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FIGURE 1. Map of type localities of the species of trionychids discussed in this manuscript. 1. Axestemys splendida (Hay, 1908), 2. Axestemys montinsana sp. nov., 3. Eugenichelys robertemryi Chkhikvadze, 2008, 4. Axestemys cerevesia sp. nov., 5. Axestemys byssina (Cope, 1872), 6. Axestemys quinni (Schmidt, 1945), 7. Conchochelys admiribalis Hay, 1905, 8. "Axestemys" puercensis (Hay, 1908).

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FIGURE 2. Most parsimonious trees resulting from analysis of fossil and recent Trionychidae based on morphological data. Axestemys is designated in black. Daggers indicate extinct species. Thick black lines represent the stratigraphic ranges of taxa. 1, The strict consensus tree topology of 315 most parsimonious trees, 288 steps, CI = 0.4063, RI = 0.6183. 2, The single most parsimonious tree topology resulting from the experimental deletion of two taxa, Nilssonia formosa and the fossil "Trionyx" egregius, 283 steps, CI = 0.4134, RI = 0.6175. Numbers to the lower right of nodes are bootstrap support values from 1000 bootstrap replicates. Abbreviations: Camp. = Campanian, Maastr. = Maastrichtian, Torrejon. = Torrejonian, Clarkfork. = Clarkforkian, Wasatch. = Wasatchian, Bridger. = Bridgerian.

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FIGURE 3. USNM 12589, skull fragments of Axestemys byssina from the Eocene Wasatch Formation of Wyoming. 1, Photograph and 2, illustration of skull roof fragment. 3, dentary fragment. Abbreviations: fr = frontal, pa = parietal, po = postorbital.

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FIGURE 4. USNM 16174, carapace and plastron of Axestemys byssina from the Eocene Wasatch Formation of Wyoming. 1, Photograph of nuchal in visceral view. 2, Photograph and 3, illustration of carapace in external view. 4, Photograph and E, illustration of plastron. Abbreviations: co = costal, ep = epiplastron, hp = hypoplastron, hy = hyoplastron, nu = nuchal.

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FIGURE 5. Photograph of Axestemys byssina material described by Hay (1908). 1, reconstruction of nuchal region. AMNH 1046, 2, partial nuchal in exterior view, 3, costal 1, 4, partial nuchal in visceral view. 5, AMNH 1034, partial hypoplastron from the Eocene Bridger Formation of Wyoming. 6. AMNH 1046, partial hypoplastron. 7, USNM 4089, holotype xiphiplastron from the Eocene Bridger Formation of Wyoming. Abbreviations: ne = neural, pne = preneural.

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FIGURE 6. USNM 12589, plastron of Axestemys byssina from the Eocene Wasatch Formation of Wyoming. USNM 12589 is also the type specimen of Eugenichelys robertemryi, a junior synonym of Axestemys byssina, 1, photograph and 2, illustration. Abbreviations: hp = hypoplastron, hy = hyoplastron, xi = xiphiplastron.

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FIGURE 7. Cervical vertebrae of Axestemys. Lateral view of Axestemys montinsana from the Paleocene Fort Union Formation of North Dakota, order unknown, 1, PTRM 5350.22, 2, PTRM 5350.30, 3, PTRM 5350.108, 4, PTRM 5350.21, seventh cervical vertebra. UMMP 27029, Axestemys montinsana from the Paleocene Melville Formation of Montana, 5-7, cervical vertebrae in lateral view, order unknown, 8, seventh cervical vertebra in lateral view, 9, eighth cervical vertebra in ventral view, 10, seventh cervical vertebra in lateral view of Axestemys byssina from the Eocene Bridger Formation, 11, seventh cervical vertebra in lateral view of Axestemys cerevisia from the Eocene Bridger Formation. Arrows point to dorsal processes.

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FIGURE 8. MRF 266, skull of Axestemys splendida from the Cretaceous Hell Creek Formation of North Dakota. 1, photograph and 2, illustration of dorsal view. 3, photograph and 4, illustration of ventral view. 5, photograph and 6, illustration of lateral view. 7, photograph and 8, illustration of posterior view. Abbreviations: bo = basioccipital, bs = basisphenoid, ex = exoccipital, fp = fenestra postotica, fpcci =foramen posterius canalis carotici intern, fr = frontal, fst = foramen stapedio-temporale, ju = jugal, mx = maxilla, op = opisthotic, pa = parietal, pal = palatine, pf = prefrontal, pm = premaxilla, po = postorbital, ppf = foramen palatine posterius, pr = prootic, pt = pterygoid, qj = quadratojugal, qu = quadrate, so = supraoccipital, sq = squamosal, v = vomer.

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FIGURE 9. Mandibles of Axestemys splendida from Cretaceous Hell Creek Formation of North Dakota. MRF 666, 1, photograph and 2, illustration of dorsal view. 3, photograph and 4, illustration of medial view. 5, photograph and 6, illustration of lateral view. MRF 849, 7. photograph and 8, illustration of dorsal view. 9, photograph and 10, illustration of lateral view. Abbreviations: ang = angular, art = articular, cor = coronoid, den = dentary, fai = foramen alveolare inferius, fna = foramen nervi auriculotemporalis, pra = prearticular, sur = surangular.

