Ontogeny of Bactrotheca and related hyoliths

Orthothecid hyoliths Bactrotheca dens (Holm) and B. quadrangidaris (Holm) from the Ordovician of the Baltic province have relatively large, smooth embryonic shells indicating a lecithotrophic type of larval development, probably inside egg capsules. Their embryonic opercula are flat, subcircular in outline. Hatching is marked by the retardation of growth of both operculum and shell, and in the appearance of adult longitudinal sculpture on the shell surface. Juvenile post-embryonic opercula have fully developed structures on their internal side. Bactrotheca probably led a sub-sessile mode of life on its postlarval stages, similar to the Recent gastropod Turritella. Representatives of other hyolith genera, Circotheca stylus (Holm) and Decoritheca excavata (Holm) from the Baltic Cambrian, bear small initial shells with mucros, suggesting a planktotrophic mode of life for the larvae. Data presented here document a high diversity of larval development within particular groups of hyoliths (Monoplacophora), similar to that of gastropods. Inclusion of Semielliptotheca, Quadrotheca, Trapezotheca, Panitheca, and possibly Nephrotheca, into Bactrotheca as its subgenera is postulated.