Dakotaraptor

Warm humid coastal plains of the North American interior during the late Maastrichtian, drained by meandering rivers crossing subtropical forests dominated by conifers (Taxodium, araucarians), palms (Arecaceae), cycadaceans, broad-leaved angiosperms (Dryophyllum) and pteridophytes. The coast of the epicontinental Western Interior Seaway still existed during Hell Creek deposition but was retreating rapidly. The unit includes active fluvial channels, hydromorphic floodplains, swamps and ephemeral ponds. The fauna includes Tyrannosaurus rex as dominant predator, Triceratops and Torosaurus among ceratopsids, Edmontosaurus annectens among hadrosaurids, Ankylosaurus and Denversaurus as ankylosaurs, Pachycephalosaurus and Thescelosaurus, and, among small theropods, Acheroraptor, Anzu, Ornithomimus and Troodon / Pectinodon. Dakotaraptor coexisted with all this fauna in the last hundreds of thousands of years of the Cretaceous.

Active predator, strict carnivore. Its intermediate size between juvenile Tyrannosaurus rex and small theropods such as Acheroraptor suggests that Dakotaraptor occupied a mesopredator niche, possibly exploiting prey beyond the reach of juvenile tyrannosaurids but at an advantage over Acheroraptor and Troodon. Likely prey included Thescelosaurus, juvenile Pachycephalosaurus, ornithomimosaurs such as Ornithomimus and juvenile hadrosaurids. The 24 cm second-toe sickle claw and the gracile hindlimbs with a very long tibia suggest both active predation on open terrain and the capacity to grasp and subdue prey, as hypothesised by Fowler et al. (2011) for Deinonychus.

No tracks or sites are unequivocally attributable to Dakotaraptor, and the reduced post-corrigendum hypodigm limits direct inferences. By analogy with other eudromaeosaurs, solitary hunting or sporadic pair behaviour is possible, with rapid running on open terrain and attack with the sickle claw to subdue prey. The presence of ulnar papillae indicating forearm plumage suggests that visual display, sheltering over nests and thermoregulation were important roles of the feathers in the adult animal, independent of any flight capability, nonexistent given the body size.

Dakotaraptor was an exceptionally large dromaeosaurid with unusually gracile anatomy among the big representatives of the family. The 678 mm tibia is the longest known in Dromaeosauridae, indicating proportionally long hindlimbs and potentially greater cursorial capacity than Utahraptor (robust) and Achillobator (massive). The ulnar papillae attest to large forearm feathers, homologous to the secondaries of modern birds. Bone histology for the taxon has not been published in detail, which is particularly sensitive given the identification controversy, but Burnham and colleagues announced in post-2015 conference talks that the femur and ulna microanatomy is consistent with dromaeosaurids.

Foundational monograph for modern Dromaeosauridae. The description of Deinonychus antirrhopus, with its second-toe 'terrible claw' and bird-like manus, revived the hypothesis of dinosaurian ancestry for birds and is an indispensable basis for interpreting the anatomy of Dakotaraptor.

Life reconstruction of Deinonychus antirrhopus, the dromaeosaurid that became the template for the whole family and anatomical reference for Dakotaraptor.

Comparative reconstruction of Velociraptor mongoliensis, a velociraptorine dromaeosaurid close to Dakotaraptor within Eudromaeosauria.

Original description of Utahraptor ostrommaysorum, the giant dromaeosaurid from the Lower Cretaceous Cedar Mountain Formation of Utah. With approximately 7 metres of body length and skull bones 2.5 times larger than those of Deinonychus, it was for two decades the largest known dromaeosaurid, until equalled by Dakotaraptor in 2015.

Life reconstruction of Utahraptor ostrommaysorum, the main comparative size reference for Dakotaraptor.

Size comparison between Utahraptor and an adult human, a baseline for scaling Dakotaraptor alongside the largest Lower Cretaceous dromaeosaurid.

Description of Achillobator giganticus, a Mongolian Upper Cretaceous dromaeosaurid (~96 to 89 Ma), with a robust skeleton, primitive vertically oriented pubis and 4.5 to 5 metre body size. Key reference for understanding the diversity of large eudromaeosaurs and for comparing the ecology of a massive predator with the gracile build of Dakotaraptor.

Mounted Velociraptor on display, a small-bodied comparative baseline for the large eudromaeosaurs such as Achillobator and Dakotaraptor.

Direct comparison between Dakotaraptor and Utahraptor, the two largest known North American dromaeosaurids, useful to contextualise Achillobator.

First direct demonstration of quill knobs (ulnar papillae) in a nonavian dromaeosaurid, based on the ulna of Velociraptor mongoliensis (IGM 100/981). The feature is identical to that observed by DePalma et al. (2015) in Dakotaraptor, and is the basis for interpreting that giant Maastrichtian raptors had feathered forearms despite being far too large to fly.

