Phorusrhacidae

Phorusrhacids Temporal range: Paleocene - Pleistocene,
Reconstructed skeleton of Titanis walleri, Florida Museum of Natural History
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Cariamiformes
Superfamily:	†Phorusrhacoidea
Family:	†Phorusrhacidae Ameghino, 1889
Type species
†Phorusrhacos longissimus Ameghino, 1887
Subfamilies
†Brontornithinae †Mesembriornithinae †Patagornithinae †Phorusrhacinae †Psilopterinae
Synonyms
Family synonymy Pelecyornidae Ameghino, 1891 Brontornithidae Moreno & Mercerat, 1891 Darwinornithidae Moreno & Mercerat, 1891 Stereornithidae Moreno & Mercerat, 1891 Patagornithidae Mercerat, 1897 Hermosiornidae Rovereto, 1914 Psilopteridae Dolgopol de Saez, 1927 Devincenziidae Kraglievich, 1932 Mesembriorniidae Kraglievich, 1932

Phorusrhacids, colloquially known as terror birds, were a clade of large carnivorous flightless birds that were the largest species of apex predators in South America during the Cenozoic, 62–2 million years (Ma) ago.

They were roughly 1–3 metres (3.3–9.8 ft) tall. Their closest modern-day relatives are believed to be the 80 cm-tall seriemas. Titanis walleri, one of the larger species, is known from Texas and Florida in North America. This makes the phorusrhacids the only known example of large South American predators migrating north during the Great American Interchange (which occurred after the volcanic Isthmus of Panama land bridge rose ca. 3 Ma ago). It was once believed that T. walleri only became extinct around the time of the arrival of humans in North America, but subsequent datings of Titanis fossils have failed to provide evidence for their survival more recently than 1.8 Ma ago. However, exists more findings that dates to 450.000 years ago and 17,000 years ago, that suggests that at least some terror birds survived until the late Pleistocene in Uruguay.

Terror birds may have even made their way into Africa, with the genus Lavocatavis recently discovered in Algeria, although its status as a true Phorusrhacid is questionable.

Kelenken guillermoi from Middle Miocene some 15 million years ago, discovered in Patagonia in 2006, represents the largest bird skull yet found. The fossil has been described as being a 71 cm (28 in), nearly intact skull. The beak is roughly 46 cm (18 in) long and curves in a hook shape that resembles an eagle's beak. Most species described as phorusrhacid birds were smaller, 60–90 cm (2.0–3.0 ft) tall, but the new fossil belongs to a bird that probably stood about 3 m (9.8 ft) tall. Scientists theorize that the large terror birds were extremely nimble and quick runners able to reach speeds of 48 km/h (30 mph).

The etymology of the name Phorusrhacidae is based on the type genus Phorusrhacos. When first described by Florentino Ameghino in 1887, the etymology of Phorusrhacos was not given. Current thinking is that the name is derived from a combination of the Greek words "phoros", which means bearer or bearing, and "rhacos", which translates to wrinkles, scars or rents. Researchers have compared Phorusrhacidae with the living families of Cariamidae and Sagittaridae, but their differences in body mass are too drastic, and thus, one cannot depend on these living families for answers.

Behavior

Phorusrhacidae was very fast and agile. It possessed a vicious beak, Powerful neck and sharp talons. However even with these attributes, Terror Bird usually preyed on smaller prey that could be eaten with less of a struggle.This is due to the fact that the Terror bird lacked a strong jaw. Due to the beaks proportions, the jaw could not generated a great deal of force. Although occasionally the Terror bird would attack large prey. It would do so using its beak purely as a spear with quick precise attacks. Then, use its feet to hold the prey down and finish him off. Although Terror Bird had a weak beak, its bones were tightly fused together, making them more resilient to force from the front to back direction.

Neck Structure

Based on Claudia P. Tambussi, Ricardo de Mendoza, Federico J. Degrange, and Mariana B. Picasso’s work, the Terror Bird's neck can be divided into three main regions (region 1, region 2, and region 3). In the higher regions of the neck, the Terror bird has bifurcate neural spines (BNS) while it has high neural spines in its lower regions. This suggests that the Terror Bird had a highly flexible and developed neck allowing it to carry its heavy head and strike with terrifying speed and power. Although the Terror Bird externally looks like it has a short neck, its flexible skeletal neck structure proves that it can expand farther beyond the expected reach and intimidate it's prey using its height, allowing it to strike more easily. Once stretched out into its full length in preparation for a downward strike, its developed neck muscles and heavy head can produce enough momentum and power to cause fatal damage to the Terror Bird’s prey.

