23-17 Understanding Lateral Arm Plate Variation in The Mississippian Ophiuroid Sulphaster odellettorum

Session: Paleontology of North America (Posters)

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Abstract:

As taxonomic research in Paleozoic ophiuroids progresses, the diagnostic significance of lateral arm plates (LAPs) is becoming increasingly recognized. Yet, a gap remains in our understanding of LAP morphological change along the arm from proximal to distal. This study addresses that gap by examining these changes within Sulphaster odellettorum, a newly described species of Mississippian ophiuroid. Here we examine overall plate shape, height-to-length ratios, number and placement of spines, and the nature of the tentacle notch/pore. Fossil material was collected from the upper Mississippian (uppermost Chesterian/ latest Viséan) Pella Formation in Mahaska County, Iowa, at the Russell Wildlife Area. Samples were processed using standard micropaleontological methods, and ophiuroid material was isolated by hand-picking under a microscope. LAPs selection was based on arm position and preservation quality under a light microscope.  Selected specimens were mounted on stubs, gold-coated, and imaged using a scanning electron microscope (SEM) to document gross morphology, fine-scale surface features, and spine articulation structures. Preliminary observations show obvious morphological gradients along the arm. Proximal plates bear up to five spine bases that are prominently placed on a ridge, the dorsal side is strongly arched, the proximal side is strongly flared, and the tentacle notch is relatively large. The medial plates are more elongate, with up to four spine bases without any ridge, straight dorsal margins, slight proximal flare, and a relatively smaller tentacle notch. Distal plates are more elongate than medial LAPs, bearing up to two spine bases, the tentacle notch shifted distally, and a slight proximal flare. Overall, the height-to-length ratio grades from about 0.673 proximally to 0.281 distally. This change mostly results from plate height loss as plate length changes little (about 0.45 mm proximally and 0.35 mm distally). Furthermore, juvenal plates in the medial and distal arm have a tentacle pore instead of a notch. These changes likely reflect functional transitions along the arm and may hold taxonomic value, offering insight into the species’ ecology, phylogenetic placement, and morphological variability.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

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