatic changes in the taxonomic placement and paleoecological habits of a large, extinct group. It is also not often that invertebrate fossils make headlines!
Moysiuk, J., Smith, M.R. and Caron, J.B. 2017. Hyoliths are Palaeozoic lophophorates. Nature doi:10.1038/nature20804