Titanis

Genus of terror bird (Phorusrhacidae)
the Greek mythological figure

Titanis (meaning "Titan" for the mythological Greek Titans) is a genus of phorusrhacid ("terror birds", a group originating in South America), an extinct family of large, predatory birds, in the order Cariamiformes that inhabited the United States during the Pliocene and earliest Pleistocene. The first fossils were unearthed by amateur archaeologists Benjamin Waller and Robert Allen from the Santa Fe River in Florida and were named Titanis walleri by ornithologist Pierce Brodkorb in 1963, the species name honoring Waller. The holotype material is fragmentary, consisting of only an incomplete right tarsometatarsus (lower leg bone) and phalanx (toe bone), but comes from one of the largest phorusrhacid individuals known. In the years following the description, many more isolated elements have been unearthed from sites from other areas of Florida, Texas, and California. It was classified in the subfamily Phorusrhacinae, which includes some of the last and largest phorusrhacids like Devincenzia and Kelenken. Like all phorusrhacids, Titanis had elongated hind limbs, a thin pelvis, proportionally small wings, and a large skull with a hooked beak. It was one of the largest phorusrhacids, possibly similar in size to Phorusrhacos based on preserved material. More recent estimates placed Titanis at in height and in body mass. Due to the fragmentary fossils, the anatomy is poorly known, but several distinct characters on the tarsometatarsus have been observed. The skull is estimated to have been between and in length, one of the largest known from any bird. 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 has been suggested to have been agile and capable of running at high speeds. Studies of the related Andalgalornis show that large phorusrhacids had very stiff and stress resistant skulls; this indicates they may have swallowed small prey whole or targeted larger prey with repetitive strikes of the beak. Titanis is known from the Pliocene deposits of Florida, southern California, and southeastern Texas, regions that had large open savannas and a menagerie of mammalian megafauna. It likely preyed on mammals such as the extinct armadillo relatives Holmesina and Glyptotherium, equids, tapirs, capybaras, and other Pliocene herbivores. Titanis is unique among phorusrhacids in that it is the only one known from North America, crossing over from South America during the Great American Interchange.

The earliest discovery of Titanis fossils was in the winter of 1961/1962, when amateur archaeologists Benjamin Waller and Robert Allen were searching for artifacts and fossils using 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 from the late Pliocene and latest Pleistocene. Waller and Allen's avian fossils consisted of only a distal tarsometatarsus (lower leg bone) and a pedal phalanx (toe bone), deposited under specimen numbers UF 4108 and 4109 respectively. They remained without analysis in the museum's donations until they were recognized as unique by paleontologist Clayton Ray in 1962. He noticed the avian features and giant size of the fossils, which led him to believe they were from a phorusrhacid (or "terror bird", a group of large, predatory birds). Despite the many fossils, only 12 of them belonged to Titanis, including cervical vertebrae, a carpometacarpus, and a metatarsal. This was the first description of Titanis fossils from outside Florida. The pit was largely disorganized, with fossils dating to the Early Pliocene and Late Pleistocene jumbled together. The description followed Brodkorb's erroneous Late Pleistocene age assessment. Later analyses of rare earth elements within the fossil demonstrated 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, and therefore from the Blancan age. The wing bone was referred to the teratorn Aiolornis at that time, an assessment backed by ornithologist Hildegarde Howard in 1972. This was supported by later studies, A lineage of related predatory birds, the bathornithids, occupied North America before the arrival of phorusrhacids, living from the Eocene to Miocene and filling a similar niche to cariamids. The oldest phorusrhacid fossils come from South America during the Paleocene (when the continent was an isolated island) and survived until the Pleistocene, eventually spreading to North America through Titanis. Though 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. Titanis itself coexisted with a variety of placental mammalian predators, including carnivorans like the saber toothed cat Smilodon, cheetah like Miracinonyx, wolf like Aenocyon, and the short faced bear Arctodus. All of these genera, including the last phorusrhacids, went extinct during the Late Pleistocene extinctions. Though for many decades the internal phylogenetics of Phorusrhacidae were uncertain and many taxa were named, they have received more analysis in the 21st century. Titanis, however, has consistently been regarded as being within the subfamily Phorusrhacinae along with Phorusrhacos, Kelenken, and Devincenzia. 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, or an unnatural grouping (topology 2). Topology 1: Alvarenga et al. (2011) results