Causes of Extinction

From 27 million years to 2.5 million years ago, there was an increase in the Terror Bird population size in South America, suggesting that in that time frame it flourished as an apex predator in the savannah environment. However as the Panamanian Land Bridge emerged, 2.5 million years ago, carnivorous dogs and cats from North America were able to cross into South America, causing an increase of competition. Due to the growth of competition, the population of Terror Birds gradually decreased. This suggests that competition with other predators is a major influence to Phorusrhacidae extinction. Some researchers believe that the Phorusrhpacidae started to go extinct at the beginning of the Pleistocene 2.5 million years ago. South America was also home to a Sebecidae, Madtsoiidae and Sparassodonta, All of which were large carnivores as well that shared similar eating habits to the Phorusrhacidae. This meant even more competition for the Terror Bird. This competition drove the Terror bird out of South American and eventually to extinction.

Recent Skull Discoveries

These birds have been in the past thought to have high beaks, round orbits, and vaulted braincases though there was never enough empirical evidence that could support this. However, new fossils have been discovered in Cormollo, Argentina. These skulls reveal that the terror bird has a triangular dorsal view, a rostrum that is hooked and more than half the length of the actual skull, and a more compact caudal portion. The external nares and antorbital fenestras (areas found in the nose) were found to be more square than triangular. These all contribute to a skull that is more rectangular in view rather than triangular. . The structure of the fossils also suggest that these birds may have been more swift than originally thought.

A skull from a smaller subspecies of this bird was found recently as well. With this fossil, it was found that the internal structure of the beak is hollow and reinforced with thin-walled trabeculaa. There is also an absence of both zona flexoria palatina and zona flexoria arcus jugalis which are key features that relate to the evolution of cranial akinesis. This discovery of the skull allows for the establishment of primary osteological homologies which are useful in comparative anatomy, functional morphology, and phylogenetic studies.

Classification

Following the revision by Alvarenga and Höfling (2003), there are now 5 subfamilies, containing 14 genera and 18 species: These species were the product of adaptive radiation.

• Subfamily Brontornithinae — gigantic species, standing over 2.3 metres (7.5 ft) high. Placement in Phorusrhacidae and/or monophyly disputed.
  • Genus Brontornis (Early - Middle Miocene)
  • Genus Paraphysornis (Late Oligocene/Early Miocene of São Paulo State, Brazil)
  • Genus Physornis (Middle - Late Oligocene of Santa Cruz Province, Argentina)
• Subfamily Phorusrhacinae — giant species 3.2 metres (10 ft) high, but somewhat slender and decidedly more nimble than the Brontornithinae
  • Genus Devincenzia (Late Miocene - Early Pliocene of NE Argentina and Arroyo Roman, Uruguay)
  • Genus Kelenken (Middle Miocene of Río Negro province, Argentina (largest known phorusrhacid))
  • Genus Phorusrhacos (Early - Middle Miocene)
  • Genus Titanis (Early Pliocene - Early Pleistocene)
• Subfamily Patagornithinae — intermediate sized and very nimble species, standing around 1.7 metres (5.6 ft) high
  • Genus Patagornis (Santa Cruz Early - Middle Miocene of Santa Cruz Province, Argentina) - includes Morenomerceraria, Palaeociconia, Tolmodus
  • Genus Andrewsornis (Middle - Late Oligocene of S Argentina)
  • Genus Andalgalornis (Late Miocene - Early Pliocene)
• Subfamily Psilopterinae — small species, standing 70–100 centimetres (2.3–3.3 ft) high
  • Genus Psilopterus (Deseado Middle Oligocene - Arroyo Chasicó Late Miocene of S and E Argentina)
  • Genus Procariama (Late Miocene - Early Pliocene of Catamarca Province, Argentina)
  • Genus Paleopsilopterus (Middle Paleocene of Itaboraí, Brazil)
• Subfamily Mesembriornithinae — medium-sized species, standing between 1.2–1.5 metres (3.9–4.9 ft) high
  • Genus Mesembriornis (Late Miocene - Late Pliocene)

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) that they may belong to a separate group of birds (the Cariamae) and their closest living relatives, according to the last nuclear studies, are a clade conformed by Falconidae, Psittaciformes and Passeriformes

External links

• Hooper Museum
• Terror Birds: Bigger and Faster (Science)
• Darren Naish: Tetrapod Zoology: "terror birds"
• Darren Naish: Tetrapod Zoology: "Raven, the claw-handed bird, last of the phorusrhacids" includes links to other articles on phorusrhacids