Researchers have found evidence of a previously unrecognized soft tissue structure in the cheek region of many dinosaur species, which they've called the "exoparia." The finding deepens our understanding of dinosaur anatomy and highlights the limitations of current methods of reconstructing parts of dinosaur anatomy that can't be well preserved.
Because muscles and tissues degrade over time, there are very few examples of soft anatomy like this from dinosaurs, says Henry Sharpe, a master's student in the Department of Biological Sciences and first author on the study describing the exoparia, published in the Journal of Anatomy.
Although bones can be unearthed and reassembled into semi-complete skeletons, "for a long time no one really had a method for how to figure out what muscles and tissues dinosaurs may have had," explains Sharpe, who has grappled with the question first-hand as a self-described "paleoartologist" whose portfolio includes lifelike renderings of dinosaurs for research papers.
That changed in the 1990s with the creation of the Extant Phylogenetic Bracket, a method that uses dinosaurs' closest modern relatives — crocodiles and birds — to gain insight into dinosaurs' tissues and muscles.
There's just one problem with this approach, Sharpe notes: "Every muscle you would ever reconstruct in a dinosaur would only be one that's in a crocodile or a bird. What if dinosaurs had their own muscles that either weren't present in the dinosaurs that led to birds, or that birds lost or adapted into something else?"
It's this question that ran through Sharpe's mind as he was examining the skull of an Edmontosaurus named Gary. There was a peculiar flanged structure on the bone near the cheek that caught his eye. When he began looking into what it could be, he couldn't find any answers.
"There were these big, corrugated parts of the skull. If we were looking at a mammal skull, we'd say that's the cheek muscle. But reptiles aren't supposed to have a cheek muscle," he says. "This got us thinking: What if there's something here that is against the currently thought of model of dinosaur muscles?"
In their quest to better understand this piece of dinosaur anatomy, Sharpe and his collaborators began examining the same area of the skull in other dinosaur species and found evidence of the same structure across the board.
"It was always in the same spot, which to us was a pretty good indication that this was one muscle or one ligament."
To confirm their hypothesis that this area of the bone was formerly the site of some type of soft tissue structure, the researchers cut thin sections of dinosaur bone. As Sharpe explains, soft tissues like muscles or ligaments are anchored to the bone by collagen fibres.
"It helps anchor that muscle or that ligament into the bone, to prevent it from pulling away from the bone and causing the animal injury."
After the soft tissue has decayed and all that's left is the bone, bits of these collagen fibres remain and can be analyzed when looking at thin slices of bone by shining polarized light through it.
"It looks like someone's taken the bone right beneath the surface and scratched it with an X-Acto knife," says Sharpe.
The researchers also used a technique they called THLEEP to look at multiple angles of bone slices from the cheekbone and lower jaw, examining the 3D orientation of the fibres. This was because, as Sharpe explains, "These collagen fibres won't just insert at any odd angle; they'll follow the angle that the muscle's inserting at."
In every dinosaur species the researchers looked at, the collagen fibres showed there was a connection between the cheek and lower jaw, verifying that the soft tissue structure was something akin to a cheek muscle or ligament.
There was variation in the sizes and angles of attachment between different dinosaur species, which Sharpe says indicates this new soft tissue was used for something specialized, like stabilizing the jaw or contributing to how dinosaurs consumed food. "We don't know exactly what it's being used for, but we know that clearly it's important to how these dinosaurs are chewing differently, because they're modifying it in different ways."
This discovery also highlights the need to look beyond comparing dinosaur fossils with modern relatives to get a more comprehensive and accurate understanding of the extinct creatures' anatomy, says Sharpe.
"There's a ton of diversity in dinosaurs that we're just missing because we're trying to explain the past only in the terms of the present."
Sharpe and his collaborators made all the slides and images they took of the bone slices available to other researchers, and also outlined and explained the techniques they used in the study, envisioning that other scientists might continue exploring the exoparia in even more species.
"We hope our study is a cautionary tale for how much we can assume and how much we have to dig deep and verify our assumptions, or challenge them when they're not right."
