Mammoth evolution in the late Middle Pleistocene: The Mammuthus trogontherii-primigenius transition in Europe

The late Middle Pleistocene (Marine Isotope Stages 11-6, ca. 425-125 ka) saw the transition from the so-called steppe mammoth (Mammuthus trogontherii) to the woolly mammoth (M. primigenius) in Europe. A metrical study of molariform teeth from more than 40 localities in Britain and continental Europe allows this transition to be traced in detail. The morphological trend began as early as MIS 10, with the final transition to M. primigenius around the MIS 7/6 boundary where several samples (Marsworth, Crayford, La Cotte) span both species' morphology. The 'small mammoth' of MIS 7 is not homogeneous, as those from Stanton Harcourt (Oxfordshire, UK) are smaller and more advanced than those from Ilford (Essex, UK), likely corresponding to their younger age within MIS 7. Contrary to expectation, the thinning of molar enamel and increase in lamellar packing from MIS 16 to MIS 7 appear not to have been driven by increase in lamellar number since they precede it, nor is lamellar number primarily correlated to molar size. The adaptive significance of size changes is unclear, but they affected molar function via their effects on lamellar spacing. This study underlines the importance of adequate sample sizes and bivariate analysis of metric variables to take account of size change. The implications of the study for taxonomy, biostratigraphy and the effects of insularity are discussed.