Fossil History of Marine Invertebrates
To trace the invertebrate lines we must also look for fossils where animals were deposited continuously and the fossil remains to have survived in a relatively undistorted condition such as has occurred in the Atlas Mountains of Morocco. From this fossil record and from other sites scattered around the world there appears a clear dichotomy in the history of earth where fossils are found and they are not found. This period of transition corresponds with about 600 million years and records the first annuals which are characterized by the presence of shells. It is conceivable that before this period the animals were soft bodied and did not fossilize. It has also been suggested that seas were not at the right temperature and or chemical composition to favour deposition of lime from which most marine shells and skeletons are constructed.
FOSSILS OF MARINE INVERTEBRATES