Which of the following periods is the earliest during

which animals may have appeared?

a. Ordovician period

b. Cambrian period

c. Ediacaran period

d. Cryogenian period

The fossil record of complex, multicellular animals appeared in the Ediacaran period which had thrown light on the animal fossil data.

The time between 620-550 million years ago is considered as the Ediacaran period. The Ediacaran fauna is found after the discovery of the fossils of large, soft-bodied, complex, multicellular animal traces.

The Ordovician period is the time between 485.4 -443.8 million years ago. The marine invertebrates' diversity was known to be lived in this period including graptolites, trilobites, conodonts, etc.

The Cambrian period was the time between 541 to 485 million years ago and from the end of the Ediacaran period. The appearance of many kinds of invertebrates and the first form of vertebrate fishes was recorded during this period.

The Cryogenian period is the time between 720 to 635 million years ago. This is the period when dynamic environmental changes are recorded.