Mystery of upside-down dinosaurs resolved

Remains of ankylosaurs, a popular family of heavily armoured dinosaurs, occur in sedimentary sequences that range in age from early Jurassic to the close of the Cretaceous. Their defences seem almost impregnable, being constructed of thick, fused scales, often bearing formidable spines, which covered them completely. They bore a crude resemblance to modern armadillos apart from the fact that they were unable to roll-up defensively. In some species the rigid tail bears a large, knob of scale tissue. At up to 3 m long, were the tail to be swung it would have packed bone-cracking momentum. Interestingly, to a poorly sighted predator the club may have been mistaken for the animal’s head at the end of a long neck, so perhaps it lured a potential assailant within range of its devastating power. The largest ankylosaur, from the Cretaceous of western Canada, was the size of a small bus, up to 8m long, 1.5 m wide, standing 1.7 m high and weighing in at around 5 to 8 t. Such dimensions would have made it almost impossible to be bitten, even by the largest predatory dinosaurs, and difficult to turn over. Their teeth show that ankylosaurs were herbivorous, and their somewhat bulbous bodies almost certainly contained a massive digestion system.

Polski: JuraPark Bałtów - Park Dinozaurów - An...

Recfonstruction of Ankylosaurus at the JuraPark Bałtówin Poland (credit: Wikipedia)

The mystery lies in the fact that most ankylosaur fossils are found lying on their backs. Early dinosaur aficionados suggested a tendency for the lumbering beasts to tumble down slopes and become stranded on their backs, so to die miserably. Such clumsiness is hardly a positive characteristic of evolutionary fitness for such a long-lived group, and, besides, the sedimentary formations in which they are found indicate very gentle slopes. So, were they flipped by dextrous predators, as imagined in some early films purporting to look to the distant past? Probably not, for most well preserved fossils show no sign of bites or gnawing. From a study of 36 late-Cretaceous ankylosaurs from Alberta four Canadian and US palaeontologists (Mallon, J.C. et al. 2018. A “bloat-and-float” taphonomic model best explains the upside-down preservation of ankylosaurs. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 497, p. 117-127; doi:10.1016/j.palaeo.2018.02.010 support the idea of their carcases or even living animals having been picked up by flood waters when their high centre of gravity would have flipped them upside down. Bloating through decay might then allow them to be transported large distances. Unsurprisingly, their conclusions rest on model simulations.

Advertisements

One response to “Mystery of upside-down dinosaurs resolved

  1. Don’t tell the Genesis Flood people about this!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

w

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.