Titanis

For Greek myth figure, see Titanis (mythology).

Titanis Temporal range: Early Pliocene-Late Pliocene (Hemphillian-Blancan) ?
Reconstructed skeleton, Florida Museum of Natural History
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Cariamiformes
Family:	†Phorusrhacidae
Subfamily:	†Phorusrhacinae
Genus:	†Titanis Brodkorb, 1963
Type species
†Titanis walleri Brodkorb, 1963

Titanis (from Greek for "titan") is a genus of phorusrhacid ("terror birds"), an extinct family of large, predatory birds, in the order Cariamiformes (an order including phorusrhacids and the extant Seriemas) that inhabited the United States during the Pliocene. Titanis is very unique among phorusrhacids in that it is the only phorusrhacid known from North America, crossing over during the Great American Interchange. The holotype (name-bearing) specimen was first unearthed by amateur archaeologist Benjamin Waller from the Santa Fe River in Florida, United States, and was named Titanis walleri by ornithologist Pierce Brodkorb in 1963. The specimen was fragmentary, consisting of only an incomplete right tarsometatarsus shin bone and phalanx toe bone, but comes from one of the largest phorusrhacid individuals known. The genus name references the Greek titans due to its large size and the specific name is after the collector Benjamin Waller. In years following the description, many more isolated elements have been unearthed from sites from other areas of Florida, Texas, and possibly California. However, Titanis remains poorly known and undescribed. Titanis was in the subfamily Phorusrhacinae, which includes some of the last and largest phorusrhacids like Devincenzia and Kelenken.

Titanis, being a phorusrhacid, had elongated hind limbs, thin pelves, proportionally small wings, and huge skulls, with a tall, long, sideways compressed hooked beak. Titanis was one of the largest phorusrhacids, rivaling Kelenken and Phorusrhacos in size based off of preserved material. A 2005 estimate placed Titanis at 2 to 2.5 meters (6.6 to 8.2 ft) in height and weighing 200 kilograms (440 lb), though it may have been more lightly built than the other phorusrhacids Devincenzia. Due to the fragmentary fossils, the anatomy is poorly known, but several distinct characters on the tarsometatarsus and wing bones have been observed. The skull has been estimated to have been between 321 millimetres (12.6 in) and 542 millimetres (21.3 in), one of the largest known from a bird. Its cranium was likely taller than that of Kelenken, making it more vertically robust.

Phorusrhacids are thought to have been ground predators or scavengers, and have often been considered apex predators that dominated Cenozoic South America in the absence of placental mammalian predators, though they did co-exist with some large, carnivorous borhyaenid mammals. Titanis co-existed with many placental predators in North America and was likely one of several apex predators in its ecosystem. The tarsometatarsus was long and slender, like that of its relative Kelenken, which suggests that it could run faster than had previously been assumed for large phorusrhacids, and would have been able to chase down small animals. Studies of the related Andalgalornis show that large phorusrhacids had very rigid and stiff skulls; this indicates they may have swallowed small prey whole or targeted larger prey with repetitive strikes with the beak. Titanis is known from the Pliocene deposits of Florida and southeastern Texas, regions with large open savannas and a menagerie of mammalian megafauna. Titanis was an apex predator in this ecosystem, likely preying on mammals like the extinct armadillo relatives Holmesina and Glyptotherium, equids, tapirs, capybaras, and other herbivores of Pliocene Florida.

Discovery and age

Pierce Brodkorb with the tarsometatarsus of Titanis (dark) and another bird

The earliest discovery of Titanis fossils originates from the winter of 1961/1962, when amateur archaeologists named Benjamin Waller and Robert Allen were hunting for fossils through the use of scuba gear in the Santa Fe River on the border of Gilchrist and Columbia Counties in Florida, United States. The two collectors donated their discoveries to the Florida Museum of Natural History (UF) later along with bones of equids, proboscideans, and many other Floridan fossils. Waller and Allen's fossils consisted of only a distal tarsometatarsus (shin bone) and a pedal phalanx (toe bone), deposited under specimen number UF 4108, but were instantly recognized as unique by paleontologist Clayton Ray who recognized the avian affinity of the material after going through the museum's donations. Ray also noted their stratigraphic origin, being found in a sedimentary layer containing the equid Nannippus and "bone-crushing" dog Borophagus, indicating that they originated from the upper part of the Blancan stage (2.2-1.8 million years old). Ray presented the Santa Fe fossils to the museum's ornithologist Pierce Brodkorb, who mistakenly believed that they were from Rancholabrean strata and came from a relative of the South American rhea. Brokorb created a manuscript assigning it to the Struthioniformes, though Ray pushed Brokorb to change his assessment. Brodkorb published his description in 1963, dubbing the genus and species Titanis walleri, the generic name originating from the Greek titans due to the size of the bird and walleri after one of the type specimen's collector, Benjamin Walker. Titanis was grouped with the subfamily Phorusrhacinae within Phorusrhacidae, along with Phorusrhacos and Devincenzia, as was suggested by Ray.

