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A new kinosternoid from the Late Cretaceous Hell Creek Formation of North Dakota and Montana and the origin of the Dermatemys mawii lineage

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Abstract

A nearly complete turtle shell from the Late Cretaceous (Maastrichtian) Hell Creek Formation of Slope County, North Dakota, represents the most complete remains to date of a Mesozoic kinosternoid turtle and a new species, Hoplochelys clark nov. sp. The new taxon is diagnosable from other representatives of Hoplochelys by the plesiomorphic placement of the humeral/femoral sulcus behind the hyo/hypoplastral suture and the autapomorphic development of an interrupted median (neural) keel. All six previously named Paleocene (Puercan and Torrejonian) representatives of Hoplochelys lack diagnostic characters and are synonymized as Hoplochelys crassa. A phylogenetic analysis reveals that Hoplochelys spp. and Agomphus pectoralis are most parsimoniously placed within Kinosternoidea along the phylogenetic stem of the extant Mesoamerican River Turtle Dermatemys mawii, extending that taxon’s stem lineage from the early Eocene to the late Maastrichtian. The two primary crown lineages of Kinosternoidea are thus known from the Mesozoic and split prior to the late Campanian. The presence of a thickened cruciform plastron, true costiform processes, only three inframarginals, and the reduction of the medial contact of the abdominals are synapomorphies of Chelydroidea, the clade formed by Chelydridae and Kinosternoidae.

Kurzfassung

Eine fast komplette Schildkrötenschale, die in der spätkretazischen (Maastrichtium) Hell Creek Formation in Slope County, Norddakota, gefunden wurde, stellt den bisher vollständigsten mesozoischen Rest einer kinosternoideen Schildkröte dar und wird hier als neue Art Hoplochelys clark beschrieben. Anhand der plesiomorphen Lage des humero/femoralen Sulcus hinter der hyo/hypoplastralen Sutur und des autapomorphisch unterbrochenen medianen Kiels kann die neue Schildkröte von anderen Hoplochelys Arten unterschieden werden. Allen sechs bisher bekannten Arten aus dem Paläozän (Puercium und Torrejonium) fehlen diagnostische Merkmale. Daher werden sie als Hoplochelys crassa synonymisiert. Die phylogenetische Analyse ergibt, dass es am sparsamsten ist, Hoplochelys spp. und Agomphus pectoralis entlang der Stammlinie der heute lebenden Tabascoschildkröte Dermatemys mawii zu platzieren. Das Alter dieser Stammliniengruppe wird daher vom Früheozän ins Spätmaastrichtium zurückverlegt. Die zwei primären Stammlinien der Kinosternoidea sind somit aus dem Mesozoikum bekannt und spalteten sich voneinander vor dem Spätcampanium ab. Die Ausbildung eines verdickten, kreuzförmigen Plastrons, echte rippenartige Fortsätze des Nuchale, nur drei Inframarginalschilder und der Verlust des medianen Kontaktes der Abdominalscuta sind Synapomorphien des Monophylums Chelydroidea, welches von Chelydridae und Kinosternoidea gebildet wird.

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Acknowledgments

We thank Dan Brinkman (YPM), Don Brinkman (TMP), Matt Carrano and Kevin de Queiroz (USNM), Pat Holroyd (UCMP), Carl Mehling (AMNH), Alan Resetar (FMNH), and Greg Watkins-Colwell (YPM) for access to material under their care. R. Sullivan is particularly thanked for providing us access to unpublished material collected by him in New Mexico. Thanks to the Horse Creek Grazing Association for allowing PTRM to collect and curate fossils from the dinosaur site PTRM V95018 where the holotype of H. clark was collected. John Brown and family are also thanked for allowing PTRM to conduct surveys of their land and to collect numerous fossils sites, including two that produced the specimens referred herein to H. clark. Additional referred specimens were collected from public lands managed by the Bureau of Land Management in Montana and North Dakota and the United States Forest Service in North Dakota; we would like to thank numerous staff members for their assistance with obtaining access. Yvonne Lichtenfelt is thanked for preparation of the holotype. Numerous PTRM and YPM students and volunteers helped recover the described material. Don Brinkman and Jim Parham helped improve the quality of this manuscript. Funding for this project was provided by the Doris O. and Samuel P. Welles Research Fund (UCMP) and the University of Iowa Department of Geoscience Littlefield Fund to G.K., a National Science Foundation Graduate Research Fellowship to T.R.L., and by a grant from the University of Tübingen to W.G.J.

