Echinoids
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.
