|Borealopelta, photo borrowed from Wikimedia Commons.|
Genus and species: Borealopelta markmitchelli, "boreal" for northern in reference to the locality, "pelta" as "shield" for the scutes and other osteoderms (continuing "pelta" as a standard part of ankylosaurian names), with the species name honoring Mark Mitchell, who spent more than 7,000 hours preparing this delicate fossil. Unusually in this day and age, it has no cute nickname. Before it was described, it was often called "the Suncor [dinosaur, ankylosaur, nodosaur]," which I suppose kind of works, like an old sports nickname.
Citation: Brown, C. M., D. M. Henderson, J. Vinther, I. Fletcher, A. Sistiaga, J. Herrera, and R. E. Summons. 2017. An exceptionally preserved three-dimensional armored dinosaur reveals insights into coloration and Cretaceous predator-prey dynamics. Current Biology (advance online publication). http://dx.doi.org/10.1016/j.cub.2017.06.071.
Stratigraphy and Geography: Wabiskaw Member (lowest member) of the Clearwater Formation, Albian stage (Lower Cretaceous), Suncor Millennium Mine near Fort McMurray, Alberta, Canada. The description reports the stage as Aptian at one point, but later in the text refers to Borealopelta as Albian and depicts it as early Albian on the chrono-cladogram, so presumably there's a slip of the pen in there somewhere. I'm going with Albian because other recent publications use Albian.
Holotype: Royal Tyrrell Museum of Paleontology (RTMP) 2011.033.0001, a partial articulated nodosaurid including the skull through the sacrum, the right arm, part of the left arm, and a foot, with some chunks missing from the abdominal part of the torso.
Borealopelta has not been suffering from lack of media exposure. (Bony exposure is something else; although plenty of bones are there, we are in the unusual situation that we can't see many of them because there is something much more interesting in the way that we'd rather keep.) I fully expect there to be a high-quality toy version by 2019. Perhaps because the extraordinary preservation received a lot of attention earlier this year, for example in National Geographic, this time around the focus has been on potential coloration. We'll get to that in a bit; first I'd like to focus on some other aspects.
First off, to recap the history, Borealopelta was discovered March 21, 2011, at the Suncor Millennium Mine by heavy-equipment operator Shawn Funk. Athabasca oil sands are mined here from the McMurray Formation, which underlies the Clearwater Formation, so to get at the sands, the Clearwater Formation is removed. The Clearwater Formation is marine to estuarine in origin, and usually produces marine fossils, including some nice marine reptiles. The Borealopelta specimen, found flat on its back in lithified marine sediments, is presumably another in the grand tradition of deceased ankylosaurians doing the "bloat-and-float" into marine waters, like previous subjects Nodosaurus and Stegopelta. It has sometimes been described as a "mummy", although it is unusual in that the other "dinosaur mummies" dried out on land and were buried in terrestrial settings. Brown et al. (2017) mention that the specimen was preserved in a dense but brittle siderite concretion, siderite being an iron carbonate, or something like limestone with iron taking the place of calcium in the compound; the obvious conclusion is that the presence of the dinosaur "inspired" the formation of the surrounding concretion. The specimen broke into blocks during excavation, which wasn't ideal but does have a silver lining of making it easier to examine parts of the skeleton that would have been concealed. Preparation was also difficult because the stuff that was desired (the bones, scutes, skin impressions, etc.) was softer than the stuff that wasn't (the concretion).
Borealopelta is a nodosaurid, and the type individual would have been of fairly typical dimensions. Brown et al. (2017) estimate it at about 5.5 m (18 ft) long and 1,300 kg (2,900 lb), or about as long as the Sauropelta at the American Museum of Natural History and about as heavy as a rhino. The phylogenetic analysis found it to be closest to Pawpawsaurus from somewhat younger rocks in Texas, and indeed the skulls look broadly similar, although Pawpawsaurus's skull has more of a T-shape when viewed from above and Borealopelta's skull widens less abruptly. Scutes over the eyes lead into short horn-like projections behind the eyes, not as big as the horns of ankylosaurids, but big bony "eyebrows" do give you a stern appearance, so there's that. As I mentioned, most of the bones aren't visible, but the specimen is fractured into several blocks, so some things can be seen in cross section or viewed from beneath if the block is turned over. Aside from the hard parts of the skeleton, there are also some small non-dinosaurian objects lying against the underside interior that may represent gut contents.
