Terror Birds: Apex Predators of the Cenozoic
Terror bird
Cenozoic Apex Predators
In the wake of the Late Cretaceous mass extinction event, which killed off approximately 75% of the planet’s species–including all non-avian dinosaurs–a group of carnivorous flightless birds, incredibly fast and efficient predators, would rise to dominance in South America.
This will be a brief history of the rise and fall of the terror birds.
Terror Birds
Phorusrhacidae, a family of animals commonly referred to as “terror birds,” comprises 14 genera and 20 species. Paleopsilopterus, the earliest known terror bird, first appeared in Brazil about 53 million years ago, during the Early Eocene Epoch. Though phorusrhacids would eventually evolve into imposingly large birds, Paleopsilopterus stood at only three feet tall.
Terror birds had excellent vision and sharp hearing even at low frequencies, which undoubtedly aided them in locating prey. They were fast runners, capable of reaching speeds up to 30 mph. A huge skull equipped with a large hooked bill allowed the phorusrhacid to quickly dispatch its prey by jabbing at it relentlessly.
However, despite the intimidating size of both their skull and bill, biomechanical studies suggest that these flightless birds might not have possessed the strength to tackle larger prey and that their diet was more likely made up of small rodents, reptiles and mammals.
It’s possible that they were occasionally opportunistic scavengers as well.
Terror birds were first discovered by Argentinian paleontologist Florentino Ameghino in 1887, in Patagonia’s Santa Cruz Formation, which dates back to 17 million years ago. He named the specimen Phorusrhacos longissimus. It was estimated to have been over seven feet tall and roughly 300 lbs.
Phorusrhacos (Greek for "wrinkle" or "scar bearer") was one of South America’s apex predators and, like other terror birds, is believed to have had a preference for woodland and grassland habitats.
Additionally, fossil evidence shows that these apex predators lived in other parts of the world as well, including Antarctica, Europe, and Africa.
It’s believed that the ancestors of Lavocatavis, a terror bird who lived in Algeria during the Eocene Epoch, likely made it to Africa by island hopping.
Eleutherornis cotei lived in France and Switzerland during the Middle Eocene and remains the only phorusrhacid known to have resided in Europe.
A recent study suggests that terror birds might have been the apex predators of Eocene Antarctic as well. The Antarctica of the Eocene was very different from the one we know today, as it was much warmer and contained temperate rainforests.
Kelenken
Largest Phorusrhacids?
Among the largest phorusrhacids yet discovered are Kelenken and Devincenzia.
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Kelenken, an almost ten foot tall bird weighing approximately 220 lbs, derives its name from the bird spirit in Tehuelche mythology.
The Tehuelche, an indigenous peoples from eastern Patagonia, were called “Patagones” by Spanish explorers, in reference to their supposedly large feet. The earliest account of them comes from the 1520’s and was written by Antonio Pigafetta, Ferdinand Magellan’s expedition chronicler.
Out of this, and similar stories, grew a myth that Patagonia (“Land of the Bigfeet”) was occupied by 10-foot-tall savage giants–coincidentally the same size as the Kelenken birds who lived in the area millions of years earlier.
It was later learned that, while the Tehuelche were not actual giants, they were quite a bit taller than the average European. Varying estimates placed their true height at anywhere from 6’ to 6’7’’. Conversely, the average 16th century European male was only 5’5’’ tall. And the tracks which had inspired such fear in early Spanish explorers had been enhanced by the animal furs the Tehuelche used as footwear.
Kelenken had “the largest known skull for terror birds. As a matter of fact, it’s the largest known bird skull, period,” said paleontologist Luis Chiappe, who described the bird in 2007.
Devincenzia, a phorusrhacid from Uruguay and Argentina, reached a height of just over eight feet tall.
Titanis, the only terror bird found in North America, lived in Texas, Florida, and California. Only fragmentary fossils of Titanis have been discovered thus far, making it difficult to determine its exact size. It was originally believed to have been about the same size as Kelenken, but more recent estimates place it at 4.6 - 6.6 feet tall and 330 lbs.
Great American Biotic Interchange
By approximately three million years ago, the volcanic Isthmus of Panama had fully formed, creating a land bridge between North and South America. This resulted in a significant period of migration between the two continents, which would become known as the Great American Biotic Interchange (GABI).
South America had been an island continent for millions of years following the breakup of Gondwana in the latter part of the Mesozoic Era. As is typical for island ecosystems, many unique species evolved and thrived in the isolation of South America.
However, that would change dramatically during the Great American Biotic Interchange. Not only did many of the northward migrants fare less well than their counterparts heading in the opposite direction, but a lot of the native animals who remained behind in South America suffered as well.
The new residents in South America included horses, tapirs, deer, and large predators like bears, saber-toothed cats, cougars and jaguars. One reason for the success of the southward migrants appears to be that they managed to establish themselves in large numbers and subsequently diversified, ultimately outcompeting much of the native fauna in their new home.
Juan Carrillo, research fellow at National Museum of Natural History in Paris:
“This faunal exchange can be seen as a natural experiment: two continents, each with its own kind of animals were connected by a narrow land bridge, allowing massive migrations in both directions. Our study shows how these migrations happened and that South American mammals had more extinctions. The effect of this exchange can still be seen today.”
In addition to the ground sloths, glyptodonts, capybaras, and pampatheres (relatives of armadillos) who found their way to North America during this event, phorusrhacids made it there too. Interestingly, terror birds were the only large South American predator to migrate to North America during the Interchange. However, Titanis is the only phorusrhacid known to have lived in North America.
Half of the mammal genera living in South America today descend from North American species.
Terror bird tracks
Tracks Discovered
The discovery of the first-ever terror bird tracks was announced in 2023. The tracks, which were found at a coastal outcrop in Argentina, appear to have been made by a medium-sized phorusrhacid, perhaps from the Mesembriornithinae subfamily, and have been dated to eight million years ago.
A seriema, the closest living relative of terror birds
Downfall
As with the reign of the non-avian dinosaurs, that of the terror birds would eventually come to an end. However, while the extinction of the non-avian dinosaurs has an obvious explanation, the reasons behind the downfall of the phorusrhacids is somewhat less clear.
While the increased competition that came with the arrival of predators such as saber-toothed cats and Arctotherium (also known as the “short-faced bear”) almost certainly played a role in their extinction, it’s probably not the whole story.
In fact, terror birds were still around until relatively recently–fossils have been discovered in a formation dating to 18,000 years ago, during the Pleistocene.
Another factor in the demise of these apex predators of the Cenozoic was likely climate change. The birds, who were more adapted to a warmer climate, probably struggled to survive as the planet went through another cooling period.
Mesembriornis (“southern bird” after its discovery in southern Argentina), a medium-sized terror bird from Argentina, was among the last of its kind.
Their closest living relatives are seriemas, the last remaining members of the Cariamidae family. These long-legged birds live in South America and are only capable of short-distance flight.
Though the fascinating terror birds are no longer with us, each new discovery brings us closer to gaining a better understanding of them.