Titanis has been found in three more sites within Florida since the discovery of the genus: Inglis 1a, Citrus County; Port Charlotte, Charlotte County; and a shell pit in Sarasota County. However, 27 of the 40 Floridan specimens of Titanis have been unearthed from the Santa Fe River, many of them collected in the 1960s and 70s folliwing Brodkorb's description. The Santa Fe River specimens come from two localities within the river, 1a and 1b, the former locality being more productive, producing elements including vertebrae, limb bones, and even parts of the skull. As for Inglis 1a, it was originally a sinkhole during the Pliocene, but became a sedimentary layer of clay that uncovered during construction of the Cross Florida Barge Canal by the federal government. A pair of graduate students from the University of Florida were the first to discover fossils in the clay sediments in 1967, sparking a wave of large-scale excavation by curator David Webb of the Florida Museum of Natural History. Work on the site lasted from 1967 to 1973, in which over 18,000 fossils were accrued. Despite the large number of fossils, only 12 of them belonged to Titanis walleri, including cervical vertebrae, a carpometacarpus, and a metatarsal. As for Port Charlotte, a single fossil, a partial pedal phalanx from the fourth digit, was donated to the UF in 1990. Another partial tarsometatarsus was reportedly found in a shell pit in Sarasota County, making it the only other tarsometatarsus known from Titanis.

Texan and Californian discoveries

A new discovery of Titanis came in 1995, in which the description an isolated pedal phalanx that had been recovered from a sand and gravel pit near Odem along the Nueces River in San Patricio County, Texas. This was the first description of Titanis fossils that referred it to the genus from outside of Florida. The pit was largely dissorganized, with fossils coming from the Early Pliocene and Late Pleistocene jumbled together inside the pit. This led to an incorrect age assessment, following Brodkorb's aging error. Later analyses of rare earth elements within the fossil did demonstrate that the Texan Titanis derived from Pliocene rocks of the Hemphillian stage, a period preceding the formation of the Isthmus of Panama. This would make it the oldest estimate of a Titanis fossil, at 5 million years old, compared to the Floridan fossils which are around 2.2-1.8 million years old from the Blancan.

In 1961, while fossil collecting, G. Davidson Woodward acquired several avian fossils from sediments in the Pliocene-aged (3.7 million year old) strata of the Olla Formation in Anza-Borrego Desert State Park, California. A wing bone from a large, carnivorous teratorn called Aiolornis incredibilis, though Aiolornis was considered a species of Teratornis for many years, was unearthed in addition to the premaxilla (ABDSP/LACM 6747/V26697) of a giant bird. Later in 1972, ornithologist Hildegarde Howard referred all of the fossils to A. (=Teratornis) incredibilis, an assessment supported by Campbell et al's description of the remains in 1999. However, biologist Robert E. Chandler noted its similarities to phorusrhacid premaxillae and a description of the premaxilla followed in 2013 that stated it came from Titanis based on its geographic and stratigraphic location in addition to its phorusrhacid affinities. The age of the Anza-Borrego premaxilla is estimated at 3.7 million years old, making it the oldest confirmed individual of Titanis, though the Texan specimen may be older.

Taxonomy

During the early Cenozoic, after the extinction of the non-bird dinosaurs, mammals underwent an evolutionary diversification, and some bird groups around the world developed a tendency towards gigantism; this included the Gastornithidae, the Dromornithidae, the Palaeognathae, and the Phorusrhacidae. Phorusrhacids are an extinct group within Cariamiformes, the only living members of which are the two species of seriemas in the family Cariamidae. While they are the most speciose group within Cariamiformes, the interrelationships between phorusrhacids are unclear due to the incompleteness of their remains. A lineage of related predatory birds, the bathornithids, occupied North America prior to the arrival of phorusrhacids, living from the Eocene to Miocene and filled a similar niche to phorusrhacids. Only one genus belongs in the family, Bathornis, according to a 2016 analysis by paleontologist Gerald Mayr, who noted that Bathornis was more lightly built with longer limbs proportionally and carried skulls more akin to those of Cariama.

