SHELL MICROSTRUCTURES OF CAMBRIAN HYOLITHOMORPH HYOLITHS

Hyoliths are a group of problematic calcareous fossils that were particularly diverse in the early Paleozoic; their skeleton consisted of a long conical shell or conch, an operculum, and two spines extending from the conch aperture (present in one group of hyoliths, the hyolithomorphs, but absent from the other, the orthothecimorphs). Their phylogenetic affinities have long been controversial; many have regarded them as a group of molluscs, while others have interpreted them as an independent group of protostomes that convergently evolved shells. One reason for this uncertainty is that the relatively simple shapes of hyolith conchs present few characters for comparison with other groups. Shell microstructures represent a potential source of data that could address these questions, but as hyoliths were aragonitic their fossils are usually coarsely recrystallized. Consequently, few data on hyolith microstructure are available, particularly for hyolithomorphs, which are otherwise the better-known group and were more common for most of the Paleozoic. In this study, we examined conchs of several species of hyolithomorphs from the middle Cambrian (Drumian; local Floran Stage) Gowers Formation in the eastern Georgina Basin of Queensland. They are preserved as phosphatic internal molds, often with the inner layers of the shell partly replaced by phosphate; this early diagenetic phosphatization preserves microstructural details. All specimens show similar microstructures: conchs are composed of fibrous crystallites, ca. 0.5 µm wide; sometimes these fibers are grouped together into larger elements ca. 10–30 µm in width. In most conchs, fibers are transversely oriented in the inner part of the wall and longitudinally oriented in the outer part; some specimens show further thin layers in which fibers can be longitudinal, transverse, or oblique. Thus our material agrees with previous data from orthothecimorphs that hyolith conchs are composed of fibrous elements that are transverse in an inner layer and longitudinal in an outer layer, but our specimens do not show any evidence for the existence of the system of organic tubules that has been hypothesized for orthothecimorphs. Our hyoliths thus had a lamello-fibrillar microstructure reminiscent of similar microstructures found in both mollusc shells and annelid tubes.