Phorusrhacids, commonly known as terror birds, were a group of large, meat-eating birds that mostly could not fly. They were among the biggest top predators in South America during the Cenozoic era. Fossil evidence shows they lived from the Middle Eocene to the Late Pleistocene, about 43 million to 25,000 years ago. Some fossils suggest they may have existed even earlier, during the Early Eocene.

These birds ranged in height from 1 to 3 meters (3 to 10 feet). A large specimen from the Early Pleistocene in Uruguay, possibly from the species Devincenzia, could have weighed up to 350 kilograms (770 pounds). Their closest living relatives are the 80-centimeter-tall (31-inch) seriemas. Titanis walleri, a larger species, lived in Texas and Florida in North America. This shows that phorusrhacids were among the few large South American predators that migrated north during the Great American Interchange, which began about 2.6 million years ago. Titanis migrated earlier, around 5 million years ago.

It was once thought that T. walleri went extinct in North America when humans arrived, but later studies show no evidence they survived past 1.8 million years ago. The most recent known phorusrhacid is Eschatornis, found in Brazil during the Late Pleistocene, dated to about 25,326–25,733 years ago.

Some fossils from Algeria (Lavocatavis) and France and Switzerland (Eleutherornis) suggest phorusrhacids may have lived in Africa and Europe. However, scientists are unsure if these fossils belong to phorusrhacids, as the remains are too incomplete for detailed study. Fossils from Antarctica also hint that these birds had a broader range during the Paleogene period.

A related group, the bathornithids, filled a similar role in North America during the Eocene to Early Miocene. Some, like Paracrax, were as large as the biggest phorusrhacids. One study suggests Bathornis is closely related to phorusrhacids based on shared jaw and bone features, but this idea is debated, as these traits may have evolved separately for similar lifestyles.

Description

The neck of the phorusrhacid can be divided into three main parts. In the upper part of the neck, the phorusrhacid had split neural spines, while the lower part had tall neural spines. This suggests the phorusrhacid had a very flexible and strong neck, which helped it support its heavy head and strike quickly and forcefully. Even though the phorusrhacid looked like it had a short neck, its flexible skeletal structure allowed it to stretch its neck longer than expected. This helped it appear taller to scare prey and strike more effectively. When the neck was fully extended for a downward attack, its strong neck muscles and heavy head created enough force to harm its prey.

Kelenken guillermoi, a bird from the Langhian stage of the Miocene epoch, about 15 million years ago, was discovered in 2006 in the Collón Curá Formation in Patagonia. It has the largest bird skull found so far. The fossil is nearly complete and measures 71 centimeters (28 inches) long. Its beak is about 46 centimeters (18 inches) long and curved like an eagle's beak. Most phorusrhacid birds were smaller, standing 60–90 centimeters (2.0–3.0 feet) tall, but Kelenken guillermoi likely stood about 3 meters (9.8 feet) tall. Scientists believe large terror birds were fast runners, possibly reaching speeds of 48 kilometers per hour (30 miles per hour). Studies of their habitats suggest they competed with other predators, such as borhyaenids and thylacosmilids, forcing these mammals to live in forests instead of open plains.

The feet of the phorusrhacids had four toes. The first toe, called the hallux, was small and did not touch the ground. The other three toes (second, third, and fourth) were on the ground. Studies of the second toe and its claw show it was modified into a "sickle claw," which was curved and large. This shape suggests the claw needed to stay elevated to avoid damage, which was achieved through strong muscles and soft tissue pads. The second toe was shorter and had fewer bones, making it easier to lift the claw while running and holding prey. This is similar to modern seriemas, though less specialized than dromaeosaurid dinosaurs. Footprints from the Late Miocene in the Río Negro Formation show a terror bird with two main toes on the ground, with the sickle claw raised, like those of Mesozoic birds.

Previously, scientists thought these birds had high beaks, round eye sockets, and high braincases, but there was no strong evidence to support this. New fossils found in Comallo, Argentina, show the terror bird had a triangular shape when viewed from above, a hooked beak longer than half the skull, and a shorter, more compact back part of the skull. The openings in the nose were more square than triangular, giving the skull a more rectangular shape. These features suggest the birds may have been faster than previously believed.

A skull of Psilopterus shows the beak was hollow and reinforced with thin, bony supports. It lacks two structures, the zona flexoria palatina and zona flexoria arcus jugalis, which are important for the evolution of skull movement. This discovery helps scientists compare bone structures, study how the skull functioned, and understand the bird's evolutionary relationships.

Palaeobiology

All phorusrhacids had a large, hooked beak and a large skull. The beak’s bones were tightly joined, making it strong against force from front to back. This suggests the beak could cause harm through pecking. However, earlier studies showed they had weak bite force. This led some scientists to think phorusrhacids were not well-suited to hunt large prey, as they might need repeated pecks to harm smaller prey like rodents. However, similar skull and neck features that combine weak bite force with strength against vertical and front-to-back stress are found in other animals, such as certain felids, sabre-toothed mammals, and theropod dinosaurs, which are usually considered hunters of large herbivores. Later studies found phorusrhacids had much stronger bite forces than previously believed. These findings suggest larger phorusrhacids may have hunted larger mammals, while smaller ones with less robust skulls likely focused on small prey.

Some phorusrhacids, like Andalgalornis, were fast runners in straight lines but struggled to make sharp turns at high speeds. This contradicts the idea that they were agile hunters of small prey and instead suggests they may have targeted larger prey.

All phorusrhacids are believed to have been carnivorous. The downward curve of their beak suggests it was used to tear flesh from other animals. Many modern birds with similar beak shapes are also carnivorous. CT scans of a phorusrhacid’s skull showed it could not move prey side to side but could apply strong downward force.

