Write a brief paragraph as to how each condition affects blood flow

What are two reasons for why a species might disappear from the fossil record?

Explain how paleontologists use trilobite fossils as index fossils for various geologic time periods.

Which of the following is a reason why early living organisms on earth could not have survived on the surface? 1. the lack of an ozone layer 2. the lack of oxygen in the atmosphere 3. the fact that these organisms were single-celled