Author information

Authors and Affiliations

  1. SWCA Environmental Consultants, Inc., Sheridan, WY, 82801, USA

    Georgia E. Knauss

  2. Department of Geoscience, University of Iowa, 121 Trowbridge Hall, Iowa City, IA, 52242, USA

    Georgia E. Knauss

  3. Institut für Geowissenschaften, University of Tübingen, Sigwartstraße 10, 72076, Tübingen, Germany

    Walter G. Joyce

  4. Division of Vertebrate Paleontology, Yale Peabody Museum of Natural History, 170 Whitney Avenue, New Haven, CT, 06511, USA

    Walter G. Joyce

  5. Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT, 06511, USA

    Tyler R. Lyson

  6. Marmarth Research Foundation, Marmarth, ND, 58643, USA

    Tyler R. Lyson

  7. Pioneer Trails Region Museum, 12 First Avenue Northeast, Bowman, ND, 58623, USA

    Dean Pearson

Authors
  1. Georgia E. Knauss
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  2. Walter G. Joyce
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  3. Tyler R. Lyson
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  4. Dean Pearson
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Corresponding author

Correspondence to Walter G. Joyce.

Appendices

Appendix 1: Materials

Extant turtles

Chelydra serpentina; YPM R 16622, YPM R 17755, YPM R 10821, YPM R 10835, YPM R 15180.

Claudius angustatus; FMNH 4165, AMNH 119937, AMNH 65865, USNM 7072.

Dermatemys mawii; FMNH 4163; FMNH 98950, USNM 66669.

Kinosternon flavescens; FMNH 6849, YPM R 11305.

Macrochelys temminckii; YPM R 13495.

Sternotherus carinatus; YPM R 11301.

Staurotypus triporcatus; AMNH 108906, USNM 40045, USMN 524353.

Fossil turtles

Agomphus pectoralis Cope, 1868; ANSP 15359, YPM 774, YPM 671.

Baltemys staurogastros Hutchison, 1991; UCMP 12700 (holotype).

Baptemysgarmanii Cope, 1872; YPM PU17402, AMNH 6109, AMNH 6110, UCMP 132074.

Baptemys wyomingensis Leidy, 1870; ANSP 10074 (holotype), AMNH 1494, AMNH 4913, AMNH 5934, AMNH 5967, YPM 3754, USNM 13437, UCMP 45477, UCMP 158845.

Hoplochelys crassa Cope, 1888; AMNH 6091 (lectotype, scored from Hay, 1908a).

Hoplochelys crassa; USNM 8525, USNM 8252.

Hoplochelys bicarinata Hay, 1910; USNM 8524.

Hoplochelys laqueata Gilmore, 1919; USNM 8527 (holotype).

Hoplochelys elongata Gilmore, 1919; USNM 8553 (holotype).

Hoplochelys saliens Hay, 1908a; AMNH 1200 (holotype).

Hoplochelys clark n. sp.; PTRM 16173 (holotype), PTRM 10015, PTRM 6794, PTRM 8558, PTRM 2953, PTRM 2961, PTRM 10009, PTRM 9475, PTRM 2949, PTRM 2920, YPM 57288, YPM 57648, YPM 57706SMP 1578, SMP VP 1650, SMP VP-2109, SMP VP 2514.

Xenochelys lostcabinensis Hutchison, 1991; UCMP 1122341 (holotype).

Xenochelys formosa Hay, 1906; AMNH 1097 (holotype); SDSM 08662.

Appendix 2: Characters

(All characters highlighted with an asterisk are multistate characters that were run ordered.)

  1. (1)

    Presence and development of projecting costal keel or tenting in adults (modified from Hutchison and Bramble 1981, 11; Hutchison 1991, 13): 0 = absent; 1 = present, spans majority of shell as “tenting”; 2 = present, spans majority of shell as a low, rounded, but projecting keel; 3 = present, spans majority of shell as sharp, distinct, projecting keel.

