The Echinoid Directory

The surface of heart urchin plates is covered in a mass of small tubercles marking the attachment sites for spines and pedicellariae. In the specimen illustrated tubercles are of various sizes, the largest being slightly sunken. All are constructed in the same way. The spine base rests and articulates on the central ball-like mamelon. This is composed of a polycrystalline calcite and has a smooth, glassy appearance. It is always perforate. Surrounding the mamelon is the platform, which may be crenulate or smooth. This too is composed of solid calcite and has a glassy appearance. The mamelon and platform are on top of a mound, termed the boss. This is where an inner ring of catch apparatus is inserted. Catch apparatus is a special type of connective tissue that can clamp the spine firmly down onto the tubercle. Finally, there is a flat platform of very fine stereom surrounding the boss termed the areole. The outer ring of spine muscle attaches here. In many cases the muscle attachment area is much wider and better developed to one side of the tubercle, and in those cases the power-stroke of the spine lies in that direction. The smaller tubercles generally have only a mamelon and boss differentiated.

Heart urchins with heterogeneous tuberculation, as illustrated, are characteristically restricted to living either epifaunally or infaunally within rather coarse substrates. Species that burrow into finer sands and muds always have a very dense and uniform coating of aboral spines and tubercles.

Tubercles bearing spines that have different functions are generally morphologically distinct. It is therefore possible to use tubercle structure and distribution to be more precise about how fossil heart urchins used their spines. The structure and function of echinoid spines is fully documented in Smith, A. B. 1980. The structure and arrangement of echinoid tubercles. Philosophical Transactions of the Royal Society, London, Series B, 289, 1-54.