OMNH 67133, a partial shell (Figs. 2, 4a).

OMNH locality V1332, Dick Canyon, Fall River County, South Dakota (more detailed locality information is on file at OMNH); Unit 2, Chilson Member, Lakota Formation, Berriasian–Valanginian (see Geological settings above).

This publication and its nomenclatural acts were registered at ZooBank on 12 February 2020, prior to publication. The LSID of the publication is urn:lsid:zoobank.org:pub:66C2A3D5-1E5C-42E8-8137-9B6253ACDA25, that of the new genus urn:lsid:zoobank.org:act:85E563EB-2F61-4560-8DB1-11F59261825E, and that of the new species urn:lsid:zoobank.org:act:00A9D083-7874-4022-AB3C-41B9FF71841E.

Lakotemys australodakotensis gen. et sp. nov. can be diagnosed as a representative of Paracryptodira by the presence of a finely sculptured skull and shell and the likely location of the foramen posterius canalis carotici interni midline along the contact of the basisphenoid and pterygoid and as a representative of Baenidae by the absence of epiplastral processes and the development of well-developed axillary and inguinal buttresses. Among named Early Cretaceous baenids, Lakotemys australodakotensis can be differentiated from Protobaena wyomingensis and Trinitichelys hiatti by having an irregularly shaped vertebral V that does not lap onto neural VIII and that forms two anterolateral processes that partially hinder vertebral IV from contacting pleural IV and can be differentiated from Arundelemys dardeni and Trinitichelys hiatti by having a broader skull and triturating surfaces (see Discussion for additional differences).

The compound word Lakotemys is derived from the combination of the word Lakota, in reference to the indigenous Lakota people of the greater Black Hills region, and the fossil-bearing Lakota Formation, and emys, ancient Greek for turtles. The species epithet australodakotensis is formed in allusion to the state of South Dakota through the combination of the word auster with the suffix -alis, Latin for south, and Dakota. The suffix -ensis is used to indicate a location.

OMNH 66106, a poorly preserved skull collected from the type locality (Fig. 3); OMNH 63615, a poorly preserved shell collected from OMNH site V1382 (Fig. 4). All referred material originates from Unit 2 of the Chilson Member of the Lakota Formation.

The only available skull, OMNH 66106, is partially crushed and encrusted by a thick layer of metal oxides. The vast majority of sutures are obscured externally and in the CT scans. We therefore here only provide 3-D-rendered CT images of the skull in the primary six views and briefly highlight the most important features (Fig. 5).

Overall, the skull is characterized by being slightly longer than wide and by having intermediately developed lower and upper temporal emarginations, dorsoventrally oriented orbits, and a broad internarial bar. The size and orientation of the external nares are unclear, as is the size of the cavum tympani relative to the orbit. The overall shape nevertheless resembles the skull of most other baenids.

The frontal, postorbital, parietal, and squamosal are the only bones that can be at least partially recognized in the CT scans. The frontals and postorbitals broadly contribute to the dorsal margin of the orbit and therefore exclude the parietal from the orbit. The posterior tip of the short supraoccipital crest is not roofed by the parietal. As the remaining sutures are unclear, it is not possible to clarify on the dorsal skull surface the presence or size of nasals, the size of the dorsal lappets of the prefrontals, the contribution of the jugal to the orbit, or contact between the squamosal and parietal.

We are unable to discern any sutures on the ventral side of the skull. The anterior region is furthermore damaged, making it impossible to rigorously assess the morphology of the triturating surfaces. The posterior remnants of the maxilla nevertheless suggest that the triturating surfaces are expanded posteriorly. The external pterygoid processes are well developed and deeply protrude into the temporal fossa.

