Tavachelydra stevensoni, Lyson & Petermann & Bastien & Toth & Tamez-Galvan & Sherman & Joyce, 2025

Tavachelydra stevensoni , gen. et sp. nov.

( Figs. 2 View Fig , 3 View Fig , 4 View Fig , 5 View Fig , 6 View Fig , 7 View Fig ; also see 3D models (MorphoSource project ID: 000730318)).

Type specimen: DMNH EPV.141854, a disarticulated, but associated, skeleton that includes a nearly complete carapace and plastron and a complete pelvis ( Fig. 2 View Fig , 3 View Fig , 4 View Fig , 7 View Fig ). 3D model of the anterior region of the carapace is available at MorphoSource (project ID: 000730318) .

Type locality and age: DMNH Loc. 19,258, Corral Bluffs Study Area , El Paso County, Colorado, USA. Te Corral Bluffs Study Area is situated east of Colorado Springs (qualified researchers can obtain more detailed locality information from the vertebrate paleontology collections manager at the Denver Museum of Nature & Science; Fig. 1 View Fig ); Denver Formation, Danian, early Paleocene. Te locality is 110.5 m above the pollen defined K/ Pg boundary ( Fuentes et al., 2019), within magnetochron 29n, and with a calculated age of 65.34my ( GPTS2012 age model) or 65.37my ( Clyde et al., 2016 age model). Te holotype was collected from the top of the Puercan II NALMA within pollen biozone 2 ( Fig. 1 View Fig , point 1) and was found in a friable, silty mudstone unit rich in fossil plant fragments interpreted to be an overbank pond deposit that was frequently flooded.

Etymology: Te name ‘Tavachelydra’ is derived from the combination ‘chelydra’ (Greek word ‘khéludros” which means “amphibious serpent”) and the morpheme ‘tava’ referring to one of the names that the Ute or Nuuchiu – a Native American people indigenous to the region of discovery – use for Pikes Peak (Tavá-Kaavi, direct translation “Sun Mountain” ( Briggs, 2010)), which towers over the Corral Bluffs Study Area, having been combined following classical word formation for euphony. Te species epithet honors the late Brandon D. Stevenson, a dear friend of T. R. Lyson and long-time supporter of the Corral Bluffs project.

Nomenclatural acts: Tis publication and its nomenclatural acts were registered at ZooBank on February 5, 2025, prior to publication. Te LSID of the publication is urn:lsid:zoobank.org:pub:, that of the new genus urn:lsid:zoobank.org:act: and that of the new species urn:lsid:zoobank.org:act:.

Diagnosis: Tavachelydra stevensoni can be diagnosed as a member of Chelydroidea (i.e., the clade consisting of Kinosternoidea and Chelydridae ) based on the presence of a reduced cruciform plastron, strap-like epiplastra that cover the lateral margins of the hyoplastra, absence of extragular scales, absence of pectoral scales, absence of a midline contact between the abdominal scales, and less than four inframarginal scales; and as a representative of Pan-Chelydridae based on the presence of true rib-like costiform processes (sensu Joyce, 2007) that are not completely embedded in bone and that cross peripheral I and variably terminate in peripheral II or III on the visceral shell surface, the presence of a low carapace formed by thin neurals and costals, presence of intergular scales, strap-like xiphiplastra that cover the lateral margins of the hypoplastra, and an anal scale that is restricted to the posterior portions of the hypoplastra. Tavachelydra stevensoni differs from all other pan-chelydrid taxa in the following unique combination of characters: large size (straight carapace length of 45–50 cm), absence of plastral or carapacial fontanelles, smooth shell lacking radial plications, an upturned lip on the anterior margin of the nuchal, a thin and gracile plastron, an osseous bridge consisting of pegs and sockets but no sutures, a single, large intergular scale, a distinctive gutter in peripherals IV-VI, thin carapace lacking lateral keels on the costals, mid-line keel restricted to the posterior quarter of the carapace, presence of an iliac notch in the acetabulum, and a fan-shaped ilial shaft that is angled posteriorly and adorned by an incipient thelial process.