Elements originally identified as Dakotaraptor furculae, later reidentified as turtle entoplastra. Anatomical context for the quill knobs described in the same skeleton.

Preserved claw from the Dakotaraptor holotype, near the forearm elements where the ulnar papillae were described.

Description of Hesperonychus elizabethae, the smallest known nonavian dromaeosaurid from North America, from the Dinosaur Park Formation (Campanian, Alberta). Extends the temporal range of Microraptorinae by 45 million years and sets the context for discussing the body-size diversity of Late Cretaceous North American dromaeosaurids, of which Dakotaraptor represents the opposite extreme.

Dakotaraptor size scale next to a human, contextualising the dromaeosaurid range between Hesperonychus and the giants.

Hell Creek Formation fauna, with small and large predators coexisting at the end of the Cretaceous.

Monographic review of Dromaeosauridae systematics and paravian phylogeny. Defines Eudromaeosauria as the clade comprising Dromaeosaurinae, Velociraptorinae and Saurornitholestinae, the taxonomic framework within which Dakotaraptor was originally placed in 2015. Matrix foundation for later analyses by Jasinski (2020) and Currie and Evans (2019).

Comparison of the largest known dromaeosaurids, including Utahraptor, Dakotaraptor, Achillobator and Austroraptor at scale.

Skeletal reconstruction of Dakotaraptor with preserved material highlighted in colour, the post-2012 systematic view.

Description of Acheroraptor temertyorum based on cranial material from the Hell Creek Formation of Montana. Previously considered the only Hell Creek dromaeosaurid, it was joined two years later by Dakotaraptor, demonstrating that the formation hosted at least two contemporaneous dromaeosaurids in very different size classes.

Holotype of Acheroraptor temertyorum (ROM 63777), small dromaeosaurid contemporary of Dakotaraptor in the Hell Creek Formation.

Size comparison of Acheroraptor, the second known Hell Creek dromaeosaurid, much smaller than Dakotaraptor.

Original description of Dakotaraptor steini based on holotype PBMNH.P.10.113.T, collected by Robert DePalma in 2005 in the upper part of the Hell Creek Formation, Harding County, South Dakota, less than 20 metres below the Cretaceous-Palaeogene boundary. Key points: body size of about 5.5 metres and mass of about 350 kg, tibia of 678 mm (the longest known in Dromaeosauridae), about 15 ulnar papillae indicating large forearm feathers, and three elements described as furculae. Phylogenetic analysis placed Dakotaraptor as a eudromaeosaur close to Dromaeosaurus. This description is the primary reference and must be cited together with the 2016 corrigendum and the Arbour et al. (2016) critique.

Life reconstruction of Dakotaraptor steini in hunting pose, with plumage over the ulna inferred from the ulnar papillae described by DePalma et al. (2015).

Lateral skeletal silhouette of Dakotaraptor with preserved elements and total length estimate of about 5 metres, as per the diagnostic figure of the original paper.

Crucial taxonomic correction. The authors demonstrate that the three 'furculae' attributed to Dakotaraptor steini in the original paper, including the element incorporated into the holotype, are not theropod bones but rather entoplastra (central plates of the plastron) of the trionychid turtle Axestemys splendida, common in the Hell Creek Formation. The identification is based on the flattened V/U shape, the absence of a distinct epicleidial head, the spongy microanatomy lacking laminated pattern, and the perfect congruence with Axestemys known from the same site. The paper compelled the original authors to issue the 2016 corrigendum.

Figure 1 of Arbour et al. (2016): 'furculae' attributed to the holotype and referred specimens of Dakotaraptor compared with the entoplastron of the trionychid turtle Axestemys splendida, showing morphological congruence.

Alternative life reconstruction of Dakotaraptor steini, updated after the Arbour et al. (2016) correction that removed the 'furculae' from the hypodigm.

Figure 1: Purported furculae for the holotype and referred specimens of Dakotaraptor steini compared with the entoplastron of the trionychid turtle Axestemys splendida ; anterior is up. (A–D), Axestemys splendida plastra in ventral view, showing the entoplastron in articulation with the other elements of the plastron. (A) and (B) ROM 1430; (C) and (D) TMP 2015.012.0011. NCSM 13170 trionychid entoplastron (referred to D. steini by DePalma et al., 2015 ) in (E) dorsal and (F) ventral views. (G) PB

Formal corrigendum by the original authors, published 2 December 2016. Accepts the Arbour et al. (2016) reidentification of the 'furculae' as entoplastra of Axestemys splendida. Excludes from the Dakotaraptor hypodigm the furcula previously attributed to the holotype and the referred specimens KUVP 152429 (exclusively turtle material) and NCSM 13170. The reduced hypodigm remains the basis for all subsequent discussion of the taxon.

