Taxon of the Week: So Many Arms

I haven't covered a fossil taxon for Taxon of the Week before, but that's exactly what I'm going to do today. Behold, therefore, the majesty of the Rhodocrinitidae! The photo here shows fossils of Rhodocrinites kirbyi and Cribanocrinus watersianus, both Rhodocrinitidae, and comes from the Smithsonian.

The Rhodocrinitidae are a family of the Camerata, a clade of crinoids restricted to the Palaeozoic. For those not in the know, crinoids (or "sea lilies") are a class of echinoderms, the phylum including such beasties as starfish and sea urchins, and like all echinoderms they have a skeleton of calcareous plates. The majority of fossil species were permanently attached to the sea-bed by a stalk, but the majority of recent species belong to a clade that has lost the stalk as adults and is free-living (actually, even stalkless forms start their lives attached to the substrate, but before they reach maturity they break off their stalk - Breimer, 1978a). Crinoids are filter-feeders, usually with large numbers of feathery arms that are used to sieve the surrounding water (the common name for the stalkless forms is "feather star"). Even the stalkless forms seemingly never move more than is strictly necessary to occupy the optimum position for filter-feeding - as commented by Breimer (1978b), "Seemingly, the stemless crinoids have only gained the vagile capacity of active movement in order to gain efficiency as sedentary animals".

The Camerata were characterised by the development of a calyx (the cup-shaped part of the body that the arms come off) with the skeletal plates rigidly sutured together. The tegmen (the upper covering of the central body - calyx below, tegmen above, if I understand correctly) forms a vaulted ceiling that conceals the mouth and the proximal parts of the ambulacra (the tube-foot-lined grooves that run down the arms and transport food particles to the mouth). The only external opening to the tegmen is the anus, which is usually raised on a tube (doubtless to carry food particles further from the mouth). Extra plates between the bases of the arms incorporated them into the calyx (Ubaghs, 1978b), though in later camerates these were reduced, freeing the arms (Ubaghs, 1978a). The Camerata are divided into two orders, Diplobathrida (including Rhodocrinitidae) and Monobathrida, distinguished by the number of rings of plates making up the calyx.

Rhodocrinitidae were around from the Middle Ordovician to the Lower Carboniferous. They were a heterogenous group encompassing a variety of forms (Eckert & Brett, 2001). The calyx was globular (Ubaghs, 1978), obconical or bowl-shaped (Eckert & Brett, 2001), and the arms could be uniserial or biserial (Eckert & Brett, 2001).

The largely immobile calyx and increased numbers of pinnules (side-branches of the arms) in camerates appear to be adaptations to a rheophilic lifestyle - living in high-current environments (Breimer, 1978c). Also as a probable adaptation to high currents, rhodocrinitids were attached to the substrate by coiling the end of the stalk around an anchoring object. This may have allowed for a less rigid attachment, allowing a certain degree of slippage around the anchor.

The Rhodocrinitidae were the only family of Diplobathrida to survive the Devonian, along with a number of families of Monobathrida. While the Rhodocrinitidae became extinct during the Carboniferous, the Monobathrida trickled along until the end of the Permian, which sounded the final death-knell for the camerates (Ubaghs, 1978a).

REFERENCES

Breimer, A. 1978a. General morphology - recent crinoids. In Treatise on Invertebrate Paleontology pt. T. Echinodermata 2. Crinoidea (R. C. Moore & C. Teichert, eds.) vol. 1 pp. T9-T58. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

Breimer, A. 1978b. Ecology of recent crinoids. In Treatise on Invertebrate Paleontology pt. T. Echinodermata 2. Crinoidea (R. C. Moore & C. Teichert, eds.) vol. 1 pp. T316-T330. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

Breimer, A. 1978c. Paleoecology - Autecology. In Treatise on Invertebrate Paleontology pt. T. Echinodermata 2. Crinoidea (R. C. Moore & C. Teichert, eds.) vol. 1 pp. T331-T343. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

Eckert, J. D., & C. E. Brett. 2001. Early Silurian (Llandovery) crinoids from the Lower Clinton Group, western New York State. Bulletins of American Paleontology 360: 1-88.

Ubaghs, G. 1978a. Evolution of camerate crinoids. In Treatise on Invertebrate Paleontology pt. T. Echinodermata 2. Crinoidea (R. C. Moore & C. Teichert, eds.) vol. 1 pp. T281-T292. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

Ubaghs, G. 1978b. Camerata. In Treatise on Invertebrate Paleontology pt. T. Echinodermata 2. Crinoidea (R. C. Moore & C. Teichert, eds.) vol. 2 pp. T408-519. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).