Referred material and distribution: DMNH EPV. 143100 ( Fig. 5 View Fig ), an articulated complete carapace and partial right hyo- and hypoplastron found in situ 56 m from the holotype ( DMNH EPV.141854). It originates from the same stratigraphic horizon ( DMNH Loc. 20,053; Fig. 1 View Fig , point 2) and is thus inferred to have the same age as the holotype specimen. A 3D model of the shell is available at MorphoSource (project ID: 000730318) . DMNH EPV. 143200 (not figured), hypo- and xiphiplastra found in situ 97 m above the pollen calibrated K/Pg boundary ( DMNH Loc. 6284; Fig. 1 View Fig , point 5), within magnetochron 29n, and with a calculated age of 65.43 mya ( GPTS, 2012 age model) or 65.49 mya ( Clyde et al., 2016 age model). Te specimen originates from the top of Puercan II NALMA and the middle of pollen zone P2 ( Fig. 1 View Fig , point 5). DMNH. EPV.134087

( Fig. 6A, B, C View Fig ), a poorly preserved, but complete cranium found as float and preserved in a fine grained phosphatic concretion. Te specimen was found at the base of the bluffs, with approximately 45 m of section above (DMNH Loc. 7082; Fig. 1 View Fig , point 3). Given that the specimen was fully intact, it likely did not travel far. Te maximum age and stratigraphic position (i.e., the specimen was moved down section by gravity) was calculated for the specimen as 52 m above the pollen calibrated K/Pg boundary, within magnetochron 29r and with a calculated age of 65.74 mya (GPTS, 2012 age model) or 65.84 mya ( Clyde et al., 2016 age model). Tis corresponds biostratigraphically to the top of the Puercan I NALMA and within the P1 pollen biozone ( Fig. 1 View Fig , point 3). DMNH. EPV.136265

( Fig. 6D, E, F View Fig ), a poorly preserved, but complete cranium and lower jaws found as float and preserved in a fine grained phosphatic concretion. Te specimen was found on a knife ridge with approximately two meters of Paleocene section above (DMNH Loc. 18,852; Fig. 1 View Fig , point 4). Given that the specimen was fully intact, the specimen likely did not travel far. In addition, there was only two meters of section above the specimen. Te maximum age and stratigraphic position was calculated as 99 m above the pollen calibrated K/Pg boundary, within magnetochron 29n and with a calculated age of 65.41 mya (GPTS, 2012 age model) or 65.46 mya ( Clyde et al., 2016 age model). Biostratigraphically the specimen is dated to the top of the Puercan II NALMA and within the P2 pollen biozone ( Fig. 1 View Fig , point 4).

Description of material

Shell: Te description of the shell is based primarily on the holotype ( DMNH EPV.141854; Figs. Fig. 2 View Fig , 3 View Fig , 4 View Fig ) and the referred carapace ( DMNH EPV.143100; Fig. 5 View Fig ). Te anterior quarter of the holotype shell is preserved, but highly fragmented due to erosion, while the posterior three quarters of the shell is mostly complete. Te complete left peripheral series is present, while the right peripheral series is missing peripherals XI and XII. Te nuchal is preserved, but is damaged posteriorly. Costals I and II on both sides are preserved, but are highly fragmented, as are neurals I and II. Te carapace was found partially articulated with the costals, neurals, and suprapygals found in articulation, but the elements on the perimeter (i.e., nuchal, peripherals, and pygal) were separated from the rest of the carapace. Several of the peripherals were articulated with one another. Te hyoplastra, hypoplastra and xiphiplastra were all found in articulation, but the right epiplastra and entoplastron were separated from the rest of the plastron. Te anteromedial portion of both hyoplastra is damaged due to erosion, as is the posterior portion of the entoplastron. Te right epiplastron is complete and the left epiplastron is missing. A small portion between the plastron and articulated costals was preserved in a phosphatic concretion, but not the rest of the specimen. Te carapace is less crushed and flattened where it was preserved in concretion and this portion suggests that the carapace was flat to slightly domed. A complete, disarticulated pelvis was found mixed in with the carapace and plastron elements. Te referred carapace ( DMNH EPV.143100; Fig. 5 View Fig and 3D View Fig models) was preserved intact and is more complete than the holotype carapace ( DMNH EPV.141854). Te left side of the referred carapace ( DMNH EPV.143100) is largely complete, whereas the right side of the carapace is more fragmentary due to erosion. Te right lateral portion of the hyo-hypoplastron was found as float alongside the carapace .

