Micraster

Continuous evolution

In the classic text, The Science of Life (1931), H. G. Wells, Julian Huxley and G. P. Wells use Micraster as an example of a fossil whose continuous evolution can be traced over some 10 million years through 450–500 feet of chalk beds of the Late Cretaceous. They point out:

"During this long period the fossil Micrasters are so abundant that hundreds of thousands can be collected and a gradual evolution can be traced as we pass upward. The changes are apparently trivial. There is a slow alteration of shape from rather flattened to rather arched, and from rather elongated to about as broads as long. The mouth creeps steadily forward, its distance from the front border of the lower surface decreasing from about a third of the body-length in the early types to a sixth in the latest, on a total length of fifty to sixty millimetres ... though the changes involved are small, they are absolutely continuous, the urchins found at one level grading quite imperceptibly into those of the rest; a single specimen, indeed, may show characters of one "species" in some of its tube-feet, characters of another in the rest."