Branched Setae or Attached Macroalgae: A Case Study of an Exceptionally Preserved Brachiopod from the Early Cambrian Chengjiang Lagerstätte

The remarkable conservation of soft tissues within Cambrian fossils has significantly enhanced our comprehension of the origins and evolutionary trajectories of animals, in addition to the progression of ecological intricacy. Here, we report an exceptionally preserved specimen of the lingulid brachiopod <i>Xianshanella haikouensis</i> from the lower Cambrian Chengjiang Lagerstätte, exhibiting branched fringes along the distal ends of its marginal setae. These structures may represent either branched setae or attached macroalgae. The diameter of the branched fringes is slightly larger than that of the marginal setae, and they exhibit third- or fourth-order bifurcations, forming a complex structure comparable in length to the shell. Both the branched fringes and marginal setae are preserved as iron oxides, as revealed by SEM and Micro-XRF analyses, a characteristic preservation mode in Chengjiang fossils. The results of Micro-CT scanning suggest that these branched fringes are preserved along almost the entire distal end of marginal setae. Comparable branched fringes are reminiscent of those found in annelids, and such structural analogs between annelid and brachiopod setae support the homology of brachiopod and annelid setae, representing a lophotrochozoan synapomorphy. An alternative explanation involving attached macroalgae is proposed, given that branched setae have never been documented in either extinct or extant brachiopod taxa. If these structures represent macroalgae, this association could represent a mimicry strategy to deceive predators, although comparable macroalgal fossils remain undocumented in the Chengjiang Lagerstätte. Our research highlights the potential for brachiopod setae to serve roles in sensory function or ecological interactions, offering a new perspective on early animal adaptation and community dynamics.