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Giant fossil soft-shelled turtles of North America

Natasha S. Vitek

Plain Language Abstract

Axestemys is a name that historically was used to refer to multiple species of extinct soft-shelled turtles. However, no one has ever done a rigorous phylogenetic analysis to test whether Axestemys is a natural group of species that all share a common ancestor or if Axestemys is an artificial group of unrelated soft-shelled turtles lumped together. I confirm that Axestemys is a natural group containing five species.  In addition, two other species were historically referred to Axestemys, but there is no support for continuing to include them in the group. I name two new species of Axestemys and provide a revised description based on new fossils of two previously known species. It is apparent that a group of gigantic soft-shelled turtles lived in North America from about 70 to about 46 million years ago. The size of all of the known species of Axestemys is so large that it is matched today only by soft-shelled turtle species in tropical climates. Such gigantic size reflects a western North America in the time of Axestemys that was both much warmer and much more diverse in terms of turtles than it is today. 

Resumen en Español

Tortugas de caparazón blando gigantes fósiles de Norteamérica

Axestemys byssina, una tortuga de caparazón blando (Trionychidae) de gran tamaño y potencialmente pedomórfica, fue descrita, junto con algunos otros trioníquidos gigantes, hace más de un siglo, aunque desde entonces no se ha llevado a cabo ningún análisis riguroso para establecer sus relaciones. Estos trioníquidos son poco frecuentes y se conocen a lo largo de un amplio intervalo temporal, desde el Cretácico al Eoceno (70-46 Ma). Para este trabajo he examinado todo el material previamente asignado a Axestemys, así como material no descrito con características similares a las de Axestemys, lo que ha llevado a una concepción de Axestemys coherente tanto en un contexto sistemático como taxonómico. Se definen dos nuevas especies. Se hace una revisión de las especies Axestemys byssina y Axestemys splendida comb. nov. a partir de la descripción de material nuevo y se confirma la pertenencia de Axestemys quinni al género Axestemys. Eugenichelys robertemryi Chkhikvadze, 2008 se considera sinónimo de Axestemys byssina. El análisis filogenético y la descripción de nuevo material no justifican la inclusión en este clado de “Axestemys” puercensis y Conchochelys admiribalis, previamente atribuidas a modo de hipótesis al género Axestemys. El taxón ‘Axestemys’ se define filogenéticamente como un taxón basado en ramas (setem-based) en lugar de basado en muchos de los caracteres tradicionalmente atribuidos a él pero que no están presentes de forma sistemática en todas las especies de Axestemys. Todos los miembros de Axestemys alcanzan un tamaño gigante como el que presentan actualmente unas pocas especies topicales de trioníquidos que evolucionaron de forma independiente hacia el gigantismo. La presencia de Axestemys refleja, por una parte, un clima más cálido en tiempos pasados en Norteamérica y, por otra, una mayor diversidad y disparidad en los trioníquidos, comparables a las que muestran en la actualidad las especies confinadas a climas tropicales.

Palabras clave: nueva especie; biodiversidad; filogenia; Trionychidae; Axestemys

Traducción: Miguel Company

Résumé en Français

Un fossile de tortue à carapace molle géante d’Amérique du nord

Axestemys byssina, une tortue à carapace molle (Trionychidae) grande et peut-être paedomorphique, et plusieurs autre trionuchidés géants, on été décrits il y a un siècle, mais aucune analyse rigoureuse n’a été entreprise depuis pour étudier leur parenté. Ces trionuchidés sont inhabituels et proviennent d’un large éventail temporel partant du Crétacé jusqu'à l’Eocène (70-46 m.a.). J’ai examiné tout le matériel précédemment attribué à Axestemys de même que le matériel non décrit présentant des caractéristiques traditionnellement considérées comme proches de “Axestemys” et j’ai établi un concept de Axestemys utile pour à la fois les contextes systématique et taxonomique. Deux nouvelles espèces sont créées. Axestemys byssina et Axestemys splendida comb. nov. sont révisés suite à la description de nouveau matériel, et Axestemys quinni est confirmé en tant que membre de Axestemys. Eugenichelys robertemryi Chkhikvadze, 2008 est mis en synonymie avec Axestemys byssina. L’analyse phylogénétique et la description du nouveau matériel ne supporte pas l’inclusion dans le clade de “Axestemys” puercensis et Conchochelys admiribalis, précédemment supposés appartenir à Axestemys. Le nom de taxon ‘Axestemys’ est mieux supporté phylogénétiquement sur la base d’un clade souche plutôt que sur la base des nombreux caractères qui définissent habituellement son attribution au genre, caractères qui ne sont pas toujours présents parmi les différentes espèces de Axestemys. Tous les membres de Axestemys atteignent une taille gigantesque qui n’est aujourd’hui connue que chez quelques espèces tropicales de trionuchidés qui ont évolués indépendamment vers le gigantisme. La présence de Axestemys reflète un climat chaud dans le passé de l’Amérique du Nord. De plus, il reflète une forte diversité et disparité des trionuchidés comparable a celles des espèces récentes confinées aujourd’hui dans les climats tropicaux.

