(B and C) Skull in dorsal (B) and left lateral (C) views.

Schematic Line Drawing of TMP 2011.033.0001, the Holotype of Borealopelta markmitchelli, Illustrating Preservation of the Different Tissue Types

Figure 2 Schematic Line Drawing of TMP 2011.033.0001, the Holotype of Borealopelta markmitchelli, Illustrating Preservation of the Different Tissue Types

Additional characters of the Ceratopsidae, with notice of new Cretaceous dinosaurs.

The generic name Borealopelta is derived from “borealis” (Latin, “northern”) and “pelta” (Greek, “shield”), in reference to the northern locality and the preserved epidermal scales and dermal osteoderms. The specific epithet markmitchelli honors Mark Mitchell for his more than 7,000 hours of patient and skilled preparation of the holotype.

The holotype is Royal Tyrrell Museum of Palaeontology (TMP) 2011.033.0001: an articulated specimen preserving the head, neck, most of the trunk and sacrum, a complete right and a partial left forelimb and manus, partial pes ( Figure 1 ). In situ osteoderms and nearly complete soft tissue integument are preserved across dorsal and lateral surfaces of the axial skeleton, posterodorsal surface of forelimbs, and plantar surfaces of a manus and a pes. Specimen is preserved in multiple large blocks, including slabs and counter-slabs in the sacral region.

The new taxon can be further differentiated from Pawpawsaurus based on: dermal plate in frontonasal region (central dermal plates) flat [22:1]; absence of ciliary osteoderm [41:0]. Can be further differentiated from Sauropelta based on: parietals flat to slightly convex [51:0]; cervical half ring has 4–6 osteoderms only [164:1]; medial cervical osteoderms subequal, hexagonal, and bear prominent median ridge with posterior margin projecting beyond the basal footprint.

A nodosaurid ankylosaur characterized by the following autapomorphies ( ∗ ) and suite of characters [character/state]: cranial: dorsal skull ornamentation expressed as a large hexagonal dermal plate in frontoparietal region [52:1] and multiple (>20) small dermal plates in frontonasal region [21:2] ∗ ; external nares excluded from view dorsally (shared with Pawpawsaurus) [16:1]; supraorbital ornamentation forming sharp lateral rim dorsal to orbits (shared with Gargoyleosaurus and Kunbarrasaurus) [38:2]; jugal (suborbital) horn triangular with pointed apex (shared with Gastonia, Gargoyleosaurus, and Polocanthus) [47:2]; jugal (suborbital) horn base longer than orbit length [49:2] ∗ ; osteoderms: cervical and thoracic osteoderms form continuous (abutting) transverse rows completely separated by continuous transverse rows of polygonal basement scales; parascapular spine is the largest osteoderm, recurved, and projects posterolaterally and horizontally (potentially shared with Sauropelta); osteoderm count for transverse rows: cervicals: C1-3, C2-3, C3-3, transition: TR-2, thoracic: T1-6 ∗ ; third and sixth transverse thoracic osteoderm rows expressed medially but pinch out laterally ∗ .

For a morphological description of the head, osteoderms, epiosteodermal scales (osteoderm/spine horn sheaths), and epidermal basement scales, see the Supplemental Morphological Description

To determine the phylogenetic position of Borealopelta markmitchelli, we scored it into the morphological character/taxon matrix of Arbour et al. [] (see also STAR Methods for full phylogenetic methods, Data S1 for data, and Supplemental Phylogenetic Results ). The resulting strict consensus tree positions Borealopelta in a clade with other Albian-aged nodosaurids Pawpawsaurus campbelli and Europelta carbonensis, with the Santonian-aged Hungarosaurus tormai as a sister taxon ( Figures 3 and S2 ).

Bottom: time-calibrated strict consensus tree illustrating position of Borealopelta markmitchelli within Ankylosauria scaled to Jurassic and Cretaceous stages. Top: line drawings of representative well-preserved ankylosaur specimens with in situ armor and/or skin. Scale bars, 1 m. See also Figure S2 and Data S1

Hence, we argue that the integument was pigmented reddish-brown by pheomelanin-rich melanin. This may also explain the lack of melanosome preservation, as pheomelanin-rich melanosomes have been shown to be less stable in heat/pressure autoclave experiments [], as well as in enzymatic extraction procedures [].

