The Mystery of the Upside-down Ankylosaurs

Many ankylosaurs, the armored dinosaurs, have been found upside down when they’ve been discovered, especially when they’ve been found in river beds or marine sediments. Researchers have had different ideas as to why this might be the case.

An anklosaur. From here.

A new [study] tested several hypotheses to figure out which is the most likely to explain the upside-down fossils. The authors tested 4 hypotheses. The first was the ankylosaurs were just clumsy and would fall down hills and land upside down. This idea is hard to test, and seems unlikely, so the authors moved on.

Turtles can actually flip themselves over when they’re on their backs (though it’s nice to have help from a friend). Ankylosaurs would not have been so successful if they could not do the same.

The second hypothesis was that predators would flip the ankylosaurs over and eat them, leaving the remains to be preserved upside-down. The authors examined 36 ankylosaur specimens and only found 1 with any tooth marks on it. They rejected this hypothesis, since it did not seem that the upside-down ankylosaurs were eaten.

A picture of an ankylosaur being threatened by a T. rex. By Eldar Zakirov.

The third hypothesis was that dead ankylosaurs would wash into a river. Once there, it would start to decompose. The process of decomposition releases a lot of gas, which gets trapped inside the body of the animal, causing it to float. The armor on the back of an ankylosaur would weight more than the gas-filled belly, and the whole dinosaur would flip upside-down. Eventually it would settle to the bottom and be preserved upside-down. To test this, the authors created computer models of a bloated ankylosaur and a bloated nodosaur (an ankylosaur with no tail club). They found that the nodosaur would tip very easily and would flip over more readily. The ankylosaur had a more difficult time flipping over.

Figure 3 from the paper showing the bloated ankylosaur and nodosaur. The plus and diamond symbols are place markers for the computer simulations to test how easily they would flip over.

The last hypothesis was that the ankylosaurs would roll over as the decomposed on land. The authors tested this by examining road-kill armadillos and by watching 22 armadillos decompose in a more controlled setting. They did not find support for this hypothesis as the armadillos were equally likely to be found on their sides, stomachs, and backs.

Armadillos are pretty cute.

The authors concluded that the most likely scenario is that the ankylosaurs would get washed into a stream or into the ocean, bloat from decomposition, and roll over as they floated along. Once the gases escaped, they’d sink to the bottom and get fossilized. This study took it upon itself to test a long-standing idea that had never been tested. Now we have a better notion for how ankylosaurs were preserved and more about their biology.

The Statue-Like Ankylosaur

Back in May, I told you about an [ankylosaur] that was found by an oil company in 2011. It took 6 years of careful preparation to remove the hard rock around the fossil, but it is finally complete. This week, its [official announcement] was published in the form of a scientific paper.

Figure 1 from the paper showing the amazing fossil.

This ankylosaur is a nodosaur, an armored dinosaur without a club tail. It is from Alberta, Canada, from 110 million years ago. The authors named it Borealopelta markmitchelli, meaning “Mark Mitchel’s Northern Shield” after the person who spent hours uncovering it from its rock, and after its armor and the location where it was found. It seems that when the animal died, it got washed out to sea, sank, and was quickly buried. It’s preserved in three dimensions, and its armor is beautiful.

In life, horns and armor are covered by a keratin sheath. Keratin is a hard substance that makes up our fingernails and hair, but also what gives rhinos their horns, covers the horns of other animals, and makes up the baleen that whales use to filter-feed.

A rhino skull without the keratin on the left (from Bone Clones) and a rhino skull with the keratin on the right (from Wikipedia). The keratin makes up most of the horn.

Keratin normally breaks down very quickly after the animal dies, so it’s not usually preserved. In the case of this nodosaur, much of the keratin is preserved, giving us a complete view of the armor it had.

Figure 2a from the paper showing the fossil in a top view. The dark grey is showing all the keratin, the yellow is showing the bone underneath.

Additionally, the authors found a unique chemical signature in the keratin. These chemicals are not melanosomes (the cells that make color), but are the products of the breakdown of melanosomes. By examining these chemicals, the authors were able to figure out what colors were present on different parts of the body.

A reconstruction of Borealopelta by the Royal Tyrrell Museum.

They found that the back of Borealopelta was a redish-brown color, and that the belly was a lighter color. This combination – a darker back and lighter belly – is called countershading, and it’s used by animals who are trying to hide themselves from predators. Today, you can see a similar color pattern in an antelope.

A pronghorn antelope showing counter shading. From Wikipedia.

Some mammals, like rhinos and elephants, are so big that they don’t have to hide from predators. Their size keeps them safe. Even though Borealopelta was very large (1300 kg), the fact that it has countershading means that theropod predators were still a threat.

There are some who think that the chemicals the authors found may be from the sediment or from bacteria. As always, science continues searching for answers! Even so, Borealopelta makes a welcome addition to the ankylosaur family tree.

Kneel Before Zuul

This week, a new [akylosaur] was described! Ankylosaurs are great. They were like living tanks that roamed the Mesozoic, calmly eating plants that were low on the ground, and smashing threats with their fearfully great tail clubs (note: not all ankylosaurs had tail clubs, but the ones that did surely smashed things with them [I can only assume]).

A reconstruction of Zuul crurivestator by D. Dufault.

This new ankylosaur is the most complete akylosaurid ever found in North America. The paper described its skull and tail club, but a larger paper will come out later describing the rest of the fossil, once it gets prepared out of the rock.

The skull of Zuul. Photo from the Royal Ontario Museum.

The authors named it Zuul crurivestator, which means ‘Zuul’ (after the 1984 Ghostbusters creature) and ‘Destroyer of Shins’ because its tail club was at shin height. Its skull has a bunch of horns that aren’t present on other ankylosaurids, reminding the authors of Zuul.

A drawing of Zuul‘s head on the left and an image of Zuul from Ghostbusters on the right.

The tail club is the longest one from any ankylosaurid from North America and it preserves osteoderms (bone inside the skin layer) along the tail.

A photo of the tail showing the preserved skin, osteoderms, and the tail club. From B. Boyle (Royal Ontario Museum).

It’s from around 76 million years ago – a time for which we did not have ankylosaurs in the fossil record, even though they were present. And, to top it all off, the specimen has soft tissue preserved with it! There is a layer of black, shiny tissue on top of some of the osteoderms in the tail, which could be preservation of the keratin sheath that would have covered the osteoderms in life.

So, to wrap up, Zuul crurivestator is a brand new ankylosaurid. It’s one of the most complete ankylosaur fossils from North America, with a beautiful skull and tail club. And it has soft tissue preservation.