Clutching Crinoids

The Lower Carboniferous Parisocrinus labyrinthicus. The inclusion of Parisocrinus in Euspirocrinidae is uncertain - it was excluded by Eckert and Brett (2001), but included by Waters et al. (2003). Photo from The Virtual Fossil Museum.

This happens to be the third Taxon of the Week post on crinoids. In the earlier posts (see here and here), I mentioned the basic divisions within crinoids and some details of structure, so I'm just going to take those as read for this post.

The specific topic of this post is the cladid family Euspirocrinidae. I could start by saying that the Euspirocrinidae were found from the late Silurian to the early Carboniferous, but to be honest I already be fudging issues. As explained in the second of the posts linked to above, the Cladida have been the most successful of the three major crinoid clades. However, relationships within the Cladida are subject to a great deal of uncertainty. While a detailed subdivision between suborders, superfamilies and families was recognised in the Treatise on Invertebrate Paleontology by Moore et al. (1978), its apparent authority was largely an illusion. Apart from the two clades that have been recognised in the past as separate subclasses (the Flexibilia and Articulata), very few of the various cladid "families", "suborders", etc. that have been recognised are well-defined. The situation was bad enough that Kammer and Ausich (1996) apparently felt the need to abandon all attempts at subdividing the Cladida and simply listed all genera alphabetically, recognising at most a purely pragmatic division between primitive and advanced grades. As such, it is suspected that many of the cladid "families" represent polyphyletic groupings, and the Euspirocrinidae is one such grouping.

Such as it was, the Treatise Euspirocrinidae comprised crinoids with cone- or bowl-shaped cups, five large oral plates, slender isotomously-branching (i.e. branching into two equal parts) arms and stout round stems. A brief revision of the group by Eckert & Brett (2001) removed some of the more distinct taxa and added a few more defining features, most notably restricting the family to taxa with a large, non-porous anal sac. For sessile filter-feeders like most crinoids, excretion is often a serious matter, especially if you live in a low-energy environment. If indigestable wastes are released too close to the mouth, the poor animal could end up re-ingesting its own wastes. Many crinoids solved this problem by developing sizeable anal sacs or tubes that carried wastes a reasonable distance from the calyx before releasing them. The unusual stout, relatively inflexible columns of the Euspirocrinidae could indicate that they lived in habitats with relatively low currents (Breimer, 1978), as such a column provides extra support but would be prone to breakage in higher-energy environments. Euspirocrinids would have fed by passive capture of small food particles settling from above.

The type genus of the Euspirocrinidae, Euspirocrinus, is a particularly noteworthy genus. Uniquely among Silurian cladids, Euspirocrinus developed the ability to tightly coil its arms. Eckert & Brett (2001) suggest that rather than being a passive capturer of food particles like other euspirocrinids, Euspirocrinus was probably an active grabber and trapper of larger food items, such as small animals. The tightly coiled arms formed a chamber above the mouth in which prey could be captured, broken down and digested. A similar feeding style has been suggested for the living Holopodidae.

REFERENCES

Breimer, A. 1978. 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.

Kammer, T. W., & W. I. Ausich. 1996. Primitive cladid crinoids from Upper Osagean-Lower Meramecian (Mississippian) rocks of east-central United States. Journal of Paleontology 70: 835-866.

Moore, R. C., N. G. Lane, H. L. Strimple, J. Sprinkle & R. O. Fay. 1978. Inadunata. In Treatise on Invertebrate Paleontology pt. T. Echinodermata 2. Crinoidea (R. C. Moore & C. Teichert, eds.) vol. 2 pp. T520-T759. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).