In the ancient Hell Creek Formation, spanning the Dakotas, Montana, and Wyoming, a prehistoric menagerie thrived, featuring iconic dinosaurs such as Triceratops and Tyrannosaurus rex. Notably, SUE, one of the largest, most intact T. rex specimens ever discovered, once roamed this terrain. However, these colossal creatures weren’t the sole inhabitants of this ecosystem.
In a paper published in the journal PLOS ONE, researchers unveiled the existence of two previously unknown bird species that coexisted with these dinosaurs 68 million years ago. Astonishingly, these avian species were christened based on a solitary bone each – a formidable foot bone hinting at their ability to capture and transport 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,” says Alex Clark, a PhD student at the Field Museum and the University of Chicago and the study’s lead author. “While they might not be the first birds of prey to ever evolve, their fossils are the earliest known examples of predatory birds.”
The three fossils Clark meticulously studied in this paper had been collected over several years by researchers at other institutions, yet they had remained largely unexplored. When Clark first laid eyes on the fossils, they didn’t immediately captivate him. They were simply footbones known as tarsometatarsus, lacking the allure of skulls and claws. Despite being larger than typical bird tarsometatarsi, they were merely the size of an adult human thumb. However, these seemingly unremarkable bones held a wealth of invaluable information waiting to be unearthed.
“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. On these bones, there was an especially noteworthy bump– a muscle attachment point called a tubercle. On each bone, it was larger and farther down than in most birds. “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 embarked on a fascinating journey of biomechanical analyses, comparing the ancient foot bones to those of modern birds. Their findings revealed a remarkable strength in the ankles of these hawk-sized birds, suggesting they were capable of capturing small mammals and even baby dinosaurs. From these discoveries, two new bird species were introduced to the world of science: Avisaurus darwini, named after the renowned Charles Darwin, and Magnusavis ekalakaenis, paying tribute to the town of Ekalaka, Montana, where the fossil was discovered.
These incredible birds, part of the avisaurids group, were part of the enantiornithines, a group of birds that met their fate along with the dinosaurs when the asteroid hit 66 million years ago.
“These discoveries have effectively doubled the number of bird species known from the Hell Creek Formation 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,” Jingmai O’Connor, the Field Museum’s associate curator of fossil reptiles in the Negaunee Integrative Research Center, Clark’s advisor, and a co-author of the paper.
Journal reference:
- Alexander D. Clark, Jessie Atterholt, John B. Scannella, Nathan Carroll, Jingmai K. O’Connor. New enantiornithine diversity in the Hell Creek Formation and the functional morphology of the avisaurid tarsometatarsus. PLOS ONE, 2024; DOI: 10.1371/journal.pone.0310686