UCMP MYSTERY FOSSIL #38 is . . .
Carya

Yes, these are the fossilized fruits of a Quaternary tree of the genus Carya, in other words, hickory nuts! The fossil fruits of hickories have been found in sediments dating from the Oligocene (about 34 million years ago). The genus disappeared from Europe about two million years ago, probably because of glaciation. Find out more about flowering plants and their mysterious origins here.