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Betulaceae, with 120-150 species in six genera, are a family of Fagales that occurs mainly in the Northern Hemisphere. Previous studies of the evolution of Alnus, Betula, Carpinus, Corylus, Ostrya and Ostryopsis have relied on a relatively small number of sequence data and molecular clocks with fixed-point calibrations. We exploited Gen Bank to construct Betulaceae matrices of up to 900 sequence accessions and 9300 nucleotides of nuclear and plastid DNA; we also computed species consensus sequences to build 46- and 29-species matrices that strike a balance between species sampling and nucleotide sampling. Trees were rooted on Ticodendraceae and Casuarinaceae, and divergence times were inferred under relaxed and strict molecular clocks, using alternative fossil constraints. The data support the traditional two subfamilies, Betuloideae ( Alnus, Betula) and Coryloideae, and show that Ostryopsis is sister to Ostrya/ Carpinus. The fossil record and molecular clocks calibrated with alternating fossils indicate that the stem lineage of Betulaceae dates back to the Upper Cretaceous, the two subfamilies to the Palaeocene and the most recent common ancestors of each of the living genera to the mid- to late Miocene. A substitution rate shift in Coryloideae between 25 and 15 Mya preceded the mid- Miocene climatic optimum and may be linked to temperate niches that became available following the mid-Miocene. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 172, 465-477. [ABSTRACT FROM AUTHOR]