Predatory birds from the same fossil formation as SUE the T. rex
Unusual foot bones suggest that the newly-discovered species may have hunted like modern hawks and owls
image:
The three fossil specimens (from left to right), Avisaurus darwini, Avisaurus sp., and Magnusavis ekalakaensis, all of which are represented by a tarsometatarsus. They are all shown to scale with one another.
view moreCredit: Alex Clark
The Hell Creek Formation in what’s now the Dakotas, Montana, and Wyoming was once home to some of the world’s most beloved dinosaurs, like Triceratops and Tyrannosaurus rex (including SUE, one of the largest, most complete, and best-preserved T. rex specimens ever found). But these giant dinosaurs weren’t alone in their ecosystem, and in a paper in the journal PLOS ONE, scientists have described two new species of birds that lived alongside these dinosaurs 68 million years ago. The researchers were able to name these new species from just one bone each: the powerful foot bone that suggests these birds could have captured and carried 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,” 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 studied in this paper had been collected in the past several years by researchers at other institutions, but there hadn’t been much work done on them. Clark recalls that when he first saw the fossils, they weren’t especially dazzling-- they were all a foot bone that the toes attach to, called the tarsometatarsus, and they’d been found on their own, without other, flashier body parts like skulls and claws. And while the bones were large for bird tarsometatarsi, they were still only about the size of an adult human thumb. However, these isolated bones proved to be a treasure trove of information.
“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 colleagues conducted a series of biomechanical analyses comparing the fossil foot bones to those of a variety of 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,” says Clark. The math corroborated the researchers’ hypothesis that these feet would have been strong enough for these hawk-sized birds to pick up small mammals and even baby dinosaurs.
From the three foot bones, Clark and his team described two new species to science: Avisaurus darwini, after Charles Darwin, and Magnusavis ekalakaenis, in honor of the town of Ekalaka, Montana, where the fossil was found. (The third bone may be another new species, but the fossil’s degraded condition made it difficult to tell for sure.) All of these birds are part of a group called the avisaurids. They belong to a larger group of birds called the enantiornithines, which went extinct with most of their fellow 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.
“'I’m really proud and very impressed with what Alex was able to do with these specimens. They're each just a single bone. But he brings his background as an ecologist into his paleontological work to tell more than the average paleontologist about what an animal’s life would have been like,” says O’Connor. “Alex has done a superb job with being able to extract so much incredible ecological information from just a single bone.”
An illustration of the newly described Avisaurus darwini, whose unusual foot bones indicate that it was one of the earliest birds of prey known to science, shown here carrying a small mammal.
Credit
Illustration by Ville Sinkkonen
Lead author Alex Clark with the fossil foot bone of Avisaurus darwini, with SUE the T. rex, a fellow Hell Creek fossil, in the background.
Credit
Photo by Kate Golembiewski, Field Museum.
Journal
PLoS ONE
Article Title
New enantiornithine diversity in the Hell Creek Formation and the functional morphology of the avisaurid tarsometatarsus
Article Publication Date
9-Oct-2024
Newly discovered Late Cretaceous birds may have carried heavy prey like extant raptors
New fossil species reveal diverse body sizes and lifestyles of birds before mass extinction
PLOS
image:
Reconstruction of an avisaurid (e.g., A. darwini). Morphology of the tarsometatarsus suggests that these large birds engaged in raptorial behavior and could carry proportionally large prey. Illustration done by Ville Sinkkonen.
view moreCredit: Clark et al., 2024, PLOS ONE, CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)
Newly discovered ancient birds from Late Cretaceous North America were hawk-sized and had powerful raptor-like feet, according to a study published October 9, 2024 in the open-access journal PLOS ONE by Alexander Clark of the University of Chicago, U.S. and colleagues.
The most diverse birds during the Cretaceous Period were a now-extinct group called enantiornithines, known from all over the world during this time. However, enantiornithines and other Mesozoic birds are mainly known from Lower Cretaceous deposits, with a relatively poor record from the Late Cretaceous. Thus, there is a general lack of understanding of trends in bird evolution toward the end of the Mesozoic Era.
In this study, Clark and colleagues describe three new enantiornithine birds from fossils found in the Hell Creek Formation of Montana, dating to the latest Cretaceous Period (68 - 66 million years ago, shortly before the mass extinction that wiped out non-avian dinosaurs and enantiornithines). All three fossil birds are identified from lower leg bones. Two are new species named Magnusavis ekalakaensis and Avisaurus darwini, while the third is an unnamed species of Avisaurus.
These birds are all larger than Early Cretaceous enantiornithines, with Avisaurus darwini estimated to have weighed over one kilogram, roughly the size of a large hawk. Analysis of the leg bones of Avisaurus and its relatives reveals proportions and adaptations similar to hawks and owls, indicating powerful leg muscles and feet that could grip and potentially carry proportionally large prey, similar to some modern raptorial birds.
These fossils expand the known diversity of Late Cretaceous birds, confirm the trend toward large body size, and highlight how, over time, enantiornithines evolved a greater diversity of ecological roles. This study emphasizes how even fragmentary fossils can reveal important ecological information and be used to clarify evolutionary trends.
The authors add: “Avisaurids, a group of enantiornithine birds from the latest Cretaceous, exhibit hindlimb features indicating strong ankle flexion, which suggests the ability to carry heavy prey and behaviors similar to living raptorial birds.”
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In your coverage please use this URL to provide access to the freely available article in PLOS ONE: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0310686
Citation: Clark AD, Atterholt J, Scannella JB, Carroll N, O’Connor JK (2024) New enantiornithine diversity in the Hell Creek Formation and the functional morphology of the avisaurid tarsometatarsus. PLoS ONE 19(10): e0310686. https://doi.org/10.1371/journal.pone.0310686
Author Countries: U.S.A.
Funding: The author(s) received no specific funding for this work.
Journal
PLoS ONE
Method of Research
Observational study
Subject of Research
Animals
Article Title
New enantiornithine diversity in the Hell Creek Formation and the functional morphology of the avisaurid tarsometatarsus
Article Publication Date
9-Oct-2024
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