Of the skull, only the premaxilla, frontal (top orbit bone), pterygoid (palate bone), quadrate (skull joint bone), orbital process, and two quadratojugals (cheek bones) have been mentioned in scientific literature. The skull is estimated to have been between and in length, one of the largest known from any bird. These sizes are based on the size of quadratojugals from Titanis and the cranium of Phorusrhacos. The spinal column is poorly known from Titanis, though several vertebrae have been collected. The cervical vertebrae are elongated anteroposteriorly and somewhat flexible, whereas the dorsal, sacral, and caudal vertebrae were more boxy and rigid. It was later pointed out by Gould and Quitmyer in a 2005 study that demonstrated that this wing joint is not unique and is present in seriemas, which do not have specialized grasping hands. The wing bones articulated in an unusual joint like structure, suggesting the digits could flex to some degree. Evidence of elongated quill feathers are known from Patagornis and Llallawavis, with large tubercles called quill knobs present on their ulnae. These quill knobs would have supported long flight feathers. 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 the 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. 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.

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 Titanis. 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" where the head returned to its normal position. 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 swallowing it whole. Degrange et al. also postulated that it used a series of well targeted repetitive strikes with the beak in an "attack and retreat" strategy. Struggling prey could also have been restrained with the feet, despite the lack of sharp talons. 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 skulls. Despite the differences, studies have shown the two types handled prey similarly; the more rigid skulls and resulting larger bite force of the "Terror Bird" type would have been an adaptation to handling larger prey. The Blancan age strata of Florida from Titanis sites preserve over a hundred species and many different mammals. This includes extinct proboscideans and perissodactyls represented by grazing equids and browsing tapirs. A wide array of artiodactyls existed, including peccaries, camelids, pronghorns, and the extant white tailed deer. Large rodents are represented by capybaras and porcupines. and turkeys. 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 phorusrhacids and sparassodonts as predators in contrast to the North American placental carnivores. The fauna of North America was composed of living groups like canids, felids, ursids, tapirids, antilocaprids, and equids populating the region alongside now extinct families like the gomphotheres, amphicyonids, and mammutids. Flight capable birds could more easily migrate between continents, creating a more homogenous avian fauna. The Great American Interchange took place between the Paleogene and Pliocene, though most species crossed at around 2.7 million years ago. The momentous final stage witnessed the movement of glyptodonts, capybaras, pampatheres, and marsupials to North America via the Isthmus of Panama, which connected South America to the rest of the Americas, and a reverse migration of ungulates, proboscideans, felids, canids, and many other mammal groups to South America. The oldest fossil of Titanis is estimated to be 5 million years old, at least half a million years older than the earliest date for the Isthmus' formation about 4.5–3.5 million years ago. How Titanis was able to traverse the gap to North America is unknown. A hypothesis made by a 2006 article stipulated that it could have island hopped through Central America and the Caribbean islands. The period following the Isthmus' foundation saw the extinction of many groups, including the South American phorusrhacids, the last phorusrhacids went extinct in the Pleistocene. Human settlement in the Americas, climate change, and other factors likely led to the extinction of most of the remaining native South American mammal families.

The extinction of Titanis 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 occupied the same ancient terrestrial ecosystems during the Great American Interchange. However, this has been contested as Titanis had competed successfully against both groups for several million years upon entering North America. Notably, some phorusrhacid material from South America dates to the Late Pleistocene, younger than Titanis, close to the time of human arrival.