Morphology and mode of life of a peculiar Devonian microconchid tubeworm <i>Aculeiconchus</i> from Wyoming, USA

Acetic-acid-treated samples from the Upper Devonian Maywood Formation of Wyoming provided numerous microconchids of the peculiar species Aculeiconchus sandbergi. Preserved in three dimensions, isolated specimens allowed for the first detailed analysis of the morphology and mode of growth of this taxon. The Aculeiconchus tube coils with a wide range of degrees of whorl expansion and translation. Many specimens start out with no spines and a small degree of translation (nearly planispiral), and then rapidly increase translation, in some cases leading to a central void. As observed previously, all individuals bear spiny structures on the underside of the coiled tube, which protrude from the underside of whorls that detach from previous whorls. In such instances, the spines are much longer, reaching the preceding whorl, thus supporting the detached whorl. On a few specimens, the substrate morphology was preserved in the process of bioimmuration on the underside of the tubes. The polygonal pattern preserved is very similar to the surface morphology of modern and fossil Codium-like algae, supporting the previous assumption that these microconchids originally colonized non-calcified algal thalli, to which they were efficiently fixed by the basal spines. Evidence of a repaired tube injury preserved in one specimen may point to attempted predation, possibly by fish. However, the extremely low frequency of such damages may indicate that the attacks on these microconchids were mostly lethal. The abundant tube fragments obtained from the residues may indeed represent the crushed specimens induced by predation.