The transition from bilateral to pentaradial symmetry in echinoderms has long proved controversial.

Until now, the paucity of characters unambiguously uniting radial and non-radial fossil forms had hampered efforts to investigate the early evolution of the phylum.

Our novel data on growth in the bilaterally symmetrical echinoderm Atlascystis confirm the presence of ambulacra in non-radial echinoderms, establishing homologies between all known fossil groups and enabling rigorous analyses of their phylogenetic relationships.

This reinforces the critical importance of the fossil record for elucidating the evolutionary assembly of animal body plans.