Plant systematics and differentiation of species over trans-Beringian land connections including a newly recognized cupressaceous conifer Ditaxocladus Guo & Sun

Rich paleobotanical localities of eastern Asia and western North America provide ample opportunities for documentation and analysis of trans-Beringian migration and differentiation. Species-level comparisons are made for cupressaceous conifers Mesocyparis and Ditaxocladus, as well as for the platanoid genera Macginitiea and Platimeliphyllum. Asiatic and American species (forms) of these genera are interpreted as representing successive speciation events en route from northeastern Asia to North America (Mesocyparis rosanovii - M. beringiana - M. borealis and Platimeliphyllum valentinii - P. palanense - Platimeliphyllum sp. from Fushun and Clarno) or, alternatively, a divergent speciation over the trans-Beringian ranges, as in Ditaxocladus. A new species of cupressaceous conifer from the Paleocene of Amur Province is recognized: Ditaxocladus kivdensis sp. nov. The dominant latest Cretaceous-Paleocene group of trochodendrocarpoids (Trochodendroides type leaves, Trochodendrocarpus, Joffrea and Alasia reproductive structures) shows a generic-level differentiation across Beringia attesting to periodic rather than permanent floristic connections. The aquatic/semiaquatic congeners had appeared on both sides of the bridge with the rise of floating plant biomass in the mid-Cretaceous, then synchronously entering the Late Cretaceous (Quereuxia) and terminal Cretaceous (Limnobiophyllum, Cobbania) stages. The trans-Beringian continuity of aquatic vegetation might have been sustained by dinosaur migrations. Cobbania was lost to the end-Cretaceous extinction, but trochodendrocarpoids culminated at the boundary attesting to a widespread temperization of global climates.