Mots clés : nouvelles espèces, biodiversité, phylogénétique; Trionychid

Translator: Olivier Maridet

Deutsche Zusammenfassung

Riesen-Weichschildkröten aus Nordamerika

Axestemys byssina, eine möglicherweise paedomorphe Weichschildkröte und zahlreiche andere Riesen-Trionychiden wurden vor mehr als einem Jahrhundert beschrieben, jedoch wurden die Verwandtschaftsbeziehungen nicht gründlich untersucht. Diese ungewöhnlichen Trionychiden sind über einen langen Zeitraum hinweg von der Kreide bis in das Eozän (70 -46 Millionen Jahre) bekannt. Ich habe das gesamte früher Axestemys zugeordnete Material, ebenso wie bisher unbeschriebenes Material mit traditionellen Axestemys – artigen Merkmalen untersucht. Außerdem lieferte ich ein sowohl in systematischen als auch taxonomischem Kontext sinnvolles Konzept von Axestemys. Es wurden zwei neue Arten aufgestellt. Axestemys byssina und Axestemys splendida comb. nov. sind aufgrund von neuem Material revidiert und Axestemys quinni wird als Mitglied von Axestemys bestätigt. Eugenichelys robertemryi Chkhikvadze, 2008 wird mit Axestemys byssina synonymisiert. Phylogenetische Analysen und die Beschreibung neuen Materials befürworten keinen Einschluss innerhalb der Klade “Axestemys” puercensis und Conchochelys admiribalis, von denen vorher angenommen worden war, dass sie zu Axestemys gehören. Der Taxonname ‘Axestemys’ ist phylogenetisch am besten als stammbasierte Klade definiert und nicht auf die vielen ihm traditionell zugeschriebenen Merkmale festgelegt, die nicht bei allen Arten von Axestemys gleichmäßig vorhanden sind. Alle Mitglieder von Axestemys erreichen einen gewaltige Größe, die heute nur noch bei einigen tropischen Trionychiden-Arten zu finden sind, bei denen der Gigantismus jedoch unabhängig entwickelt wurde. Das Vorhandensein von Axestemys spiegelt ein wärmeres Klima im früheren Nordamerika wieder. Zusätzlich weist es auf eine hohe Diversität an Trionychiden hin und eine Disparität die mit der von rezenten Arten der tropischen Klimate vergleichbar ist.

SCHLÜSSELWÖRTER: neue Arten; Biodiversität; Phylogenese; Trionychidae; Axestemys

Translators: Eva Gebauer and Anke Konietzka

Arabic

299 arab

Translator: Ashraf M.T. Elewa

Polski Abstrakt

GIGANTYCZNE KOPALNE ŻÓŁWIAKI PÓŁNOCNEJ AMERYKI

Axestemy byssina, duży, potencjalnie pedomorficzny żółwiak (Trionychidae), i wiele innych trionychidów były opisane ponad sto lat temu, jednak żadna szczegółowa analiza ich pokrewieństw nie została przeprowadzona. Trionychidy te są nietypowe i są znane z szerokiego przedziału czasowego od kredy do eocenu (70-46 mln lat temu). Przebadałam cały materiał do tej pory zaliczany do rodzaju Axestemys, jak również jak dotąd nieopisane materiały wykazujące typowe cechy zbliżone do rodzaju Axestemys oraz zaproponowałam koncepcję Axestemys prawidłową zarówno w kontekście systematycznym jak i taksonomicznym. Ustanowiłam dwa nowe gatunki. Axestemys byssina i Axestemys splendida comb. nov. zostały zrewidowane jako rezultat opisania nowego materiału, a przynależność gatunku Axestemys quinni do rodzaju Axestemys została potwierdzona. Eugenichelys robertemryi Chkhikvadze, 2008 został zsynonimizowany z gatunkiem Axestemys byssina. Analiza filogenetyczna opisanie nowego materiału nie potwierdzają włączania do kladu składającego się z gatunków „Axestemys" puercensis i Conchochelys admiribalis, do tej pory uważanego za należący do rodzaju Axestemys. Nazwa taksonu 'Axestemys' jest najlepiej zdefiniowana filogenetycznie jako klad typu stem-based niż jako zdefiniowany na podstawie wielu cech zwykle mu przypisywanych, a nieobecnych u wszystkich gatunków zaliczanych do rodzaju Axestemys. Wszystkie gatunki rodzaju Axestemys osiągają gigantyczne rozmiary dzisiaj obecne u kilku tropikalnych trionychidów, które niezależnie wyewoluowały gigantyzm. Obecność rodzaju Axestemys odzwierciedla zarówno cieplejszy klimat w dawnej Ameryce Północnej. Na dodatek, wskazuje to na duże zróżnicowanie trionychidów porównywalne do tego u dzisiejszych gatunków ograniczonych współcześnie do klimatu tropikalnego.

