Flipside of a dinosaur mystery: 'Bloat-and-float' explains belly-up ankylosaur fossils

A scientist with the Canadian Museum of Nature has answered a long-standing mystery about why fossils of ankylosaurs -- the "armoured tanks" of the dinosaur world -- are mainly found belly-side up. In doing so, he has ruled out three other competing theories involving clumsiness, predation, and the effects of bloating as seen in armadillo roadkills.

Illustration of Euoplocephalus, an ankylosaur. Ankylosaurids are sometimes called 
the 'tanks of the Cretaceous' given their squat bodies and armored hides 
[Credit: Brett Booth Brett Booth]

Palaeontologist Dr. Jordan Mallon says the evidence points to a phenomenon called "bloat-and-float," whereby the bloating carcasses of ankylosaurs would end up in a river, flip belly-side up due to the weight of their heavy armour, and then float downstream. The remains would wash ashore, where decomposition and then fossilization would seal the dinosaur remains in their upside-down death pose.

"Textbooks have touted that ankylosaur fossils are usually found upside down, but no one has gone back and checked the records to make sure that's the case," explains Mallon. The observations date from the 1930s. Indeed, the fossils of two star ankylosaurs described in 2017, Borealopelta from Alberta and Zuul from Montana, were found upside down.

Mallon examined 32 ankylosaur fossils from Alberta (of which 26 were found belly up), photos of specimens, field notes, and other signs such as erosion of the exposed surface, sun bleaching, and the presence of lichens.

The results are published in the online journal Palaeogeography, Palaeoclimatology, Palaeoecology. Collaborators included armadillo experts Drs. Colleen McDonough and Jim Loughry of Valdosta State University in Georgia, and Dr. Don Henderson, with Drumheller Alberta's Royal Tyrrell Museum of Palaeontology.

Mallon ruled out three other theories before settling on "bloat-and-float" to explain the preponderance of the belly-up remains.

"One idea was that ankylosaurs were simply clumsy, tripping over themselves or rolling down hills and ending up dying that way," he says. But since ankylosaurs existed for about 100 million years, clumsy habits would not fit with their apparent evolutionary success.

Another theory was that ankylosaurs were prey for carnivores, such as hungry tyrannosaurids, which would flip the armoured dinosaurs onto their backs to get at the soft underbelly. "If this was true, we would expect to see signs of bite marks, especially on upside-down ones, but we saw marks on only one specimen," explains Mallon. "Since they were armoured, it makes sense that ankylosaurs were not regularly preyed upon, and the fossil evidence in museum collections supports this."

The third idea, proposed in the 1980s, is an analogy to what happens with some armadillo roadkills -- as the carcass rots and bloats, gas accumulates, and the limbs would splay out, eventually rolling the animal onto its back.

The challenge was to test this hypothesis. Enter McDonough and Loughry who are experts on modern armadillos, which also have an armoured shell. Over the summer of 2016, they studied 174 examples of dead armadillo. "Sure enough, the data show that they do not occur more often on their backs," says Mallon. The pair even examined dead armadillos placed in plexiglass cases in their backyard to keep away scavengers. Regardless of the positioning of the carcasses, bloating did not cause them to roll over onto their backs.

That left the "bloat-and-float" hypothesis as the most likely explanation for the presence of upside-down fossils. To study this, Mallon turned to computer simulations developed by Dr. Don Henderson, who specializes in the floating behaviour of animals in water.

Ankylosaur fossils in North America are found in river channel deposits, and in the Late Cretaceous Period these animals would have been living along a coastline of what is known as the Western Interior Seaway.

"We designed these models of ankylosaurs, both clubless and clubbed, and looked at their floating behavior," explains Mallon. The computer modelling showed that the animals would tend to flip upside down quite easily in water. Nodosaurids, which are ankylosaurs with no tail clubs, would flip most easily at the slightest tilt; the ankyosaurids (with clubbed tails), were more stable but could still be flipped.

"So 'bloat-and-float'" fits with their known environment, and this research helps inform about the transport behavior of dead dinosaurs, which is important to know when studying fossil ecosystems. Ultimately, this is a classic case study of the scientific method: examining alternative hypotheses, finding ways to test them, and ruling them out one-by-one. What you are left with at the end is the most likely explanation."

Source: Canadian Museum of Nature [February 28, 2018]

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Geologists solve fossil mystery by creating 3-D 'virtual tour' through rock

Have you ever wished you could travel inside a rock? It may sound more like magic than science, but Princeton scientists have found a way to make it (almost) true.