Carapace: Te carapace is large, with a straight carapace length of approximately 45 cm for the holotype and 49.5 cm for the referred carapace ( DMNH EPV.143100). Te carapace is composed of a nuchal, eight neurals, two suprapygals, one pygal, eight pairs of costals and eleven pairs of peripherals ( Figs. 2–5 View Fig View Fig View Fig View Fig ). A supernumerary postneural is present in the holotype and possibly in the referred specimen as well. Te perimeter elements (nuchal, peripherals, and pygal) are distinctly thickened while the remaining carapace is thin, only 3–4 mm in thickness. Te carapace is oblong in shape and lightly embayed anteriorly in the nuchal region. Te sides of the shell are nearly straight and the posterior region is broadly rounded and lacks a caudal notch. Te concreted portion, costals IV-VI, in the holotype specimen indicate that the shell is low domed. Narrow sulci are weakly impressed into the carapacial bones, but plications are absent. Peripherals IX, X, and XI are lightly scalloped posteriorly. Of the midline series, only the posterior midline neurals ( VI – IX) and the suprapygal have a slight, continuous midline keel. Lateral keels are absent on the costals .

Nuchal: Te nuchal is damaged posteriorly as well as on the left side in the holotype specimen, but is more complete in the referred carapace. Te nuchal is a large trapezoidal structure, with an embayed anterior edge, two short margins that form a notched contact with peripheral I laterally, and longer margins that form a broadly-sloping contact with costal I, and a rounded contact with neural I posteriorly ( Figs. 2A, B View Fig , 5A, B View Fig ). Te anterolateral margin of the nuchal is upturned to form a distinctive lip on either side of the midline cervical scale (see 3D models). Te anterior edge of the nuchal is distinctively thickened but thins out posteriorly. Although both sides are damaged in the holotype, a long groove and remnants suggest that the costiform processes were rib-like and superficially crossed peripherals I and II and the anterior half of peripheral III, but the open nature of the groove precludes full confidence ( Fig. 3C View Fig ). In the referred carapace, by contrast, the costiform processes are more deeply imbedded into the peripherals and unambiguously terminate in the lateral portion of peripherals II ( Fig. 5A, B View Fig , and 3D View Fig models). A broken portion of the left costiform process in the holotype shows that the process is oblong in cross section .

Neurals: Eight neurals are present in both specimens. A postneural is partially fused to neural VIII in the holotype and may be present in the referred specimen as well ( Figs. 2A, B View Fig , 3A, B View Fig , 5A, B View Fig ). A low, rounded median keel originates at neural III that becomes increasingly pronounced towards the posterior. Neural I is preserved completely in DMNH EPV.143100, has a rectangular shape, and contacts the nuchal anteriorly, costal I laterally, and neural II posteriorly ( Fig. 5A, B View Fig ). Neural II in the same specimen is an elongate subrectangular structure that forms a broadly-rounded contact with neural I and costal I anteriorly, contacts costal II laterally, and forms a straight contact with neural III posteriorly

( Fig. 5A, B View Fig ). In both specimens, neural III is hexagonal in shape with short anterior sides that contact costals II and long sides that contact costals III. Te vertebral II / III sulcus is situated along the middle of neural III. Neural IV is hexagonal in shape with short anterior sides that contact costals III and long sides that contact costals IV. Neural IV is wider anteriorly and tapers slightly posteriorly where it forms a straight contact with neural V. Neural V is hexagonal in shape with short anterior sides that contact costals IV and long sides that contact costals V. Neural V is slightly shorter in length than neurals I– IV. Te vertebral III / IV sulcus is situated on the posterior third of this neural. Neural VI is hexagonal in shape with short anterior sides that contact costals V and long sides that contact costals VI. Neural VI has a slight taper posteriorly where it forms a posteriorly-rounded contact with neural VII. Neural VII is hexagonal in shape and its width approximates its length. Te short anterior sides contact costals VI and the long sides contact costals VII. A hexagonal neural VIII and a square postneural are fused in the holotype into an elongate, polygonal element. Although the two elements cannot be distinguished externally ( Fig. 2A, B View Fig ), a suture is apparent between the two in visceral view ( Fig. 3A, B View Fig ). Te posterior of the two elements is identified as a postneural, as it is associated with dorsal vertebra X (see Menon and Joyce, in review, for discussion on homology). Te neural VIII of the referred specimen has an irregular, elongate shape as well, which is highly suggestive of a fused element, but we cannot discern sutures in dorsal or ventral view. In either case, the fused neural VIII/postneural has short anterior sides that contact costal VII, elongate lateral sides that contact costal VIII laterally, and a short posterior contact with suprapygal I. Te vertebral IV / V sulcus is situated on the middle ( holotype) or anterior third of neural VIII (referred specimen) .

