70-11 New Micro-CT Data on the Morphology of Lateral and Groove Spine Base Structures on the Lateral Arm Plates of the Mississippian Brittle Star Furcaster

Session: Paleontology, Biogeography/Biostratigraphy & Phylogenetic/Morphological Patterns (Posters)

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Abstract:

Brittle stars are taphonomically fragile organisms that rapidly disaggregate into thousands of skeletal plates upon death. Historically, isolated plates were considered of little importance from a taxonomic perspective. However, new methodologies utilizing detailed morphological analyses of lateral arm plates have revolutionized our understanding of brittle star systematics,  evolution, and diversification. While modern brittle star lateral arm plates are relatively well understood, archaic taxa are comparatively poorly documented. In particular, details of the spine articulations and how the muscle pits and nerve canals articulate remain unknown in fossil brittle stars, limiting our understanding of their deep evolutionary history. This project uses micro-CT to investigate the morphology of a proximal lateral arm plate of Furcaster wardi from the Upper Mississippian (lowest Viséan) Glen Dean Formation, near Hopkinsville, Kentucky. This locality preserves an open marine fauna dominated by fenestellate bryozoans and echinoderms, especially blastoids and crinoids. We focused on a proximal right lateral arm plate, which contains shale plugs and slight pyritization in the nerve canals, providing sufficient X-ray density contrast to enable segmentation of the internal features. The specimen was scanned with a Zeiss Xradia Versa 520 micro-CT scanner at the Natural History Museum, London. Dragonfly Software was used to segment the scan, informing on how the nerve canals articulate with the spine bases. Several previously unknown morphological features of the lateral arm plate were revealed by this analysis, including the depth of the muscle pits associated with the spine bases, the presence of two previously unknown openings for the nerve canals on the inner surface (one each for the lateral and groove spine bases), and the presence of internal canals running through the arm plates. These findings allow for a more refined understanding of spine base morphology, which will aid in unraveling evolutionary and diversification patterns among brittle stars.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9778

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