NEW MORPHOLOGICAL DATA FOR THE EARLY ONTOGENY OF PENTREMITES (BLASTOIDEA, ECHINODERMATA) USING SYNCHROTRON-BASED TOMOGRAPHY

Blastoids from the Upper Chesterian (Mississippian) strata near Hopkinsville, Kentucky are abundant, well preserved, and exhibit a high degree of morphological variation. There are upwards of four species of Pentremites falling into two major morphotypes: godoniform and pyriform, and these morphotypes progress along very different ontogenetic trajectories. Particularly abundant at this locality are early post-larval passalocrinid stage thecae with specimen densities estimated to be up to 35,000 per kilogram of washed fossil residue. Extensive work at this location has shown that while thousands of macroscopic specimens of Pentremites were recovered, only a single specimen of Diploblastus was collected, suggesting that these ‘Passalocrinus’ specimens are juvenile Pentremites

New technological advances allow for fine detail on minute specimens to be analyzed. Herein, we utilize synchrotron tomography to examine the external anatomy of ten passalocrinid stage blastoids, between 0.25 and 0.45 mm in thecal height. Although the interior of the specimens is not well preserved, we provide a detailed exterior morphological analysis following the Universal Elemental Homology framework to provide a thorough assessment of juvenile plate arrangements with varying gross morphology. Compared to adult morphology, the forms are exceptionally similar with differences found in the growth rate of specific plates. Furthermore, we assessed the utility of advanced imaging techniques on echinoderm specimens. Echinoderms produce calcitic skeletons that are usually infilled by a sparry or micritic calcite. This results in very low X-ray density contrast between the skeleton and infill, making examination of internal structures difficult for extinct echinoderms. By utilizing propagation-based phase-contrast synchrotron radiation X-ray tomographic microscopy, we are able to distinguish fine density differences that are unattainable from other non-destructive imaging techniques.