Final answer:
The most abundant fossil evidence in the geologic record first appeared during the Cambrian Period, about 521 million years ago, with the Cambrian Explosion marking a significant diversification of life. Stromatolites, some of the oldest known fossils, date back to approximately 3.7 billion years ago.
Step-by-step explanation:
This era is renowned for the Cambrian Explosion, a time of rapid diversification of life forms on Earth. Trilobites, which are some of the most well-known early fossils, first appeared around this time. The oldest known fossils, however, are stromatolites, dating back to approximately 3.7 billion years ago, which are evidence of ancient photosynthetic bacteria. Other significant fossils such as echinoids first appear about 460 million years ago, notably diversifying much later during the Jurassic.