Echinoderms (starfish, sea urchins, sea cucumbers, crinoids, etc.) are well-known for their pentamerous (5-fold) radial symmetry. This makes them unique among higher Metazoa, but makes studying their relationships particularly problematic. The parts of an echinoderm just don’t match those of other animals very well, so questions about the origin of those parts (i.e., their homologies) are difficult to address. Many living echinoderms pass through a bilateral larval stage, evidence for the well-worn adage “ontogeny recapitulates phylogeny” (also known as the Biogenic Law, which as a general hypothesis is now largely unaccepted). However, no fossil evidence – that is, an actual bilateral echinoderm – has ever been found (although a few asymmetric fossil echinoderms are known). Until now. Samuel Zamora, of The Natural History Museum in London, and colleagues have just described Ctenoimbricata spinosa, a new genus and species from the Murero Formation (earliest middle Cambrian Period) in northeastern Spain. Its externally plated body exhibits features similar to those of known early echinoderms, such as Courtessolea and members of the extinct class Ctenocystoidea. Ctenoimbricata thus provides a root for all echinoderms and also confirms that the earliest echinoderms were deposit feeders (like sea cucumbers) rather than suspension feeders (like crinoids).
Read the open-access original article in the online journal PLoS One.