Titanis
Titanis is a genus of phorusrhacid, an extinct family of large, predatory birds, in the order Cariamiformes that inhabited the United States from the early Pliocene to early 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 and phalanx, 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. The species 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 over 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 before the Great American Interchange.
Discovery and age
The earliest discovery of Titanis fossils occurred 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 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 and a pedal phalanx, 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 . Ray also noted their stratigraphic origin; they were 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.Ray presented the Santa Fe fossils to the museum's ornithologist Pierce Brodkorb, who mistakenly believed that they were from Rancholabrean strata, an error which made it to the final publication. In that publication, Brodkorb erroneously classified it as a relative of rheas, though Ray pushed Brodkorb to assign the fossils to Phorusrhacidae. Brodkorb published his description in 1963, naming the new genus and species Titanis walleri. The generic name, Titanis, references the Greek Titans, due to the bird's large size, and the specific name, walleri, honors Waller, one of the collectors of the type specimen. As suggested by Ray, Brodkorb grouped Titanis with the subfamily Phorusrhacinae within Phorusrhacidae, along with Phorusrhacos and Devincenzia. This was the first discovery of phorusrhacids outside South America.
Titanis has been found in five locales in Florida: Santa Fe River sites 1a and 1b and Inglis 1b, Citrus County; Port Charlotte, Charlotte County; and a shell pit in Sarasota County. Of the 40 Floridan specimens of Titanis, 27 have been unearthed from the Santa Fe River, many of them collected in the 1960s and '70s following Brodkorb's description. The Santa Fe River specimens come from two localities within the river, 1a and 1b. The former locality is more productive, producing elements of Titanis including vertebrae, limb bones, and even parts of the skull. Inglis 1b was originally a sinkhole during the Pliocene, but became a sedimentary layer of clay that was uncovered during construction of the Cross Florida Barge Canal by the federal government during the 1960s. 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 excavations by curator David Webb of the Florida Museum of Natural History. Work on the site lasted from 1967 to 1973, during which over 18,000 fossils were collected. Of the many fossils, only 12 belonged to Titanis, 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 newer discovery of Titanis was described in 1995; 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 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.In 1961, while fossil collecting, G. Davidson Woodward acquired several avian fossils from sediments in the Pliocene-aged strata of the Olla Formation in Anza-Borrego Desert State Park, California, including a wing bone found in association with the premaxilla of a giant bird. 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, but a 2013 paper by paleontologist Robert Chandler and colleagues assigned the premaxilla to Titanis, the authors citing the bone's age and phorusrhacid features. The age of the Anza-Borrego premaxilla is estimated at 3.7 million years old, making it the oldest confirmed fossil of Titanis, though the Texan specimen may be older. The paper assigning the Anza-Borrego premaxilla suggested the locality was 3.5 to 3 million years old. On the other hand, another paper estimates the age of the Olla Formation equivalent to the late early to early late Blancan.
Classification
During the early Cenozoic, after the extinction of the non-avian 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. Although phorusrhacids are the most taxon-rich group within Cariamiformes, their interrelationships are unclear due to the incompleteness of their remains. 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 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. Since phorusrhacids survived until the Pleistocene, they appear to have been more successful than the South American metatherian thylacosmilid predators, 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 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. Brazilian paleontologist Herculano Alvarenga and colleagues published a phylogenetic analysis of Phorusrhacidae in 2011 that did not separate Brontornithinae, Phorusrhacinae, and Patagornithinae, resulting in Titanis in a 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, or an unnatural grouping.
Topology 1: Alvarenga et al. results
Topology 2: Degrange et al. results