Suprapygal: Two suprapygals (i.e. bones not associated with the vertebral column) are present in both available carapaces ( Figs. 2A, B View Fig , 5A, B View Fig ). Suprapygal I is nearly square, with its width just barely exceeding its length. Suprapygal I contacts costals VIII laterally, the postneural anteriorly, and suprapygal II posteriorly. Suprapygal II is a large crescent shaped element. On the right side of the holotype specimen, suprapygal II has a broad contact with peripheral XI and nearly contacts peripheral X, whereas on the left side it clearly only contacts peripheral XI. Corner contacts appear to be present on both sides of the referred specimen. Te low, rounded midline keel that originates on the neurals becomes highly distinct in suprapygal I, fades along suprapygal II, and is absent on the pygal. Te posterior margin of vertebral V is located on suprapygal II near its posterior suture with the pygal .

Pygal: Te pygal is nearly square, with its length only slightly exceeding its width ( Figs. 2 View Fig , 5 View Fig ). Te posterior half forms a distinctive trough or ridge, which is best seen in cross section ( Fig. 4 View Fig ). Te pygal is a thin element that tapers to a rounded point posteriorly.

Costals: Eight pairs of costals are present ( Figs. 2 View Fig , 3 View Fig , 5 View Fig ). As in other turtles, the anterior five costals are larger than the posterior three costals. Te costals are thin, no more than 3–4 mm thick. Te intramembranous portion of the costals is tightly sutured with the peripherals leaving no space for carapacial fontanelles between the costals and peripherals. Te endochondral rib portion of the costals inserts into peripherals III through X, best seen in the referred specimen ( Fig. 5C, D View Fig ). While most of the distal ends of the ribs are missing in the disarticulated holotype specimen, they are entirely preserved in left costals VI –VIII and about 1 cm in length, suggesting they insert deeply into each peripheral ( Fig. 3A, B View Fig ) .

Costal I is the largest carapacial element, contacting peripherals I– III laterally, the nuchal anteromedially, neural I medially, and neural II posteromedially. Te costal rib I terminates in peripheral III. Te costal I/ II suture contacts peripheral III. Ventrally, costal rib I (i.e., dorsal rib II) underlies the costiform process groove in peripheral III, suggesting that these two structures nearly touch in the holotype ( Fig. 3C View Fig ). In the referred carapace, the costiform process terminates at least on the surface in peripheral II and there is thus greater distance between it and the endochondral rib portion of costal I ( Fig. 5C, D View Fig ). Ventrally, the first dorsal rib, a thin and long strap-like structure that spans about half the width of costal I, is closely attached to the anteroventral portion of costal rib I ( Figs. 3A, B View Fig , 5C, D View Fig ). Costal II expands distally, contacts peripherals III and IV laterally, and its rib deeply inserts into the posterior half of peripheral IV. Te costal II / III suture contacts peripheral V and the costal III / IV suture contacts peripheral VI. Te width of costal III and IV does not change and their ribs insert into peripheral V and VI, respectively. Costal V expands distally and its rib inserts into the posterior half of peripheral VII. Costals VI –VIII are much smaller than the anterior costals and become successively smaller and shorter towards the posterior. Teir ribs insert into the posterior halves of peripherals VIII, IX, and X respectively. Te rib heads of dorsal ribs IX (i.e. costal rib VIII) and X are fully formed and medially contact the vertebral column. A pair of oblong depressions present on the ventral side of costal VIII are interpreted as the iliac scars, where the ilia articulate with the carapace ( Fig. 3A, B View Fig ) .

