Paracomatula: Feather Star, or Feather Star Wannabe?

Fossilised accumulation of Paracomatula helvetica, from here.

Earlier posts on this site have discussed examples of the feather stars, the most successful representatives in the modern environment of the crinoids. Originally a group whose members lived permanently attached to the substrate by a stalk, at some point crinoids diversified to a more mobile (or least shiftable) lifestyle, discovering the joys of travel. This was not, it should be noted, an entirely direct process. Many stalked crinoids are also mobile, able to detach themselves from their substrate and crawl to a new position. Other crinoids than the feather stars lost their stalks. And at least one group within the feather stars, the Mesozoic Thiolliericrinidae, reverted back to retaining as adults the larval stalk that most feather stars lose in the course of development.

Nevertheless, the feather stars had definitely made their appearance by the early Jurassic. One of the earliest taxa that has been assigned to the feather stars is Paracomatula, which is known from the very late Triassic to the middle Jurassic (Hess 2013). Paracomatula would have largely resembled a modern feather star in appearance, but had one significant difference. In modern feather stars, the base of the central cup is formed by a large conical plate known as the centrodorsal. In Paracomatula, however, the centrodorsal is replaced by a stack of five narrow plates. These correspond to much-shortened versions of the columnals that make up the stalked in other crinoids, and Rasmussen (1978) and other authors suggested that Paracomatula's separate columnals became fused to form the centrodorsal of the feather stars proper.

However, not all authors have accepted this interpretation. Hess (2013) has argued that details of the development of modern feather stars from a stalked juvenile to a free-living adult indicate that the centrodorsal is derived from the enlargement of a single columnal rather than the fusion of a series. In the earliest definitive feather star, the Jurassic Palaeocomaster, the cirri (tentacle-like appendages) on the centrodorsal are arranged in a haphazard fashion consistent with their development on a single expanding plate, while Paracomatula has a more orderly array of one ring of cirri per columnal without the development of supernumerary cirri. Hess therefore argues that Paracomatula species were not the forebears of feather stars, but their rivals: a closely related group that was independently experimenting with a stalk-free way of life.

REFERENCES

Hess, H. (in press, 2013) Origin and radiation of the comatulids (Crinoidea) in the Jurassic. Swiss Journal of Palaeontology.

Rasmussen, H. W. 1978. Articulata. In: Moore, R. C., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt T. Echinodermata 2: Crinoidea, vo. 3, pp. T813-T927. The Geological Society of America, Inc., and The University of Kansas Press.