23-3 A Morphological, Ecological, and Biogeographic Assessment of the Evolution and Extinction of Spire-bearing Brachiopods

Session: Phylogenetic and Computational Approaches in Paleobiology and Paleoecology, Part I

Presenting Author:

Authors:

Abstract:

The four spire-bearing articulate brachiopod orders (Atrypida, Athyridida, Spiriferida, and Spiriferinida) are characterized by a spiralia or mineralized spirolophe lophophore. They were some of the most diverse in the Late Paleozoic, survived the end-Permian extinction, and went extinct in the Jurassic. In contrast, Rhynchonellida, a likely paraphyletic taxon from which the spire-bearers evolved, maintained a lower diversity through the Paleozoic, survived the end-Permian extinction, and are extant. Our recent phylogenetic analyses, which have illuminated the evolutionary relationships among these groups, provide a framework to test hypotheses that explain this different evolutionary trajectory. Specifically, are there identifiable morphological or ecological traits that led to the preferential extinction of brachiopods with spiralia?

To address this, we examined relative diversity, body size, overall shell shape, environmental affinity, and biogeographic distribution of these taxa over the Phanerozoic. Data on body size are extracted from a published dataset, shell shape determinations were made using images in the primary literature and the Treatise on Invertebrate Paleontology, and environmental and biogeographic data were determined from occurrences in the Paleobiology Database, vetted with the primary literature. All data were combined into a morpho-ecospace and analyzed within the context of the phylogenetic tree.

Compared to the spire-bearers, rhynchonellides are less diverse and have smaller body volumes with less variation in both size and shape. For both groups, body size and shape variability increased through the Paleozoic, decreased across the end-Permian extinction, and rebounded in the Early Triassic. The spire-bearer size increase reflects the spiralia occupying an ever-greater volume in the mantle cavity, increasing in number of whorls and lophophore arm length. The end-Triassic extinction had minimal impact on the rhynchonellides but was severe for the spire-bearers, restricting geographic range and making them “dead-clades walking” until their Jurassic extinction. Following the spire-bearer extinction, the mineralized loop-bearing terebratulides diversified and are the most diverse group of extant articulated brachiopods. Loops occupy a smaller overall volume in the mantle cavity and provide an intermediate degree of structural support between crura and spiralia. Possible reasons for this extinction selectivity pattern will be examined in greater phylogenetic detail and are likely related to the functional role of mineralized spiralia and the oxygen demands of different lophophore geometries in different environments and paleogeographic regions.

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

doi: 10.1130/abs/2025AM-10877

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