Impression of a fossil specimen of the hyolithid Hyolithus cecrops Walcott, 1917 (19 mm height, early middle Cambrian, northwestern Montana). Found with specimens of trilobites [Albertella helena], brachiopods, and trace fossils.
Hyolithids are a group of small, somewhat flat-shaped, shells whose biologic affinities have long been uncertain. The most common assumption was that they were gastropods, which had a planktonic (floating) existence, like the pteropod gastropods (so-called "sea butterflies") found in modern seas. Their most perplexing morphologic feature were the two "oars," which are now named "helens." In the image shown above, only the left helen is preserved.
Most of the several hundred species belong to genus Hyolithus, which had a worldwide distribution, starting in Early Cambrian time. They became diminished during Ordovician and lingered thereafter until the Middle Permian.
Side view of exterior of H. cecrops. The operculum (lid) could open slightly, and the feeding apparatus (probably similar to the lophophore of some modern animals), could then be extended into the water column. The helens were used as stilts/props for holding the animal above the muddy bottom substrate.
Dorsal view of interior of H. cecrops, showing critically important soft-tissue impressions: the presumed lophophore, the location of the mouth, and the U-shaped gut. These impression are indicative that hyolithids were not planktonic mollusks. Instead, they were benthonic (bottom dwellers) animals that had their closest relatives being the lophophore-bearing brachiopods and bryozoans.
For more information see:
Moysiuk, J. et al. 2017. Nature v. 54:394–397 (like other pay-for- subscription journals, the abstract is free, but it costs to read or download the actual paper).
