Fig. 1

Phylogenetic distribution of the spiral cleavage geometry in the Spiralia. Green circles indicate taxa that exhibit a spiral cleavage geometry (alternating oblique cell divisions). Red squares indicate groups whose cleavage geometry is not spiral. White squares with question mark indicate taxa where the cleavage geometry is unknown. The nodes mark the presumed ancestral cleavage pattern of the branch. Green circles with question mark indicate preliminary, but not conclusive evidence of a spiral cleavage geometry in Gnathostomulida [183]. Spiralian relationships based on [27, 29–32], and cleavage data based on [5] (most clades), [184] (Phoronida), [185, 186] (Entoprocta), [33] (Bryozoa), [34] (Brachiopoda), [35] (Gastrotricha), and [36] (Rotifera). Dashed lines indicate alternative placements for Bryozoa