FIGURE 1. Sequence of mammoth species in Europe. Left: heightening and shortening of skull; middle: increase in plate number and crown height of molars; right, artists’ reconstructions, taxonomy and chronology. Skull diagrams from ref. 46: a - Mammuthus meridionalis, b - M. primigenius, reproduced by permission of Oxford University Press. M. meridionalis molar from Yukarısöğütönü, no. 2306 (ref. 47); M. trogontherii molar from Süssenborn, Institut fur Quartärpaläontologie, Weimar, no. 1966/6906 (photo A.V. Sher); M. primigenius molar from Siberia, NHMUK PV M1235 (© The Trustees of Natural History Museum). Artists’ reconstruction of M. meridionalis by Richard Phipps, from Lister and Bahn (2007), with permission; of M. trogontherii by Richard Kiwit, © Urmensch Museum Steinheim an der Murr, with permission; of M. primigenius by Peter Snowball, © The Trustees of Natural History Museum.
FIGURE 2. Mammoth evolution according to Lister and Sher (2015). Time is represented on the vertical axis, geographical spread on the horizontal. The shaded blue area hence represents the spread and distribution of named species in time and space, and the arrows their proposed evolutionary relationships. Note the derivation of both Mammuthus columbi and M. primigenius from M. trogontherii of NE Siberia. From Lister and Bahn (2007), with permission.
FIGURE 3. Morphology of the earliest northeast Siberian steppe and woolly mammoth third molars. a, upper molar from Chukochya (PIN-3341-737); b, upper molar from Krestovka (PIN-3491-3, flipped horizontally); c, partial lower molar from Adycha (PIN-3723-511), occlusal view flipped horizontally. Note the more closely spaced lamellae and thinner enamel in a (primigenius- like) than b and c (trogontherii -like). d, hypsodonty index vs lamellar length index of upper M3s; e, Enamel thickness index vs basal lamellar length index of lower M3s. Olyorian specimens that yielded DNA are labelled by site name. Green dashed line: convex hull summarising Early to early Middle Pleistocene (ca. 1.5-0.5 Ma) North American Mammuthus samples (data points not shown). Green and blue squares: Early and Late Olyorian North-East Siberian samples, respectively; red and green circles: European M. meridionalis and M. trogontherii, respectively; blue circles, M. primigenius from North-East Siberia and Alaska. Note (i) similarity of Krestovka and Adycha to other Early Olyorian molars and to European steppe mammoths (M. trogontherii), (ii) similarity of early North American mammoths to these (Early Olyorian in particular), and (iii) similarity of Chukochya to M. primigenius. From van der Valk et al. (2021), with permission.
FIGURE 4. Simplified scheme of relationships, gene flow and dispersal revealed by the DNA analysis of van der Valk et al. (2021). The colours of connecting lines indicate geographical location: pink, Eurasia; blue, North America. Circle icons indicate morphology: green, trogontherii- (steppe mammoth)-like; blue, primigenius- (woolly mammoth)-like. Right-pointing black arrows indicate two inferred episodes of introgression through hybridisation. The identification of European trogontherii with either of the two trogontherii-like Siberian lineages is uncertain. Modified after Roca (2021); photo sources as in Figure 3.
FIGURE 5. Origin and spread of Holarctic mammoths in two stages, according to Lister and Sher (2001). A: Early Pleistocene origin of M. trogontherii in eastern Asia and its spread westward into Europe and eastward into North America (becoming M. columbi). B: Middle Pleistocene origin of M. primigenius in NE Siberia & its spread westward into Europe (meeting endemic M. trogontherii) and eastward into North America (meeting endemic M. columbi). Black arrows indicate evolutionary transformations; solid, long-dashed, and short-dashed lines indicate successively later dispersals within (A) to (C). Double-ended arrows indicate the Eurasian range. The position of species names is significant only at continental scale. The location of the three genomically analysed molars is marked (star symbols).
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).
FIGURE 7. Schematic diagram illustrating some of the issues raised by the discovery of the hybrid origin of Late Pleistocene North American mammoths (nominal Mammuthus columbi). First, genomic analysis indicates hybridisation, in North America, of endemic mammoths with M. primigenius in the Middle Pleistocene, but there are no fossils of that species identified from the Middle Pleistocene of North America. Second, if the hybridisation is taken to be the ‘origin’ of M. columbi, then earlier North American mammoths should be a different species (‘xxx’). However, it would be hard to diagnose such a species or choose a type specimen, because we have not yet morphologically differentiated it from later M. columbi, we have no DNA from it, and the date of its transition to M. columbi is uncertain.
