Evolutionary Development of Pseudomonocycly among Paleozoic Crinoids

Most crinoids posses either one or two plate circlets below the arm-bearing plates of the cup (e.g. radials). Irrespective of homology, these morphotypes are defined as monocyclic and dicyclic, respectively. Pseudomonocycly (sensu stricto) is defined as the loss of the lowermost cup-plate circlet (e.g. infrabasals) in an otherwise dicyclic clade. However, the term has been applied broadly to describe any monocyclic crinoid that has originated from a dicyclic ancestor regardless of the mode of evolutionary development.

Beginning with a dicyclic morphotype, psudomonocycly (sensu lato) could occur through the loss of any one of the cup plate circlets, (typically infrabasals, basals, or radials). All three forms of loss have been documented among Paleozoic crinoids: (1) articulates and some flexibles lost the infrabasal circlet, (2) some cladids lost the basal circlet, and (3) some disparids lost the radial circlet.

Loss of the infrabasal and radial circlets almost invariably occurs through atrophy, and ultimately, total resorption of those plates. However, the development of pseudomonocycly via the loss of basal plates exhibits numerous pathways. In such crinoids pseudomonocycly is observed to occur via loss of the basal plate circlet (1) with or (2) without compensatory growth of surrounding plates, (3) fusion with the infrabasal circlet, and (4) possibly from torsional plate growth.

Such variability among dicyclic to monocyclic developmental trajectories impedes efforts to establish a clear plate homology system for crinoids. Nevertheless, studies of evolutionary development may provide clues for unravelling the puzzle. For example, whereas in most crinoids plate circlets are offset by 36° relative to sub- and superadjacent circlets, many of the earliest crinoids exhibit mid-cup rotational instability. This may indicate that pseudomonocycly via loss of the basal circlet was common in the early evolutionary history of the group.