Exceptionally Preserved Setae: A Possible MorphologicalSynapomorphy of Cambrian Lophotrochozoans

Abstract

Cambrian Lagerstätten yield exceptionally preserved fossils that have greatly improved our understanding of the origin andevolution of animal groups. Brachiopoda, a phylum of bivalved marine invertebrates nested firmly within the lophotrochozoanprotostomes, are widely recovered in such Lagerstätten. The marginal chitinous setae (or chaetae) of brachiopods are the mostcommonly described soft tissue and have been interpreted as performing a defensive and/or sensory role. Despite their relativelycommon appearance in Cambrian Lagerstätten, the origin, function, and evolution of setae in the Brachiopoda is poorly known.Here, we document exquisitely preserved setal structures from South China and Laurentia paleocontinents giving new insightsinto their formation, microstructure and preservation mode. New setae typically make their appearance within the follicle of aneighbouring older seta and then branches off laterally forming its own follicle. Setal microstructure is likely to be composed ofmany canals, highly comparable to setae of their recent counterparts. Moreover, setae recovered from these palaeo‐continentspresent different preservation: aside from the normal preservation of iron oxides and carbonaceous ingredients, some compositions of calcium are also detected in this originally chitinous organization. Investigating the evolutionary origins of chitinous setae, a specialized type found notably in lophotrochozoans such as brachiopods and annelids, reveals its presence inearly Cambrian stem groups. This character likely serves as a morphological synapomorphy in lophotrochozoan evolution.However, the dearth of morpho‐ultrastructure and comparative studies in Cambrian fossils presents a challenge in fullyunderstanding this evolutionary development.This contribution was funded by research projects from the National Key Research and Development Program of China (2023YFF0803601), the National Natural Science Foundation of China (42302010, 42072003), the China Postdoctoral Science Foundation (2023M732836), the Postdoctoral Fellowship Program of CPSF (GZB20230590), Department of science and technology of Shaanxi Province 2022TD-11, and 111 Project (D17013). Research of L.E. Holmer is supported by the Swedish Research Council (VR2018-03390), and by a Zhongjian Yang Scholarship from the Department of Geology, Northwest University. Research of Topper T.P. is supported by the Swedish Research Council (VR2017-05183 and VR2021-04295).</p