Phorusrhacids, also called terror birds, were a group of large, meat-eating birds that mostly could not fly. They were among the top predators in South America during the Cenozoic era. Fossil records of these birds date back to the Middle Eocene and end in the Late Pleistocene, roughly 43 million to 25,000 years ago. Some evidence suggests they may have existed even earlier, during the Early Eocene.

These birds ranged in height from 1 to 3 meters (about 3 to 10 feet). One of the largest, found in Uruguay, likely weighed up to 350 kilograms (770 pounds). Their closest living relatives are seriemas, which are about 80 centimeters (31 inches) tall. Titanis walleri, a large species, lived in Texas and Florida in North America. This shows that phorusrhacids were among the few large South American predators that moved north during the Great American Interchange, which began about 2.6 million years ago. Titanis migrated earlier, around 5 million years ago.

People once thought T. walleri became extinct in North America when humans arrived, but later studies show no evidence of them surviving after 1.8 million years ago. The most recent known phorusrhacid is Eschatornis, found in Brazil and dated to about 25,326 to 25,733 years ago.

Some scientists suggest phorusrhacids may have lived in Africa and Europe, based on fossils like Lavocatavis in Algeria and Eleutherornis in France and Switzerland. However, these fossils are too incomplete to confirm their connection to phorusrhacids. 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 bathornithids, like Paracrax, were about the same size as the largest phorusrhacids. Some studies suggest Bathornis is closely related to phorusrhacids because of shared jaw and bone features, but this connection is debated, as these traits may have evolved independently 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. These features suggest that the phorusrhacid had a very flexible and strong neck, which helped it support its heavy head and attack prey 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 easily. When the neck was fully extended for an attack, its strong muscles and heavy head created enough force to harm prey.

Kelenken guillermoi, a species from the Langhian stage of the Miocene epoch about 15 million years ago, was discovered in 2006 in the Collón Curá Formation in Patagonia. This fossil is the largest bird skull ever found. It is nearly complete and measures about 71 centimeters (28 inches) long. The beak is approximately 46 centimeters (18 inches) long and curves like an eagle’s beak. Most phorusrhacid species were smaller, standing about 60 to 90 centimeters (2.0 to 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 that phorusrhacids competed with predatory mammals like borhyaenids and thylacosmilids, leading these mammals to avoid open plains and live in forests instead.

The feet of phorusrhacids had four toes. The first toe, called the hallux, was small and did not touch the ground. The other three toes were used for walking. Analysis of the second toe and its claw shows it was modified into a "sickle claw." This claw was curved, large, and likely kept elevated to avoid wear or damage. Structures like a well-developed extensor tubercle and soft tissue pads on the fingers helped keep the claw raised. The second toe was shorter and had fewer bones, making it easier to hold the claw off the ground while running or capturing prey. This is similar to how modern seriemas use their feet, though less specialized than dromaeosaurid dinosaurs. Footprints from the Late Miocene in the Río Negro Formation show a trackway made by a mid-to-large terror bird with two toes on the ground and the third toe (with the sickle claw) raised, much like some Mesozoic dinosaurs.

In the past, scientists thought phorusrhacids had high beaks, round eye sockets, and arched braincases, but there was no strong evidence to support this. New fossils found in Comallo, Argentina, show that the terror bird’s skull had a triangular shape when viewed from the top, a hooked beak that was more than half the length of the skull, and a more compact back part. The external nostrils and areas near the nose were more square than triangular, giving the skull a more rectangular shape overall. These features suggest the bird may have been faster than previously believed.

A skull of Psilopterus reveals that the beak was hollow and reinforced with thin, bony structures. It also lacks two specific features—zona flexoria palatina and zona flexoria arcus jugalis—that 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 bones were joined tightly, making the beak strong against force from front to back. This suggests the beak could cause harm through pecking. Earlier studies showed their bite force was weaker than expected. This led some scientists to think phorusrhacids could not easily harm larger prey and likely hunted small 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 machaidorontine felids, sabre-toothed mammals, and allosauroid theropods, which are known to hunt larger herbivores. Later studies found phorusrhacids had much stronger bite forces than previously thought. 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 quick turns at high speeds. This contradicts the idea that phorusrhacids were agile hunters of small prey and suggests they may have targeted larger prey instead.

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 skull show the animal could not move prey side to side but could apply strong downward force.

Florentino Ameghino wrote to Édouard Trouessart that he found fossilized remains in Argentina, including crushed bones of large rodents, small notoungulates, and deer-sized litopterns, arranged in a spherical mass with a skull in the center. This resembled owl pellets, which birds regurgitate after eating prey. Ameghino suggested phorusrhacids may have swallowed prey whole and regurgitated indigestible parts like owls. However, Ameghino never formally described these remains, and they have not been relocated, making it unclear if they are truly 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, he did not explain the meaning of its name. Today, scientists believe the name combines the Greek words "phoros," meaning "bearer" or "bearing," and "rhakos," which refers to "wrinkles," "scars," or "rents." Researchers have compared Phorusrhacidae to the living families Cariamidae and Sagittariidae, but differences in body size are too large to rely heavily on these families for answers.

During the early Cenozoic Era, after non-bird dinosaurs went extinct, mammals evolved into many different forms. Around the same time, some bird groups grew very large, including the families Gastornithidae, Dromornithidae, Palaeognathae, and Phorusrhacidae. Phorusrhacids are an extinct group within Cariamiformes, a group that now only includes two living species of seriemas in the family Cariamidae. Although Phorusrhacidae has many species, scientists do not fully understand how these species are related because their fossils are incomplete. A group of predatory birds called bathornithids lived in North America before phorusrhacids arrived. These birds lived from the Eocene to the Miocene and 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 a skull more similar to seriemas than to phorusrhacids.

