There are some fossils it’s difficult to get enthusiastic about. Don’t get me wrong: as someone whose PhD focussed on the fossil equivalent of tea-leaves, my threshold for getting excited is a lot lower than most people. But when you’re studying undergraduate palaeontology there’s an awful lot of extinct shelly things you must learn about. Contrary to the popular conception of what palaeontologists study (dinosaurs!), marine invertebrates with mineralised hard parts are the mainstay of working with fossils.
There are some familiar groups that, although they are extinct, we do know a fair bit about. Trilobites and ammonites fall into this category: we have a good sense of their evolution and diversity through time, and have inferred a fair bit about their biology. There are other groups that are less well understood. We’ve got lots of shells, but we don’t know what the soft bits of the animal looked like, and we don’t know where they fit on the tree of life. In short, they are a bit of an embarrassment. Luckily, there are palaeontologists who rise to the challenge.
Hyoliths are (or were) one of these awkward groups of shelly fossils. They appear at the base of the Cambrian, about 540 million years ago, where they are diverse and abundant, and dwindle away until they disappear in the Permian Period, some 300 million years later. They have a small, conical calcium carbonate shell, with a lid called an operculum. Happily, the main subgroup, the hyolithids, have an additional feature which ensures my attention: a pair of rigid, curved spines with a memorable name.
We all tend associate certain qualities to people’s names, usually on the basis of people we have known. Helen, for example, is a very sensible name. I associate it with practical, dependable people I have known. You can rely on a Helen. A quick look at the ONS data for girls’ names in England and Wales tells me that it reached a high point of number 8 in the list of baby names in both 1964 and 1974. It’s also the technical term for a hyolith appendage: a hyolithid has a pair of helens. I think this is utterly brilliant. The original paper from 1975 says “We term these … structures helens because the word has no functional connotations, and they were first described under the generic name Helenia by Walcott”. Really? Or did they know a Helen?
Hyoliths had traditionally been classified with the molluscs, based on the way that the shell material of the cone was accrued, or else stuck in their own awkward phylum. New research by Joseph Moysiuk and colleagues has solved the mystery of how they fit in the tree of life, helens and all. A study of over 1500 specimens of Haplophrentis, a mid-Cambrian hyolithid from the celebrated Burgess Shale in the Canadian Rockies and the Spence Shale from Utah, has revealed the details of its soft parts. They reconstruct Haplophrentis as a suspension-feeder on the sea bed, using its helens as stilts to raise its body off the sediment, and using tentacles to sweep organic debris from the water into its mouth.
The exceptionally preserved soft tissues of some specimens show an extendable, gullwing-shaped, tentacle-bearing organ surrounding a central mouth. They interpret this as an organ as a lophophore, the feeding structure found in another ancient, but still living, group of marine shelly animals: the brachiopods (lamp shells). Like bivalve molluscs (such as clams and mussels), brachiopods have a two-valved shell, but their internal biology shows that they evolved separately and they form part of a group called the lophophorates, along with bryozoans and horseshoe worms. The presence of a lophophore, combined with other features including a U-shaped gut, means that hyoliths are also lophophorates, and not molluscs.
Yet again, fossil material from the Burgess Shale, a wonderful window on life half a billion years ago, has contributed to our understanding of the epic evolutionary history of life on our planet. Hyolithids may not be as exciting as some of the more weird and wonderful inhabitants of mid-Cambrian seas, but seeing a long-standing palaeontological mystery like this being solved is intensely satisfying.
