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Hyoliths are extinct invertebrates with calcareous shells that were common constituents of Cambrian fauna and formed a minor component of benthic faunas throughout the Palaeozoic until their demise in the end-Permian mass extinction. The biological affinity of hyoliths has long been controversial and the group has been compared with a number of animal phyla, most frequently the Mollusca or the Sipuncula, although other researchers have considered hyoliths as a separate "extinct phylum." However, recent discoveries of a tentaculate feeding apparatus ('lophophore') and fleshy apical extensions from the shell ('pedicle'), have resulted in hyoliths being placed within the lophophorates with a close relationship to the brachiopods.
A new article by Zhifei Zhang and his research group at Northwest University, China, together with Dr. Christian Skovsted from the Swedish Museum of Natural History have questioned this phylogenetic placement, after analyzing hundreds of hyolith fossils from the lower Cambrian (520 million years ago) Chengjiang Biota of South China (Liu et al.). In their material from South China, the first credible soft parts of an orthothecid hyolith other than the gut has been preserved in the species Triplicatella opimus.
Part - Morphology - Triplicatella - Opimus - Presence
The soft part morphology of Triplicatella opimus confirms the presence of a tentaculate feeding organ in orthothecids, demonstrating that both recognized orders of hyoliths possessed a tentaculate feeding organ. The tuft-like arrangement of the tentacles of T. opimus differs from that of hyolithids suggesting a different function of the feeding organ between orthothecid (collecting food directly from the substrate) and hyolithid hyoliths (filter feeding strategy).
A comparative study was undertaken by Liu et al., investigating the structure of the feeding organ between hyoliths and other recognized fossil and modern lophophore-bearing animals. This analysis indicated that the structure lacked many morphological features that are distinctive of a lophophore...
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