FIGURE 6. Molar morphology in North American mammoths, modified from Lister (2017). Specimens above dashed line are Early and Middle Pleistocene plus some Late Pleistocene; this represents the approximate range of 'typical' Mammuthus columbi. Note that all Early and Middle Pleistocene specimens fall within the range of Late Pleistocene ones. Late Pleistocene specimens below and to the left of the dashed line represent more advanced, 'jeffersonian' M. columbi, some of them from the same locality as the 'typical' individuals and therefore variants, not a different species. Plotted are lamella length (inverse of lamellar frequency) and enamel thickness of upper third molars (indexes standardized to crown width 100 mm).