Peripherals: Eleven pairs of peripherals are present

( Figs. 2 View Fig , 3 View Fig , 4 View Fig , 5 View Fig ). As in other pan-chelydrid turtles, the peripherals are robust, thick elements that form a broad rim around the otherwise thin carapace. Peripheral I is broadly triangular structure that is slightly longer than wide and contacts the nuchal medially, costal I posteriorly, and peripheral II laterally. Te rib-like costiform process is loosely attached to the ventral side of peripheral I and II in a shallow groove in the holotype specimen ( Fig. 3A, B, C View Fig ) and is more deeply embedded in bone in the referred carapace ( Figs. 5C, D View Fig ), which we attribute to the larger and more ossified nature of the referred carapace. Ventrally the skin sulcus is visible midway along the length of peripheral II and the anterior portion of peripheral III. As in all turtles, costal ribs do not insert into peripherals I or II. Te edge of the carapace is slightly upturned starting in peripheral II, becomes increasingly guttered through peripheral VI, but this flaring tapers to an end in peripheral VII ( Fig. 4 View Fig ). Te distal rib end of costal I inserts into the middle of peripheral III, cuts across this peripheral along a broad transverse groove ( Fig. 3C View Fig ), and ends at the posteriormost margin of this peripheral along the peripheral III / IV suture. Anteroventrally to this costal rib groove is a deep pit that receives the hyoplastral axillary buttress. Te ventromedial edges of peripherals IV through VI have small, distinctive pits for articulation with the lateral pegs formed by the hyo- and hypoplastra ( Fig. 3A, B View Fig ; see 3D model for better view). Te hypoplastral inguinal buttress ends in a pit in the middle of peripheral VII. Te bridge peripherals are V- to C-shaped in cross section

( Fig. 4 View Fig ). Peripherals VII through XI are long rectangular structures that are slightly upturned ( Fig. 4 View Fig ). Peripherals III through X have distinct pits along their medial suture for articulation with the costal ribs. Such pits are not present on peripherals I, II, and XI. Te posterior edges of peripherals IX through XI are weakly scalloped. Musk duct grooves are absent.

Carapacial scales: Te carapacial sulci are overall only weakly impressed on the carapacial bones, but indicate the presence of five vertebrals, one undivided cervical, four pairs of pleurals, and twelve pairs of marginal scales. Te cervical scale is small and approximately three times wider than it is long. It is entirely situated on the nuchal bone. Vertebral scale I is the smallest of the vertebral series. Its anterior width is slightly greater than its posterior width. Te anterior two thirds of vertebral scale I is positioned on the nuchal, while the posterior third covers the anteromedial portion of costal I and the anterior two thirds of neural I. Te vertebral I/ II contact is located on the posterior portion of neural I. Vertebrals II, III, and IV are squarish in the holotype ( Fig. 2 View Fig ), but slightly more hexagonal in the referred specimen ( Fig. 5 View Fig ). Te vertebral II / III and vertebral III / IV contacts are located on neural III and V, respectively. Vertebral V is trapezoidal in shape with its anterior width approximately half of its posterior width. Te four pairs of pleural scales are large and rectangular to trapezoidal in shape. Te pleural I/ II, II / III, and III / IV sulci are situated on costals II, IV, and VI, respectively. Te pleural IV and vertebral V sulcus transversely crosses costal VIII and the lateral-most corner of the suprapygal before contacting marginal XI. Te marginal scales span the sutures between the peripheral series, starting with the nuchal/peripheral I suture through the peripheral XI /pygal suture. Te marginal I– XI scales are restricted to the peripherals and do not extend onto the costals. Te marginal XII scales, however, clearly extend anteriorly onto the suprapygal .

Plastron: Te plastron in the holotype specimen ( DMNH EPV.141854) consists of an entoplastron and paired epi-, hyo-, hypo-, and xiphiplastra ( Fig. 2C, D View Fig , 3D, E View Fig ). Te plastron is mostly complete and is only missing the anterior portions of the hyoplastra and the complete left epiplastron. Te plastron was found mostly articulated, with only the epiplastra and entoplastron disarticulated. It is approximately 30 cm long, significantly shorter than the straight midline length ( 45 cm long) of the associated carapace. Te plastron lacks fontanelles and is thicker than the carapace, the lateral edges of the xiphiplastra having a thickness of 8–9 mm. Te bridge is narrow, 35 mm in length at its narrowest point. Te bridge has a loose osseous connection with the carapace via short pegs that insert from the middle of peripherals III to the middle of peripheral VII but does not extend onto the costals. Musk duct foramina are not present, but at least one deep, rounded depression along the lateral margin of the right hyoplastron is highly suggestive of a notch for passage of the musk duct. While the anterior plastral lobe is distinctly triangular, the posterior plastral lobe is more rounded .

