Early ontogeny and other possible molluscan traits in hyolith biology and anatomy

Hyoliths were extinct invertebrates with embryonic and larval conchs closely similar to those of the conchiferan molluscs. Yet, they lacked radula and were equipped with a tentacular oral apparatus. To resolve their phylogenetic affinities, it is necessary to determine taxonomic strength of particular aspects of their anatomy. Along with the early Palaeozoic ‘taphonomic windows’, offering data on the hyolith soft anatomy, the phosphatized ‘small shelly fossils’ assemblages with a high contribution from hyoliths are the main source of evidence on the ontogeny of their skeletal aspects. In this study, one of the latest comes from the Late Ordovician Mójcza Limestone, Poland, and one of the earliest from the Early Cambrian Sellick Hill Formation at Mayponga Beach, South Australia. It appears that irrespective of the adult anatomy, the ontogeny of various hyoliths was closely similar to that of the early Palaeozoic gastropods and orthoceratid nautiloids. Their embryonic trochophore stage conchs were bulbous, with apical mucro. After hatching, the larva secreted a cylindrical conch. The mode of larval development seems to be independent of whether the post-larval tentacular apparatus and simple U-shaped intestine was used to filtration of organic matter (hyolithids), or rather the mud was collected and digested in the complexly folded intestine (orthothecids). A smooth cup-like operculum developed already at the trochophore stage. The post-larval hyolith conch shapes, ranging from strongly convex to concave venter, apparently represented various adaptations to passive lying on the sea bottom. Instead, the laterally compressed Turcutheca-like conchs suggest an erect pose and free life, offering a link to the more-or-less conical conchs of the helcionellid molluscs. The change from such epibenthic vagile mode of life to a passive filtration, which is not necessarily inconsistent with the molluscan affinity of hyoliths, may explain the loss of radula by analogy with bivalves.