Despite the availability of CT scans, we are not able to trace the path of most cranial nerves and vessels. Faint traces of canals near the basisphenoid–pterygoid contact suggest that the internal carotid canal entered the skull halfway along the basisphenoid–pterygoid contact, but we cannot comment on the presence or absence of the palatal branch of the internal carotid or on the exact path of the facial nerve. The stapedial canals open into the upper temporal fossa on top of the otic capsule, but we are not able to clarify if the opisthotic contributes to its formation, both at the surface and within the skull.

Although both shells are incomplete, by reference to other paracryptodires we conclude with high confidence that they once consisted of a nuchal, eight neurals, two suprapygals, a pygal, eight pairs of costals, and 11 pairs of peripherals (Figs. 2, 3).

The nuchal is only partially preserved in OMNH 63615 and its preservation is poor (Fig. 3). The partially preserved anterior margin suggests that a distinct nuchal notch may have been present, while the partially preserved posterior margin highlights the contact of the nuchal with costal I. The likely lateral contact of the nuchal with peripheral I is not preserved.

OMNH 63615 preserves the anterior six neurals (Fig. 3), while OMNH 67133 preserves the posterior six neurals (Fig. 2). OMNH 67133 exhibits two minor abnormalities in the form of a doubled neural VII and the inclusion of an asymmetric ossification between neurals V and VI. The neurals form a continuous row of elements that fully separate the costals from one another. Neural I is rectangular and slightly shorter anteroposteriorly than neural II. The remaining neurals are hexagonal with short anterolateral sides. Although neurals II–VIII have a similar width, their anteroposterior lengths decrease gradually towards the posterior of the shell.

Suprapygal I is preserved in OMNH 67133 (Fig. 2). It is an octagonal element having a short anterior contact with neural VIII, an elongate anterolateral contact with costal VIII, a short lateral contact with peripheral X, a short posterolateral contact with peripheral XI, and a broad posterior contact with suprapygal II.

Suprapygal II is not preserved in either specimen, but the concave articular facet preserved along the posterior border of suprapygal I in OMNH 67133 (Fig. 2) is highly suggestive of its former presence. This element likely had a broad anterior contact with suprapygal I, a short posterolateral contact with peripheral XI, and a broad posterior contact with the pygal.

The pygal is not preserved in either specimen.

OMNH 67133 (Fig. 2) and OMNH 63615 (Fig. 3) combined document the presence of eight pairs of costals. From front to back, the costals gradually decrease in anteroposterior and mediolateral dimensions. While costal I medially contacts a single neural, all remaining elements are in contact with two neurals. In addition, costal I contacts the nuchal anteriorly, while costal VIII contacts suprapygal I posteriorly. The distal contacts of the costals are only partially preserved. Costal I likely contacted peripherals I–III. Costal II laterally contacts peripherals IV and V, costal III contacts peripherals V and VI, costal IV contacts peripherals VI and VII, costal V contacts peripherals VII and VIII, and costal VI contacts peripherals VIII and IX. Costal VII mostly contacts peripheral IX but also shows a minor contact with peripheral X. Finally, costal VIII only exhibits a lateral contact with peripheral X.

The specimens only preserve peripherals III–X, but the large gap at the back of OMNH 67133 (Fig. 2) suggests that 11 pairs were once present, instead of 10 pairs with an anomalously large pygal. Poor preservation obscures the morphology of the anterior peripherals, but peripherals I and II were likely flat. The bridge stretches from peripheral III to VIII. All bridge peripherals are laterally decorated by a well-developed, bead-like thickening. The posterior peripherals are once again flattened. Peripheral I likely contacted the nuchal medially. The likely or preserved medial contacts with the costals are described above. While peripheral X shares a short contact with suprapygal I, peripheral XI has a short contact with suprapygal I and likely has a broad anterior contact with suprapygal II and a medial contact with the pygal.

OMNH 67133 and OMNH 63615 both preserve some traces of carapacial scutes, but as the observed pattern on the shell of OMNH 63615 is highly abnormal by showing pronounced asymmetry (Fig. 3), we restrict our description to the carapace of OMNH 67133 (Fig. 2).

