41-13 Anatomical ramifications of the loss of respiratory structures in Pleurocystitid echinoderms

Session: Paleontology, Stratigraphy, Sediments, and Soils (Posters)

Poster Booth No.: 34

Presenting Author:

Zac Matthews

Authors:

Matthews, Zac¹, Register, Hannah², Bitner, Lucian³, Koniecki, Joseph M.⁴, Bauer, Jennifer E⁵, Deline, Bradley⁶

(1) Earth and Environmental Sciences, University of West Georgia, Carrollton, GA, USA, (2) Environmental Protection Division, Georgia Department of Natural Resources, Atlanta, GA, USA, (3) Geosciences, Georgia State University, Atlanta, GA, USA, (4) Milford, MI, USA, (5) Museum of Paleontology, University of Michigan, Ann Arbor, MI, USA, (6) Earth and Environmental Sciences, University of West Georgia, Carrollton, GA, USA,

Abstract:

Quantifying the morphology of organisms within a phylogenetic context provides insight into the relationship between developmental changes, physiology, and anatomy. Blastozoan echinoderms provide an ideal system exploring these relationships in that their respiratory structures are distinctive, variable, and evolutionarily plastic. Pleurocystites, like most pleurocystitids, have respiratory pectinirhombs that grow allometrically and have been estimated to supply approximately half of an individual’s respiratory needs. Pectinirhombs consist of a series of slits that cross plate suture boundaries, which allows for the diffusion of oxygen from the water flowing through the canals. Amecystis, which is closely related and co-occurs with Pleurocystites, lost their respiratory structures while retaining the standard plating and thecal anatomy of the family. A collection of Pleurocystites and Amecystis from the Late Ordovician Brechin Lagerstätte of Southern Ontario were morphometrically characterized to explore how the shape of individual plates and the theca differed following the loss of pectinorhombs. The shape and variability of the five homologous plates, with and without pectinirhombs, and the theca were compared between the two genera using outline analysis. Even with consistent thecal architecture, the shapes of the individual plates and the theca are distinctive between the two genera. In addition, Amecystis shows a much higher amount of variation in the shape of most of the individual plates and the thecal outline compared with Pleurocystites. This difference in variation may be the result of the need for more uniform plate shapes allowing the alignment of the slits in the pectinirhombs across plate boundaries. These respiratory structures may have a constraining effect on plate shape to retain the function of the respiratory structures, representing an evolutionary trade-off between respiratory rates, intraspecific variability, and the potential for evolutionary change. To further evaluate these results, additional thecal plates and species from other localities will be incorporated into future analyses.

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

doi: 10.1130/abs/2026SE-13205

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