The Echinoid Directory

Evolutionary history

The oldest echinoids come from the Late Ordovician Period and are approximately 450 million years old. The closest sister group to the echinoids are the holothurians and the two groups must have diverged from starfishes and ophiuroids between 500 and 450 million years ago.

Echinoids remained only a very minor element of the benthos throughout the Palaeozoic and are never common as fossils. Although all had a lantern of some sorts it was not a particularly strong or efficient structure and these early members must have been generalist scavengers of macrodetritus. Diversity increased progressively through the Palaeozoic, and in the Devonian forms started to appear with massive numbers of oral tube-feet - possibly specialist detritus feeders. Archaeocidarids, the precursors to all modern forms, also appeared at this time. Archaeocidarids had formidable spines and more effective lanterns than their contemporaries. The Palaeozoic fauna shows a remarkable variation in the plating structure of the test.

The roots of the modern echinoid fauna can be traced back to the Permian about 250 million years ago, when the first cidaroids appeared. Both cidaroids and the ancestors to the euechinoids passed from the Palaeozoic into the Mesozoic, but we know very little about this period in the group's history. In the Triassic both cidaroids and euechinoids started to diversify, but it is really not until 180 million years ago in the early Jurassic that a major radiation in form got underway.

The Jurassic saw the differentiation of most of the major lines of echinoid. Irregular echinoids appeared for the first time and rapidly specialised for deposit feeding, possibly triggered by ever rising levels of ocean productivity. Echinoids generally become a major constituent of the shallow-water benthos from this time onwards. Whereas cassiduloids dominated the Jurassic and Cretaceous shallow-water clastic settings, atelostomates diversified in more offshore settings. In the Cretaceous heart urchins make their first appearance and thrived in the deeper water settings provided by chalk seas. The end Cretaceous extinction 65 million years ago had a major influence on the structure of echinoid communities and the Tertiary saw the rise to dominance of clypeasteroids over cassiduloids, spatangoids over holasteroids, and camarodonts over stirodonts. Sea urchins are more diverse today than they have ever been.

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