The dorsal integument is well preserved as an organic film derived from the keratin sheaths over the osteoderms, integumentary scales, and the epicuticle of hinge regions between scales. The distribution of the film correlates well to the expected distribution of melanin, a pigment that has been found to preserve in a number of vertebrate integumentary structures []. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDAX) analyses reveal that the organic material is present as solid to finely granular material with desiccation cracks scattered in a matrix largely composed of siderite cement ( Figure S3 ii; see also STAR Methods and Supplemental Observations Under the Electron Microscope ). No apparent melanosomes are preserved. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) reveals spectra of negative secondary ions that have an overall relative secondary ion intensity similar to previously studies of fossil melanins ( Figure S3 iiiA; see also STAR Methods and Supplemental Results from the TOF SIMS ). Principal-component analyses demonstrate that there is a significant contribution of sulfur-bearing secondary ions to the organic material in TMP 2011.033.0001, which spread the nodosaur samples distinctly from other fossil melanin samples ( Figure S3 iiiB). These secondary ions have been identified for sulfur-bearing pheomelanin (benzothiazole) previously []. Pyrolysis-gas chromatography-mass spectroscopy (py-GC-MS) afforded pyrolysates with assemblages of small nitrogen-, oxygen- and sulfur-containing heterocyclic and aromatic molecules characteristic of eumelanin (e.g., pyrrole, indole, N-methylpyrrole, and methylphenol). Of special note is the presence of significant amounts of benzothiazole ( Figure S3 iv), which is diagnostic for pheomelanin. Although sulfur may be incorporated into melanin secondarily [] to yield thiophenes, which are also observed and could similarly be derived from pheomelanin, this process is not known to give rise to benzothiazoles [] (see also STAR Methods and Supplemental Results of Pyrolysis GC-MS ).

The specimen was discovered in the Suncor Millennium Mine (open pit, oil sands) in northeastern Alberta, Canada during overburden removal and was subsequently collected by staff of the Royal Tyrrell Museum and Suncor. The hosting rock is the Albian-aged Wabiskaw Member of the Clearwater Formation overlying the bitumen-rich McMurray Formation. Several plesiosaurs and ichthyosaurs have been recovered from the Wabiskaw previously [], but never a dinosaur. This member records a lower shoreface or proximal offshore marine environment []. The carcass arrived at the seabed on its back and with sufficient force to impact and deform the immediately underlying sedimentary layers. Despite the trace fossils left by burrowing animals in the hosting sediments, implying at least a partially oxygenated environment, the specimen lacks any evidence of scavenging. When found, the fossil was completely encased in a very dense and strong but brittle siderite concretion that ranged in thickness around the carcass from 20 cm on the upper side to 40 cm on the lower, seabed side. Broken surfaces through the concretion reveal sedimentary features above the fossil that allow for inferring the natural collapse of the body after burial and before consolidation ( Figure S4 i). About 15 cm of sediment was laid down prior to release of internal body fluids and collapse ( Figure S4 i), evidenced from a collapse/fluid escape structure in the sacral region ( Figure S4 i). The body cavity was injected with almost homogeneous sand, with no apparent sedimentary features. Formation of the concretion must have commenced shortly after the carcass arrived at the seabed, preventing any scavenging and allowing all of the scales and osteoderms to retain their original configurations and morphology, with minimal dorsoventral compression.

A small ichthyosaur from the Clearwater Formation (Alberta, Canada) and a discussion of the taxonomic utility of the pectoral girdle.

The countershading transition can be traced from cross-sectional views of the sacrum ( Figure S4 ii) and neck. The organic film terminates a little beyond the ventralmost lateral osteoderms. Projecting the melanin distribution to a retrodeformed body outline suggests a transition from highly pigmented to less pigmented integument on the lateral flank ( Figure S4 ii; see also Supplemental Discussion on the Chemical Preservation of Melanin ).

