Echinoid ecology and evolution

Echinoids exploit three main life habits represented by three very distinctive morphologies (Table 7.3, Fig. 7.9). Although regular echinoids are known from the Ordovician they are generally uncommon in the Palaeozoic. Lower Palaeozoic forms tended to be small. Size generally increased through the era. Echinoids declined significantly in the late Carboniferous and only a few taxa survived the end-Permian mass extinction event.

Echinoid abundance increased in early Mesozoic times. There was a major radiation in the early Jurassic. Irregulars first appeared during this period, and by the Cretaceous echinoids were exploiting a wide range of infaunal habitats. Flattened sand dollars first appeared in the Palaeocene and these highly modified echinoids quickly became widely distributed.

Table 7.3 Echinoid life habits and morphology.


Shallow infaunal

Deep infaunal

Mode of life Echinoids living on the substrate surface as scavangers or grazers in intertidal or shallow subtidal environments

Morphology Regular echinoids with a rounded test and radial symmetry. The anus is on the upper surface and the mouth is directly opposite on the underside of the test

Echinoids that are able to burrow rapidly in high energy, shifting sands

Irregular echinoids with very flattened, bilaterally symmetric tests. Ambulacra are petal shaped. The anus and mouth are on the lower surface

Echinoids that construct structured, semipermanent burrows in low energy environments

Irregular echinoids with heart-shaped, bilaterally symmetric tests. Ambulacra are petal shaped. The anus is on the posterior margin of the test

Epifaunal echinoids

Slow-moving echinoids that graze the substrate surface using a complex jaw apparatus. Spines are used for locomotion and protection. Tube feet aid the movement and also help anchor the echinoid in crevices or to the substrate. Some forms cover the test with debris for camouflage


Deep infaunal

Echinoids with a wedged profile and deep anterior groove producing a distinctive heart-shaped plan. These highly adapted echinoids construct a complex burrow with a respiratory funnel and sanitary tube. Specialized tube feet, with brush-like ends, build and maintain these structures. Currents are generated within the burrow by cilia attached to modified spines. Food is passed to the mouth via the anterior groove by the tube feet


Shallow infaunal

Extremely flattened echinoids that live in high energy environments. Such echinoids are frequently washed out of their shallow burrows but are able to dig rapidly into the mobile sediment. Some forms have large holes in the test called lunules. These allow water and sediment to flow through the test preventing it from being lifted up and carried away by strong currents. The mouth and anus are on the undersurface. During feeding the test protrudes at an angle from the sediment, directing food-rich currents towards the mouth. Spines are reduced to increase burrowing efficiency and ambulacra are limited to the upper surface of the test to enhance respiration

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