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Tuesday, June 6, 2023

What Did Pterosaurs Eat? Look Very Closely at Their Teeth

The pterosaurs had no way of knowing it, but they would one day become a bit of a headache for scientists. The flying reptiles lived alongside the dinosaurs between 210 million and 66 million years ago, ranging from the size of a sparrow to the height of a damn giraffe in the case of Quetzalcoatlus northropi, whose wings stretched 33 feet.

If they flew like birds but were in fact reptiles, which group did they take after when it comes to diet? Paleobiologists often look to living animals for clues: A Komodo dragon, for instance, packs serrated teeth for slashing through flesh, whereas crocodilians use their peg teeth to grasp prey and swallow it whole. So even though the pterosaurs are long gone, today scientists can analyze the shapes of their skulls and teeth to suggest whether a certain species was likely to hunt insects, fish, or the flesh of terrestrial animals.

Now, one group of researchers has a newfangled tool for divining not only what a pterosaur ate, but how that prey, in a sense, bit back. It turns out that chewing on different materials creates characteristic patterns of “microwear” on the tooth, on the scale of millionths of a meter. (The same thing also happens to your teeth, and to modern reptiles like crocodiles and monitor lizards.) These patterns offer clues to an animal’s eating habits.

Writing today in the journal Nature Communications, the researchers describe how they imaged the teeth of pterosaurs using a fancy technology called infinite focus microscopy, which measures in three dimensions. Then they compared the teeth to those of modern beasts, whose diets we know in great detail. They found that between all the species of pterosaurs they examined, there wasn’t much that the ancient animals didn’t eat, giving the scientists new insights that skull and tooth morphology alone could never provide. “We found carnivores, we found piscivores—fish eaters—and also invertebrate eaters,” says paleobiologist Jordan Bestwick of the University of Leicester and the University of Birmingham, lead author on the new paper. “We found pterosaurs that might have been eating slightly softer insects, so a similar hardness to dragonflies and crickets, and then those who might have been eating more crunchy items along the vein of crabs, beetles, and snails.”

They could also see how the group’s dietary preferences changed throughout tens of millions of years of evolution, painting a more vivid picture of the roles pterosaurs played in ecosystems all over the world. The researchers even unraveled clues as to how an individual pterosaur’s diet may have changed as it grew up.

Prior to this new work, paleobiologists had a few ways of resolving the diet of pterosaurs. For one thing, a few rare fossils have some soft tissues preserved, so scientists can look inside their stomachs for bones and fish scales. Fossil pterosaur feces, known as coprolites, also help. And the fact that so many pterosaur fossils are found in what were once coastal environments is a solid clue that they ate fish and other seafood.

But Bestwick and his colleagues could pick apart the pterosaur diet like never before thanks to the infinite focus microscope, which bombarded each pterosaur tooth with photons and measured how long it took for them to return to the device. A photon hitting the bottom of a groove in the tooth texture takes ever so slightly longer than one hitting a peak. They then ran this data through software that engineers use to determine the smoothness of machined parts, giving them a quantitative measure of the roughness of pterosaur teeth.

They began with fossil teeth from 17 different pterosaur species—more specifically, the front teeth the animals used to grasp their prey. Like modern reptiles, pterosaurs snagged their prey and swallowed it whole. So those front teeth would develop a unique microwear pattern depending on whether a particular species was after, say, fish or crabs. Even though their teeth would fall out periodically and be replaced with new ones, the chompers lasted long enough to accumulate microwear.

If you take a look above, you can see six of these images, each corresponding to an area a tenth of a millimeter square. At top are three extant species: the gharial, which eats fish; the American crocodile, which eats harder invertebrates like snails and crustaceans; and Grey’s monitor lizard, which is an omnivore. At bottom are three pterosaur species.

Based on the characteristic wear patterns, the researchers reckon that Istiodactylus was a meat eater. Coloborhynchus probably took a broader range of prey, maybe both fish and softer invertebrates. And Austriadactylus, which paleobiologists have speculated might have fed on fish or meat, now appears to have been a consumer of invertebrates—critters without backbones. “So this one might have been feeding on crunchy insects and such,” says Bestwick. “This can really help us understand what these animals were doing in ancient food webs.”

