Ammonicrinus

Nothing too heavy today; I just wanted to draw your attention to a very unusual crinoid, Ammonicrinus from the Early and Middle Devonian of Europe and north Africa. While we're used to the idea that crinoids are either stalked things with spindly arms, or free-floating things with spindly arms, not all of them stuck to this body plan. Some of them evolved an enrolled body plan, such as Myelodactylus, which as a fossil looks a bit like a curled-up millipede. Some of them went even farther, like the subject of today's post (and see Bohatý 2011 for much more information).

Yes, this is a crinoid, just... different. Figure 1 in Bohatý (2011). CC-BY-4.0.

Ammonicrinus came in three great flavors: stalked and with a shielded but exposed crown ("exposed roller-type", seen only in the early history of the genus); stalked and with an entirely enrolled crown, most of the animal laying on the seafloor ("encased roller-type"); and barely stalked with an entirely enrolled crown, perched on a brachiopod shell ("settler type"). For the remarkable "encased roller-type", the base was a holdfast attached to something, which was followed by several large, bead-like columnals. Then the columnals began to widen and flatten into broad concave-convex structures, shaped something like brackets in cross-section. These bracket-shaped columnals then turned into a much narrower section which connected to a stocky crown. The crown and the thinner columnals were wrapped up within the broader bracket-shaped segments, with just enough space on either side to pass water through. For good measure, the segments were also decorated with long, articulated, echinoid-like spines (Bohatý 2011).

An early Ammonicrinus, of the "exposed roller-type". Figure 6 in Bohatý (2011). CC-BY-4.0.

What could have possessed the ammonicrinids to go in this direction? One possibility is that everything is tied to making an end-run around other crinoids. Typical crinoids would filter higher in the water column; Ammonicrinus could have the lower levels all to itself. The drawbacks are that the crinoid would be more exposed to predation from benthos (it's hard to believe that anything would willingly eat a crinoid, but there's no accounting for taste), and would be more vulnerable to fouling from the muddy bottom. Ammonicrinus addressed the issue of predation by protecting its crown via enrolling; the spines would have also offered protection. The spines would have also helped to brace the animal against the unstable bottom environment. To keep water flowing and to clear itself of sediment, it could rock the enrolled part of the skeleton, which would force water through the crown (Bohatý 2011).

An Ammonicrinus doing its thing, rocking to promote a current. Figure 14C in Bohatý (2011). CC-BY-4.0.

References

Bohatý, J. 2011. Revision of the flexible crinoid genus Ammonicrinus and a new hypothesis on its life mode. Acta Palaeontologica Polonica 56(3):615–639.