In the 1970s there was already a debate about whether the emergence of the modern phyla was "explosive" or gradual but hidden by the shortage of Pre-Cambrian animal fossils.

These provide firm data points for the "end" of the explosion, or at least indications that the crown groups of modern phyla were represented.

Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all.

The Cambrian Period witnessed the most rapid and widespread diversification of life in Earth's history, known as the Cambrian explosion, in which most modern phyla first appeared.

Besides including the earliest known representatives of some modern phyla, they have the great advantage of presenting a nearly continuous record of Early Cambrian organisms whose bodies include hard parts.

By the end all modern phyla had evolved, and since then no new phyla have appeared in the fossil record.

But the Burgess motor of invention had been shut off by then, and nearly all the beautiful fossils of Mazon Creek fit comfortably into modern phyla.

Such a ring of tentacles would strongly resemble a lophophore--the feeding structure of several modern phyla, notably the bryozoans and brachiopods.

After all, they include representatives of nearly all modern phyla, and what could be more different, one from the other, than a trilobite, a snail, a brachiopod, and an echinoderm?

Gephyrea is a now-dismantled taxon that was considered to be a class of annulated worms, containing the three modern phyla Echiura, Sipuncula, and Priapulida.