HOW MANY OSSICLES DO BLASTOIDS AND OTHER ECHINODERMS ACTUALLY HAVE?

Echinoderms are common marine animals throughout the Phanerozoic and at times (such as the Lower Carboniferous) produced copious amounts of sediment derived from the disarticulation of their multielemental skeleton. Often, echinoderms are described in the non-specialist literature as bearing a small number of skeletal ossicles surrounding the visceral mass with only a mention of a stem and peripheral feeding appendages. This leads to the impression that most echinoderms, such as blastoids, have only 13 - 18 ossicles per animal plus stem material that can be incorporated into sediments. This notion is misleading because echinoderms have arguably the most complex animal skeletons with among the largest number of ossicles. In the blastoid, Pentremites godoni, the 18 major thecal plates are attached to a stem typically 10 cm in length bearing ~100 columnals. The peristome and spiracles are covered by ~60 and ~50 ossicles respectively. Each of the five ambulacra has two side plates on each of ~100 secondary food grooves, summing to ~1000 ossicles. These secondary food grooves in turn are covered by a series of 20 to 24 cover plates summing to ~10,000 - 12,000 ossicles. Each brachiole (feeding appendage) has on average 150 brachiolar plates and 200 cover plates over the brachiolar food groove summing to ~350 ossicles per brachiole. Because each of the ~100 side food grooves on each of five ambulacra supports one brachiole, the sum of brachiole-borne elements is a staggering 175,000 ossicles! Thus, the total number of ossicles in a typical Pentremites godoni is ~190,000. Although the vast majority of the ossicles are extremely small, the volume of brachiolar and stem ossicles is comparable with the main thecal plates. Similar counts for the modern crinoid Oxycomanthus japonicus, the modern sand dollar Mellita quinquiesperforata, and the Mississippian edrioasteroid Neoisorophusella lanei sum to ~406,000; 367,000; and 920 ossicles respectively. The crinoid, like the blastoid, has over 99% of ossicles in the feeding appendages. All but ~1000 of the ossicles in the echinoid are spines and pedicellaria. The relatively low number of ossicles in the edrioasteroid results from the rudimentary ambulacra lacking free appendages.