Top 10 Open Access Fossil taxa of 2017: Borealopelta markmitchelli

The second ankylosaur in our Top 10 of 2017, Borealopelta markmitchelli, comes in at spot number 2! Weighing in at around 1300kg, about the same as an adult rhino, Borealopelta is one of the first dinosaurs to have been discovered with evidence of possible camouflage.

The heavyweight clubbing champion was described by Caleb M. Brown and colleagues in Current Biology. Ankylosaurs, often thought of as fleshy but heavily protected tanks, were typically adorned with tough body armour and spikes. Some even had giant tail clubs, thought to be used of to terrify the kneecaps of predators everywhere. Borealopelta did not have such a club, however, and seems to have had an entirely different mode of defence.

Borealopelta is a beautiful, gargoyle of a dinosaur. It looks like it would have been perfectly at home, adorning the walls of a gothic cathedral, with each spike preserved in exquisite detail. The fossil itself was discovered encased in a hard, thick siderite nodule, enabling this exceptional level of preservation.

Fact file:

• Etymology: borealis (Latin) for the northern locality of discovery, and pelta (Greek) referring to its bony armour. The species name is honour of Mark Mitchell, who spent more than 7000 hours over more than 5 years preparing the specimen!
• Discovered: Suncor Millennium Mine, Fort McMurray, Alberta, Canada.
• Discoverer: Shawn Funk, a construction worker, in 2011.
• When: Early Cretaceous (Aptian, around 110 million years ago), Clearwater Formation (Wabiskaw Member).
• What: Nodosaurid ankylosaur. Preserves the head, neck, and a partial skeleton.
• Size: 5.5 metres in length.
• Distinguishing features: Three-dimensional, statuesque preservation. Layers of organic residue fossilised alongside scales and horn sheaths capping the body armour.

It can’t see us if we don’t move

Now, remember again when Jurassic World made us all groan in despair when it transpired that Indominus rex could use camouflage as part of its genome was assembled using frog camo DNA? Yet again, such fantasies weren’t really needed. Dinosaurs were already pretty good at hiding themselves! Animals have been doing this for millions of years, as part of the typical predator-prey dynamic that we see in historical and modern ecosystems.

Borealopelta was no exception to this. Brown et al. used mass spectroscopy, a method of chemical identification, to discover the degraded traces of melanin in the preserved organic residues – a chemical fossil, if you like. What the research team found is that the largest spines spines covering the shoulder region were lightly coloured, and therefore likely used for some sort of display function. The presence of sulfur ions allowed the team to infer the actual colour of the pigment, which they think was a reddish-brown, based on the chemical structure of the preserved melanin.

Across the rest of the body, the team think they found evidence of what is called a countershading colour scheme – darker on top, lighter below. Countershading is a common camouflage strategy used by many modern land-dwelling herbivores, for example deer, rabbits, and mice. It works by helping the animal to appear all one colour, due to the shadow its own body casts – if it was all one colour, the lower side would be darker, therefore making it easier to spot. As such, it represents an important pattern driven by natural selection for helping herbivores to visually conceal themselves from animals with pointier teeth.

We know that by looking at modern mammals, in larger animals any form of countershading is typically lost as predation risk decreases. Therefore, the presence of this camouflage type tells us something important about Borealopelta. Despite having a tough armour casing, an adult Borealapelta was still under predation pressure enough for countershading to be necessary to help reduce risk.

Intriguingly, Borealopelta represents the largest recorded terrestrial animal to date with this sort of self-concealment, which tells us something was still keeping Borealopelta up at night.

Tankbusting theropods

So what would have been big enough to try and take on this armoured behemoth? Sadly, there is little direct evidence here, but the fossil record has left us some clues.

Large theropod footprints are known from rocks of similar ages and nearby localities in Alberta. South of where Borealopelta was found, big theropods are also known, including some of the largest like Acrocanthosaurus. The presence of camouflage in Borealopelta suggests that some of these large predators were actively hunting them, and terrifyingly effective at it.

This is quite different to modern mammalian ecosystems. No large cats, no matter how daring, ever try to take on a rhino or an elephant – the modern equivalent of a big ankylosaur. That just leads to bad things. So were theropods different? Bigger, badder, hungrier, triggering an evolutionary arms race between the big herbivorous dinosaurs and the theropods?

New research techniques and rarely discovered fossils are beginning to help shed new light on our understanding of dinosaur predator-prey dynamics. For now, we have to remain patient, but also imaginative and inquisitive!

Reference

Brown, C. M., Henderson, D. M., Vinther, J., Fletcher, I., Sistiaga, A., Herrera, J., & Summons, R. E. (2017). An Exceptionally Preserved Three-Dimensional Armored Dinosaur Reveals Insights into Coloration and Cretaceous Predator-Prey Dynamics. Current Biology, 27(16), 2514-2521. http://dx.doi.org/10.1016/j.cub.2017.06.071