PERAMORPHISM IN THE AMBULACRAL SYSTEM OF EDRIOASTEROID ECHINODERMS

Heterochrony is common within isorophid edrioasteroids. Although paedomorphism has been documented in isorophids, the more common peramorphism has not been studied in detail. Acceleration is seen several times in the evolution of isorophid ambulacral systems. It is recognized by over-developed ambulacra in taxa that retain plesiomorphic body size. Four styles of ambulacral over-development are recognized: terminal branching, terminal meandering, terminal coiling, and pin wheeling. These ambulacral growth strategies result in proportionately longer ambulacra at maturity that occupy much of the interambulacral fields. Terminal branching is seen in Thresherodiscus where new, short, straight ambulacral segments are added throughout ontogeny. The straight ambulacra suggest that this condition was derived from a primitively paedomorphic form. Terminal coiling is seen in New Genus A from the Ordovician of Morocco, where the distal ambulacra curve back into the centers of the interambulacra. Terminal meandering is seen in Pyrgopostibulla where up to three meander loops can develop at maturity. Pin wheeling is seen in Streptaster where ambulacra spiral up to 180 degrees around the oral surface. Here, all ambulacra curve in the same direction so that they do not cross distally. Preserved ontogenetic series in Pyrgopostibulla, New Genus A, and Streptaster show that over-developed ambulacral systems are mature modification to the ancestral ontogeny trajectory. Hypermorphosis is recognized by over-developed ambulacra at maturity in taxa that are much larger than the ancestral condition. Here, only three styles of over-developed ambulacra are seen. Distal meandering is seen in Torquerisediscus where the distal ambulacra cover much of the oral surface. Preserved ontogeny documents this as a mature modification to the ancestral ontogenetic series. Distal coiling as in Bostryclavus and pin wheeling in Giganticlavus are interpreted to represent hypermorphosis but no preserved ontogenetic information is available. Predisplacement has not been recognized in edrioasteroid ambulacra. This demonstrates that acceleration and hypermorphosis can result in similar strategies for ambulacral lengthening.