Not all of the extinct groups of animals are large, charismatic, dinosaur-like creatures. Although these may capture our imagination, much of the life that has perished over Earth’s history is represented by more minuscule creatures. One such group is known as the hyoliths.
Hyolith fossils have been found on all seven continents and appear to have lived from ~541 to 252 million years ago, perishing in the same extinction (end Permian) that claimed the glossopterids, sea scorpions and captorhinids . They were marine dwellers and are characterized by their tiny, conical shells. Hyoliths typically had an operculum, a sort of ‘cap’ to their shell, and many species also possessed distinctive tusk-like features known as helens.
A reconstruction of Haplophrentis with its operculum to the right (purple) and its helens coming out from both sides of the operculum.
Little is known about their biology, but it is thought that they dwelled on sea floors and fed on the organic material found within sediment. Their shells have muscle scars that suggest they could open and close their operculum and move part of their body out of their shell. Additionally, given their worldwide distribution, it is likely that they had a planktonic life stage before settling on the sea floor, just like many marine creatures alive today.
Though they had shells like mollusks, particularly some kinds of snails, many researchers think they were a distinct evolutionary lineage. They may have been related to mollusks and their kin, but without the preservation of soft tissue anatomy it will likely be difficult to resolve their relationships to living organisms with any confidence.
Hyoliths hold a special place in my heart given my first foray into evolutionary biology research involved a fossil known as Cupitheca. My undergraduate research adviser and I thought these were likely hyoliths, and a recent study  provided additional evidence of this possibility after the authors discovered opercula that likely belonged to these little guys.
An assortment of Cambrian microfossils including Cupitheca (#3) and another hyolith, Parkula (#6-7).
Questions for Creationists
Where did the hyoliths go? If these were small marine creatures distributed across every continent, shouldn’t they have survived Noah’s flood? Why would mollusks, brachiopods and other shelled animals have survived a catastrophic flood but hyoliths did not? Is it just a coincidence that they disappear from the fossil record in rocks dated to the same time that glossopterids, sea scorpions and captorhinids disappeared?
2. Skovsted, C. B., Pan, B., Topper, T. P., Betts, M. J., Li, G., & Brock, G. A. (2016). The operculum and mode of life of the lower Cambrian hyolith Cupitheca from South Australia and North China. Palaeogeography, Palaeoclimatology, Palaeoecology, 443, 123-130.