Comments: We score chelydrids as not having keels per se because these forms typically have pleural prominences that are not connected with one another. We furthermore distinguish between projecting keels and “tenting.” Projecting keels are longitudinal structures that are formed by a buildup of bony material. In contrast, tenting is the result of a sharp bend in the shell without significant buildup of material, which produces a longitudinal structure as well. The four character states developed herein do not form a logical morphocline and we thus run this character unordered.

  1. (2)

    Presence and extent of median keel: 0 = absent; 1 = present, only developed along the posterior half of shell; 2 = present, developed along majority of shell, even if faint on the anterior half of the shell.

Comments: This character only refers to true keels that consist of a buildup of bony material. Keels that are the result of “tenting” (i.e., a sharp bend in the shell) are not included in this character definition. Although this character can be ordered into a morphocline, we do not feel that character state 2 must evolve via character state 1 and thus leave this character unordered.

  1. (3)

    Development of median keel (new character): 0 = consisting of multiple posteriorly fading subkeels that abut at intervertebral sulcus; 1 = consisting of a single, more or less uninterrupted keel.

Comments: Even though the median keel is incomplete in Baptemys wyomingensis, we nevertheless score this taxon as having a continuous keel because the keel spans two vertebral scales without interruption.

  1. *(4)

    Costiform processes (modified from Hutchison and Bramble 1981, 12; Joyce 2007, 63): 0 = costiform processes rib-like, span two peripherals to insert in peripheral III; 1 = costiform processes tapered, span peripheral I to insert into peripheral II; 2 = costiform processes short, but tapered, inserts into peripheral I; 3 = costiform processes absent.

Comments: Joyce (2007) scored Dermatemys mawii as sometimes lacking a costiform process, but observations of FMNH 98950 reveal that the process is developed, but hidden in bone. Our scorings for Baltemys staurogastros and Xenochelys lostcabinensis follow the description of Hutchison (1991). All four character states form a logical morphocline, and we thus run this character ordered.

  1. (5)

    Relative position of costal I/II suture relative to peripheral series (modified from Hutchison 1991, 17): 0 = contacts peripheral III; 1 = contacts peripheral IV.

  2. (6)

    Relative position of costal II/III suture relative to peripheral series (modified from Hutchison and Bramble 1981, 14; Hutchison 1991, 34): 0 = contacts peripheral IV; 1 = contacts peripheral V.

  3. (7)

    Relative position of costal III/IV suture relative to peripheral series (modified from Hutchison and Bramble 1981, 34; Hutchison 1991, 35): 0 = contacts peripheral VI; 1 = contacts peripheral V.

  4. (8)

    Termination of costal I rib (modified from Hutchison 1991, 32): 0 = costal rib I terminates within peripheral III; 1 = costal rib I may enter peripheral III, but terminates within peripheral IV.

  5. (9)

    Costal rib VIII (reworded from Hutchison 1991, 7): 0 = medially contacts vertebral column; 1 = medial portion of rib not developed.

  6. *(10)

    Reduction of neural column (Hutchison and Bramble 1981, 9, 25, 48; Hutchison 1991, 3): 0 = eight neurals present; 1 = seven neurals present; 2 = six neurals present.

Comment: Our three character states form a morphocline, and we consequently run this character ordered. The best-preserved specimens of Agomphus pectoralis unfortunately only preserve seven neurals, but more may have been there. We thus score it as 0/1, because we are confident that it has more than six neurals.

  1. (11)

    Number of suprapygals (Hutchison and Bramble 1981, 44; Hutchison 1991, 12): 0 = two present; 1 = one present.

  2. (12)

    Anterior contacts of neural II (reworded from Hutchison 1991, 14): 0 = costal I and II; 1 = costal II only.

  3. (13)

    Anterior contacts of neural III (reworded from Hutchison 1991, 9): 0 = costal III only; 1 = costal II and III.

  4. (14)

    Anterior contacts of neural IV (reworded from Hutchison 1991, 25): 0 = costal III and IV; 1 = costal IV only.

  5. (15)

    Anterior contacts of neural V (reworded from Hutchison 1991, 28): 0 = costal V only; 1 = costal IV and V.

