The zone is 170 m thick, and ammonoids and other fossils occur as imprints in mudstones and as shell-rich layers in sandy turbidites.
The illustrations are mostly high quality black-and-white photos but there are also 16 pages of excellent colour photos of fossils and their living counterparts.
A biostratigraphic correlation was possible as the fauna was not associated with other fossils and was unique.
The ages of these inferred foreign successions, which lack fossils, have been a matter of debate.
The use of trace fossils as proxies for the presence of animals allows palaeoecological data to be gathered in the absence of body fossils.
The fossils were found in the middle part of the road-cut.
The conditions leading to preservation of these fossils are undoubtedly rare but not entirely unique.
However, this technique only provides an external morphological description of putative fossilized cells and is incapable of distinguishing between abiotic fossils of similar morphologies.
