Brachiopod Morphospace and Character Evolution of Lophotrochozoans

Morphology usually serves as an effective proxy for functional ecology, and evaluating morphological, anatomical, and ecological changes permits a deeper understanding of the nature of diversification and macroevolution. Lingulid (order Lingulida) brachiopods are both diverse and abundant during the early Palaeozoic but decrease in diversity over time, with only a few genera of linguloids and discinoids present in modern marine ecosystems, resulting in them frequently being referred to as “living fossils”. The dynamics that drove this decline remain uncertain, and it has not been determined if there is an associated decline in morphological and ecological diversity. Here, we apply geometric morphometrics to reconstruct global morphospace occupation for lingulid brachiopods through the Phanerozoic, with results showing that maximum morphospace occupation was reached by the Early Ordovician. At this time of peak diversity, linguloids with a sub-rectangular shell shape already possessed several evolutionary features, such as the rearrangement of mantle canals and reduction of the pseudointerarea, common to all modern infaunal forms. The end Ordovician mass extinction has a differential effect on linguloids, disproportionally wiping out those forms with a rounded shell shape, while forms with sub-rectangular shells survived both the end Ordovician and the Permian-Triassic mass extinctions, leaving a fauna predominantly composed of infaunal forms. Morphospace occupation over time, when considered using anatomical and ecological analyses, suggests that the limited morphological and ecological diversity of modern lingulid brachiopods reflects evolutionary contingency rather than deterministic processes.To further understand the anatomical innovations underpinning these evolutionary patterns, exceptionally preserved fossils from Cambrian Lagerstätten provide critical evidence. Among soft tissues, chitinous setae are the most commonly reported structures in brachiopods and have been interpreted as serving sensory and/or defensive functions. New material from South China and Laurentia reveals exquisitely preserved setae, offering insights into their microstructure, formation, and preservation pathways. These data indicate that chitinous setae, a key feature of lophotrochozoans (e.g., brachiopods and annelids), were already present in early Cambrian stem groups and may represent a morphological synapomorphy.