Echinoids first appeared about 440 million years but were a relatively minor group for at least the first 200 million years of their existence. By Jurassic times, around 180 million years ago, the first of the 'irregular' echinoids had appeared. They were able to diversify into many more niches than the more primitive 'regular' echinoids through the development of new feeding strategies and the ability to burrow into soft sediment. Remains of 'regular' echinoids are sometimes found in Carboniferous and Lower Jurassic rocks in Northern Ireland, but mostly they occur just as isolated fragments, since the calcite plates of which they were constructed were not rigidly fused together. In Upper Cretaceous rocks, such as the Hibernian Greensand and Ulster White Limestone, echinoids are among the more common fossils although nearly all of those found in Northern Ireland are the more rigidly constructed 'irregular' echinoids.





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