SYMMETRY IN EDIACARAN STEM-GROUP CTENOPHORES

The Ediacaran fossil Eoandromeda octobrachiata is interpreted as an early stem-group ctenophore, characterized by the synapomorphies ctenes, comb rows, and octoradial symmetry but lacking crown-group synapomorphies such as tentacles, statoliths, polar fields, and biradial symmetry. The early appearance in the fossil record of octoradial ctenophores is most consistent with the Planulozoa hypothesis (Ctenophora is the sister group of Cnidaria+Bilateria) of metazoan phylogeny. The Ediacaran occurrences of triradially symmetrical forms, such as Tribrachidium, sharing unique characters with Eoandromeda suggests that n-radial symmetry in early ctenophores fluctuated with regard to the value of n. An analogous pattern is seen in early cnidarians.