Hyolith relationships and paleobiology continue to be active research topics, which isn't bad for animals that had their heyday more than 500 million years ago and are primarily known from diminutive shells. Let's get right down to the issue that has haunted study of Hyolitha, as posed in the title of Smith (2020): "Finding a home for hyoliths". For a couple of years recently, they'd been drifting into the welcoming arms of brachiopods and other lophophores (Moysiuk et al. 2017; Sun et al. 2018), as discussed in previous entries. However, cracks quickly began to emerge. A group of authors has published several papers challenging a close relationship with lophophores, instead emphasizing shell structural similarities with mollusks (Li et al. 2019, 2020) or being a bit more conservative and placing them nearer the base of Lophotrochozoa (lophophores, annelid worms, mollusks, etc.; Liu et al. 2020a).
Liu et al. (2020a) questioned the evidence presented by Moysiuk et al. (2017) and Sun et al. (2018). Liu et al. interpreted the putative pedicles of Sun et al. (2018) as crushing of the tip of the shell, with no evidence for a pedicle or an opening for one in intact hyolith shells. They concurred with Moysiuk et al. (2017) that hyoliths had a tentaculate feeding structure, but they did not regard this as homologous with a true lophophore. Smith (2020), in a commentary on Liu et al (2020a), illustrated two potential options: that hyoliths are closer to the mollusks and annelids, in which case the tentaculate feeding apparatuses of hyoliths and lophophores are not related but shells are an ancestral trait of lophotrochozoans (which also means that the Cambrian record of these groups is more complete than otherwise thought); or that hyoliths are closer to brachiopods, making the shell something that appears multiple times while limiting the tentaculate apparatus to the brachiopod line. (I suppose there could be an option where shells are ancestral *and* the hyolith tentacles are related to the lophophore feeding structure, but this wasn't explored.)
|Dueling options for hyolith relationships, from Smith (2020). CC-BY-4.0.|
Another area of interest in recent years has been the feeding methods and internal anatomy, bolstered by Cambrian fossils with soft tissue traces. Liu et al. (2020a, 2020b) have documented a tentaculate feeding apparatus in a second hyolith genus, the orthothecid Triplicatella. This hyolith had a more "tuft"-like array of tentacles than the broader "gull-wing" spread of the hyolithid Haplophrentis, and like other orthothecids did not have helens (long curved rods; see Skovsted et al. 2020 for the latest on helens). Liu et al. proposed that Triplicatella was not a filter/suspension feeder, as proposed for Haplophrentis, but was instead a stationary deposit feeder, indicating distinct lifestyles for the orthothecid and hyolithid orders of hyoliths.
|Interpretation of the internal anatomy of Triplicatella, an orthothecid (so no helens; compare to Figure 7 in Liu et al. 2020c). Figure 8 in Liu et al. (2020b). CC-BY-4.0.|
Li, L., X. Zhang, C. B. Skovsted, H. Yun, B. Pan, and G. Li. 2019. Homologous shell microstructures in Cambrian hyoliths and molluscs. Palaeontology 62(4):515–532. doi:10.1111/pala.12406.
Li, L., C. B. Skovsted, H. Yun, M. J. Betts, and X. Zhang. 2020. New insight into the soft anatomy and shell microstructures of early Cambrian orthothecids (Hyolitha). Proceedings of the Royal Society B 287(1933):20201467. doi:10.1098/rspb.2020.1467.
Liu, F., C. B. Skovsted, T. P. Topper, and Z. Zhang. 2020b. Revision of Triplicatella (Orthothecida, Hyolitha) with preserved digestive tracts from the early Cambrian Chengjiang Lagerstätte, South China. Historical Biology. doi:10.1080/08912963.2020.1747059.
Liu, F., C. B. Skovsted, T. P. Topper, and Z. Zhang. 2020c. Soft part preservation in hyolithids from the lower Cambrian (Stage 4) Guanshan Biota of south China and its implications. Palaeogeography, Palaeoclimatology, Palaeoecology. doi:10.1016/j.palaeo.2020.110079.
Moysiuk, J., M. R. Smith, and J.-B. Caron. 2017. Hyoliths are Palaeozoic lophophorates. Nature 541:394–397. doi:10.1038/nature20804.
Skovsted, C. B., M. Martí Mus, Z. Zhang, B. Pan. L. Li, F. Liu, G. Li, and Z. Zhang. 2020. On the origin of hyolith helens. Palaeogeography, Palaeoclimatology, Palaeoecology 555:109848. doi:10.1016/j.palaeo.2020.109848.
Sun, H., M. R. Smith, H. Zeng, F. Zhao, G. Li, and M. Zhu. 2018. 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.