Titanis

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.^[1][2][3] 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.^[1][4] 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).^[5][1] 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 (2.2–1.8 million years old).^[6][7][1][8]

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. He belonged to a relative of the South American rhea.^[5][1] Brodkorb authored a manuscript that classified it as a relative of rheas, though Ray pushed Brodkorb to assign the fossils to Phorusrhacidae.^[1] 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.^[3][1] This was the first discovery of phorusrhacids outside of South America.^[9]

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.^{[10][3][1][5][2]} 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.^[3][5][2] 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.^[2] Inglis 1b was originally a sinkhole during the Pliocene,^[11][12][5] 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.^[13][5] 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.^[5][2][10] Work on the site lasted from 1967 to 1973, during which over 18,000 fossils were collected.^[14] Despite the many fossils, only 12 of them belonged to Titanis, including cervical vertebrae, a carpometacarpus, and a metatarsal.^[2][15] As for Port Charlotte, a single fossil, a partial pedal phalanx from the fourth digit, was donated to the UF in 1990.^[5][2] Another partial tarsometatarsus was reportedly found in a shell pit in Sarasota County, making it the only other tarsometatarsus known from Titanis.^[5]

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Fossil localities of Titanis
Legend: Early Pliocene-aged Early Pleistocene-aged

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.^[16] This was the first description of Titanis fossils from outside Florida.^[2][17] 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.^[8]

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, including a wing bone found in association with the premaxilla of a giant bird.^[18][19] The wing bone was referred to the teratorn Aiolornis at that time, an assessment backed by ornithologist Hildegarde Howard in 1972.^[20] This was supported by later studies,^[19] 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.^[17] 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.^[16][6][17]

Classification

During the early

The oldest phorusrhacid fossils come from South America during the

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 36 centimetres (14 in) and 56 centimetres (22 in) 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.^[2] The premaxilla of Titanis is incomplete, consisting of its frontmost end including the characteristic long, sharp beak tip of Phorusrhacidae that would have been used for hunting.^[37] Its preserved length is 9 centimetres (3.5 in) with a height of 5.5 centimetres (2.2 in) with a triangular 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 (upper arc) of the exposed premaxilla is identical to that in Patagornis marshi, an Argentine phorusrhacid.^[38][17] The pterygoid is enlarged, as seen in other phorusrhacids, at 10 centimetres (3.9 in) in complete length with a medially placed joint for its articulation to the basipterygoid process. Two quadratojugals are preserved, both with different anatomies. The larger of the two has a more pronounced crest cranial to the articulation tubercular, whereas the smaller quadratojugal has a deep fossa instead of a crest. Potential sexual dimorphism has been suggested due to the lack of signs of unfinished ontogenetic development in the smaller quadratojugal, meaning they both come from adults.^[15][2] In the lower jaw, a partial mandible is known but it is unfigured and undescribed in scientific literature. Being a phorusrhacine, it would have had a long and narrow symphysis ending in a sharp tip that was pointing downward.^[26]

Postcranial skeleton

As for the postcranial anatomy, Titanis and other phorusrhacines were heavily built. They all preserve an elongated, thin tarsometatarsus that was at least 60% the length of the tibiotarsus. Titanis is distinguished from other phorusrhacines by the anatomy of its tarsometatarsus; the distal end of the mid-trochlea is spread out onto its sides and its slenderness compared to related genera of the same size.^[26][3] The pes is large and had three digits, the third of which had an enlarged ungual akin to that of dromaeosaurid dinosaurs.^[9][39] 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.^[40] The dorsal vertebrae have tall neural spines atop the centra. The dorsal ribs connected to the sacral ribs, creating a basketed underbelly.^[40][2]

The wings are small and could not have been used for flight, but were much more strongly built than those of living

Little is known about the paleobiology of Titanis due to a scarcity of fossil remains. Many of its habits are inferred based on related taxa like Kelenken and Andalgalornis.[9] Features such as the pointed premaxillary beak tip and recurved pedal unguals are direct evidence of its carnivorous lifestyle.^[2]

Feeding and diet

Phorusrhacids are thought to have been terrestrial predators or

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

During the Blancan stage, Titanis lived alongside both endemic mammals as well as new immigrants from Asia and South America. 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.^[16][14][17] In Inglis 1a specifically, longleaf pine flatwoods and pine-oak scrub are known to have occupied the area, similar to the modern flora.^[14][11][12] During the Pliocene-Miocene climatic transition, the climate was cooler but temperatures did not reach those of the Pleistocene, creating a warm period. Sea levels were higher, but this was reversed at the end of the Pliocene during the beginning of large glaciations that fostered the Pleistocene's "Ice Age".^[44][45]

The Blancan age strata of Florida from Titanis sites preserve over a hundred species and many different mammals. This includes extinct proboscideans and

South America, the continent where phorusrhacids originated, was isolated after the breakup of the landmass Gondwana at the end of the Mesozoic era.^[53] 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.^[10][53] Phorusrhacids evolved in South America to fill gaps in niches otherwise filled by placentals in other continents, such as that of apex predator.^[54] Flight-capable birds could more easily migrate between continents, creating a more homogenous avian fauna.^[55]

The Great American Interchange took place between the

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.[31]^[6] Brodkorb's description of Titanis as being from the latest Pleistocene, an error followed by later studies, postulated that it went extinct as recent as 15,000 BP (about 13000 BC).^[3] The rare earth element analysis of Titanis fossils by MacFadden and colleagues in 2007 dispelled this, demonstrating that the genus lived during the Pliocene and earliest Pleistocene.^[8] Notably, some phorusrhacid material from South America dates to the Late Pleistocene, younger than Titanis, close to the time of human arrival.^[31][62]

References