Fig. 2

In all Articulata, stem growth proceeds in a double mode: Nodals (black) form serially at the base of the calyx, followed by the intercalation of internodals in hierarchical order. In the benthic Encrinus, the latter process stoped after three generations of internodals and accretionary growth led to drum-shaped columnals and a solid stem that thickens distally. In the pseudoplanktonic Pentacrinites, no more internodals were added after two generations, with little (mainly transversal) secondary accretion. Seirocrinus had no limit to either mode of stem-lengthening. It also shows how driftwood dwellers accelerated growth by “brooding” several generations of columnals simultaneously. The “factory” for nodals is protected by spines extending from the radial plates of the calyx (black), while the ones for internodals are protected by the thickened rims of already mineralized neighbors. (Modified from Seilacher and Hauff (2004)