  6. (16)

    Presence and development of musk duct groove (modified from Hutchison and Bramble 1981, 26, 45, 49; Hutchison 1991, 11, 18): 0 = musk duct groove absent; 1 = musk duct groove present and terminates on peripheral II; 2 = groove present and terminates on peripheral I.

Comment: The characters in Hutchison and Bramble (1981) and Hutchison (1991) address how deep the musk duct grooves are developed, yet, we omit this set of characters because it is difficult to replicate the results, especially with fossil taxa. Our three character states can be arranged in a morphocline; however, given that we do not feel that character state 2 necessary derived from character 0 via character state 1, we do not order this character.

  1. (17)

    Number of peripherals (Hutchison and Bramble 1981, 27; Hutchison 1991, 1): 0 = 11 pairs of peripherals present; 1 = 10 pairs of peripherals present.

  2. (18)

    Shape of vertebrals II–IV in adult specimens (modified from Hutchison and Bramble 1981, 29; Hutchison 1991, 5): 0 = more or less square; 1 = more or less rectangular, at least one and a half times longer than wide; 2 = distinctly hexagonal.

Comment: We run this character unordered because the three character states do not form a logical morphocline.

  1. (19)

    Elevation of marginal X relative to marginal IX (modified from Hutchison 1991, 6): 0 = even or only slightly elevated; 1 = distinctly elevated.

  2. (20)

    Posterior plastral lobe shape (modified from Hutchison and Bramble 1981, 20; Hutchison 1991, 20): 0 = posterior lobe tapers to a distinct point; 1 = posterior lobe rounded, anal notch absent; 2 = posterior lobe rounded, distinct anal notch present.

  3. (21)

    Hyoplastral buttress (modified from Hutchison and Bramble 1981, 15): 0 = elongate hyoplastral buttress absent; 1 = hyoplastron forms long buttress that runs along the visceral surface of peripherals and terminates on peripherals; 2 = hyoplastron forms buttress that runs along the visceral side of the peripherals and terminates on costals.

  4. (22)

    Anterior extent of surficial ossified hyoplastral bridge (modified from Hutchison and Bramble 1981, 13, 37; Hutchison 1991, 29): 0 = hyoplastron contacts peripheral IV on the shell surface; 1 = hyoplastron contacts peripheral V on the shell surface.

Comment: This character does not refer to contacts formed by a hyoplastral buttress that runs along the visceral side of the peripherals, but rather only those portions of the bridge that are surficial and covered by scales. All taxa with ligamentous bridges are scored as inapplicable.

  1. *(23)

    Hypoplastral buttress (modified from Hutchison and Bramble 1981, 13, 37; Hutchison 1991, 8): 0 = elongate hypoplastral buttress absent; 1 = hypoplastron forms buttress that runs along the visceral surface of peripherals and terminates on peripheral VII; 2 = hypoplastron forms long buttress that runs along the visceral surface of peripherals and terminates on peripheral VIII; 3 = hypoplastron forms long buttress that runs along the visceral surface of peripherals and terminates on peripheral IX.

Comment: The four character states of this character form a logical morphocline, and we thus run it ordered.

  1. (24)

    Posterior extent of surficial ossified hypoplastral bridge (modified from Hutchison and Bramble 1981, 13, 37; Hutchison 1991, 8): 0 = hypoplastron contacts peripheral VI on the surface; 1 = hypoplastron contacts peripheral VII on the surface; 2 = hypoplastron contacts peripheral VIII on the surface.

Comment: Similar to character 22, this character is only intended to reveal the extent of those portions of the bridge that are covered by scales. All taxa with unossified bridges are scored as inapplicable.

  1. (25)

    Entoplastron (Hutchison and Bramble 1981, 52 in part; Hutchison 1991, 31): 0 = present; 1 = absent.

  2. (26)

    Number of inframarginals (modified from Hutchison and Bramble 1981, 5, 10, 19; Hutchison 1991, 4): 0 = four or more; 1 = three; 2 = two.

  3. (27)

    Overlap of hyo/hypoplastral suture by inframarginal series (modified from Hutchison and Bramble 1981, 6): 0 = axillary overlaps hyo/hypoplastral suture; 1 = inframarginal II or III overlap hyo/hypoplastral suture; 2 = inguinal overlaps hyo/hypoplastral suture.

