Fig. 1

In these two Toarcian driftwood dwellers, peramorphic development of the arms was combined with a switch to heterotomous, but unlimited, branching at the 20-arm stage. Branches on the inner side (ramuli) remain thinner, run parallel, and do not bifurcate any more, while those on the outer side keep branching at regular intervals. This allows neighboring arms to be coupled by pinnular adhesion and to act like the radial spokes of an umbrella. Differences between the two forms regard overall size and the distancing of nodal cirri, which became “winged” in Pentacrinites. If one deals with developmental stages rather than different taxa, the distal root cirri of Seirocrinus should also be winged (see Fig. 6), while its proximal cirri became reduced to a “mini-skirt” protecting only the non-filtrating base of the crown. (modified from Seilacher and Hauff (2004)