Phorusrhacidae

Phorusrhacids, often called terror birds, were a group of large, meat-eating birds that could not fly. They lived in South America during a time called the Cenozoic era, from about 43 million to just over 25,000 years ago. These birds stood between 1 to 3 meters tall, and some of the biggest could weigh as much as 350 kilograms.

Their closest living relatives today are much smaller birds called seriemas. One big species, Titanis walleri, lived in Texas and Florida in North America, showing that some of these birds moved north from South America long ago. Scientists used to think they lived until humans arrived in North America, but newer studies show they were gone by about 1.8 million years ago.

Some scientists think phorusrhacids might have lived in Africa and Europe too, based on a few old fossils, but these are not very clear. These large birds were top hunters in their environments, and they were part of a fascinating group of animals that lived long before humans.

Description

Phorusrhacids, often called terror birds, were large, meat-eating birds that could not fly. They lived in South America many millions of years ago. These birds had very flexible necks, which helped them swing their heavy heads quickly to strike their prey.

One of the largest known terror birds, Kelenken guillermoi, lived about 15 million years ago. Its skull was almost 70 centimetres long with a curved, hooked beak. Most terror birds were smaller, about 60 to 90 centimetres tall, but this one could stand up to 3 metres tall. Scientists think these big birds could run very fast, up to 48 kilometres per hour. Their feet had four toes, with a special curved claw on one toe that helped them catch and hold onto their prey. Recent fossil discoveries show that their skulls were more rectangular than previously believed, with a hooked beak and a swifter build.

Palaeobiology

Phorusrhacids had large hooked beaks and big skulls. Their beaks were very strong and could peck hard, which might have helped them hunt. Some scientists first thought they could only catch small animals, but newer studies show that bigger ones could hunt larger animals too. Smaller phorusrhacids probably focused on smaller prey.

All phorusrhacids likely ate meat. Their beaks curved downward, which is common in meat-eating birds today. Studies of their skulls show they could apply strong downward pressure but not shake their prey side to side.

Diet

Researchers have wondered if phorusrhacids swallowed their prey whole and later spit out undigested bits, like some owls do. One scientist mentioned finding piles of crushed animal bones that looked like owl pellets, but these specimens were never fully described, so it's still unclear.

Classification

The name Phorusrhacidae comes from the type genus Phorusrhacos. When first described in 1887, the meaning of Phorusrhacos was not clear. Today, we believe it comes from Greek words meaning “bearer” and “wrinkles” or “scars.”

During the early Cenozoic, after the extinction of the non-bird dinosaurs, mammals began to diversify, and some bird groups grew very large in size. This included groups like the Gastornithidae, Dromornithidae, Palaeognathae, and the Phorusrhacidae. Phorusrhacids are an extinct group within Cariamiformes, which today only includes the two species of seriemas in the family Cariamidae. Though they are the richest group in Cariamiformes, the relationships between phorusrhacids are unclear due to incomplete fossils.

There are 5 subfamilies with 13 genera and 22 species. These species evolved to fill different roles in their environments.

A 2024 study found that one group, Bathornis, was closer to seriemas than to phorusrhacids.

Family Phorusrhacidae

• Incertae sedis
  • Genus ?Lavocatavis – Middle Eocene Glib Zegdou Formation of Algeria (likely more related to a possible paleognath Eremopezus)
  • Genus ?Patagorhacos – Early Miocene Chichinales Formation of Rio Negro Province, Argentina.
  • Genus ?Paleopsilopterus – Lower Eocene (Itaboraian) Itaboraí Formation of Itaboraí, Brazil (identity as a phorusrhacid dubious)
  • Genus ?Brontornis – Early to Middle Miocene (Santacrucian–Laventan) Santa Cruz and Monte León Formations, Argentina – gigantic species, standing on average 8.6 feet (2.6 m) high. Placement in Phorusrhacidae and/or monophyly disputed.
  • Genus ?Eleutherornis – Middle Eocene (Bartonian) of Rhône, France and Baselland, Switzerland (a cariamiform, probably more related to Strigogyps)
• Subfamily Physornithinae — equivalent to Brontornithinae, if Brontornis is included within the family
  • Genus Paraphysornis (Late Oligocene to Early Miocene (Deseadan) Tremembé Formation of São Paulo State, Brazil)
  • Genus Physornis (Middle to Late Oligocene (Deseadan) Sarmiento Formation of Santa Cruz Province, Argentina)
• Subfamily Phorusrhacinae — giant species 8.3 feet (2.5 m) high (Kelenken up to 9.8 feet (3.0 m) high), but somewhat slender and decidedly more nimble than the Brontornithinae
  • Genus Devincenzia – Miocene to Early Pliocene, possibly up to Early Pleistocene
  • Genus Kelenken – Middle Miocene (Colloncuran) Collón Curá Formation of Río Negro Province, Argentina; largest known phorusrhacid
  • Genus Phorusrhacos – Early to Middle Miocene (Santacrucian) Santa Cruz Formation of Argentina
  • Genus Titanis – Early Pliocene to Early Pleistocene (Blancan) of Florida, California, and Texas
• Subfamily Patagornithinae — intermediate sized and very nimble species, standing around 5.4 feet (1.6 m) high
  • Genus Patagornis – Early to Middle Miocene (Santacrucian–Laventan) Santa Cruz Formation of Santa Cruz Province, Argentina – includes Morenomerceraria, Palaeociconia, Tolmodus
  • Genus Andrewsornis – Middle to Late Oligocene (Deseadan) Agua de la Piedra Formation of southern Argentina
  • Genus Andalgalornis – Late Miocene to Early Pliocene (Huayquerian) Ituzaingó Formation of northwestern Argentina
• Subfamily Psilopterinae — small species, standing 3.2 feet (0.98 m) high
  • Genus Eschatornis - Late Pleistocene of Brazil
  • Genus Psilopterus – Middle Oligocene (Deseadan) Santa Cruz Formation and Late Miocene (Chasicoan) Arroyo Chasicó Formation of southern and eastern Argentina respectively (Possible Late Pleistocene (Lujanian) records from Uruguay)
  • Genus Procariama – Late Miocene to Early Pliocene (Huayquerian–Montehermosan) Cerro Azul and Andalhualá Formations of Catamarca Province, Argentina
• Subfamily Mesembriornithinae — medium-sized species, standing 4.4 feet (1.3 m) high
  • Genus Mesembriornis – Late Miocene to Late Pliocene (Montehermosan) Monte Hermoso Formation of Argentina
  • Genus Llallawavis – Late Pliocene (Chapadmalalan) Playa Los Lobos Allo Formation of northeastern Argentina

Alvarenga and Höfling did not include the Ameghinornithidae from Europe in the phorusrhacoids; these have meanwhile turned out to be more basal members of Cariamae. Though traditionally considered as members of the Gruiformes, based on both morphological and genetic studies (the latter being based on the seriema) Cariamiformes may belong to a separate group of birds, Australaves, and their closest living relatives, according to nuclear sequence studies, are a clade consisting of Falconidae, Psittaciformes and Passeriformes.

Extinction

During the Miocene and early Pliocene, the number of these large birds grew in South America, showing they were successful hunters in the open lands.

As land connected North and South America about 2.7 million years ago, new hunters like dogs, bears, and cats arrived from the north. Some scientists thought this caused these birds to disappear because of competition. But newer studies show that these birds died out before many of these new hunters arrived. Instead, changes in the environment likely played a bigger role in their disappearance. Some smaller types of these birds may have lived until around 25,000 years ago, but most were gone long before humans arrived in South America.