  4. (28)

    Intergular (Hutchison and Bramble 1981, 22; Hutchison 1991, 19): 0 = absent; 1 = large intergular present.

  5. (29)

    Pectorals (Hutchison and Bramble 1981, 2): 0 = present; 1 = absent.

  6. *(30)

    Abdominals (Hutchison and Bramble 1981, 23, 28; Hutchison 1991, 2): 0 = absent; 1 = present, but medial contact absent; 2 = present, partial medial contact present; 3 = present, medial contact along almost the entire anteroposterior length (i.e., the abdominal medial length is nearly equal to or possibly greater than its lateral length).

Comment: The character states of this character form a morphocline, and we thus run this character ordered.

  1. (31)

    Contribution of the abdominal to the axillary notch (new character): 0 = present, pectorals or humerals do not contact inframarginals; 1 = absent, pectorals or humerals contacts inframarginals.

  2. (32)

    Anal scales (Hutchison and Bramble 1981, 33): 0 = paired; 1 = fused.

  3. (33)

    Orientation of epi/hyoplastral suture (modified from Hutchison and Bramble 1981, 35, 40, 50; Hutchison 1991, 21): 0 = oriented sloping backwards; 1 = oriented transverse or rising towards the front.

  4. (34)

    Direct overlap of gular/humeral sulcus with epi-ento/hyoplastral suture (modified from Hutchison 1991, 33): 0 = absent; 1 = present.

  5. *(35)

    Placement of humeral/femoral sulcus (modified from Hutchison and Bramble 1981, 32, 53; Hutchison 1991, 26, 36): 0 = placed over hyo/hypoplastral suture; 1 = placed over hypoplastron; 2 = placed over hypo/xiphiplastral suture.

Comment: All three character states form a morphocline, and we thus run this character ordered. An overlap of the humeral/femoral sulcus with the hypo/xiphiplastral suture generally correlates with the presence of posterior lobe kinesis.

  1. (36)

    Frontal contribution to orbit (Hutchison 1991, 38; Meylan and Gaffney 1989, 16): 0 = absent; 1 = present.

  2. (37)

    Maxilla/quadratojugal contact (Hutchison 1991, 39; Meylan and Gaffney 1989, 17): 0 = absent; 1 = present.

  3. (38)

    Lingual ridges (i.e., maxillary tooth of Meylan and Gaffney 1989) on the palate (Hutchison and Bramble 1981, 16; Hutchison 1991, 40; Meylan and Gaffney 1989, 5): 0 = absent; 1 = present.

  4. (39)

    Enlarged nose scale (Hutchison 1991, 41): 0 = absent; 1 = present.

  5. (40)

    Parietal contribution to processus trochlearis oticum (taken from Hutchison 1991, 42; Meylan and Gaffney 1989, 44): 0 = absent, or very slightly present; 1 = significant contribution present.

  6. *(41)

    Foramen stapedio-temporale (Hutchison and Bramble 1981, 4; Meylan and Gaffney 1989, 1): 0 = large foramen for thick blood vessel present; 1 = stapedial foramen reduced to the size of a cranial nerve foramen; 2 = absent.

Comment: We run this character ordered because the character states can be arranged in a morphocline.

  1. (42)

    Foramen posterius canalis carotici interni (modified from Gaffney and Meylan 1989, 3; Hutchison 1991, 43): 0 = not fully formed by bone; 1 = fully surrounded by pterygoid; 2 = fully surrounded by the pterygoid ventrally and the opisthotic dorsally.

Comment: The three character states of this character cannot be arranged in a logical morphocline, and we thus run this character unordered.

  1. (43)

    Ventral process of cervical vertebra VIII (Hutchison and Bramble 1981, 31; Hutchison 1991, 44; Meylan and Gaffney 1989, 26): 0 = single; 1 = double, split lengthwise.

  2. (44)

    Size of pectineal process (modified from Hutchison 1991, 47): 0 = short; 1 = long.

  3. (45)

    Orientation of pectineal process (modified from Hutchison 1991, 47): 0 = anteriorly oriented; 1 = laterally oriented.

  4. *(46)

    Thelial process (modified from Meylan and Gaffney 1989, 37; see Joyce 2007 for comment regarding primary homology): 0 = absent; 1 = low process developed; 2 = clear process developed.

