Brachiopods are a phylum of bilaterally symmetrical animals whose direct ancestors are unknown in the fossil record. As a result, the origin of their body plan can only be reconstructed based on indirect evidence. Currently, there are two conflicting hypotheses developed by paleontologists and zoologists. The paleontological hypothesis is based on fossil remains of small-shelled organisms known as tommotiids, the soft body morphology of which remains unknown. However, their skeletal remains (sclerites) bear similarities to the valves of brachiopod shells. According to paleontological hypotheses, the ancestor of brachiopods was a tube-dwelling. Zoologists, on the other hand, base their conclusions on the soft-body structure and ontogeny of modern brachiopods. They identify morphological features suggesting that the ancestor of brachiopods had a worm-like body and underwent ventral folding. This paper compares and analyzes both hypotheses, concluding in favor of the folding hypothesis.
Madison et al. (2025) studied this question.