You searched for: “cryptozoic
1. A reference to small terrestrial animals (cryptozoa) inhabiting crevices, living under stones, in soil, or in litter.
2. Used to describe invertebrates that live in dark or concealed places such as under stones or in caves or holes.
3. In geology, the geologic time interval for which the corresponding rocks show limited evidence of primitive life forms [see the excerpts that follow].

The earliest rocks on earth with elaborate fossils are said to belong to the Cambrian age; and the entire four-billion-year history of our planet that preceded it has been, until recently, dismissed as the pre-Cambrian age. Now that the traces of life have unmistakably been found in it, the more appropriate name Cryptozoic eon (hidden life) is used, while the last 600 million years make up the Phanerozoic eon (visible life).

The Cryptozoic eon is even divided into two sections:
The earlier Archeozoic era (ancient life), to which the first traces of unicellular life belong; and the later Proterozoic era (early life).

Asimov’s New Guide to Science by Isaac Asimov
(New York: Basic Books, Inc., 1984), p. 782.