Comment: We run this character ordered because the character states form a morphocline.

  1. *(47)

    Iliac notch (modified from Meylan and Gaffney 1989, 36): 0 = absent; 1 = slight notch developed; 2 = deep notch developed.

Comment: We run this character ordered, because the character states form a morphocline.

  1. (48)

    Medial pectoral processes and ridges (new character): 0 = absent; 1 = present.

Comments: In all living kinosternoids, a number of unique features can be found in the pectoral girdle. In particular, a distinct accessory ridge arises near the glenoid from the medial side of the acromial process of the scapula that appears to correlate with a small process that arises from the medial side of the scapula. These ridges are highly distinct in Dermatemys mawii and Baptemys wyomingensis but have previously not been noted for kinosternids. We are unaware of the function of these structures.

Characters omitted from this study

Hutchison and Bramble (1981)

  1. (3)

    Posterior lobe reduced in width: subjective and other less subjective characters appear to correlate with plastral size.

  2. (7)

    Loss of sculpturing: uninformative, absent in outgroup and all ingroup taxa, an artifact from the use of trionychids as the outgroup.

  3. (17)

    Femoral scale contacts inguinal: uninformative, autapomorphic for Dermatemys mawii.

  4. (21)

    Increase in plastral size: see character #3.

  5. (24)

    Megacephaly and probably increase in durophagy: not objective and largely uninformative as few fossil taxa have preserved skulls.

  6. (30)

    Tendency to develop secondary palate: not discrete.

  7. (36)

    Increased width of nuchal scale: character not discrete (must be measured to be objective).

  8. (38)

    Scapular attachment transferred from entoplastron to epiplastra and marked by distinct pits: difficult to reproduce, as these are often not visible in fossils; in addition this character appears to correlate with the loss of the entoplastron.

  9. (39)

    Usual loss of neural 7; anterior suprapygal elongated and often contacting neural 6 (or neural 7, if present): uninformative, autapomorphic for Staurotypus.

  10. (42)

    Fusion of hyoplastra with hypoplastra: uninformative, autapomorphic for Claudius.

  11. (43)

    Great reduction in plastral size: see character #3.

  12. (47)

    Distinct notch or step on margin of xiphiplastra at set 6–7 scale (anal-femoral) sulcus: subjective, not discrete.

  13. (51)

    Upturned spurs developed on set 1 (gular) areas of epiplastra: uninformative, autapomorphic for Xenochelys lostcabinensis, contradictory information exists pertaining to Baltemys staurogastros (see Hutchison and Bramble 1981 versus Hutchison 1991).

  14. (54)

    Head size decrease: see character #24.

Hutchison (1991)

  1. (10)

    Relative height of M10 to M12: not discrete.

  2. (15)

    Gular cusp present: see Bramble and Hutchison 1981 character #51.

  3. (16)

    Femoral cusp present: see Bramble and Hutchison 1981 character #51.

  4. (23)

    Width of anterior lobe: see Hutchison and Bramble 1981, character #3.

  5. (24)

    Intergular (completely) separating gulars: uninformative, absent in all taxa used in analysis.

  6. (30)

    Ventral scapular ligament attached to epiplastra only: see Hutchison and Bramble 1981, character #38.

  7. (37)

    Foramen intermaxillaris: uninformative, feature was absent in all specimens viewed for this analysis.

  8. (45)

    Ectepicondylar foramen open: uninformative, often varies with ontogeny.

  9. (46)

    Pubis and ischium closely opposed or in contact: vague, and distribution was not confirmed as these elements are often connected by small pieces of calcified cartilage that may not be preserved in fossils.

  10. (48)

    Ilia expanded at dorsal end: uninformative, expansion was not observed in any individual within the ingroup.

Appendix 3: Character matrix

See Table 1.

Table 1 Extant and fossil turtles studied and their characters
Full size table

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Knauss, G.E., Joyce, W.G., Lyson, T.R. et al. A new kinosternoid from the Late Cretaceous Hell Creek Formation of North Dakota and Montana and the origin of the Dermatemys mawii lineage. Paläontol Z 85, 125–142 (2011). https://doi.org/10.1007/s12542-010-0081-x

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