Słowa kluczowe: nowy gatunek, bioróżnorodność, filogenetyka, Trionychidae, Axestemys

Translators: Dawid Mazurek, Robert Bronowicz, and Daniel Madzia

 

Anke Konietzka
 

Appendix 1

List of characters

1-77: The first 77 characters are taken directly from Vitek's (2011) analysis. This analysis is based off of the analysis of Joyce et al. (2009) with some revisions to character wording and scoring.

78-84: Characters 78-84 are the seven new characters that Joyce and Lyson (2011) added to Joyce et al.'s (2009) matrix, taken directly from Joyce and Lyson (2011).

Character 85: Maximum adult carapace size greater than 600 mm: 0 = no; 1 = yes.

Character 86: hyo-hypoplastral callosity, if present, lacks sculpturing on more than 90 percent of its surface: 0 = no; 1 = yes.

Character 87: xiphiplastral callosity, if present, lacks sculpturing on more than 90 percent of its surface: 0 = no; 1 = yes.

Character 88: Nuchal sutures to preneural, and/or neural 1, before it sutures to anterolateral edge of costal 1: 0 = no 1 = yes.

Appendix 2

Data matrix

a=0/1 b=1/2 c=2/3 d=1/2/3

outgroup 00000 00100 00010 00000 00000 00000 00000 00000 00001 00011 00000 00000 00000 00000 00010 ?0000 00000 000

aubryi 10103 00111 1a0b0 10000 10021 10111 03110 20000 0111- 01011 00010 00101 10100 00000 00010 10000 00000 000

bibroni 21113 00300 0a011 00110 00020 00100 11000 10002 00200 10111 00111 11000 01011 00000 00000 20000 000a0 000

cartilaginea 21113 00200 00011 02110 00000 10111 01000 10010 00100 10211 10101 00010 01001 00000 00000 20000 00010 000

elegans 1110 30041 01a0b 00011 000020 01111 02110 20000 0001- 01111 11000 00101 01110 00000 00010 10000 00000 000

euphraticus 21113 01400 010b1 00110 00000 10100 01000 20010 00000 00221 11102 00001 01101 00000 00000 20000 00a10 000

ferox 21113 01300 012b1 00110 01000 20100 01000 20000 00000 00221 11002 00000 01101 00000 0a0a0 10000 00110 0-0

formosa 11113 00300 00010 00110 00120 10111 01000 20020 00001 00211 00--1 00011 01--- 00000 000?? ?0000 00010 000

frenatum 10003 00110 1a0b0 10000 10021 00111 03110 20001 1021- 01111 10010 00101 10100 00000 00010 10000 00010 000

gangeticus 21103 00200 0a1b0 01110 00000 10111 01000 20020 00001 00111 10002 00010 01010 00000 00000 20000 00a10 000

hurum 21103 00200 0a1b0 01110 00010 20111 01000 20020 00001 10211 00000 00010 01011 00000 00000 20000 00a10 000

indica 21213 00300 00010 00110 01021 00100 01000 10001 00200 20200 00111 11001 01011 00000 00000 20000 00a11 000

leithii 21103 00200 0a010 01110 00110 10111 01000 20020 00000 00211 00001 00010 01010 ????? ????? ????? ??a10 000

mutica 31113 01100 0a2b2 00111 01110 20101 01000 20000 0001- 00122 11001 00000 01010 00000 000a0 10000 00110 000

nigricans 21103 00200 000b- 01110 00100 10111 01000 10020 00201 00211 01001 00010 01010 ????? ????? ????? ??a10 000

punctata 10002 10111 1b0b0 10000 10010 00110 01110 20000 00101 00011 00000 00101 00110 00000 00011 10000 00010 000

senegalensis 21103 10010 12-40 00000 00021 10100 02110 20000 0021- 01011 00000 00101 11100 00000 00010 20000 00--0 000

