New Observations on the Last Pterocarya (Juglandaceae) Occurrences in Eastern North America

Premise of research. Pterocarya (Juglandaceae) was widespread in the Cenozoic of the Northern Hemisphere, including western and eastern North America, but presently survives only in the Caucasus Mountains and eastern Asia. We describe two macrofossil fruit occurrences, one from the Miocene of Maryland and one from the Pliocene of Alabama. These augment existing palynological evidence of its survival in eastern North America to the Pliocene. We also explore hypotheses for its demise in what should have been a promising modern-day refugium.Methodology. The morphology of both fossils was observed using a dissecting microscope, and characters of extant and previously described fossil species of Pterocarya, both from the literature and from living specimens, were examined. Measurements of wing size and fruit size were made with imaging software to establish a wing-to-fruit ratio.Pivotal results. The fossils from the Miocene of Maryland can be assigned to Pterocarya section Platyptera, aff. P. rhoifolia, of Japan. The new Pterocarya fruit from the Pliocene of Alabama cannot be assigned to a modern section. It possesses an abnormally large nut and the lowest wing-to-fruit ratio of any living or fossil species. We describe this fruit as P. magnifructa, a new species.Conclusions. Pterocarya was a common constituent of the Miocene flora of the mid-Atlantic coast but became restricted to southeastern North America in the Pliocene, where it was apparently less common. The size and wing-to-fruit ratio of the Pliocene specimen suggest that it may have been evolving toward biotic dispersal, perhaps in response to climate change and/or competition from the radiating diversity of accompanying Carya species.