Meeting Abstract

83.6  Sunday, Jan. 6  The Gomphoid Synarthrosis: a New Joint in Echinoderms CROCE, H.M.*; TURNER, R.L.; Florida Institute of Technology; Florida Institute of Technology hcroce@my.fit.edu

Certain ossicles in crinoid, echinoid, asteroid, and ophiuroid echinoderms have long been thought to fuse. The development of these ossicle systems has not been well studied, often due to obstruction by other ossicles. Here, the development of vertebral ossicles in the ophiuroid Ophiophragmus filograneus was examined. Arm tips, cleaned of soft tissue, were studied using scanning electron microscopy. Vertebral ossicles originate under the ocular in halves. The two ambulacral ossicles grow towards each other; the stereom branches and eventually interdigitates like a three-dimensional jigsaw puzzle. As vertebrae grow, ambulacral ossicles interlock tightly, creating a suture line, which has been taken before as evidence of fusion. In mature vertebrae, the suture line was not always visible at articular surfaces, indicating fusion. This study suggests that interdigitation of ambulacral ossicles forms an immobile joint, a gomphoid synarthrosis, joining vertebral ossicles in ophiuroids. We examined the gomphoid synarthrosis in vertebrae for the percentage by weight of magnesium making up the stereom. A higher magnesium content imparts greater strength to the ossicle; thus, this area is an excellent candidate for strengthening with magnesium. The gomphoid synarthrosis in O.filograneus vertebrae is not, however, strengthened in this way. Other ossicle systems in echinoderms reported to fuse include compound plates and auricles of Aristotle’s lantern in echinoids, genital plates of irregular echinoids, the circumoral ring of brisingid asteroids, and infrabasal calyx plates and juncture of the stem and calyx in crinoids. These ossicle systems will also be examined for the presence of a gomphoid synarthrosis or fusion.