Why do zoologist think that pentaradial symmetry was not present in the ancestor of echinoderms?

Zoologists believe that pentaradial symmetry was not present in the ancestor of echinoderms because this symmetry is a derived characteristic that evolved later in their evolutionary history. The ancestral echinoderms likely exhibited bilateral symmetry, similar to their closest relatives, such as chordates and other deuterostomes. Fossil evidence and comparative studies of living echinoderms suggest that the transition to pentaradial symmetry occurred after the divergence from their bilateral ancestors, as a response to their marine environments and ecological adaptations.