Insects pollinating plants have recently been confirmed from 100 million years ago (Physorg, May 14, 2012):
Amber from Cretaceous deposits (110-105 my) in Northern Spain has revealed the first ever record of insect pollination. Scientists have discovered in two pieces of amber several specimens of tiny insects covered with pollen grains, revealing the first record of pollen transport and social behavior in this group of animals.
Given that 80% of today’s flowering plants depend on insect pollination, there must have been a good deal of co-evolution and horizontal gene transfer in the meantime.