Borealopelta markmitchelli found its way back into the sunlight in 2017, millions of years after it had died. This armored dinosaur is so magnificently preserved that we can see what it looked like in life. Almost the entire animal—the skin, the armor that coats its skin, the spikes along its side, most of its body and feet, even its face—survived fossilization. It is, according to Dr. Donald Henderson, curator of dinosaurs at the Royal Tyrrell Museum, a one-in-a-billion find.
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Borealopelta is a nodosaur, a type of four-legged ankylosaur with a straight tail rather than a tail club. Its finding in 2011 in an ancient marine environment was a surprise, as the animal was terrestrial.
A land-based megaherbivore preserved in an ancient seabed is not as uncommon as one might think. A number of other ankylosaurs have been preserved in this manner, albeit not as well as Borealopelta. Scientists suspect its carcass may have been carried from a river to the sea in a flooding event; it may have bobbed at the surface upside-down for a few days before sinking into the ocean depths.
It would have been kept at the surface by what’s referred to as "bloat-and-float," as the buildup of postmortem gasses would keep it buoyant. Modeling done by Henderson indicates its heavy armor would have rolled it onto its back, a position he suspects may have prevented ocean predators from scavenging its carcass.
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