23-4 Spirolophes, spiralia, and the evolution and extinction of the spire-bearing brachiopods

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

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Abstract:

Through the Paleozoic, spire-bearers were among the largest and most diverse neoarticulate brachiopods, surviving the end-Permian extinction, becoming extinct in the Jurassic.  Each of the three groups of articulated brachiopods alive today possess lophophores with mineralized support in one of three different geometries:  crura in rhynchonellides, loops in terebratulides, and brachial ridges in thecideides.  Extant rhynchonellides have spirolophe lophophores supported hydrostatically, suspended from relatively short prong-like crura; terebratulides have plectolophe lophophores; thecideides have ptycholophe lophophores.  Why are there no extant neoarticulate brachiopods with spirolophe lophophores supported by mineralized spiralia?  To begin to answer this question from a phylogenetic perspective, we coded 100 morphological characters for 43 Paleozoic spire-bearing brachiopods, 15 rhynchonellides, and eight orthide and pentameride outgroups.  We analyzed them with parsimony and Bayesian methods, with and without stratigraphic range data, focusing on patterns in shell microstructure, hinge line length, and lophophore support, testing the purported monophyly of spire-bearers.  Our analyses indicate that endopunctae evolved from impunctate taxa multiple times, never the reverse; astrophic valves evolved from strophic valves and then returned to strophic morphologies multiple times; spire-bearing clades evolved from rhynchonellides once or twice, depending on the evolutionary model employed.

We then added five Triassic spire-bearing genera to this matrix, including impunctate Thecospira, hypothesized to be ancestral to all endopunctate thecideide brachiopods, which do not possess spiralia.  This hypothesis traditionally places thecideides within a strophic spire-bearing clade because of numerous other morphological similarities Thecospira shares with thecideides; if this hypothesis cannot be rejected, the spire-bearing lineage is not extinct.  In the Maximum Clade Compatibility consensus tree resulting from our analysis using the Fossilized Birth-Death Range Process model, Thecospira nests on a long branch in a clade with other atypical strophic spire-bearers.  However, new analysis of genomic data (BUSCO v. 6.0.0) from a range of metazoans, including broader sampling of brachiopod species, nests extant thecideides within terebratulide brachiopods, not basal to the other neoarticulates, casting doubt on the Thecospira spire-bearing ancestry of thecideides.  Having established a phylogenetic basis for temporal patterns in morphology that support apparent spire-bearer extinction beyond plotting taxonomic diversity, we can begin to investigate several functional and ecological hypotheses to explain their evolution and extinction from the perspective of body size evolution, in the context of changes in environment and paleobiogeographic distribution.

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

doi: 10.1130/abs/2025AM-10410

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