What if? suppose a molecular clock dates the divergence of two taxa at 80 million years ago, but new fossil evidence shows that the taxa diverged at least 120 million years ago. explain how this could happen.

The gene/genes which was applied for the molecular clock determination may have evolved at a slow pace in these two taxa when compared to species used to calibrate the clock; this will lead to the clock underestimating the time of divergence of the two taxa apart from each other.

explanation:

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.

this is math not biology xd