POSTERIOR PLATE REDUCTION IN LOWER MISSISSIPPIAN ADVANCED CLADID CRINOIDS: LINKED TO PAEDOMORPHOSIS?

The posterior interray of Paleozoic cladid crinoid cups contains anal plates that helped support the anal sac, which contained much of the digestive system. Lower Mississippian advanced cladids (those with arm pinnules) typically have three anal plates, although the number and arrangement of plates may vary. Among 43 North American genera, there are nine with only one anal plate. These include: Acylocrinus, Armenocrinus, Bursacrinus, Corythocrinus, Graphiocrinus, Holcocrinus, Lebetocrinus, Nactocrinus, and Tropiocrinus. Remarkably, seven of the nine genera occur in the Burlington Limestone, which suggests unique conditions for this formation.

In the nine genera with only a single anal plate, eight other characters are closely associated: high aboral cup, plenary radial facets, primibrachial-one axillary, 10 arms, A-ray identical to other rays, round column, smooth cup plates, and cuneate/subcuneate brachials. The frequency distributions of these characters were evaluated using the binomial distribution function, which indicates that the characters high aboral cup, plenary radial facets, primibrachial-one axillary, 10 arms, and A-ray identical to other rays are linked with a single anal plate at the 90% or higher cumulative distribution level; the remaining three characters are not significant in their association. The five characters that are linked represent either a more primitive state (high aboral cup), or a reduction in arm branching complexity. Along with the reduction in number of anal plates, this suggests progenesis caused an overall reduction in the number of cup and arm plates resulting in paedomorphosis. The relatively high energy conditions, and unstable substrate of the Burlington Limestone packstones/grainstones, may have selected for shortened ontogeny and rapid maturity.

In spite of sharing many morphologic similarities, these nine genera show evidence of belonging to a variety of clades. Traditional crinoid classification schemes have placed great emphasis on the number and arrangement of anal plates, often placing genera together on the basis of a shared single anal plate. These data suggest that the presence of a single anal plate should be used cautiously in evaluating cladid crinoid phylogenetics as it may be a homoplastic rather than a homologous character.