Phylogenetic analysis of Cariamiformes and their relatives according to Mayr (2016) in his redescription of Bathornis:

	Anhimidae Cathartidae Falconidae Opisthocomidae Paracrax Elaphrocnemus  Cariamiformes  Bathornis Ameghinornis Dynamopterus Cariama  Phorusrhacidae  Psilopterus Patagornis Llallawavis

Skeletal reconstruction of the Titanis holotype individual, showing its size.

Phorusrhacids originated in South America during the Paleocene (when the continent was an isolated island) and survived until the Pleistocene, eventually spreading to North America through Titanis. While fossils from Europe and Africa have been assigned to the group, their classification is disputed. It is unclear where the group originated; both cariamids and phorusrhacids may have arisen in South America, or arrived from elsewhere when southern continents were closer together or when sea levels were lower. Since phorusrhacids survived until the Pleistocene, they appear to have been more successful than for example the South American metatherian thylacosmilid predators (which disappeared in the Pliocene), and it is possible that they competed ecologically with placental predators that entered from North America in the Pleistocene. Titanis itself coexisted with a variety of mammalian predators and contended with them, including placental carnivorans like the saber-toothed cat Smilodon, cheetah-like Miracinonyx, "bone-crushing" dog Borophagus, and the short-faced bear Arctodus. All of these genera went extinct by the middle Holocene during the Quaternary Extinction Event, in which the last phorusrhacids also died out during.

The internal phylogenetics of Phorusrhacidae have recently received more analysis in the 21st century, though for many decades they were uncertain, with many subfamilies and genera being dubbed in quick succession. Titanis, however, has consistently been regarded as being within the subfamily Phorusrhacinae along with Phorusrhacos, Kelenken, and Devincenzia. Brazilian paleontologist Herculano Alvarenga and colleagues published a phylogenetic analysis of Phorusrhacidae in 2011 that did not separate Brontornithinae, Phorusrhacinae, and Patagornithinae, ending up with Titanis in polytomy. In their 2015 description of Llallawavis, the Argentinian paleontologist Federico J. Degrange and colleagues performed a phylogenetic analysis of Phorusrhacidae, wherein they found Phorusrhacinae to be polyphyletic (an unnatural grouping). The following cladogram shows the position of Titanis following the 2015 analysis:

Phorusrhacidae

Mesembriornithinae   Mesembriornis incertus  Mesembriornis milneedwardsi  Llallawavis scagliai  Procariama simplex  Psilopterinae   Psilopterus affinis  Psilopterus bachmanni  Psilopterus colzecus  Psilopterus lemoinei  Kelenken guillermoi  Devincenzia pozzi   Titanis walleri  Phorusrhacos longissimus  Andalgalornis steulleti  Andrewsornis abbotti  Patagornis marshi  Physornis fortis  Paraphysornis brasiliensis

Description

Lsife reconstruction

Phorusrhacids were large, flightless birds with long hind limbs, narrow pelvises, proportionally small wings, and huge skulls, with a tall, long, sideways compressed hooked beak. Overall, Titanis was very similar to the South American Phorusrhacos and Devincenzia, its closest relatives. However, it differs from these in having a shorter, thicker neck, and an overall more heavily built bodily structure. Little is known of its body structure, but it seems to have been less wide-footed than Devincenzia, with a proportionally much stronger middle toe. Titanis size has been estimated several times, with older guesses placing it at 2 to 2.5 meters (6.6 to 8.2 ft) tall, but more accurate scaling after the discovery of new material downsized it to 1.4 to 1.87 meters (4.6 to 6.1 ft) tall and 200 kilograms (440 lb) in weight.

