FIRST 3D RECONSTRUCTION OF INTERNAL HYDROSPIRE RESPIRATORY STRUCTURES IN FISSICULATE BLASTOIDS (ECHINODERMATA)

Respiratory structures of Paleozoic echinoderms have been used as a basis of classification, but have rarely been reconstructed three-dimensionally for morphological examination. Blastoids possess internal respiratory structures (hydrospires) that are lightly calcified and well-preserved. While external hydrospire expression provides the basis for classification of the two major groups (spiraculates and fissiculates), internal morphology of these structures has largely been ignored. Our research aim is to generate new internal hydrospire models to provide additional data for future analyses such as inferring phylogenetic relationships and functional morphology simulations.

We have been utilizing scanned acetate peels from the Naturalis Biodiversity (Leiden, Netherlands) to generate internal anatomical models of hydrospires. The reconstruction process begins with alignment of slices from a single specimen, outlining the exterior of the specimen, and tracing the hydrospires, all of which is done in Adobe Photoshop. After creating vector images from the line drawings in Adobe Illustrator, we use Rhinoceros to create 3-D objects that can be manipulated in space, allowing for a thorough examination and description of the hydrospires.

Spiraculates had a complex water flow pathway through the hydrospires, first travelling through small pores along the ambulacral edge into internal canals, and then exiting through spiracule openings at the summit. Alternatively, fissiculates had a simple water flow pathway through slits on the exterior to the internal hydrospire folds. Fissiculates account for nearly half of all blastoid genera and this work presents the first reconstructed hydrospire model for the morphotype. Prior to this study, six complete models of spiraculate hydrospires had been created. Our study provides two additional spiraculates (Orbitremites derbiensis and Pentremites pulchellus) and presents two complete fissiculate models (Heteroschisma subtruncatum and Cryptoschisma schulzii). These models will be utilized alongside a pre-existing dataset of external character data to test the evolutionary relationships of blastoids.