Although only vertebrals II–V, pleurals I–IV, and marginals IV–X are preserved, the shell was likely covered by five vertebrals, four pairs of pleurals, and 12 marginals. The former presence of cervicals cannot be ascertained as the surface of the nuchal is poorly preserved.

Vertebral II is only partially preserved but appears to have similar dimensions to that of vertebral III (Fig. 2). Vertebral III is six-sided, but the parallel lateral margins give it an overall square outline. Vertebral III contacts vertebral II anteriorly along a sulcus that crosses neural III, pleurals II and III laterally, and vertebral IV posteriorly along a sulcus that crosses neural V. Vertebral IV is approximately as wide as vertebral III, but is slightly longer and has a more hexagonal outline. In addition to contacting vertebral III anteriorly, it contacts pleurals III and IV laterally and forms a highly sinuous posterior sulcus with vertebral V that posteriorly circumnavigates neural VIII to cross the suprapygal. Only the anterior half of vertebral V is preserved. This portion of this scute documents a highly unusual morphology consisting of two fingerlike projections that intrude between vertebral IV and pleural IV. The highly symmetric arrangement suggests that this is not an irregularity. The sulcus with pleural IV is otherwise located on suprapygal I. As the posterior elements are not preserved, it is not clear if vertebral V contributed to the posterior margin of the carapace, the condition seen in baenodds. However, the plesiomorphic, expanded nature of the most posteriorly preserved peripherals makes this unlikely.

The pleurals are slightly wider than the vertebrals (Fig. 2). The interpleural sulci are straight and located on costals II, IV, and VI. Pleural I at least contacted marginal IV and V laterally, pleural II contacted marginals V–VII, pleural III contacted marginals VII–IX, and pleural IV contacted marginals IX–XI. The contacts of marginals I–III are not preserved. The marginals alternatingly have rectangular to pentagonal outlines. The tips of the pentagons partially subdivide the pleurals and rise to the level of the costals.

The anterior plastral lobe is better preserved in OMNH 63615 (Fig. 3), and the posterior lobe is better preserved in OMNH 67133 (Fig. 2). The two specimens therefore jointly provide a nearly complete picture of this part of the shell.

The plastron consists of an entoplastron and pairs of epi-, hyo-, meso-, hypo-, and xiphiplastra. The arrangement and contacts of these bones resemble those of other turtles. The epiplastra are notable by having a broad midline contact. The anterior portion of the large entoplastron is smaller than the posterior portion. The anterior plastral lobe is broad, elongate, and rounded and mostly formed by the enlarged hyoplastra. It likely protruded anteriorly beyond the anterior margin of the carapace. The hyo-, meso-, and hypoplastral jointly form the bridge that articulates with peripherals III–VIII. The exact nature of the buttress is obscured by matrix but can nevertheless be ascertained to be more expansive than in Glyptops ornatus, by articulating with the costals. The mesoplastra are large elements that form an irregular, offset contact with one another and the neighboring elements along the midline. They laterally contact peripherals V and VI. The posterior plastral lobe is shorter and narrower than the anterior lobe, is constricted towards the posterior, and only partially covers the carapace. It is jointly formed by the hypoplastra and xiphiplastra. An anal notch is present but shallow.

The plastron is covered by pairs of gulars, extragulars, humerals, pectorals, abdominals, femorals, and anals and four pairs of inframarginals. The gulars and extragulars are only partially preserved, but it is possible to ascertain that the gulars were paired, triangular elements that had a broad midline contact and lapped onto the entoplastron and that the extragulars did not have a midline contact or lap onto the entoplastron. The extragular–humeral sulcus is located on the epiplastron, the humeral–pectoral sulcus on the hyoplastron just anterior to the axillary notch, the pectoral/abdominal sulcus on the mesoplastra, the abdominal–femoral sulcus on the hypoplastra just posterior to the inguinal notch, and the femoral–anal sulcus on the xiphiplastra far from its contact with the hypoplastron. The pectoral and abdominal scutes form the deepest portions of the axillary and inguinal notches, respectively. Inframarginal I medially contacted the pectoral and laterally contacted at least marginals IV and V.

