An analysis of fossilized soft tissues confirms that hyoliths - distinctive creatures, found throughout the Palaeozoic era, with a cone-shaped shell and two unusual appendages that curve out from under its body - belong to the group of invertebrates known as Lophophorata. The findings, reported in a paper published online this week in Nature, provide new insights into the diversification of body plans that took place during the earliest phases of the Cambrian explosion.
Hyoliths were present from the onset of the Cambrian period approximately 540 million years ago, during a time of rapid and marked evolutionary change that gave rise to representatives of most of the major animal phyla. First described 175 years ago, they are thought to resemble molluscs, among other groups, and have even been assigned to their own phylum. However, their unusual anatomy and an incomplete fossil record have eluded definitive classification.
Joseph Moysiuk and colleagues analysed more than 1,500 hyolith specimens obtained from the Burgess Shale in British Columbia, Canada, and the Spence Shale in Utah, USA. Some of these fossils contain well-preserved soft tissues, which allowed the authors to identify a tentacle-bearing feeding apparatus that is characteristic of a group of animals called lophophorates, whose living members include brachiopods (‘lamp shells’) and phoronids (‘horseshoe worms’). This discovery shows that hyoliths are members of Lophophorata, and not closely related to molluscs as was previously hypothesized. Their reconstruction of the hyolithid Haplophrentis reveals that it used the pair of long, curved structures called helens to elevate its body above the sea floor.
The authors conclude that their work settles a longstanding palaeontological debate and emphasizes the importance of soft tissue preservation for resolving the evolutionary history of certain taxa.