sinensis 31113 00100 0b2b1 02111 00110 20111 01101 20020 00002 00211 10001 00000 01010 00000 00000 20000 00110 000

spinifera 21113 01100 012b2 00111 01000 20100 01000 20010 0001- 00222 11002 00000 01010 00000 000a0 10000 00110 000

steindachneri 11113 00300 0a0b0 02111 00020 20111 01001 20020 0001- 20212 00--1 00000 -1--0 00000 000?? ?0000 00010 000

subplana 31113 00300 00102 02111 00010 20111 01001 21010 0000- 00112 10001 00010 01011 00000 00000 20000 00010 000

swinhoei ---13 01400 0---- 00110 00100 10100 01100 20000 00000 00221 11--- 00001 -1--1 ????? ????? ????? ??--1 000

triunguis 21113 00300 010b1 00110 00000 20011 01000 20100 00000 0011? 11001 00000 01010 00000 00000 20000 10a10 0-0

thomasii 31?0 30010 11b-c 0??00 00?00 ???01 -01111 21000 00??? ???-2 00??? ???0? ????? 00000 01110 21001 ?1--0 000

rememdium 31?03 00101 110b0 1?100 0???? ????? ????? ????? ????? ????? ????? ????? ?10?? 11110 01120 1???? ?a010 000

arctochelys 31?03 00101 110b0 1?000 0???? ????? ????? ????? ????? ????? ????? ????? ????? 10111 11121 1???? ?1010 000

foveatus 31103 00300 011b1 00110 0?000 21110 ?1??0 10?00 00??1 ?2?12 1???? ????? ????? 000?0 00000 10??0 ?0a10 000

uintaensis 31113 01300 01?ca 00110 0?010 10100 00000 000?0 00a00 a0211 0110? ???0? 01000 -0000 00000 200?0 00a10 010

tetraneton 31?03 00?00 11010 ???01 0???? ????? ????? ????? ????? ????? ????? ????? ????? 11010 01?20 ????? ??010 000

sterea 21?03 00101 11020 10001 0???? ????? ????? ????? ????? ????? ????? ????? ????? 10010 01120 ????? ?1110 000

lancensis 31?03 00?02 01020 ??110 0?121 20a11 02111 2000? 0001- 20211 00??? ????? ????? 00000 00?10 211?1 100a0 000

allani 31103 00d10 0a0b1 00110 0???? ????? ????? ????? ????? ????? ????? ????? ????? -00?0 ?0000 2???? ?0010 110

byssina 311?3 01c00 0??22 0?110 0???? ????? ????? ????? ????? ????? ???0? ????? 0???? -0?00 00000 1???? ?0??1 110

puercensis 3110 301d0 0?1?2 1??11 00??? ?????? ????? ????? ????? ????? ????? ????? ????? 00000 00??? ????? ?00a0 001

quinni 3??03 01??? ?1?22 ????0 0???? ????? ????? ????? ????? ????? ????? ????? ????? 00000 0???? ????? ?0001 ??1

cerevisia 31?03 00300 00?12 00?10 0???? ????? ????? ????? ????? ????? ???0? ????? ???0? -0000 00000 1???? ?0001 1-1

montinsana 31103 0?300 0???a 00?10 ???0? 101?? 0??00 ?0?0? 00??? ????1 ?1?1? ???0? 01001 -000? ?0000 100?0 0???1 100

splendida 31103 00300 011b1 00110 0?000 10110 00000 b0000 00??? ??211 11??? ????? ????? 000?0 00000 b00?0 ?0aa1 000

egregius 21113 01??? ?1?2a ????0 0???? ????? ????? ????? ????? ????? ????? ????? ????? 00-00 0???? ????? ?0010 ??0

Appendix 3

Specimen List

Apalone ferox: YPM R 10574, skeleton.

Apalone mutica: YPM R 10890, skeleton.

Axestemys byssina: USNM 4089, xiphiplastron, cervical vertebra, isolated ilium, several fragmentary appendicular elements; USNM 12589, skull fragments, partial left dentary, left hyoplastron, right hypoplastron, left and right xiphiplastron; USNM 16174, fragmentary carapace, left and right hyo- and hypoplastra, partial epiplastron; AMNH 1034, partial medial hypoplastron; AMNH 1046, partial medial hypoplastron, partial nuchal, partial costal 1, costal fragments.

