The Hell Creek Formation, stretching across present-day North and South Dakota, Montana and Wyoming, was once the stomping grounds of some of the planet’s most iconic dinosaurs, including Triceratops and Tyrannosaurus rex.
Among them was SUE, one of the largest and most complete Tyrannosaurus rex specimens ever discovered.
However, these colossal creatures weren’t the only inhabitants of their ancient ecosystem. In a recent study published in PLOS ONE, researchers have identified two new species of birds that lived alongside these dinosaurs 68 million years ago.
Despite only having a single bone for each species to work with – the robust foot bone – the scientists uncovered evidence that these birds were skilled hunters, capable of capturing and carrying off large prey.
"Based on clues in their foot bones, we think these birds would have been able to catch and carry prey, similar to what a modern hawk or owl does,” explains Alex Clark, lead author of the study. “While they might not be the first birds of prey to ever evolve, their fossils are the earliest known examples of predatory birds.”
The fossilised bones analysed in the study had been discovered by other researchers over the last few years, but little work had been done on them. Clark recalls his initial impression: the bones, while large for bird foot bones, were far from flashy – small, tarsometatarsal bones about the size of a human thumb. However, these seemingly modest fossils held a wealth of clues.
“Every nook and cranny and bump that occurs on a bone can tell us something about where the muscles or tendons attached and how big they were," says Clark.
One particular feature caught his attention – a large tubercle, or muscle attachment point, located further down the bone than usual. “When we see tubercles this large and this far down in modern birds, they're in birds of prey like owls and hawks,” says Clark.
“That’s because when they hunt and pick up their prey with their feet, they're lifting proportionally heavy things and holding them close to their bodies to stay as aerodynamically efficient as possible. These fossil ankle bones look like they're built to do something similar.”
Clark and his team ran biomechanical tests, comparing the fossils to modern bird species. "The muscles and bone of the ankle work like a lever, and by comparing how far down on the bone the muscle attaches, we can get a good idea of how it would have moved and how strong it would have been,” explains Clark.
The results confirmed that these birds, about the size of a hawk, could have snatched up small mammals or even baby dinosaurs.
From these bones, the team identified two new species: Avisaurus darwini, named in honour of Charles Darwin, and Magnusavis ekalakaenis, after Ekalaka, Montana, where the fossils were found. Both birds belong to the avisaurids, a group within the larger enantiornithines, which perished in the mass extinction event that wiped out most dinosaurs 66 million years ago.
“These discoveries have effectively doubled the number of bird species known from the Hell Creek Formation," says Jingmai O’Connor, co-author of the paper, "and will be critical for helping us to better understand why only some birds survived the mass extinction that wiped out T. rex and the avisaurids described here.”
Find out more about the study New enantiornithine diversity in the Hell Creek Formation and the functional morphology of the avisaurid tarsometatarsus.
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