Skull

Of the skull, only the premaxilla, frontal, pterygoid, quadrate, orbital process, and two quadratojugals have been mentioned in scientific literature as being known. The premaxilla is incomplete, consisting of its anteriormost end including the caracteristic long sharp beak tip of phorusrhacidae that would have been used for predatory purposes. Its preserved length is 90 mm with a height of 55 mm, with a traingular shape in vertical cross section. Sides of the fossil are flat bearing a large dorsal crest, as in other thin-skulled phorusrhacids like Phorusrhacos. The culmen (arc) of the exposed premaxilla was described as identical to that in Patagornis marshi, an Argentine phorusrhacid. The pterygoid is giant, as seen in other phorusrhacids, at 100 mm in complete length with a medially placed facet for its articulation to the basipterygoid process. Two quadratojugals are preserved, one of which has a more pronounced crest cranial to the articulation tubercular. This is the opposite in the smaller individual, which has a deep fossa anterocranial to the articulation tubercular. Potential sexual dimorphism has been suggested twice due to the lack of signs of unfinished ontogenetic development in the smaller quadratojugal, meaning they both come from adults. In the lower jaw, a partial mandible is known but it is unfigured and undescribed. Being a phorusrhacine, it would have had a long and narrow symphysis ending in a sharp tip that was anteriorly oriented.

Postcrania

As for the postcranial anatomy, Titanis and other phorusrhacines were heavily built with an elongated, thin tarsometatarsus that was around half the length of the tibiotarsus. The pes was large and had three toes, the middle being the longest, with a large killing ungual on the third digit. The spinal column is poorly known from Titanis, though several vertebrae have been collected from Florida. The necks were long and somewhat flexible, whereas the dorsal, sacral, and caudal vertebrae were more rigid. The dorsal vertebrae had tall neural spines and large robust ribs connected to the sacral ribs.

The wings were small and could not have been used for flight, but were much more strongly built than those of living ratites. The wing bones articulated in an unusual joint-like structure, suggesting the digits could flex to some degree. It also had a relatively rigid wrist, which would not have allowed the hand to fold back against the arm to the same degree as other birds. This led R. M. Chandler to suggest that the wings may have supported some type of clawed, mobile hand similar to the hands of non-avian theropod dinosaurs, such as the dromaeosaurs (also popularly known as "raptors"). However, it was later pointed out that this wing joint is not in fact unique, and is present in seriemas (extant members of the same order, Cariamae, to which Titanis and other phorusrhacids belonged), which do not have any specialized grasping hands. Evidence of large quill-feathers is known from Patagornis and Llallawavis, with large tubercles called quill knobs being found on their ulnae. These quill knobs would’ve supported long feathers, though their purpose is unknown.

Paleobiology

Feeding and diet

Restoration of a feeding group

Phorusrhacids are thought to have been ground predators or scavengers, and have often been considered apex predators that dominated Cenozoic South America in the absence of placental mammalian predators, though they did co-exist with some large, carnivorous borhyaenid mammals. Earlier hypotheses of phorusrhacid feeding ecology were mainly inferred from them having large skulls with hooked beaks rather than through detailed hypotheses and biomechanical studies, and such studies of their running and predatory adaptations were only conducted from the beginning of the 21st century.

Alvarenga and Elizabeth Höfling made some general remarks about phorusrhacid habits in a 2003 article. They were flightless, as evidenced by the proportional size of their wings and body mass, and wing-size was more reduced in larger members of the group. These researchers pointed out that the narrowing of the pelvis, upper maxilla, and thorax could have been adaptations to enable the birds to search for and take smaller animals in tall plant growth or broken terrain. The large expansions above the eyes formed by the lacrimal bones (similar to what is seen in modern hawks) would have protected the eyes against the sun, and enabled keen eyesight, which indicates they hunted by sight in open, sunlit areas, and not shaded forests.

Leg function

In 2005, Rudemar Ernesto Blanco and Washington W. Jones examined the strength of the tibiotarsus (shin bone) of phorusrhacids to determine their speed, but conceded that such estimates can be unreliable even for extant animals. While the tibiotarsal strength of Patagornis and an indeterminate large phorusrhacine suggested a speed of 14 m/s (50 km/h; 31 mph), and that of Mesembriornis suggested 27 m/s (97 km/h; 60 mph), the latter is greater than that of a modern ostrich, approaching that of a cheetah, 29 m/s (100 km/h; 65 mph). They found these estimates unlikely due to the large body size of these birds, and instead suggested the strength could have been used to break the long-bones of medium-sized mammals, the size for example of a saiga or Thomson's gazelle. This strength could be used for accessing the marrow inside the bones, or by using the legs as kicking weapons (like some modern ground birds do), consistent with the large, curved, and sideways compressed claws known in some phorusrhacids. They also suggested future studies could examine whether they could have used their beaks and claws against well-armored mammals such as armadillos and glyptodonts.