The contrast in fossilization between the ventral and dorsal surface provides a further case for melanin preservation, as this transition is best interpreted as countershading. Other epidermal structural molecules such as keratin [] or collagen [] would have had a very similar distribution in the epidermis on both top and bottom surfaces, and no unique protein markers (amides, succinimides, diketopiperazines) were recovered in the py-GC-MS data to the exclusion of markers overlapping with melanins [] ( Figure S3 iv).

A new Chinese specimen indicates that ‘protofeathers’ in the Early Cretaceous theropod dinosaur Sinosauropteryx are degraded collagen fibres.

Molecular evidence of keratin and melanosomes in feathers of the Early Cretaceous bird Eoconfuciusornis.

The largest of the horn sheaths, the parascapular spines, are distinct from the remaining sheaths and epiosteodermal scales in being both lighter colored in visible light and slightly fluorescing under UV light ( Figures S4 iiiE–H). This is most simply interpreted as having lower concentrations of melanin incorporated into the horn sheath and likely reflects a distinct lighter color of these spines in life.

Several cervical osteoderm sheaths show underlying longitudinal ridges that appear to have been unpigmented, as they are dramatically lighter in visible light and exhibit strong fluorescence in UV ( Figures S4 iiiA–D). These unpigmented longitudinal striae lie in the same plane as the inferred direction of growth of the sheath (i.e., parallel to the stratum germinativum) and may represent some preserved artifact of appositional growth of these epiosteodermal scales, as seen in living crocodilians [].

A comparison of the epidermis and pigment cells of the crocodile with those in two lizard species.

Hardened keratinous tissues such as claws, scales, and feathers have reinforcing calcium phosphate deposits [], which often preserve well in fossil tissues and can be identified using fluorescence imaging []. However, the keratinized tissues in TMP 2011.033.0001 are heavily pigmented, which masks calcium phosphate fluorescence []. These keratinous sheaths are inert (non-fluorescing and non-reflecting) under UV light, with two major exceptions.

Implications for Paleobiology

The discovery of a three-dimensionally preserved ankylosaurian provides new evidence for understanding the anatomy, soft tissue outline, and arrangement of dermal armor in thyreophoran dinosaurs.

30 Arbour V.M.

Burns M.E.

Bell P.R.

Currie P.J. Epidermal and dermal integumentary structures of ankylosaurian dinosaurs. 31 Vickaryous M.K.

Sire J.-Y. The integumentary skeleton of tetrapods: origin, evolution, and development. 32 Vickaryous M.K.

Hall B.K. Development of the dermal skeleton in Alligator mississippiensis (Archosauria, Crocodylia) with comments on the homology of osteoderms. The preservation of the nearly complete integument, along with a suite of in situ pre-caudal osteoderms and their horn sheaths, allows multiple novel inferences regarding the epidermis of the ancient animal. Across all preserved regions, the epidermal covering (“epiosteodermal scales” sensu []) associated with the osteoderms is highly congruous in having a 1:1 correlation in count and basal shape between epidermal scale and underlying osteoderm. This epidermal scale/osteoderm association is most analogous, and potentially deeply homologous, to that observed in extant crocodilians [].

The epidermal coverings for the thoracic and sacral osteoderms are best interpreted as single, sub-centimeter-thick, keratinized scales (scutes) that slightly exaggerate the keels and spines. In contrast, the epidermal components of the spine-like cervical, transitional, and parascapular osteoderms are sheath-like, being thick, pointed, and extending significantly beyond the bony core, substantially increasing the length of the spine. The macroscopic structure and incorporation of hardening calcium phosphate into the spine-like epiosteodermal scales is broadly similar to the keratinized horn sheaths in Bovidae.

9 Matthews W. A super-dreadnaught of the animal world: The armored dinosaur Palaeoscincus. 33 Romer A.S. Vertebrate Paleontology. 34 Carpenter K. Skeletal and dermal armor reconstruction of Euoplocephalus tutus (Ornithischia: Ankylosauridae) from the Late Cretaceous Oldman Formation of Alberta. 35 Blows W.T. Dermal armour of the polacanthine dinosaurs. 36 Hayashi S.

