Paper No. 88-4
Presentation Time: 9:00 AM
QUANTITATIVELY ASSESSING GROSS MORPHOLOGY OF HOLOCYSTITID DIPLOPORITANS (BLASTOZOA: ECHINODERMATA)
Blastozoan echinoderm ontogenetic studies are often hindered by a lack of available specimens from a full representation of growth stages, which limits what can be understood. In particular, the diploporitans stand out among blastozoans for a considerable lack of juvenile specimens found in the fossil record. This differs from other blastozoan groups, such as blastoids and rhombiferans, whose fossil record is represented by individuals at all sizes, including recognized juveniles. The Holocystites Fauna (Holocystitidae), a clade of diploporitans from the Ordovician and Silurian of North America, is unique in that its species are quite large when compared to other diploporitans, and even to other blastozoans, with its largest members reaching sizes of over 15 cm in height. However, even though these fossil holocystitids are very high in number throughout certain localities in North America, no juvenile specimens have been found to date, even with extensive sediment sieving to look for isolated plates. Recently, a collection of the smallest known specimens of Holocystites scutellus has been recovered from the Silurian Massie Formation of Napoleon, Indiana USA. These specimens were scanned utilizing a NextEngine 3D laser scanner to create digital 3D models with which to examine shape changes. As holocystitids have relatively few landmarks and their thecal plating is highly irregular, a traditional landmark ontogenetic study is not appropriate. A comprehensive arrangement of 3D "pseudolandmarks" was generated to characterize the surface of the specimens. Meshes with landmarks were imported into R and subjected to 3D geometric morphometric analyses to assess the variance and coverage of morphospace and to better understand ontogenetic trajectories. This study adds information to understanding the ontogeny of these blastozoan echinoderms.