Saturday, August 5, 2017

[Paleontology • 2017] Borealopelta markmitchelli • An Exceptionally Preserved Three-Dimensional Armored Dinosaur Reveals Insights into Coloration and Cretaceous Predator-Prey Dynamics

Borealopelta markmitchelli 
Brown, Henderson, Vinther, Fletcher, Sistiaga, Herrera & Summons, 2017

• A new armored dinosaur is described based on an exceptionally preserved specimen
• Abundant in situ osteoderms with keratinous sheaths and scales are preserved
• Reddish-brown coloration and crypsis in the form of countershading are indicated
• Crypsis indicates strong predation pressure on this large, heavily armored dinosaur

Predator-prey dynamics are an important evolutionary driver of escalating predation mode and efficiency, and commensurate responses of prey. Among these strategies, camouflage is important for visual concealment, with countershading the most universally observed. Extant terrestrial herbivores free of significant predation pressure, due to large size or isolation, do not exhibit countershading. Modern predator-prey dynamics may not be directly applicable to those of the Mesozoic due to the dominance of very large, visually oriented theropod dinosaurs. Despite thyreophoran dinosaurs’ possessing extensive dermal armor, some of the most extreme examples of anti-predator structures, little direct evidence of predation on these and other dinosaur megaherbivores has been documented. Here we describe a new, exquisitely three-dimensionally preserved nodosaurid ankylosaur, Borealopelta markmitchelli gen. et sp. nov., from the Early Cretaceous of Alberta, which preserves integumentary structures as organic layers, including continuous fields of epidermal scales and intact horn sheaths capping the body armor. We identify melanin in the organic residues through mass spectroscopic analyses and observe lighter pigmentation of the large parascapular spines, consistent with display, and a pattern of countershading across the body. With an estimated body mass exceeding 1,300 kg, B. markmitchelli was much larger than modern terrestrial mammals that either are countershaded or experience significant predation pressure as adults. Presence of countershading suggests predation pressure strong enough to select for concealment in this megaherbivore despite possession of massive dorsal and lateral armor, illustrating a significant dichotomy between Mesozoic predator-prey dynamics and those of modern terrestrial systems.

 Systematic Paleontology

Dinosauria Owen, 1842  
Ornithischia Seeley, 1888  

Ankylosauria Osborn, 1923  
Nodosauridae Marsh, 1890  

Borealopelta markmitchelli gen. et sp. nov. 

Etymology: 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.

An illustration of Borealopelta markmitchelli. The study suggests that it displayed a camouflage effect known as counter-shading.
 Illustration: Julius Csotonyi/Courtesy of the Royal Tyrrell Museum of Palaeontology, Canada. 

 Illustration: Robert Nicholls 

 Illustration: Davide Bonadonna

Figure 1. Photographs of the Holotype of Borealopelta markmitchelli, TMP 2011.033.0001 Top: anterodorsolateral view; bottom: anterodorsal view. Scale bar, 10 cm. 

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.

Locality and Horizon: Suncor Millennium Mine, Fort McMurray, Alberta, Canada. Wabiskaw Member, Clearwater Formation, Aptian stage. Detailed locality data are available at Royal Tyrrell Museum of Palaeontology.

Diagnosis: 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 and multiple (>20) small dermal plates in frontonasal region∗; 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 GastoniaGargoyleosaurus, and Polocanthus); jugal (suborbital) horn base longer than orbit length∗; 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∗.

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

Figure 2: Schematic Line Drawing of TMP 2011.033.0001, the Holotype of Borealopelta markmitchelli, Illustrating Preservation of the Different Tissue Types (A) Schematic of complete specimen in dorsal view. (B and C) Skull in dorsal (B) and left lateral (C) views. (D) Close-up view of the neck, illustrating alternating cervical osteoderm bands (and preserved keratinous sheaths) and polygonal scales. (E) Close-up view of flank illustrating lateral thoracic osteoderms (with keratinous coverings) and polygonal scales. (F) Close-up view of sacral shield counterpart illustrating osteoderms and scales. (G) Close-up view of antebrachium including osteoderms and keratinous coverings. (D’–G’) Interpretive line drawings of the corresponding panels (D)–(G). Scale bars in (B)–(G), 10 cm.

Figure 3Time-Calibrated Strict Consensus Tree Showing Position of Borealopelta markmitchelli within Ankylosauria, with Representative Well-Preserved Ankylosaurs Shown Above 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.
(A) Kunbarrasaurus, QM F18101. (B) Euoplocephalus, NHMUK 5161. (C) Sauropelta, AMNH 3035 and 3036 composite. (D) Borealopelta, TMP 2011.033.0001 (this study). (E) Edmontonia, AMNH 5665.

Figure 4:  Chart Illustrating the Loss of Countershading as Body Mass Increases in Terrestrial Mammal Herbivores 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.

The Making of a Most Extraordinary Fossil

The Making of a Most Extraordinary Fossil

Caleb M. Brown, Donald M. Henderson, Jakob Vinther, Ian Fletcher, Ainara Sistiaga, Jorsua Herrera and Roger E. Summons. 2017. An Exceptionally Preserved Three-Dimensional Armored Dinosaur Reveals Insights into Coloration and Cretaceous Predator-Prey Dynamics. Current Biology. DOI: 10.1016/j.cub.2017.06.071

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