scientists hypothesize that the family myrtaceae arose between sixty and fifty-six million years ago during the paleocene era. pollen fossils have been sourced to the ancient supercontinent gondwana. the breakup of gondwana during the cretaceous period at around forty million years ago geographically isolated disjunct taxa and allowed for rapid speciation: in particular, genera once considered members of the now-defunct leptospermoideae alliance are now isolated within oceania. generally, experts agree that vicariance is responsible for the differentiation of myrtaceae taxa, except in the cases of "leptospermum" species now located on new zealand and new caledonia, islands which may have been submerged at the time of late eocene differentiation.