Evolution & Systematics

D. savignyi falls within the phylum Echinodermata, making it a deuterostome, identifiable by its pentaradial symmetry, water vascular system, calcitic skeleton composed of ossicles, and mutable connective tissue (Ruppert et al, 2004). One speculation into the evolution of Echinoderms suggests that a bilaterally symmetrical enteropneust-like ancestor developed hydraulic outpockets of its coelomic cavity due to a shortening and widening of the body, and thus gradually became pentaraidially symmetrical (Gudo, 2005).

Within Echinodermata, D.savignyi is part of the superclass Echinozoa; taxa that lack arms and have a body mostly covered by ambulacra. The body is rounded and mostly consists of the oral surface, and has a well-developed hemal system (Ruppert, et al, 2004).Within this superclass, D. savignyi is an Echinoid, a group that contains sea urchins, sand dollars, heart urchins and sea biscuits. Echinoids are distinguished from other echinozoans by their fused tests, movable spines and pedicellariae (Ruppert et al, 2004). The oldest fossil record of echinoid pedicellariae came from the late Silurian of England,which was described as very simple, consisting of three slender spines stemming from a single tubercule (Coppard, 2012). This suggests that early Echinoids may have evolved from an ancestor that had clusters of small spines that became united at their base.

The family Diamematidae can be distinguished from related families by their long, hollow spines that are whorled with tiny barbs that point away from the test; the lack of a longitudinal median ridge on the concave inner surface of the teeth; perforate tubercles of primary spines; its 5 pairs of buccal plates, and tridentate and needle-like pedicellaria (Randall, 1964).

As a genus, Diadema differs from the other genera of the family by its tube feet that are arranged in groups of three, and by its test, which has a unique arrangement of ambulacral tubercles (Randall, 1964).

The phylogeny of Echinodermata adapted from Ruppert et al, 2004.