Although Borealopelta has held many of the secrets of its bones, it certainly doesn't do the same for the armor, or osteoderms. Behind the head are three cervical half-rings, each with six osteoderms (three on each side). These osteoderms are keeled, with the keels becoming more prominent as you move away from the midline and closer to the body, so that the biggest keels are on the lowest osteoderms of the third ring. The appearance of these pieces is of sharp, pointed plates. A fourth similar pointed plate without a corresponding ring sneaks in just posterior to the cervical half-rings on the lateral margin of shoulders, followed by the big scapular spine. Sauropelta has similar big shoulder spines, although in this dinosaur the spines are restored as equivalent to the middle osteoderm position of a cervical ring, whereas in Borealopelta the spines are located in the lower, most lateral position. (If you're looking at this picture, take the big spike and shift it to the lower row of spines to go from Sauropelta configuration to Borealopelta configuration.) The osteoderms shift into narrow transverse (side to side) bands of osteoderms, positioned between vertebrae. Including the band with the big shoulder spine, there are a dozen, although not all are complete; a couple are only found near the midline and peter out going laterally. Aside from the big spines, these osteoderms are not as strongly keeled, and can be described as having pointed ridges. The individual osteoderms meet on their lateral boundaries, but the bands are separated by thin rows of smaller scales. In fact, this is not unlike the configuration described for Nodosaurus, so if you want to see a "corduroy armadillo" as it actually existed, here's one example. By the time we get to the vicinity of the hips, the osteoderms shift into a pavement of hexagonal to circular pieces that contact multiple other osteoderms, without a strict layout of bands (not unlike the pelvic armor of Nodosaurus or Stegopelta). Unfortunately, the upper layer of bone and preserved soft tissue has been clipped off in this part of the body, so we only see the bases of the scutes. There are also a few scutes on the arms. (Incidentally, what look like little points running down the midline are actually the tops of the neural spines sticking through.)
One thing Borealopelta reinforces is that there is no one-armor-plan-fits-all for ankylosaurians. Figure 3 in Brown et al. (2017) illustrates this quite nicely by depicting five ankylosaurians with articulated armor: Kunbarrasaurus, Scolosaurus, Sauropelta, Borealopelta, and Edmontonia. Kunbarrasaurus has relatively modest osteoderms; Scolosaurus has a moderate number of large osteoderms with much space between them, set in broad transverse bands; Sauropelta has numerous medium-sized osteoderms, more tightly packed than in Scolosaurus but still not touching, arranged in lazy bands; Borealopelta has numerous medium-sized osteoderms in well-defined bands up to its hips; and Edmontonia has the relatively largest osteoderms, but only a few, confined mostly to three cervical half-rings and along the lateral margin of the body, with lots of small "filler". The only one from this group that really looks much like the classic ankylosaur grid is, oddly enough, the most recently described taxon, Borealopelta. (A skeleton long considered Saichania but now assigned to Pinacosaurus suggests that it might have been even closer to the grid ideal, and of course we don't know yet how Zuul will turn out.)
|Sauropelta, loosey-goosey with its bands. That's just how we do things in the Cloverly, man!|
|Edmontonia, whose three rows of large scutes somehow fused with Barnum Brown's cobblestone Ankylosaurus to "inspire" every bloody restoration of "Palaeoscincus" in existence. How did that happen? Am I the only one to have ever noticed that Edmontonia has practically no large or medium scutes past its shoulders? Why do people keep restoring the thing with large or medium scutes all over? Did they all fall off or something, with no one bothering to tell me?|
Now, the other stuff. Borealopelta is unusual for preserving remains of soft tissues, including a carbonaceous layer that has replaced the original keratinous sheaths of osteoderms. The sheaths show some exaggeration of the heights of the keels, with some furrowing and striations. The layer also includes chemicals that may have come from the breakdown of a red pigment. Per Brown et al., the chemicals are limited to the dinosaur, and to the upper surfaces of the dinosaur. They interpret the distribution as evidence that the dinosaur had a rusty coloration on its back and head, but was lighter in color underneath, This could in turn be countershading, a kind of camouflage. Countershading would be significant because Borealopelta is much larger than any countershaded animal today, and the implication is that even though Borealopelta was a rhino-sized mobile carpet of small to large osteoderms, it still needed camouflage against its predators, large predatory dinosaurs. This is a substantial amount of interpretation, and the conclusions are not universally supported. For me, it boils down to three major questions. The first two can be addressed by more study or more specimens, but the third is kind of hard to get at. First, is the red pigment residue definitely from the dinosaur, or could it be environmental? Brown et al. do find it limited to the dinosaur, which is helpful, but there could be microbial or decay processes to account for. Second, if the pigment is dinosaurian, is it definitely confined to the upper part of the body? Annoyingly, we run out of dinosaur without getting a chance to thoroughly survey the underside, although it is interesting to note that the big shoulder spines were seemingly unpigmented. Third, if the pigment is dinosaurian and confined to the upper part of the animal, is it necessarily countershading, or could it be attributed to other causes? For example, Borealopelta's ancestors may have been reddish on top and the coloration just stuck around because it did not hinder survival, or perhaps Borealopelta found red over light an attractive combination. This is all a long-winded way of saying that countershaded Borealopelta is plausible, but is also speculative. With that, I have once again turned into that guy who points out things, making this a perfect spot to shuffle off for another week or two and leave Borealopelta to bask in its well-earned glory.
Brown, C. M., D. M. Henderson, J. Vinther, I. Fletcher, A. Sistiaga, J. Herrera, and R. E. Summons. 2017. An exceptionally preserved three-dimensional armored dinosaur reveals insights into coloration and Cretaceous predator-prey dynamics. Current Biology (advance online publication). http://dx.doi.org/10.1016/j.cub.2017.06.071.