According to Chiappe and Bertelli in 2006, the discovery of Kelenken shed doubt on the traditional idea that the size and agility of phorusrhacids correlated, with the larger members of the group being more bulky and less adapted for running. The long and slender tarsometatarsus of Kelenken instead shows that this bird may have been much swifter than the smaller, more heavyset and slow Brontornis. In a 2006 news article about the discovery, Chiappe stated that while Kelenken may not have been as swift as an ostrich, it could clearly run faster than had previously been assumed for large phorusrhacids, based on the long, slender leg-bones, superficially similar to those of the modern, flightless rhea. The article suggested that Kelenken would have been able to chase down small mammals and reptiles. In another 2006 news article, Chiappe stated that Kelenken would have been as quick as a greyhound, and that while there were other large predators in South America at the time, they were limited in numbers and not as fast and agile as the phorusrhacids, and the many grazing mammals would have provided ample prey. Chiappe stated that phorusrhacids crudely resembled earlier predatory dinosaurs like Tyrannosaurus, in having gigantic heads, very small forelimbs, and very long legs, and thereby had the same kind of meat-eater adaptations.

Skull and neck function

Stress distribution in bird skulls during various movements, including the related Andalgalornis (left, A-C, the other skulls belong to a red-legged seriema and a white-tailed eagle), and hypothetical up and downwards range of movement of the neck in the same genus (right)

A 2010 study by Degrange and colleagues of the medium-sized phorusrhacid Andalgalornis, based on Finite Element Analysis using CT scans, estimated its bite force and stress distribution in its skull. They found its bite force to be 133 Newtons at the bill tip, and showed it had lost a large degree of intracranial immobility (mobility of skull bones in relation to each other), as was also the case for other large phorusrhacids such as Kelenken. These researchers interpreted this loss as an adaptation for enhanced rigidity of the skull; compared to the modern red-legged seriema and white-tailed eagle, the skull of the phorusrhacid showed relatively high stress under sideways loadings, but low stress where force was applied up and down, and in simulations of "pullback". Due to the relative weakness of the skull at the sides and midline, these researchers considered it unlikely that Andalgalornis engaged in potentially risky behavior that involved using its beak to subdue large, struggling prey. Instead, they suggested that it either fed on smaller prey that could be killed and consumed more safely, by for example swallowing it whole, or that when targeting large prey, it used a series of well-targeted repetitive strikes with the beak, in a "attack-and-retreat" strategy. Struggling prey could also be restrained with the feet, despite the lack of sharp talons. A 2012 follow up study by Tambussi and colleagues analyzed the flexibility of the neck of Andalgalornis, based on the morphology of its neck vertebrae, finding the neck to be divided into three sections. By manually manipulating the vertebrae, they concluded that the neck musculature and skeleton of Andalgalornis was adapted to carrying a large head, and for helping it rise from a maximum extension after a downwards strike, and the researchers assumed the same would be true for other large, big-headed phorusrhacids. A 2020 study of phorusrhacid skull morphology by Degrange found that there were two main morphotypes within the group, derived from a seriema-like ancestor. These were the "Psilopterine Skull Type", which was plesiomorphic (more similar to the ancestral type), and the "Terror Bird Skull Type", which included Titanis and other large members, that was more specialized, with more rigid and stiff skulls. Despite the differences, studies have shown the two types handled prey similarly, while the more rigid skulls and resulting larger bite force of the "Terror Bird" type would have been an adaptation to handling larger prey.

Paleoenvironment

During the Blancan, Titanis coexisted with many endemic genera from North America that had not yet gone extinct due to the evolution of modern mammal genera. Because of this, the fauna of the Blancan starkly contrasted with the fauna of the Pleistocene and Holocene. Localities in which Titanis is known are all tropical or subtropical in climate, with dense forests and a variety of flora. In Inglis 1a specifically, longleaf pine flatwoods and pine-oak scrub are known to have occupied the area based on the fossils of lizards, birds, and snakes. During the Pliocene-Miocene climatic transition, the climate was cooled but temperatures did not reach those of the Pleistocene, creating a warm period. Sea levels were higher, but this was reversed as the later end of the Pliocene witnessed the formation of glaciers at the poles.

