226 Paleontology IV: Morphology and Phylogeny

Wednesday, 12 October 2011
8:00 AM - 12:00 PM
Room 205CD

A COMPLETE RECONSTRUCTION OF THE HYOLITHID SKELETON

Paper number: 226-10
Program: Technical Sessions
Time: 10:15 AM – 10:30 AM

Authors

Abstract

Hyolithids are a group of extinct lophotrochozoans, which started to diversify in the earliest Cambrian and were important constituents of the early Paleozoic benthos. They are characterized by a mineralized skeleton consisting of 4 elements: a conical conch (with an oval or subtriangular cross section), a complex operculum that closed the conch aperture, and two lateral "spines" (named helens) which were partly internal and extended outside the conch through a pair of symmetrical slits at the conch-operculum commissure. Both the conch and operculum are relatively well known and, to a certain extent, have modern analogues in other lophotrochozoan groups. The helens, on the other hand, are less well known (partly because of their relative fragility and rarity as fossils) and do not have clear modern analogues. This has hindered the knowledge of the complete morphology of the hyolithid skeleton, as well as aspects of their biology, such as the organization of soft parts and their ability to move.

Exceptionally preserved material from the upper Silurian of Gotland (Sweden) sheds new light on the complete morphology of the hyolithid skeleton by providing a growth series of complete, three-dimensionally preserved helens as well as abundant conchs and opercula, all belonging to the same species. The different skeletal elements are all pyritized (now partly iron oxide) and were obtained by acid preparation. The helens were solid spines, likely of the same composition as the conch and operculum, and had a layered, concentric microstructure. The proximal end of the helen was rounded and thin, while the distal was pointed and more robust. Shell material was accreted at the proximal end in the form of concentric layers of uneven thickness. Thus the helen grew gradually by accretion developing into a curved spine. The complete helen has the form of a logarithmic spiral which does not lie in a plane, but bends gently describing an arch. Helens are more or less oval in cross-section, but have a bulging anterior face and a flattened posterior one. All this information indicates that the helens were massive spines with a mechanical function and that, apart from serving as stabilizers, may have allowed the hyolithids to move on the sea floor.