Epiplastra: Te epiplastron is a strap-like element that contacts its counterpart anteromedially, the entoplastron medially, and the hyoplastron posteriorly ( Figs. 2C, D View Fig , 3D, E View Fig ). Te epiplastron forms a beak-shaped process along the anterior plastral lobe. Te epiplastron medially forms broad grooves that embrace the rounded lateral margin of the entoplastron and hyoplastron. Te full element forms a thick layer of metaplastic bone that fully obscures its origin as a clavicle.

Entoplastron: Te entoplastron is an arrowheadshaped element that is damaged posteriorly ( Fig. 2C, D View Fig , 3D, E View Fig ). It forms a thick layer of metaplastic bone that hides its origin as a T-shape interclavicle. Anteriorly the entoplastron forms a rounded tongue that fits into the groove formed by the epiplastron. It otherwise sutures with the hyoplastra via a poorly preserved peg-andsocket connection. Te anterior plastron lobe thus lacks fontanelles.

Hyo-/Hypoplastra: Te anterior edge of both hyoplastra is missing, but based on the epiplastra and entoplastron, it is clear that the hyoplastra anterolaterally form a tongue that sutures with the groove in the epiplastra and anteromedially forms distinct pegs that suture with the entoplastron ( Fig. 2C, D View Fig , 3D, E View Fig ). Te hyoplastra form a straight suture with the hypoplastra across the middle of the plastron. Te hyo- and hypoplastra suture with their counterparts along the plastral midline via a deeply interfingering suture. Posteriorly, the hypoplastron forms a broad posterolateral tongue that inserts into the xiphiplastron. Tis deeply interfingered contact is both visible in dorsal and ventral view ( Fig. 2C, D View Fig , 3D, E View Fig ). Te minimum sagittal length along the hyo-/hypoplastra bridge is approximately 35 mm. Dorsally (i.e., internally), the lateral edges of the hyo-/hypoplastra are thickened to approximately 6–8 mm. Laterally, short pegs and ridges of the hyo- and hypoplastra connect with the peripherals. Te axillary buttress inserts into the middle of peripheral III and the inguinal buttress is a long finger-like structure that inserts into the middle of peripheral VII. No portion of the hyo-/hypoplastra contacts the peripherals.

Xiphiplastra: Te xiphiplastra are long, robust elements that form a sinuous contact with each other medially. Te xiphiplastra are not tightly sutured to the hypoplastra anteromedially, but rather connect anterolaterally via a complex deeply interfingering connection. An elongate process of the xiphiplastron laterally covers the hypoplastron. Medial to this interfingering connection the xiphiplastra and hypoplastra are only loosely abutting another. Posteriorly the xiphiplastra create a distinctly pointed plastral lobe.

Plastral scales: We use the plastral nomenclature of Hutchison and Bramble (1981) as implemented by Joyce (2016) for pan-chelydrids. Te plastral sulci are lightly impressed into the plastron and show the presence of an intergular scale, paired gular, humeral, abdominal, femoral, and anal scales, and three paired inframarginal scales. No anterior midline sulcus is present on the entoplastron, indicating that there is only one midline intergular. Te intergular extends over the epiplastral beak, covering half of the epiplastra as well as nearly half of the entoplastron. Te gular-humeral sulcus does not extend onto the epiplastra. Te entirety of the humeral scale is therefore on the hyoplastron. Pectoral scales are absent. Te abdominal scales do not contact another medially. Te abdominal scales broadly contribute to both the axillary and inguinal notches. Te humeral/femoral sulcus is confluent with the hyo/hypoplastron suture. Te anal scales are paired and are restricted to the xiphiplastra. A postanal scale is absent. Tree pairs of inframarginal scales are present. Inframarginal scales I and II are located on the hyoplastron and a larger third inframarginal scale on the hypoplastron. Inframarginal scale I is a small, triangular scale that covers the anterior tip of the axillary buttress. Inframarginal scale II is larger than I, is rectangular in shape, contacts inframarginal scale I and III anteriorly and posteriorly, respectively, the abdominal scale anteromedially, and participates in the axillary notch. Inframarginal scale III is a large, kite-shaped scale that contacts inframarginal scale II anteriorly, the abdominal scale anteromedially, and broadly participates in the inguinal notch posteromedially.