A 2024 study found that Bathornis is more closely related to seriemas than to phorusrhacids. After a 2003 study by Alvarenga and Höfling, Phorusrhacidae is now divided into five subfamilies, containing 13 genera and 22 species. These species evolved into many different forms through a process called adaptive radiation. The classification below is based on a 2024 study by LaBarge, Garderner, and Organ, which excluded species with uncertain placement (incertae sedis) from its analysis, except for Brontornis.

Family Phorusrhacidae
– Incertae sedis
– ? Lavocatavis – Middle Eocene, Algeria (possibly related to Eremopezus)
– ? Patagorhacos – Early Miocene, Argentina
– ? Paleopsilopterus – Lower Eocene, Brazil (identity as a phorusrhacid uncertain)
– ? Brontornis – Early to Middle Miocene, Argentina (gigantic, 8.6 feet tall; placement in Phorusrhacidae debated)
– ? Eleutherornis – Middle Eocene, France and Switzerland (related to Strigogyps)

• Subfamily Physornithinae (equivalent to Brontornithinae if Brontornis is included)
• Paraphysornis – Late Oligocene to Early Miocene, Brazil
• Physornis – Middle to Late Oligocene, Argentina

• Subfamily Phorusrhacinae (giant species, 8.3 feet tall; Kelenken up to 9.8 feet tall)
• Devincenzia – Miocene to Early Pliocene
• Kelenken – Middle Miocene, Argentina (largest known phorusrhacid)
• Phorusrhacos – Early to Middle Miocene, Argentina
• Titanis – Early Pliocene to Early Pleistocene, Florida, California, and Texas

• Subfamily Patagornithinae (intermediate-sized, 5.4 feet tall)
• Patagornis – Early to Middle Miocene, Argentina (includes Morenomerceraria, Palaeociconia, Tolmodus)
• Andrewsornis – Middle to Late Oligocene, Argentina
• Andalgalornis – Late Miocene to Early Pliocene, Argentina

• Subfamily Psilopterinae (small species, 3.2 feet tall)
• Eschatornis – Late Pleistocene, Brazil
• Psilopterus – Middle Oligocene, Argentina; Late Miocene, Argentina (possibly Late Pleistocene, Uruguay)
• Procariama – Late Miocene to Early Pliocene, Argentina

• Subfamily Mesembriornithinae (medium-sized, 4.4 feet tall)
• Mesembriornis – Late Miocene to Late Pliocene, Argentina
• Llallawavis – Late Pliocene, Argentina

Alvarenga and Höfling did not include the Ameghinornithidae from Europe in the phorusrhacoids. These birds are now considered older relatives in the Cariamae group. Though once classified in Gruiformes, recent studies suggest Cariamiformes may belong to a separate bird group called Australaves. Genetic studies indicate their closest living relatives are a group including falcons, parrots, and songbirds.

Cladogram (based on Degrange et al., 2015):
– Mesembriornis incertus
– *Mesembriornis milneedwardsi

Extinction

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

The formation of the Isthmus of Panama 2.7 million years ago allowed carnivorous dogs, bears, and cats from North America to migrate into South America, increasing competition for resources. Procyonids, a group of animals related to raccoons, had arrived in South America earlier, about 7.3 million years ago. Scientists once believed that the arrival of these new predators caused the phorusrhacid population to decline and eventually go extinct. Similar ideas were also proposed for sparassodonts and sebecid crocodiles.

In more recent years, however, the idea that competition from new predators caused the extinction of South American predators has been questioned and largely 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 match the earlier arrival of procyonids, which grew large in South America but were omnivorous. Native South American predators, including most phorusrhacids, sparassodonts, and sebecids, disappeared long before most large placental carnivores arrived. Bathornithids, which shared similar ecological roles and were likely close relatives of phorusrhacids, lived in North America during the Cenozoic era and competed successfully with large carnivores like nimravids before going extinct in the Early Miocene, about 20 million years ago. Phorusrhacids like Titanis moved north into southern North America during the Great American Interchange and survived there for millions of years. During this time, Titanis coexisted with other carnivores such as borophagines, hyaenids, and machairodonts.

This theory also relies on the assumption that phorusrhacids had traits similar to flightless birds on islands. However, South America was not an island and had different conditions. Island evolution depends on the size of the landmass, and studies of bone growth patterns in phorusrhacids show rapid, uninterrupted growth, unlike the slower growth seen in island birds. This difference suggests that phorusrhacids were better adapted to compete with predators. South America had non-avian predators like sebecids and sparassodonts, which island birds did not face. Researchers concluded that the fast growth of phorusrhacids likely made them less vulnerable to competition or predation, suggesting their extinction was caused by environmental changes.

Some scientists once thought human activity, such as hunting or habitat changes, caused the extinction of phorusrhacids, like other Pleistocene megafauna. However, this idea is no longer supported because dating of Titanis fossils shows that large phorusrhacids went extinct more than 1.5 million years before humans arrived in the Americas. Smaller phorusrhacid species, however, may have survived longer. The most clear evidence is the fossil of Eschatornis, which was dated to 25,326–25,733 calendar years before the present. Another species, Psilopterus, may have survived until about 96,040 years ago. A smaller, unidentified phorusrhacid from Uruguay was dated to around 17,620 years ago, though this dating is uncertain. The remains of Macrauchenia patachonica from the same site were more precisely dated to about 21,600 years ago using radiocarbon and gamma spectrometry methods.