We here refer three specimens to Lakotemys australodakotensis gen. et sp. nov. The holotype, a partial shell, and the only known skull were collected from the same quarry and may reasonably originate from the same specimen, but this cannot be ascertained with confidence. The second referred specimen, a shell, was collected from nearby outcrops of the same unit within the Lakota Formation. The three specimens therefore at least represent a single species, at most three species. The hypothesis that all three fossils represent a single species is supported by common provenance (the same rock layer and/or quarry), shared characteristics (overall similarity in shell morphology), and the realization that other Early Cretaceous formations are not known to be particularly diverse. The two major conclusions drawn herein, in particular the naming of a new species and its identification as a basal baenid, however, should remain unaffected if future finds indicate that the three specimens represent separate species. Indeed, as the skull and referred shell are not particularly informative, both conclusions can be justified on the basis of OMNH 67133 (the holotype) alone, as this specimen exhibits the majority of informative character information.

We summarize in the following section differences with all other currently recognized Late Jurassic to Early Cretaceous paracryptodires worldwide (Joyce and Lyson, 2015; Joyce and Anquetin, 2019).

The Late Jurassic (Kimmeridgian) Selenemys lusitanica Pérez-García and Ortega, 2011 from Portugal substantially differs from Lakotemys australodakotensis gen. et sp. nov. by having a more coarsely textured shell; a short, midline contact of peripheral 1 anterior to the nuchal; a neural reversal; a suprapygal I that is much smaller than suprapygal II; broader and shorter plastral lobes; a much larger entoplastron; broader, regularly shaped and positioned vertebrals; and a highly sinuous midline plastral sulcus.

The Late Jurassic (Tithonian) Glyptops ornatus Marsh, 1890 from North America, as described by Hay (1908) and Gaffney (1979) under the name Glyptops plicatulus, resembles Lakotemys australodakotensis in possessing a finely textured skull and shell, and in the overall configuration of the shell, but differs by being smaller, having a much more elongate skull with delicate triturating surfaces; a smaller suprapygal I that does not contact the peripherals; a shorter, but broader anterior plastral lobe; and regularly positioned, hexagonal vertebral scutes.

The Late Jurassic (Tithonian) Dinochelys whitei Gaffney, 1979 from North America differs substantially from Lakotemys australodakotensis by lacking a textured shell; by having a neural reversal; by lacking a contact of suprapygal I with the peripherals; by having much broader, regularly shaped and positioned vertebrals and shorter plastral lobes; by lacking an overlap of the gulars onto the entoplastron; and by having an overlap of the anal onto the hypoplastron.

The Late Jurassic (Tithonian) Riodevemys inumbragigas Pérez-García et al., 2015b from Spain resembles Lakotemys australodakotensis in possessing a finely textured skull and shell, but differs by having a smaller suprapygal I than suprapygal II, broader and shorter plastral lobes, and a much larger entoplastron; by lacking a midline contact of the mesoplastra; and by having broader, regularly shaped and positioned vertebrals.

The Early Cretaceous (Berriasian) Pleurosternon bullockii (Owen, 1842) from the UK, as described by Evans and Kemp (1975) and Milner (2004), resembles Lakotemys australodakotensis in possessing a finely textured skull and shell, but differs by having a much more elongate skull with narrow triturating surfaces, a suprapygal I that is much smaller than suprapygal II, wider, but regularly positioned vertebrals, a broader anterior plastral lobe, a much larger entoplastron, and a distinct anal notch.

The Early Cretaceous (Berriasian) Pleurosternon typocardium (Seeley, 1869) from western Europe, as described by Evans and Kemp (1976) and Pérez-García (2014), resembles Lakotemys australodakotensis in the overall gestalt and configuration of the skull and shell, and by having a lateral contact of suprapygal I with the peripheral series, but differs by having a neural reversal, regularly shaped and positioned vertebrals, a shorter anterior plastral lobe, a much larger entoplastron, and, likely, a deep nuchal notch.

