Evolution & The Club Tail . . .

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Dinosaurs dominated the earth for more than 130 million years until 65 million years ago. As a kid, not quite so long ago, I made my mom read the same dinosaur book, over and over. The one with a Godzilla-like T. Rex and the now-defunct brontosaurus. And they were all the color of elephants. It's mind-blowing what we've learned since then. I now know that my favorite book showed some of the last of the non-bird dinosaurs. Many of these were found in the North American northwest, which  has changed a bit since.

Alberta or Utah or New Mexico would have been sort of more tropical and humid, so similar to today's Louisiana; kind of humid and tropical with hurricanes and big storms that would come in every now and then. There would have been cypress trees and lots of lush swampy areas.

That's postdoctoral researcher Dr. Victoria Arbour, of the Paleontology Research Lab at the North Carolina Museum of Natural Sciences. Her specialty is the diverse group of dinosaurs known as ankylosaurs.

Ankylosaurs are plant-eating, four-legged dinosaurs. They're very heavily armored. As a group, they're all characterized by this sort of, like, wide, flat body plan with lots of spikes and bumps. But all of those spikes are made out of special bones in the skin called osteoderms. These were structures that were really easy to shape into lots of different sizes and shapes and textures. In different ankylosaurs, those take on all sorts of different interesting shapes. So, some of them have big spikes over their shoulders, some of them have big spikes over their hips, and one group of them, called ankylosaurid ankylosaurs, evolved a tail-club; so it's this very stiff tail with a big knob of bone at the end.

The tail-club appears in the fossil record 66 million years ago, but early Ankylosaurids, over 145 million years ago, sported flexible tails. Dr. Arbour wanted to know if the rigid tail and the club evolved together or if one came before the other.

The tail would have been, like, really rigid and quite long, so about the size of a baseball bat, really. And a baseball bat is still a really good weapon, so obviously there was some pressure that was selected toward having the stiff tail even without that big, crushing knob of bone at the end. So, what I ended up finding out was that ankylosaurs evolved kind of a stiff tail before they got sort of the full true tail-club that we usually associate with those dinosaurs.

Over time, the vertebrae in the tail fused, or locked, together. Without this adaptation, supporting and swinging the club, like a wrecking ball, could not have happened.

The Aynkylosaurids that have a tail- club are very sort of low-slung, wide dinosaurs with relatively short legs. They lived about 66 million years ago, alongside animals like tyrannosaurs and triceratops; very famous, familiar dinosaurs. So, I think that's why the armor and these tail-clubs were important to ankylosaurs, because, if you're a prey animal, there are a couple of different ways that you can avoid being eaten. You can be fast and run away. You can hide. Ankylosaurs aren't really good at either of those things because they're too big, and they're too slow, and can be unpalatable to a carnivore. I feel like, with all of that armor, they probably were just a bit too much work, and a bit too crunchy.

To try and understand an animal, extinct for 65 million years, we can look at clues from our own environment.

It's quite possible that they were using their tails against each other to battle for mates, or for territory, or to defend their food resources; all of the different things that animals will fight over in modern days.

Dr. Arbour has traced the migration of tail-club adapted Ankylosaurus.

The tail-club first shows up in China and Mongolia, and all of the ones in North America moved in via Alaska and Russia. Those two continents were connected through that way. And then they diversified into their own group of North American, tail-clubbed ankylosaurs. But, it's possible that they never actually made it to eastern North America because there was a huge body of water called the Western Interior Seaway that divided North America. So, it's quite possible that we wouldn't have found them, even if we did have rocks of the right age here.

Additional Resources:
https://news.ncsu.edu/2015/08/ankylosaur-tail/
http://www.iflscience.com/plants-and-animals/how-ankylosaur-got-its-tail

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