Hyoliths III: Season of the Hyolith

You may remember that last year we had some Paleozoic excitement with the publication of specimens showing the enigmatic hyoliths to be lophophorates, related to brachiopods, bryozoans, and phoronid worms (Moysiuk et al. 2017). Independent support for this position has just been published based on Chinese Cambrian specimens. In this case, hyoliths are not only lophophores, but they are stem brachiopods (closely related to modern brachiopods, but not within the group including modern brachiopods, the crown brachiopods), based on the presence of pedicles in a species of basal hyolith (Sun et al. 2018a).

The pedicle is a fleshy organ, found in most brachiopods, which anchors the shell to the substrate. (Curiously enough, our last visit with the brachiopods covered the strophomenids, noted for not having pedicles.) Sun et al. (2018a) describe a new hyolith taxon, Pedunculotheca diania, which has the typical elongate pointed main shell and lid-like operculum of hyoliths (a bit more oval in cross-section than the classic triangles we've seen so far), with the addition of a short nonmineralized stalk ending in a small holdfast growing from the tip of the shell.

An anchored hyolith, like Pedunculotheca diania, would be a suspension feeder, but other, more derived hyoliths were unattached (Sun et al. 2018a). Moysiuk et al. (2017) interpreted hyoliths in general as filter feeders, but it appears that hyoliths were more ecologically flexible. Recent publications have documented aggregations of hyoliths around coprolites (Kimmig and Pratt 2018; Sun et al. 2018b) and dead animals (Sun et al. 2018b), indicating scavenging/detritivore roles. Because skeletal fragments have never been reported in hyolith guts, Sun et al. (2018b) suggested that they focused on the more fluid phase of the decomposing detritus (or at least the non-mineralized phase), or on microbial films growing around the detritus. The authors also found hyoliths associated with small burrows, indicating that they could enter the uppermost substrate.

In my quest to always have different hyolith images when the topic comes up, here is Figure 2a from Plate LXIII in Walcott (1899). It is a "reproduction of photograph of a slab of sandstone, with numerous specimens of H. primordialis, from the St. Croix sandstone of Wisconsin. The small shells are identical with H. gregaria of M. and H. [Meek and Hayden]." The publication is actually on fossils of Yellowstone National Park, but Walcott had identified hyoliths there as basically the same as H. primordialis from Wisconsin, which is why he included this figure. Later, the Yellowstone hyoliths would be transferred to a new species (H. gallatinensis Resser 1938) and eventually genus (Haydenoconus Malinky 2014).

References

Kimmig, J, and B. R. Pratt. 2018. Coprolites in the Ravens Throat River Lagerstätte of northwestern Canada: implications for the Middle Cambrian food web. Palaios 33:125–140. doi:10.2110/palo.2017.038.

Malinky, J. M. 2014. Cambrian Hyolitha and problematica from West Laurentian North America: taxonomy and palaeobiology. Alcheringa 38(3):338–362.

Moysiuk, J., M. R. Smith, and J.-B. Caron. 2017. Hyoliths are Palaeozoic lophophorates. Nature 541:394–397. doi:10.1038/nature20804.

Resser, C. E. 1938. Fourth contribution to nomenclature of Cambrian fossils. Smithsonian Miscellaneous Collections 97(10).

Sun, H., M. R. Smith, H. Zeng, F. Zhao, G. Li, and M. Zhu. 2018a. Hyoliths with pedicles illuminate the origin of the brachiopod body plan. Proceedings of the Royal Society B: Biological Sciences 285(1887). doi:10.1098/rspb.2018.1780.

Sun, H.-J., F.-C. Zhao, R.-Q. Wen, H. Zeng, and J. Peng. 2018b. Feeding strategy and locomotion of Cambrian hyolithides. Palaeoworld 27(3):334–342. doi:10.1016/j.palwor.2018.03.003.

Walcott, C. D. 1899. Cambrian fossils. Pages 440-478 in Geology of the Yellowstone national park. U.S. Geological Survey, Washington, D.C. Monograph 32.