Reconstructed bust of Dakotaraptor steini. As the skull is not preserved in the holotype, the head is inferred from closely related dromaeosaurids such as Dromaeosaurus and Acheroraptor.

Study sketch of Dakotaraptor for body proportions analysis, used as a basis for post-corrigendum reconstructions.

Description of the cranial anatomy of a nearly complete Saurornitholestes langstoni skeleton from the Dinosaur Park Formation (Campanian, Alberta). Includes a new phylogenetic analysis in which Dakotaraptor is retained within Eudromaeosauria, but at a node sensitive to removal of unverifiable characters, in line with later critiques by Cau.

Acheroraptor cranial material (NMNH), direct comparative for Saurornitholestes and for Hell Creek dromaeosaurids.

Cranial reconstruction of Acheroraptor temertyorum, key to interpreting the unpreserved head of Dakotaraptor.

Description of the troodontid Hesperornithoides miessleri and a large phylogenetic analysis of Paraves. In the Hartman and colleagues scheme, Dakotaraptor was recovered within Unenlagiidae (South American and Antarctic dromaeosaurids) rather than Eudromaeosauria. This topological instability is one of the main reasons why the position of Dakotaraptor remains contested.

Artistic reconstruction of Dakotaraptor steini in a Hell Creek setting, a visual reference for phylogenetic discussions placing it in Unenlagiidae or Eudromaeosauria.

Detail of Dakotaraptor reconstruction in motion, highlighting the proportionally long tibia discussed in Paraves analyses.

Figure 1: Geographic relationship of the Jimbo Quarry and the majority of the Morrison Formation, Late Jurassic, USA. Formation outcrop and map data based on paleobiodb.org. Download full-size image DOI: 10.7717/peerj.7247/fig-1

Figure 2: Condensed stratigraphic sections demonstrating the lateral variability near the Jimbo Quarry. “?” indicate loss of direct lateral correlation due to covered section. 1 = Jimbo Quarry; 2 = Lori locality; 3 = marginal wetland deposits. Download full-size image DOI: 10.7717/peerj.7247/fig-2

Figure 3: Reconstructed quarry map of WYDICE-DML-001. Association of skeletal elements assembled from 3D scans of specimen blocks prior to final mechanical preparation. Scale bar = 6 cm. Download full-size image DOI: 10.7717/peerj.7247/fig-3

Figure 4: Primary blocks of WYDICE-DML-001. “Left” (A) and “right” (B) sides of the blocks after final preparation (B). Scale bar = one cm. Images taken by Levi Shinkle, used with permission. Download full-size image DOI: 10.7717/peerj.7247/fig-4

Description of Dineobellator notohesperus, the first diagnostic dromaeosaurid from the latest Cretaceous of the southern United States (New Mexico). The phylogenetic analysis recovers Eudromaeosauria with Saurornitholestinae, Dromaeosaurinae and Velociraptorinae, with Dakotaraptor and Acheroraptor among the Maastrichtian terminals. This is the most cited published cladogram for placing Dakotaraptor in recent trees.

Figure 1 of Jasinski et al. (2020): holotype elements and diagnostic characters of Dineobellator notohesperus, another Maastrichtian dromaeosaurid.

Figure 3 of Jasinski et al. (2020): time-calibrated phylogeny of dromaeosaurids, with Dakotaraptor in Eudromaeosauria alongside Acheroraptor and Dineobellator.

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Osteological monograph of Dineobellator notohesperus with a broad revision of Eudromaeosauria. Discusses the body-size diversity of Maastrichtian dromaeosaurids, placing Dineobellator as a medium-sized eudromaeosaur and Dakotaraptor as the size extreme of the same clade. Reinforces, with more characters, that Dakotaraptor belongs within Eudromaeosauria, while acknowledging the topological instability highlighted by Hartman et al. (2019) and Cau.

Speculative scene of Dakotaraptors interacting with a juvenile Tyrannosaurus rex, contextualising the mesopredator niche of the largest known eudromaeosaur.

Alternative Hell Creek Formation fauna panel with Dakotaraptor in the context of terminal Cretaceous dinosaurs.

High-precision geochronological calibration (U-Pb CA-TIMS on zircon and 40Ar/39Ar) of the Hell Creek Formation in Montana. Places the K-Pg boundary at 66.043 ± 0.010 Ma and constrains deposition of the upper Hell Creek to less than 1 million years. Essential context to assert that Dakotaraptor lived in the final 100 to 400 thousand years of the Cretaceous, less than 20 m below the K-Pg boundary at the holotype site.

Hell Creek Formation landscape in Harding County, South Dakota, type locality of Dakotaraptor steini.

Sandstone and mudstone outcrops of the upper Hell Creek Formation, with the K-Pg boundary near the top.