PALEOBIOLOGY OF CARBONIFEROUS MICROCRINOIDS

Carboniferous microcrinoids (crinoids with adult thecae less than 2mm high), with the exception of Amphipsalidocrinus (tentatively identified as a camerate), belong to the disparid family Allagecrinidae or to the cladid family Codiacrinidae. Clades containing microcrinoids in both families also contain larger crinoids, particularly in the Permian.

Microcrinoids were common constituents of Carboniferous benthic assemblages. They inhabited environments ranging from relatively deep, low energy basin floor, through higher energy, open shelf, to shallow water, inner shelf, and are found in both carbonates and mudrocks. Taxonomic diversity of a single assemblage may be as high as 11 species distributed among 8 genera. In a single echinoderm assemblage, microcrinoids are commonly more numerous than larger crinoids.

Where microcrinoids stems are known, those of allagecrinids were relatively long and flexible, whereas those of codiacrinids were short, rigid and attached to firm substrates.

Allagecrinids used their non-pinnulate, unbranched arms to capture food particles substantially smaller than those used by adult larger crinoids. The orals were opened during feeding to allow entry of food into the theca. Many codiacrinids were either armless or had less than five arms. The abrachiate forms probably fed using podia that were exposed when the orals were pushed open. Both types of microcrinoids may also have utilised dissolved organic molecules as a nutrient source.

There is tenuous evidence that allagecrinids had a single internal gonad. The small node on the CD oral of allagecrinids and some codiacrinids was a hydropore not a gonopore; gametes were shed by opening of the orals. Larvae of such small crinoids might have been expected to be planktotrophic but the presence of brooding structures in one allagecrinid genus suggests that at least some were lecithotrophic.