by A newly discovered leg bone from Columbia likely comes from the largest member of the Forusrhacid family – aptly known as ‘terror birds’ – that we have found so far, which could be more than 3 meters tall. The team describing the fossil believes the individual it came from was 5 to 20 percent larger than the previous record holder.
Now that we know that birds are dinosaurs, it is not surprising that some have regained some of their former status in the post-Cretaceous period. The most formidable birds of the Cenozoic era tended to be on islands, such as the elephant birds of Madagascar or the moa of New Zealand, where they faced less competition from mammals.
The Phorusrhacidson the other hand, fully deserved their nickname as “terror birds”. Not only were they carnivores, rather than subsisting on fruit or seeds like most giant birds, but they were also apex predators for at least 43 million years in South America, where large mammals were plentiful. They probably filled the same niche in Antarctica before it became frozen.
The bone that increases the size of terror birds is not new; it was found almost twenty years ago. However, when Cesar Perdomo of the Museo La Tormenta in Colombia found the fossil, he wasn’t sure what it was. Now researchers have created a 3D model of the bone and concluded that it came from a member of the Phorusrhacids.
“Terror birds lived on the ground, had limbs adapted for running, and usually ate other animals,” co-author Dr. Siobhan Cooke of Johns Hopkins University explained in a statement. Size calculations based on part of one bone are necessarily rough, but Cooke and co-authors estimate that it would have weighed 156 kg (343 pounds). Since relatives are equipped with smaller foot bones that are almost 3 meters high, it may have exceeded this height.
It’s only part of a left tibiotarsus, but it indicates that its owner was a terror bird larger than ever before.
Image credit: Degrange et al., 2024, Paleontology
The fossil is from the Miocene, 12 million years ago, and comes from the lower left leg, which corresponds to a human tibia. Its size made paleontologists doubt its origins, but scans reveal deep pits, a distinguishing feature of Phorusharcids’ leg bones. In addition to the natural pits, the scans also revealed teeth marks, likely from a member of the crocodile order.
Cooke and co-authors of a paper describing the find think the bite came from a Purussaurus. Despite the ending of the name, Purussaurus wasn’t a dinosaur, but it was just as terrifying: a 30-foot caiman.
“We suspect that the terror bird would have died as a result of its injuries, given the size of crocodilians 12 million years ago,” Cooke said.
In addition to the size and suspected cause of death, the discovery is remarkable for another reason. Most fossils of terror birds have been found in Argentina and the surrounding area. The Tatacoa Desert in Colombia, where this discovery was made, is located north of the equator. The dry canyons have been a rich source of fossils for more than a hundred years Phorus harcids have not been found there before.
The authors consider this a sign Phorus harcids were not as common in what is now the Tatacoa, a wealthy riverine province at the time this particular individual lived. On the other hand, Cooke acknowledged, “It is possible that there are fossils in existing collections that have not yet been recognized as terror birds because the bones are less diagnostic than the lower leg bone we found.”
Terror birds or not, the region would have been every nature documentary maker’s dream at the time. “It’s a different kind of ecosystem than we see today or in other parts of the world in a period before South and North America were connected,” Cooke said. Examples include glyptodonts (relatives of armadillos the size of cars), giant ground sloths, ungulates that fill the niches of zebras or impalas in Africa, and even monkeys whose ancestors had made the epic raft journey.
Although Phorusharcid fossils were much more common in the southern half of South America, and finding a specimen this far north isn’t a complete surprise. When Panama joined South America, allowing species to exchange between the two continents, the northerners had the upper hand. As far as we know, terror birds were the only large predators from South America to use the connection, subsequently found in places like Texas. They appear to have made the crossing about 5 million years ago and must have been in the region before then. Nevertheless, none of the family had previously been found in South America, north of southern Peru, more than 2,000 kilometers away.
The study is open access in Paleontology.
