A new species of Tsuga (Pinaceae) from the middle Eocene of Axel Heiberg Island, Canada, and an assessment of the evolution and biogeographical history of the genus

The fossil record of Tsuga (Endl.) Carriere (Pinaceae) indicates that the genus was distributed throughout North America and Eurasia from the Late Cretaceous to the Plio-Pleistocene. Examination of the spatial and temporal distribution patterns of Tsuga fossils indicates that the exchange of representatives between North America and Europe occurred across the North Atlantic land bridges prior to their demize at the end of the Eocene, while exchange between North America and Asia occurred throughout the Cenozoic across the Beringian Corridor. The discovery of well-preserved seed cones of Tsuga swedaea sp. nov. from the middle Eocene (Lutetian/Uintan; 41.3-47.5 Myr) sediments of the Buchanan Lake Formation at Napartulik, Axel Heiberg Island, Canadian High Arctic provides a basis for re-assessment of the fossil record of the genus and an evaluation of the variability of morphological features seen in this taxon. Assignment of these fossil seed cones to the genus Tsuga is based almost entirely on a suite of anatomical features seen in the fossils that have been deemed diagnostic from studies of extant representatives of the genus. Among living representatives of Tsuga, species discrimination is based mainly on the morphological features of the leaves, seed cones and shoots. However, the degree of inter- and intraspecific variability of the characters examined among living representatives of Tsuga is considerable, which has made species-level circumscription and interpreting phylogenetic relationships within the genus difficult. Examination of the bract-scale complexes of T. swedaea and those of all extant species of Tsuga indicates that bract morphology, together with the ovuliferous scale morphology is distinct for each species and useful for species-level circumscription. Based on anatomical and morphological characters of the seed cones and bracts, those of T. mertensiana (Bong.) Carriere most closely resemble the fossil seed cones. However, the differences between T. swedaea, T. mertensiana and all other species of fossil Tsuga cones described in the literature are significant and allow for the erection of a new fossil species. Consideration of the entirety of the fossil record of Tsuga indicates that the earliest representatives of the genus closely resembled T. mertensiana and that diversification of the genus occurred mainly during Miocene and Pliocene time as global climate cooled and new habitats formed in response to major orogenic events. (C) 2003 The Linnean Society of London, Botanical Journal of the Linnean Society, 2003, 141, 257-296.