Earliest known predatory birds lived alongside dinosaurs 

The Hell Creek Formation, spanning what are now the Dakotas, Montana, and Wyoming, is best known for housing some of the most famous dinosaurs, such as Triceratops and Tyrannosaurus rex.

In particular, this site was home to SUE, one of the largest and most complete T. rex fossils ever discovered. However, these iconic dinosaurs weren’t the only creatures inhabiting this prehistoric world. 

Skilled predatory birds 

In a recent study published in the journal PLoS ONE, scientists have described two newly identified species of birds that coexisted with these dinosaurs approximately 68 million years ago. 

Remarkably, the researchers were able to name these species based on just one bone each — specifically, a sturdy foot bone, which suggests that these birds were skilled predators capable of capturing and carrying off 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,” said lead author Alex Clark, a PhD student at the Field Museum and the University of Chicago. 

“While they might not be the first birds of prey to ever evolve, their fossils are the earliest known examples of predatory birds.”

Wealth of information in foot bones

The three fossil specimens Clark analyzed for this study were collected in recent years by other researchers but had not received much detailed attention. 

At first glance, Clark recalls that the fossils weren’t particularly remarkable — each was a single foot bone, known as a tarsometatarsus, to which the bird’s toes would have been attached. 

These bones were found without more eye-catching body parts like claws or skulls, and while they were considered large for bird foot bones, they were still only about the size of an adult human thumb. 

However, these unassuming bones turned out to hold a wealth of information about the ancient birds’ lives and behaviors.

“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,” Clark explained. 

Unique features of predatory birds 

One particularly interesting feature on these bones was a large bump, known as a tubercle, which is a muscle attachment point.

The size and location of this tubercle were unusual — it was much larger and positioned farther down the bone than is typical 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,” noted 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.”

Predatory birds with strong feet 

To further investigate their hypothesis, Clark and his team conducted biomechanical analyses of the fossilized foot bones, comparing them to the bones of various modern birds. 

“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,” explained Clark. 

The findings confirmed that the structure of these birds’ feet would have been strong enough for them to grasp and carry off small mammals and even baby dinosaurs, similar to modern hawks and owls.

Two new bird species named

From these three foot bones, Clark and his team identified and named two new species: Avisaurus darwini, named after Charles Darwin, and Magnusavis ekalakaenis, named in honor of the town of Ekalaka, Montana, where one of the fossils was discovered. 

The third bone may represent another new species, but its degraded condition made it difficult to draw firm conclusions. 

These birds belong to a group called avisaurids, which are part of a larger extinct group of birds known as enantiornithines.

Like most of the dinosaurs, the enantiornithines went extinct when the asteroid struck Earth 66 million years ago.

Diversity of ancient bird species

Study co-author Jingmai O’Connor is the associate curator of fossil reptiles at the Field Museum and Clark’s advisor.

“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,” said O’Connor.

The findings shed new light on the diversity of ancient bird species that lived alongside dinosaurs and highlight the important role that even isolated fossils can play in understanding ancient ecosystems. 

While the bones may have seemed unimpressive at first glance, they have provided a fascinating glimpse into the lives of some of the earliest predatory birds.

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