The Blancan age strata of Florida from sites Titanis has been unearthed from preserve over a hundred species and many different megafauna. This includes the gomphothere proboscidean Rhynchotherium and the American Mastodon, perissodactyls represented by the grazing equids Nannipus and Equus with the browsing tapirs Tapirus lundeliusi and Tapirus haysii. A wide array of artiodactyls existed, including; the peccaries Mylohyus and Platygonus, two species of the camelid Hemiauchenia, pronghorn relative Capromeryx, and the extant white-tailed deer. Armadillos and there are also known, like the pampathere Holmesina, giant glyptodont Glyptotherium, and an extinct species of the extant armadillo genus Dasypus, D. bellus. One of the largest groups known from the Blancan of Florida is the ground sloths, which consist of the enormous megathere Eremotherium, the megalonychid Megalonyx, and the medium-sized mylodontid Paramylodon. The carnivores include the unusual “bone-crushing” dog relative Borophagus and the “running hyena” Chasmaporthetes, in addition to the "saber-toothed" cats Xenosmilus and Smilodon gracilis. As for the rodents, the extant North American porcupine and capybaras Hydrochaeris and Neochoerus are known from sites like Inglis 1a. Many fossils of smaller mammals like soricids, lagomorphs, and Ondatra have been found associated with Titanis. In addition to mammals, a menagerie of reptiles is known from the Blancan of Florida. The shells of the giant tortoise Hesperotestudo along with extant box turtles’ are known. Many snakes are also known, most of them in the family Colubridae, and lizards like iguanas lived in the area. The Avifauna of the period has extensive preservation, with thousands of fossils known. The birds-of-prey are represented by falcons like the American kestrel and merlin; the Cooper’s and Black-tailed hawks; an unknown species of eagle; the hawk-eagle Spizaetus; and several owls including Buteo and Strix. Many additional bird groups are known, including a loon, several grebes, herons, a species of American ibis, vultures, pigeons, woodpeckers, turkeys, wading birds, and several others.

Great American Interchange

Examples of fauna that participated in the Great American Interchange.

South America, the continent where glyptodonts originated, was isolated after the breakup of the landmass Gondwana at the end of the Mesozoic era. This period of separation from the rest of the Earth's continents led to an age of unique mammalian and avian evolution, with the dominance of groups such as marsupials, xenarthrans, phorusrhacids, and notoungulates in contrast to the North American mammal fauna. Phorusrhacids evolved independently in South America to fill the apex predator and other predatory niches, with phorusrhacines specifically being rivaled only by the sparassodonts in this regard. Marsupials likely got to South America prior to its separation from the rest of Gondwana in the Late Cretaceous or Paleogene, although the origins of mammalian orders like Xenarthra and Notoungulata ended up on the continent remains a mystery. There were several movements of outside mammals to South America prior to the formation of the Isthmus of Panama, such as with primates and rodents which may have rafted to the continent from Africa and the movement of bats via flight. As for the fauna of North America, contemporary groups like canids, felids, ursids, tapirids, antilocaprids, and equids populated the region in addition to extinct families like gomphotheres, amphicyonids, and mammutids. The Great American Interchange did not enter its biggest stage until the crossing of the Isthmus of Panama 2.7 million years ago during the Blancan stage of the Pliocene, though the Isthmus itself formed 4.5-3.5 million years ago. However, the oldest fossil of Titanis is over 1.3 million years older than the Isthmus’ formation, dating to the late Hemphilian. How Titanis was able to traverse the gap to North America is unknown, though it has been theorized that island hopping through Central America and the Caribbean. Titanis is not the only animal to have done this however, as two genera of ground sloth and a procyonid made it to North America millions of years before the volcanic formation of Panama. The immigration after the full formation witnessed the movement of glyptodonts, capybaras, pampatheres, and marsupials to North America via the Central America route and a reverse migration of ungulates, proboscideans, felids, canids, and many other megafauna groups to South America. The period following the Isthmus' foundation witnessed the extinction or extirpation of many groups, including the South American terror birds, though this would not finish until the Late Pleistocene, toxodonts, macraucheniids, pampatheres, ground sloths, and glyptodonts.

Extinction

The extinction of T. walleri and other phorusrhacids throughout the Americas was originally theorized to have been due to competition with large placental (canid, felid, and possibly ursid) carnivores that radiated in the same ancient terrestrial ecosystems during the Great American Interchange. However, this has been contested as Titanis walleri had competed successfully against both groups for several million years upon entering North America. From circumstantial evidence (i.e., bone fractures), it has been suggested that the species did not become extinct until 15,000 years ago, but more precise dating by McFadden and colleagues refutes such a late date; all known Titanis fossils appear to be at least 2 million years old. Therefore, the extinction of Titanis and phrousrhacidae as a whole is a mystery.

See also

• Devincenzia
• Kelenken
• Phorusrhacos