Cranium: Te referred crania, DMNH EPV.134087 and 136,265, are complete, but are poorly preserved ( Fig. 6 View Fig ). Tis type of preservation is common among vertebrate fossils from the Corral Bluffs Study Area preserved in phosphatic concretions ( Lyson et al., 2019). Specimen DMNH EPV.134087 is the slightly larger of the two specimens and most of the description is derived from this specimen ( Fig. 6A, B, C View Fig ). Tis specimen is preserved three dimensionally, does not appear to be crushed, but the skull roof is particularly poorly preserved as the sutures on the dorsal skull roof are fully obliterated. In addition, the left squamosal, basioccipital region, and distal end of the supraoccipital are damaged. Te smaller specimen, DMNH EPV.136265, is dorsoventrally compressed and has portions of the lower jaw preserved in occlusion with the cranium ( Fig. 6D, E View Fig ). An isolated bone fragment is preserved on the anterior tip of the skull giving the false impression that an elongate snout is present. Tis specimen is particularly poorly preserved as no sutures are visible .

Te skull of DMNH EPV.134087 is large and triangular in shape with an occipital condyle to snout tip length of 96.9 mm and a maximum width of 99.8 mm. Te supraoccipital extends posteriorly beyond the occipital condyle and, while damaged, the snout tip to distal supraoccipital length is 110 mm. Te base of the supraoccipital flares out laterally to form a T-shaped cross section. Te orbits are oriented vertically and the upper temporal emargination is deep extending well anterior of the otic chamber. Te nasals slightly overhang the confluent nares. Te orbit is large, more than three times larger than the confluent nares. A distinct notch is present in the posteroventral portion of the orbit on both sides of the skull, suggesting that this represents true morphology rather than taphonomic distortion. Te cheek emargination is minor, not extending above the ventral-most portion of the orbit. Although damage makes them appear particularly large, the stapedial foramen is well-developed nonetheless. Te triturating surface is broad and flat. Te labial ridge is broadly rounded and formed by the maxillae and premaxillae. A lingual ridge of the triturating surface is absent. Te palatines contribute broadly to the triturating surfaces. Te foramen praepalatinum are situated entirely within the premaxillae. Te vomer is a single, broad-bar-shaped element that contacts the premaxillae anteriorly, the maxillae anterolaterally, the palatines laterally, and the pterygoids posteriorly. Te vomer floors the apertura narium interna.

Pelvic girdle: Te complete pelvis is preserved in the holotype specimen ( DMNH EPV.141854) and was found disarticulated and intermixed with the shell ( Fig. 7 View Fig ). Te pelvis broadly resembles that of other pan-chelydrids. Te ilium, pubis, and ischium equally participate in the acetabulum. Te acetabulum is a broad, bean-shaped structure. Te ilium forms the dorsal third of the acetabulum, has a rod-shaped shaft, and expands dorsally into a fan-shaped structure. In lateral view, the ilium is posterodorsally directed and the expanded fan-shaped portion of the ilium has distinctive longitudinal ridges. Te distal fan contacts the carapace along an elongate depression contained within costal VIII. A distinctive bump or thelial process is present midway up the rod-shaped shaft, but not associated with a kink in the ilial shaft. Like the distal fan, the thelial process has longitudinal ridges

( Fig. 7 View Fig ). A distinctive iliac notch is present between the acetabulum and articular surface with the ischium, best seen in posterior view ( Fig. 7H View Fig ).

Te ischium forms the posteroventral portion of the acetabulum. Te medial plate of the ischium forms a broad plate that broadly contacts its counterpart medially. Te lateral process of the ischium is large and distally exhibits longitudinal ridges. An anteromedial process of the ischium extends into the thyroid fenestrae. Te merged thyroid fenestrae form a large opening shaped like a horizontal figure eight.

Te pubis forms the anteroventral portion of the acetabulum. Te lateral process of the pubis is elongate, oval in cross section, and has longitudinal ridges for most of its length. Te pubes broadly meet along the midline to form a broad plate-like structure. A V-shaped and smooth groove between the pubes indicate the presence of an unossified epipubic process anteriorly. A small posterior process of the pubis extends into the thyroid fenestra, but does not contact the ischium.

Te great lateral expansion of the pelvis suggests that the only preserved sacral rib can be identified as the first. It is unclear if additional sacral ribs articulate with the pelvis.