With an industrial grinder, a super-high-resolution camera usually used for wedding photography, and high-speed neural
networks, Princeton geoscientists Adam Maloof and Akshay Mehra can deconstruct rock samples and create three-
dimensional digital versions, which they have used to analyze specimens of Cloudina fossils gathered by Mehra (left)
and undergraduates Will Van Cleve and Christian Gray (right) of the Class of 2017 from the Byng Formation,
a fossil reef formation in a glacier-carved valley on Salient Mountain in the Canadian Rockies
[Credit: Adam Maloof and Akshay Mehra, Princeton University Department of Geosciences]

With an industrial grinder and a super-high-resolution camera, Princeton geoscientists Adam Maloof and Akshay Mehra can deconstruct rock samples and create three-dimensional digital versions that scientists can look at from any angle. In addition, they have developed software that allows the computer to segment images and isolate objects without human bias.

Using this technology in conjunction with detailed field observations, they examined a thin-shelled creature that lived over much of the world about 545 million years ago, Cloudina, generally agreed to be the first-ever "biomineralizer," an organism that can create a shell or bones in addition to soft tissue.

While previous researchers had argued that Cloudina were reef builders, Maloof and Mehra were able to use their 3-D reconstruction of the creatures' delicate tube-like structures to conclude that the fossils had been transported from other areas, suggesting that Cloudina played only a minor role in the earliest reef systems. Their work appears in the current issue of the Proceedings of the National Academy of Sciences.

"I thought going in we would learn all sorts about this amazing first biomineralizer and first reef builder, but Cloudina turned out to be more like a reef dweller," said Maloof, an associate professor of geosciences. He has now turned his focus to the next-oldest potential reef builder, a sponge called Archaeocyathid that lived about 520 million years ago.

Cloudina had proven resistant to detailed study because its delicate casing is too fragile to extract physically from the surrounding limestone, and it could not be imaged remotely with traditional X-ray tomography techniques, which require density differences between the object of interest and the surrounding material. Because Cloudina is chemically identical to limestone, the fossils were effectively invisible to X-rays.

Meet GIRI

Almost five years ago, Maloof a nd Situ Studio collaborator Brad Samuels assembled the technology to create what he now calls "flipbooks," digital renderings that move through more than a thousand wafer-thin slices through a rock. Known as "GIRI" or "the grinder," the Princeton Grinding Imaging and Reconstruction Instrument is an answer to geologists' long-standing desire to know what rocks look like on the inside.

Princeton University geoscientist Adam Maloof has spent five years perfecting a combination grinder and imaging system
that can create a three-dimensional 'virtual tour' through the inside of any solid object, from rocks to batteries. Here,
a diamond wheel grinds a sample in the Princeton Grinding Imaging and Reconstruction Instrument (GIRI) in the
Grinder Lab behind Guyot Hall, Princeton University [Credit: Adam Maloof and Akshay Mehra,
Princeton University Department of Geosciences]

"Forever -- since Darwin -- people have tried to figure out how fossils look in 3-D, when they're embedded in rock and it's hard to get them out," Maloof said. "People did serial sections just like this way back then -- but perhaps not at this scale -- where they would grind away a little rock, draw it, grind a little more, draw it. ... It can be incredibly time-consuming."

Enter GIRI, which can cut slices as thin as a few microns (less than 1 percent of a millimeter) and can run 24 hours a day for weeks on end. As each slice takes about 90 seconds to cut and image, researchers have to choose between speed and scale. Most of the specimens Maloof and Mehra have imaged are cut into 30-micron slices, about a third the thickness of a human hair. A typical inch-thick, 1,500-slice sample takes about a day and a half to grind and image; during this time, the operator needs to replace machine fluids and clean the wipers (which clear the surface after each cut) only once.

"The process is destructive," Maloof said. "Dinosaur bones, lunar samples -- there are certain specimens that people are less likely to give us. It hasn't really stopped us, because most samples are not precious. Cloudina, there are zillions of them -- we could never grind them all."

GIRI can produce a 3-D rendering of any solid object, whether or not it has the density differences needed for effective X-ray computed microtomography (usually known as X-ray CT or Micro CT). In addition, because you're taking a super-high-resolution photograph with every slice, you're always seeing the rock itself, not just the density model that remote sensing can provide.

"It's destructive of course, that's the disadvantage, but what's so nice is that you get to see photographs and make direct observations," Maloof. "That's what's been so life-changing to me: I love that it's not a model. You can just see it. On any given slice, if you find something great, you can just find the slice and say, 'What did it look like?' ...We're on a virtual tour inside, rather than looking at waveforms and trying to interpret them."