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FIGURE 10. MRF 835 carapace of Axestemys splendida from the Cretaceous Hell Creek Formation of North Dakota. 1, photograph and 2, illustration of visceral view. Shaded areas indicate fragmented regions. Abbreviations: co = costal, nu = nuchal, tv = thoracic vertebrae.

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FIGURE 11. MRF 849, MRF 631, MRF 654, MRF 676, MRF 700, carapace of Axestemys splendida from the Cretaceous Hell Creek Formation of North Dakota. 1, photograph and 2, illustration of external view. 3, photograph and 4, illustration of visceral view. Abbreviations: co = costal, ne = neural, nu = nuchal, pne = preneural.

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FIGURE 12. Hyoplastra and hypoplastra of Axestemys splendida from the Cretaceous Hell Creek Formation of North Dakota. MRF 678, right hyoplastron, 1, photograph and 2, illustration. MRF 834, left hyoplastron and hypoplastron, 3, photograph and 4, illustration. MRF 675, right hyoplastron, 5, photograph and 6, illustration. MRF 567, left hyoplastron, 7, photograph and 8, illustration.

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FIGURE 13. UW 2382, holotype of Axestemys cerevisia from the Eocene Bridger Formation of Wyoming. 1, illustration of carapace. Gray area indicates limit of sculpturing. 2, Photograph and 3, illustration of plastron. Dotted line on hyoplastra and hypoplastra indicates limit of callosity. Abbreviations: en = entoplastron, ep = epiplastron, hp = hypoplastron, hy = hyoplastron. Carapace illustration modified from Gaffney, 1979.

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FIGURE 14. UMMP 27029, skull of Axestemys montinsana from the Paleocene Melville Formation of Montana. 1, photograph and 2, illustration of dorsal view. 3, photograph and 4, illustration of ventral view. 5, photograph and 6, illustration of lateral view. Abbreviations: bo = basioccipital, bs = basisphenoid, fpcci =foramen posterius canalis carotici intern, fr = frontal, fst = foramen stapedio-temporale, ju = jugal, mx = maxilla, op = opisthotic, pa = parietal, pal = palatine, pf = prefrontal, pm = premaxilla, po = postorbital, pr = prootic, pt = pterygoid, qu = quadrate, so = supraoccipital, sq = squamosal, v = vomer.

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FIGURE 15. Mandibles of Axestemys montinsana. PTRM 5350.23 and PTRM 5350.24, 1, photograph and 2, illustration of dorsal view. PTRM 5350.24, 3, photograph and 4, illustration of medial view. 5, photograph and 6, illustration of lateral view. UMMP 27029, 7, photograph and 8, illustration of right dentary pocket and coronoid in dorsal view. 9, photograph and 10, illustration of left half of dentary in dorsal view. 11, photograph and 12, illustration of medial view. 13, photograph and 14, illustration of lateral view. Abbreviations: ang = angular, art = articular, cor = coronoid, den = dentary, fai = foramen alveolare inferius, fna = foramen nervi auriculotemporalis, pra = prearticular, sur = surangular.

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FIGURE 16. UMMP 27029, carapace and plastron of Axestemys montinsana from the Paleocene Melville Formation of Montana. 1, photograph and 2, illustration of carapace external view. 3, photograph and 4, illustration of carapace in visceral view and left xiphiplastron. 5. photograph and 6. illustration of plastron. Abbreviations: co = costal, en = entoplastron, hp = hypoplastron, hy = hyoplastron, ne = neural, nu = nuchal, pne = preneural, xi = xiphiplastron

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FIGURE 17. PTRM 6030.01-6030.08, plastron of Axestemys montinsana from the Paleocene Fort Union Formation of North Dakota. 1, photograph and 2, illustration. Abbreviations: en = entoplastron, ep = epiplastron, hp = hypoplastron, hy = hyoplastron, xi = xiphiplastron.

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FIGURE 18. Pectoral and pelvic girdle of Axestemys montinsana. Pectoral girdle from the Paleocene Fort Union Formation of North Dakota, 1, PTRM 5350.91, 2, PTRM 5350.100, 3, PTRM 5350.31, pelvic girdle. 4, PTRM 5350.27, ischium. 5, UMMP 27029, pelvic girdle from the Paleocene Melville Formation of Montana.

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FIGURE 19. Illustration of YPM R 10890, carapace of a juvenile Apalone mutica.

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FIGURE 20. FMNH P 26641, holotype of Axestemys quinni from the Paleocene De Beque Formation of Colorado. A, Photograph and B, illustration of carapace. Abbreviations: co = costal, ne = neural, nu = nuchal, pne = preneural.

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FIGURE 21. The stratigraphic distribution of taxa discussed in the text. Thin lines indicate phylogenetic relationships only and do not indicate divergence date estimates. Letters indicate character transitions. Key: a = carapace length at least 60 cm, sculpturing on the skull roof; b = smooth hyoplastral and hypoplastral callosity, smooth border around carapace callosity; c = dorsal processes on the cervical vertebrae; d = suprascapular fontanelles; e = loss of xiphiplastral callosity, strong reduction of hyoplastral and hypoplastral callosity; f = reduced eighth costals, smooth xiphiplastral callosity; g = loss of smooth band around carapace callosity.

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