Axestemys cerevisia: UW 2382, a carapace, plastron, partial pectoral and pelvic girdles, and one cervical vertebrae

Axestemys montinsana: UM 27029, a skull, partial lower jaw, partial carapace, fragmentary entoplastron, hyoplastron, and hypoplastron, xiphiplastron, five cervical vertebrae, a partial pelvic girdle, two humeri, a femur, and various disarticulated appendicular elements; PTRM 5350.88, skull fragments; PTRM 5350.23, PTM 5350.24, lower jaw; PTRM 6030.07, PTRM 6030.08, epiplastron; PTRM 6030.01, entoplastron; PTRM 6030.03, hyoplastron; PTRM 6030.04, hypoplastron; PTRM 6030.02, hyo- and hypoplastron; PTRM 6030.05, PTRM 6030.06, xiphiplastron; PTRM 5350.21, PTRM 5350.22, PTRM 5350.30, PTRM 5350.108, cervical vertebra; PTRM 5350.91, PTRM 5350.100, pectoral girdle; PTRM 5350.25, humerus; PTRM 5350.21, pelvic girdle, PTRM 5350.77,5350.78, ilium; PTRM 5350.27, ischium, PTRM 5350.75, PTRM 5350.76, pubis; PTRM 5350.28, PTRM 5350.51, PTRM 5350.130, femur; PTRM 5350.122, tibia; PTRM 5350.47, PTRM 5350.110, PTRM 5350.117, 5350.117, PTRM 5350.121, phalange; PTRM 5350.43, PTRM 5350.105, claw; UCM 49231, skull fragments; UCM 49228, partial dentary; DMNH 44623, costal and plastron fragments; UCM 37755, costal fragments and partial cervical vertebra; DMNH 45130, shell fragments and phalanges; DMNH 44622, plastron fragments and ungual phalanx; DMNH 43187, partial pectoral girdle; UCM 34134, humerus; UCM 34119, phalanx.

Axestemys quinni: FMNH P 26641, carapace.

Axestemys splendida: AMNH 3952, partial carapace; MRF 266, skull; MRF 666, lower jaw; MRF 834-835, partial carapace and complete right hyo- and hypoplastron; MRF 849, partial carapace and dentary; MRF 700, preneural; MRF 631, costal 1; MRF 654, 676, costal; MRF 567, 675, 678 hyoplastron;MRF 699, partial pectoral girdle; MRF 586, MRF 661, femur; TMP 90.50.01, partial skull and skeleton; TMP 2001.12.27, carapace, partial plastron, partial skeleton;

“Trionyx” puercensis: AMNH 1202, partial carapace, partial hyo- and hypoplastron, partial xiphiplastron.

Conchochelys admiribalis: AMNH 6090, skull.

 

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).

 fig01

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.

 fig02

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.

 fig03

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.

 fig04

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.

 fig05

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.

 fig06

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.

 fig07

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.

 fig08

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.

 fig09

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.

 fig10

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.

 fig11

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.

 fig12

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.

 fig13

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.

 fig14

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.

 fig15

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

 fig16

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.

fig17 

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.

 fig18

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

fig19 

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.

 fig20

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.

fig21

 

This email address is being protected from spambots. You need JavaScript enabled to view it.
Natasha S. VitekDepartment of Geology and Geophysics
Yale University
210 Whitney Avenue
P.O. Box 208109
New Haven, Connecticut 06520
USA

Natasha Vitek is a Master's student at the University of Texas at Austin. She received her Bachelors (BS) from Yale University. There, her undergraduate research focused on fossil soft-shelled turtles (Trionychidae), and her senior research project focused on Axestemys in particular.

 

TABLE 1. Taxonomic history of North American species historically or currently considered a part of Axestemys. A dash indicates that the species was not mentioned in a given reference.

Cope, 1872 Hay, 1908 Schmidt, 1945 Gaffney, 1979 Gardner et al., 1995 Chkhikvadze, 2000 Hutchison and Holroyd, 2003 This study

-

Conchochelys

admiribalis

-

-

-

Conchochelys admiribalis

Axestemys admiribalis

Conchochelys admiribalis

Axestus byssinus

Axestemys byssina

-

Trionyx byssinus

-

Axestemys byssina

Axestemys byssina

Axestemys byssina

-

-

-

Trionychidae gen. indet.

-

-

Axestemys indet.

Axestemys cerevesia

-

-

-

-

-

-

-

Axestemys montinsana

-

Aspideretes

puercensis

Paleotrionyx puercensis

Trionyx puercensis

-

Paleotrionyx puercensis

Axestemys puercensis

"Trionyx" puercensis

-

-

Paleotrionyx quinni

Trionyx quinni

-

Paleotrionyx quinni

Axestemys quinni

Axestemys quinni

-

-

-

-

-

Eugenichelys roberte

mryi

-

Axestemys montinsana

-

Aspideretes splendidus

-

-

Aspideretoides splendidus

Eugenichelys splendidus

-

Axestemys

splendida