The ‘era’? The era/period is determined by the fossils located therein. The fossils are dated by the geological strata the are found in... Science? Looks like one of the ants we just killed in our front yard here in Florida. I thought it was summer, but it must be the cretaceous period!
“given the era”
Keep in mind that 100 million years ago is the most recent fifth of the history of Insects.