FOSSIL EVIDENCE FOR HYBRIDIZATION IN THE ROSACEAE

Hybridization has always been a focus of neontologists examining its role in the mode and tempo of evolution. Given hybridization’s only occasional significance to invertebrate and relatively rare significance to vertebrate evolution today, it is not surprising that paleontologists have rarely interpreted morphological intermediates as hybrids (e.g., Ausich and Meyer, 1994; Schulz-Mirbach, 2008). What is surprising is that while modern plants are well known to hybridize at interspecific and intergeneric levels, concrete evidence for hybridization in the plant fossil record is rarely addressed. The rose family, Rosaceae, is characterized today by a variety of reproductive strategies including hybridization, polyploidy and other speciation mechanisms. These processes occur repeatedly in several lineages, producing characteristic patterns of morphological variation that challenge plant systematists studying the family. Leaves of Rosaceae are well represented in the latest early Eocene Republic flora of northeastern Washington State. We have been able to demonstrate the presence of at least one hybrid complex at Republic, that shows the same types and ranges of leaf variation found within the modern genus Sorbus. Documenting similar patterns of variation in the fossil and extant leaf types provides a means for interpreting models of evolution for the family and suggests that for at least 50 million years lineages of Rosaceae have experienced numerous episodes of intense speciation associated with climate shifts.