Iocrinus subcrassus

This sea lily grew to about 15-20 cm and had branched arms (with up to seven branching divisions) forming an open-fan filtering system. Its stem was made of columnals (individual calcareous plates) of pentalobate or pentagonal shape, and it attached to the substrate by tight coiling of its distal part (Thomka et al, 2018). Several monospecific assemblages (i.e., containing only a single species) of this crinoid have been found, as seen in this specimen, pointing to a gregarious lifestyle (Thomka et al., 2018). Additionally, preservation of articulated stem, calyx, and arms, also seen in this specimen, implies rapid or nearly instantaneous burial at death or nearly after death, with no time for disarticulation (Brett et al., 1997). Crinoids of the genus Iocrinus have been described from Ordovician rocks from North America, the United Kingdom, Morocco, Oman, and Arabia, pointing to an unusually widespread paleogeographic distribution for this crinoid (Donovan et al., 2011; Zamora et al., 2015). This specimen was collected from the Ordovician Waynesville Formation in Indiana. Iocrinus subcrassus is a disparid crinoid traditionally classified within the order Myelodactylida (Ausich, 1998).

References:

Ausich, W.I., 1998. Phylogeny of Arenig to Caradoc crinoids (Phylum Echinodermata) and suprageneric classification of the Crinoidea. University of Kansas Paleontological Contributions, v. 9, 36 p.

Brett, C.E., Moffat, H.A., and Taylor, W.L., 1997, Echinoderm taphonomy, taphofacies, and Lagerstätten, in Waters, J.A. and Maples C.G. (eds.) Geobiology of Echinoderms. Paleontological Society Papers, v. 3, p. 147–190. DOI: https://doi.org/10.1017/S1089332600000243.

Donovan, S.K., Miller, C.G., Sansom, I.J., Heward, A.P. and Schreurs, J., 2011. A Laurentian Iocrinus Hall (Crinoidea, Disparida) in the Dapingian or Darriwilian (Middle Ordovician, Arenig) of Oman. Palaeontology, 54(3), pp.525-533. DOI: https://doi.org/10.1111/j.1475-4983.2011.01042.x.

Thomka, J.R., Brett, C.E., Bole, T.A. and Campbell, H.J., 2018. A noteworthy accumulation of disparid crinoids from the type Cincinnatian (Upper Ordovician) of southwestern Ohio, USA: implications for the palaeoecology and taphonomy of crinoid “logjam” assemblages. Swiss Journal of Palaeontology, 137, pp.259-264. DOI: https://doi.org/10.1007/s13358-018-0159-8.

Zamora et al. (2015), Palaeogeographic implications of a new iocrinid crinoid (Disparida) from the Ordovician (Darriwillian) of Morocco. PeerJ 3:e1450; DOI: 10.7717/peerj.1450.