The fossil record of echinoids in the Palaeozoic is relatively sparse: fewer than 40 genera are known. Palaeozoic echinoids differ from post-Palaeozoic forms in having a flexible (as opposed to rigid) test. Geological terms are explained .
The earliest true echinoids are known from the Upper Ordovician, including Aulechinus. The first cidaroids arose in the Upper Devonian. Though echinoids were never really abundant, an increase in diversity and establishment of the major Palaeozoic lines was seen until the Upper Carboniferous. Here, diversity declined markedly, and by the Permian only two genera are known. By the end of the Palaeozoic all perisoechinoids had become extinct and only one cidaroid genus, Miocidaris, is known to have crossed the Permo-Triassic boundary.
Most post-Palaeozoic echinoids possess a rigid test, and consequently the fossil record is good. Though only Miocidaris is known to have survived the end-Permian mass extinction, analysis of tooth structure and lantern morphology indicates that euechinoids and cidaroids diverged prior to the appearance of this genus. Two separate lineages are therefore thought to have survived into the Triassic: Miocidaris, ancestral to the modern cidaroids, and a precursor of the euechinoids (see Smith and Hollingworth, 1990).
Echinoid diversity remained low until the early Late Triassic. The first cidaroids of modern type appeared then, and by the end of the Triassic, a new radiation produced all major lineages of regular echinoids. This continued into the Jurassic and it is here that the first irregular echinoids are seen. Cidaroids have changed little since the first of modern type appeared. The regular echinoids continued to adapt and diversify, with for example improvement of the lantern, and dominated the early Mesozoic.
Early irregular echinoids such as the pygasteroids and holectypoids probably only lived semi-infaunally. Subsequent diversification is thought to relate to adaptive breakthroughs in both habit and feeding. Real extremes of functional differentiation are not seen until the Cretaceous and origin of the spatangoids. The most recent echinoids to evolve are the clypeasteroids, the order which includes the sand dollars. These arose in the early Tertiary and radiated to adapt to a quasi-infaunal niche in fine sediments.
Images at the top of this page are courtesy of Professor Mike Benton (and from his book 'Basic Palaeontology'). Sequentially from left to right they are: the regular Echinometra, spatangoid Brissus (upper and lower surface views), and the helectypoid Echinoneous.