For palaeontologists, deducing the behaviour of long-dead animals is notoriously tricky. And new evidence can rewrite long-standing hypotheses. Case in point: research published in Current Biology presents a surprising theory for the unusual teeth of an ancient bird.
Longipteryx chaoyangensis, a 120-million-old bird whose fossils are found in north-eastern China has an elongated, kingfisher-like skull with teeth found only at the tip of its beak.
As an early lineage of birds, Longipteryx is not closely related to modern kingfishers. However, using the technique of evaluation based on “convergent evolution,” which identifies similarities in body shape between distantly related animals, can sometimes indicate similar behaviours.
When scientists discovered Longipteryx in 2000, they suggested that its kingfisher-like silhouette meant that this ancient bird also hunted fish. However, this hypothesis has been challenged by scientists considering additional lines of evidence.
“There are other fossil birds, like Yanornis, that ate fish, and we know because specimens have been found with preserved stomach contents,” says Jingmai O’Connor, the study’s lead author and an associate curator at the Field Museum in Chicago.
In their study, O’Connor and colleagues present the first fossil evidence of stomach contents in Longipteryx. Instead of fish, they identified seeds from ancient trees in the stomachs of two different individuals.
Given the seasonality of seeds, the researchers suggest that Longipteryx also ate insects. But they conclude that the fish-eating hypothesis for this bird doesn’t hold water.
“Fish-eating birds had lots of teeth, all the way along their beaks, unlike how Longipteryx only has teeth at the very tip of its beak,” says O’Connor. “It just didn’t add up.”
The question remained: what did Longipteryx use its teeth for?
O’Connor and colleagues drew upon analyses of Longipteryx’s teeth as well as comparisons to hummingbirds to propose that Longipteryx used their teeth for fighting.
“Tooth enamel is the hardest substance in the body, and Longipteryx’s tooth enamel is 50 microns thick,” says Alex Clark, a PhD student at the Field Museum and the University of Chicago and a co-author of the paper.
“That’s the same thickness of the enamel on predatory dinosaurs like Allosaurus that weighed 4,000 pounds, but Longipteryx is the size of a blue jay,” says Clark. (For our Australian readers, blue jays are slightly smaller than Australian magpies, and 4000 pounds is about 1.8t).
The research team argue that the “overpowered” teeth in Longipteryx were used as a weapon, where tip-only teeth make evolutionary sense.
“Having a weaponized beak makes sense, because it moves the weapon further away from the rest of the body, to prevent injury,” says Clark.
The team invoke a different case of convergent evolution to back up this hypothesis.
“There are no modern birds with teeth, but there are hummingbirds that have keratinous projections near the tip of the rostrum [beak] that resemble what you see in Longipteryx,” says O’Connor. “And they use them as weapons to fight each other.”
These tooth-like projections have evolved at least seven different times in modern hummingbirds. This is evidence of strong selection for this beak shape associated with fighting behaviour.
In addition to formulating a stronger case for one strange fossil bird’s behaviour, O’Connor and colleagues hope their research illustrates the promises and pitfalls of using convergent evolution to infer the lives of ancient animals.
“We’re trying to… get palaeontologists to think about the complexity of the behaviours that these animals might have engaged in beyond just what they were eating,” says O’Connor. “There are many factors that could be shaping the structures that we see.”