Carpenter K.

Scheyer T.M.

Watabe M.

Suzuki D. Function and Evolution of Ankylosaur Dermal Armor. 37 Padian K.

Horner J.R. The evolution of ‘bizarre structures’ in dinosaurs: biomechanics, sexual selection, social selection or species recognition?. Historically, the function of osteoderms within Ankylosauria has largely been discussed within the context of anti-predator defense, with the term “armor” used ubiquitously to describe these dermal components of the skeleton []. More recently, other potential functions including thermoregulation [] and intraspecific combat and display [] have been proposed, often to augment the seemingly default function as defensive armor. Preserved evidence of countershading suggests that the predation pressure on Borealopelta, even at large adult size, was strong enough to select for camouflage from visual predators (see also Supplemental Discussion of Countershading and Body Mass and Data S2 ). This offers support to the idea that many of the osteoderms functioned in a defensive role, even in adults of large species. In contrast, the distinct pigmentation and enlarged keratinous sheath of the parascapular spine suggest that this particular spine may have functioned more predominantly in display. When combined with data indicating highly species-specific morphology of the parascapular spines of other taxa, this suggests that the extensive elaboration of these spines may be attributable to sociosexual display.

38 Carpenter K. Skeletal reconstruction and life restoration of Sauropelta (Ankylosauria: Nodosauridae) from the Cretaceous of North America. 39 Benson R.B.

Campione N.E.

Carrano M.T.

Mannion P.D.

Sullivan C.

Upchurch P.

Evans D.C. Rates of dinosaur body mass evolution indicate 170 million years of sustained ecological innovation on the avian stem lineage. 40 Paul G. Dinosaur models: the good, the bad, and using them to estimate the mass of dinosaurs. 41 Owen-Smith N.

Mills M.G.L. Predator-prey size relationships in an African large-mammal food web. 42 Hayward M.W.

Kerley G.I.H. Prey preferences of the lion (Panthera leo). 42 Hayward M.W.

Kerley G.I.H. Prey preferences of the lion (Panthera leo). 43 Kamilar J.M. Interspecific variation in primate countershading: effects of activity pattern, body mass, and phylogeny. 44 Arnold E.N. Indian Ocean giant tortoises: their systematics and island adaptations. 45 Jaffe A.L.

Slater G.J.

Alfaro M.E. The evolution of island gigantism and body size variation in tortoises and turtles. Figure 4 Chart Illustrating the Loss of Countershading as Body Mass Increases in Terrestrial Mammal Herbivores Show full caption Chart includes pooled data for artiodactyls, perissodactyls, and proboscideans divided into body-mass bins, showing relative proportion of species that exhibit countershading. The diagonally hatched area represents the mass above which significant predation of adults does not occur. Animals illustrated above chart are representative taxa within each mass bin; species names in italics at top indicate body masses of the largest carnivores. See also Data S2 The presence of countershading in a large, heavily armored herbivorous dinosaur provides a unique insight into the predator-prey dynamic of the Cretaceous Period. With an estimated length of 5.5 m and a conservative body mass estimate of ∼1,300 kg (similar to estimates for other nodosaurids, i.e., Sauropelta [], but also see higher estimates []), Borealopelta is much larger than any modern terrestrial animal exhibiting countershading. Modern mammalian predators do not represent a significant predatory risk to the largest mammalian herbivores (>1,000 kg) [], and herbivores above this size threshold generally do not exhibit countershading or other types of camouflage ( Figure 4 ). Additionally, herbivores below this threshold that also possess defensive weapons (e.g., horn, quills) experience lower predation than those that do not []. Similarly, large primates are less likely to be countershaded due to lower predation risk []. A parallel pattern is also seen in Testudinidae, where island forms, which are free of significant adult predation, consistently become large [] and lose the cryptic pattern seen in smaller relatives.