Florentino Ameghino wrote to Édouard Trouessart that he found fossils in Argentina resembling giant owl pellets, which contain undigested remains of prey. These fossils included crushed bones of large rodents, small notoungulates, and deer-sized litopterns, all clustered in a spherical shape with a skull in the center. This suggested phorusrhacids may have swallowed prey whole and regurgitated indigestible parts, like owls. However, Ameghino never officially described these fossils, and they have not been relocated, making it hard to confirm if they are phorusrhacid pellets. Fossilized pellets from northwestern Argentina have also been linked to small phorusrhacids like Procariama.

Classification

The name Phorusrhacidae comes from the type genus Phorusrhacos. When Florentino Ameghino first described Phorusrhacos in 1887, the meaning of its name was not explained. Today, scientists believe the name combines the Greek words "phoros," meaning "bearer" or "bearing," and "rhakos," meaning "wrinkles," "scars," or "tears." Researchers have compared Phorusrhacidae to living bird families, such as Cariamidae and Sagittariidae. However, the large differences in body size between these groups make it difficult to rely on them for answers.

During the early Cenozoic era, after non-bird dinosaurs disappeared, mammals evolved into many different forms. At the same time, some bird groups, including Gastornithidae, Dromornithidae, Palaeognathae, and Phorusrhacidae, grew very large. Phorusrhacids were an extinct group in the order Cariamiformes, which today only has two living species of seriemas in the family Cariamidae. While Phorusrhacidae has many species, the relationships between them are unclear because their fossils are incomplete. Before phorusrhacids arrived in North America, a group of predatory birds called bathornithids lived there from the Eocene to Miocene periods. These birds filled a similar role to phorusrhacids. According to a 2016 study by paleontologist Gerald Mayr, the only genus in the bathornithid family is Bathornis, which had longer limbs and skulls more similar to those of modern seriemas.

A 2024 study found that Bathornis is more closely related to seriemas than to phorusrhacids. After a 2003 revision by Alvarenga and Höfling, Phorusrhacidae is divided into five subfamilies with 13 genera and 22 species. These species evolved into many different forms over time. This classification is based on a 2024 study by LaBarge, Garderner, and Organ, which excluded species with uncertain placement from its analysis (except for Brontornis).

Family Phorusrhacidae

Alvarenga and Höfling did not include the Ameghinornithidae from Europe in their classification of phorusrhacoids. Later research showed these birds are more primitive members of Cariamae. Though once thought to belong to Gruiformes, recent studies using both physical traits and genetic data (from seriemas) suggest Cariamiformes may be part of a separate bird group called Australaves. According to nuclear sequence studies, the closest living relatives of Cariamiformes are a group including falcons, parrots, and songbirds.

The following list shows species analyzed by Degrange and colleagues in 2015:
– Mesembriornis incertus
– Mesembriornis milneedwardsi
– Psilopterus bachmanni
– Phorusrhacos longissimus
– Andalgalornis steulleti
– Paraphysornis brasiliensis

Extinction

During the Miocene and early Pliocene periods, the number of phorusrhacid species in South America increased, showing that these birds thrived as predators in the savanna environment during that time.

The formation of the Isthmus of Panama 2.7 million years ago allowed carnivorous dogs, bears, and cats from North America to move into South America, creating more competition for food and resources. Procyonids, which are a group of mammals, had already arrived in South America as early as 7.3 million years ago. Scientists once believed that the arrival of these new predators led to a decline in phorusrhacid populations and their eventual extinction. Similar ideas were also considered for sparassodonts and sebecid crocodiles in South America.

In recent years, however, the idea that competition caused the extinction of South American predators has been questioned and mostly dismissed. The timing of major changes in predator populations does not match the arrival of large carnivores like canids or saber-toothed cats. However, it does align with procyonids, which grew large in South America but were omnivorous. Native predators, including most phorusrhacids, sparassodonts, and sebecids, disappeared long before most large placental carnivores arrived. Bathornithids, which shared a similar lifestyle and may be related to phorusrhacids, lived in North America during part of the Cenozoic era. They competed with large carnivores like nimravids but became extinct in the Early Miocene, about 20 million years ago. Phorusrhacids like Titanis migrated north into North America during the Great American Interchange and survived there for millions of years. During this time, Titanis lived alongside other carnivores such as borophagines, hyaenids, and machairodonts.

This theory about competition also depends on the idea that phorusrhacids were similar to flightless birds on islands. However, South America was not like islands, as it had a much larger land area. Studies of bone growth patterns in phorusrhacids show they grew quickly and continuously, unlike island birds, which grew more slowly. This difference suggests that phorusrhacids were better at surviving competition or predation. South America also had non-avian predators like sebecids and sparassodonts, which island birds did not face. Scientists concluded that the fast growth of phorusrhacids likely made them less vulnerable to competition or predation, meaning their extinction was more likely due to environmental changes.

Some researchers once thought human activity, such as hunting or habitat changes, caused the extinction of phorusrhacids, like other large Pleistocene animals. However, this idea is no longer supported. Dating of Titanis fossils shows that the last large phorusrhacids went extinct more than 1.5 million years before humans arrived in South America. Smaller phorusrhacid species, however, may have survived longer. Fossil evidence, such as the partial leg bone of Eschatornis, dates to 25,326–25,733 calendar years before the present. Another species, Psilopterus, may have existed as recently as 96,040 ± 6,300 years ago. A smaller, unidentified phorusrhacid from Uruguay was dated to about 17,620 ± 100 years ago, though this date is questionable. A more reliable date for the site comes from the fossil of Macrauchenia patachonica, which is about 21,600 ± 1,000 years old.