The Early Cretaceous (Albian) Toremys cassiopeia Pérez-García et al., 2015a from Spain resembles Lakotemys australodakotensis by having finely textured shell sculpturing, but differs by being much smaller and having a shorter anterior plastral lobe, a large entoplastron, and a deeper anal notch.

The Early Cretaceous (Aptian–Albian) Protobaena wyomingensis (Gilmore, 1919) from Wyoming resembles Lakotemys australodakotensis in having a finely textured shell surface, but differs by having regularly shaped and placed vertebral scutes, a pygal notch that reaches peripheral X, and a narrower posterior plastral lobe.

The Early Cretaceous (Aptian–Albian) Trinitichelys hiatti Gaffney, 1972 from Texas resembles Lakotemys australodakotensis by having a finely textured shell surface, the proportions of the plastral lobes, and the overall arrangement of bones and scutes, but differs by having a more elongate skull and less distance between the triturating surfaces, regularly placed vertebral scutes with sinuous margins, and a greater overlap of the gulars onto the entoplastron.

The Late Jurassic (Tithonian) Uluops uluops Carpenter and Bakker, 1990 and the Aptian–Albian Arundelemys dardeni Lipka et al., 2006 are only known from cranial material. The poor preservational state of the only available skull of Lakotemys australodakotensis therefore makes it difficult to highlight meaningful differences. We nevertheless note that the skulls of Uluops uluops and Arundelemys dardeni are more elongate and have much narrower triturating surfaces. In addition, the ventral side of the basisphenoid of Uluops uluops is decorated with processes, while the pterygoid of Arundelemys dardeni exhibits deep pterygoid fossae.

In conclusion, the new turtle material documents a unique morphotype and we therefore feel justified designating it as representing a new species, Lakotemys australodakotensis gen. et sp. nov.

Our phylogenetic analysis hypothesizes that the early Early Cretaceous Lakotemys australodakotensis gen. et sp. nov. is the immediate sister of the late Early Cretaceous Trinitichelys hiatti at the base of Baenidae. The late Early Cretaceous Protobaena wyomingensis is the next more highly nested representative. This conclusion overall suggests that Early Cretaceous baenids form a lineage that gradually acquired characters more typical of Late Cretaceous and Paleogene forms. The placement of Lakotemys australodakotensis and Trinitichelys hiatti at the base of Baenidae is suggested by the presence of an extended midline contact of the pterygoids (not preserved in Lakotemys australodakotensis), absence of epiplastral processes, and the development of well-developed axillary and inguinal buttresses. The sister-group relationship of Lakotemys australodakotensis and Trinitichelys hiatti is not actually supported by a character, but rather appears to be a result of the use of implied weighting. More derived characters that unite Protobaena wyomingensis with all Late Cretaceous to Paleogene baenids include the fusion of the shell, but we also note that Protobaena wyomingensis has an incipient pygal notch, which appears intermediate to the deep pygal notch of baenodds.

Late Cretaceous baenids have already historically been thought to be closely related with Late Jurassic pleurosternids (e.g., Hay, 1908). This sister-group relationship was supported by more recent phylogenies as well (e.g., Joyce, 2007; Lyson and Joyce, 2011), but it also suggests an undocumented baenid ghost lineage that crosses the early Early Cretaceous into the Late Jurassic. The herein described material from the early Early Cretaceous (late Berriasian–early Barremian) Lakota Formation of South Dakota helps to partially fill this gap. We nevertheless note that the earliest-known pleurosternids are Kimmeridgian in age, which suggests that baenids must have existed during the deposition of the Late Jurassic Morrison Formation as well. The origin of the baenid lineage is therefore to be expected among Morrison fossils previously described as “glyptopsids”.