GEOLOGY

Detailed explanation-1: -The Cambrian Period marks an important point in the history of life on Earth; it is the time when many kinds of invertebrates and the first vertebrates-fishes-appeared in the fossil record. The Burgess Shale contains the best record of Cambrian animal fossils including soft-bodied forms.

Detailed explanation-2: -The beginning of the Cambrian Period is marked by the evolution of hard body parts such as calcium carbonate shells. These body parts fossilize more easily than soft tissues, and thus the fossil record becomes much more complete after their appearance.

Detailed explanation-3: -During this time, dissolved oxygen was increasing in the water because of the presence of cyanobacteria. The first animals to develop calcium carbonate exoskeletons built coral reefs. The middle of the Cambrian Period began with an extinction event.

Detailed explanation-4: -Most of the phyla featured in the debate about the Cambrian explosion are coelomates: arthropods, annelid worms, molluscs, echinoderms, and chordates – the noncoelomate priapulids are an important exception.