By comparing the microwear on pterosaur teeth to that of extant species, the researchers could build a clearer picture of what the ancient fliers were eating, but they can’t yet say how different kinds of prey created different microwear patterns on the teeth. “The mechanistic process of how a tooth interacts with food and causes chipping and scratching and indentation is not quite yet fully understood,” says Bestwick, “especially in reptiles, because these animals don't chew their food. But we can make sort of logical leaps of faith based on how we know that reptiles eat.”

Why go through all the trouble of microwear analysis? Because teeth can lie. Imagine I handed you a panda skull and a polar bear skull and had you examine the shapes of their heads and teeth. They'll look quite similar, even though a polar bear eats seals almost exclusively, and a panda specializes in bamboo. “You'd assume that these animals were eating the same things,” says Bestwick. “These types of comparisons aren't always the best. And so you need kind of a more quantitative experimental way of looking at diet.”

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Also, like modern alligators, some pterosaur species may have fed on a variety of foods. “An adult alligator can break open turtle shells, it can crush bones, and it can also eat very soft things like fish,” says Daemen College vertebrate anatomist Domenic D'Amore, who wasn’t involved in this work. “So not only do teeth not indicate necessarily the type of food they eat, they also don't indicate the breadth of food that an organism eats.”

Microwear analysis allowed Bestwick and his colleagues to even peer into the mysterious upbringings of individual pterosaurs. A tricky bit about pterosaur paleobiology is that it’s hard to tell if the animals cared for their young (like birds do) or let them fend for themselves (like reptiles tend to do). Luckily, these researchers had teeth from Rhamphorhynchus specimens of various ages. The teeth of babies indicated they fed more on invertebrates, like insects, while the adults ate fish. “If Rhamphorhynchus was looking after its young, we therefore would have expected microwear to have been the same regardless of age,” says Bestwick, because mom and dad would have been bringing home fish for the kids to eat. “So this tells us maybe pterosaurs had lifestyles more similar to reptiles than birds.”

Their analysis lends further evidence to the idea that pterosaurs were good to go, right out of the egg. “We've got eggs with near-term embryos, and the bones are really well developed—these things were probably able to fly within minutes of hatching,” says paleobiologist Don Henderson of the Royal Tyrrell Museum of Palaeontology, who wasn’t involved in this new work. “And so they were out and about, just like modern lizards today. Baby crocodiles and alligators out of the egg, they're nippy and ready to feed.”

Microwear analysis, then, could be a powerful tool for confirming what paleobiologists already suspected about the lifestyles of pterosaurs. In the case of Rhamphorhynchus, “we absolutely know the diet because we have gut contents from some of them—and in one case, we have one that ejected a coprolite upon death,” says Natural History Museum of Los Angeles paleobiologist Mike Habib, who wasn’t involved in this work. “That's a great case where, OK, check your microwear signal. If it doesn't tell you it ate the thing we know it ate, then your microwear analysis isn't working. If it tells you that it did eat that stuff, then it is working.”

The technology might also expose dietary changes among members of the same species, Habib says. One population of a pterosaur species may have split off from the main group and decamped to a new environment to exploit a different food source. While the tooth shape of the two populations might suggest they’re eating the same thing, microwear analysis might betray subtle differences. “The tooth shape may not have evolved to exactly match those specific dietary changes, but you will pick it up in microwear,” says Habib. “Because if they're eating harder things in one area versus another, you can get really fine-scale stuff that you're not going to see out of the tooth shape.”

Bestwick and his colleagues also used microwear analysis to look across evolutionary time. The teeth of early pterosaurs indicate they fed on crunchy invertebrates like insects, their study shows. Over millions of years of evolution, though, pterosaurs shifted to feeding almost exclusively on meat and fish.

At the same time, the ancestors of modern birds, like Archaeopteryx, were evolving. So might pterosaurs have evolved to eat more fish and meat because of competition from insect-eating early birds? “I want to stress that we haven't tested to see whether or not this dietary shift was because of birds,” says Bestwick. “But we did notice that the timeframe—it's a little bit more than a coincidence, at the moment. That's a story for another day, to actually test whether or not this dietary shift was directly due to the emergence of birds.”

It’s food for thought, that’s for sure.

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