Different studies often report substantial discrepancies between age estimates for groups of organisms implied by the fossil record and estimates generated by molecular clocks. (sciencedaily.com)
This is because body size not only has numerous implications for function and life history but also has necessary limits that differ between groups of organisms. (science.sciencemag.org)
We take groups of organisms put them in their environment and try and understand what's happening. (pbs.org)