HOW MANY TIMES DID TWO AMBULACRA EVOLVE IN GLYPTOCYSTITOID RHOMBIFERANS - NEW INSIGHTS FROM THE PAEDOMORPHIC AMBULACRAL REDUCTION (PAR) MODEL?

Pelsiomorphic echinoderms grow a feeding ambulacral system in three distinct phases (the development of the shared BC and DE ambulacra, the development of the A ambulacrum, and the bifurcation of the lateral shared ambulacra forming the distal B, C, D, and E ambulacra). Loss of some combination of these ambulacral elements via paedomorphic ambulacral reduction (PAR) is a common evolutionary pattern in Paleozoic echinoderms. This results in the repeated, parallel evolution of echinoderms with between one and four ambulacra in distantly related clades. No PAR evolutionary pattern is more common than retaining only the shared BC and DE ambulacra at maturity resulting from the developmental loss of the A ambulacrum combined with a loss of the bifurcation of the lateral shared ambulacra. Although this pattern independently arises in at least six classes, nowhere it is more pronounced than in glyptocystitoid rhombiferans. The four known such reductions in this clade show a wide variety of adaptations and different feeding strategies. Although this ambulacral reduction variant is commonly associated with anterio-posterior flattening as in lingulocystids, and cinctans, this only happened once in glyptocystitoids in Pleurocystitidae which have two short ambulacra bearing single, stout terminal brachioles. Similarly, in the Callocystinid “Brockocystis n. sp. A” (Broadhead and Ettensohn, 1981), two short ambulacra bearing single, stout terminal brachioles are present, but without the anterior posterior flattening of the theca. In the scoliocystinid Schizocystis, medium length ambulacra extend onto the lateral plates with upwards of five brachioles each. The theca is otherwise unmodified. In contrast, the staurocystinid Pseudocrinites has exceedingly long ambulacra that in some cases extend onto the stem with tightly spaced brachioles numbering nearly 200. Here the theca is biconvex with the two ambulacra running the margin. Consequently, adults with two ambulacra evolve repeatedly in glyptocystitoids showing a developmental predisposition for this variant on the PAR model. However, in each case, glyptocystitoids show unique morphologies suggesting this ambulacral reduction is in response to different evolutionary pressures.