Teratornis (Greek: "wonder" (teretos), "bird" (ornis)[2]) was a genus of huge North American birds of prey—the best-known of the teratorns—of which, two species are known to have existed: Teratornis merriami and Teratornis woodburnensis. A large number of fossil and subfossil bones, representing more than 100 individuals, have been found in locations in California, Oregon, southern Nevada, Arizona, and Florida, though most are from the Californian La Brea Tar Pits. All remains except one Early Pleistocene partial skeleton from the Leisey Shell Pit near Charlotte Harbor, Florida (which may represent a different species or a subspecies) date from the Late Pleistocene, with the youngest remains dating from the Pleistocene–Holocene boundary.
Another form, "Teratornis" olsoni, was described from the Pleistocene of Cuba, but its affinities are not completely resolved; it might not be a teratorn, but has also been placed in its own genus, Oscaravis. There are also undescribed fossils from southwestern Ecuador, but apart from these forms, teratorns were restricted to North America.[6]
A closely related genus, Aiolornis, was about 40% larger and lived at an earlier time; it was formerly known as Teratornis incredibilis, but is distinct enough to be placed in its own genus.
According to description of 1945, Teratornis merriami had a wingspan of around 3.5 to 3.8 m (11 to 12 ft) and a wing area of 17.5 square meters, standing an estimated 75 cm (30 in) tall. It was somewhat larger than the extant Andean condor, and by calculating the area of the organism's sternum and synsacrum, Teratornis was estimated to weigh about 22.5 kg (50 lb), which was nearly double the weight of an average Californian condor.[7] A 1983 study gives a range of wingspan of 2.938–3.379 m (9.64–11.09 ft) and a weight of 13.7 kg (30 lb).[8]
The finger bones of the Teratornis were fused as in all modern birds; however, part of the index finger forms a shelf which aided in bearing the load of long and stout primaries, enabling the bird to utilize strong upcurrents in flight. Their legs were similar to an Andean condor's, but stouter, and the feet could hold prey for tearing off pieces, but could not exert a very forceful grip like birds of prey. Its wing loading was not much larger than a Californian condor's, and Merriam's teratorn should have been able to take off by simply jumping and beating its wings under most circumstances.[6] Indeed, it seems to have been better adapted for utilizing a short run into the wind from an elevated location as condors do, as its legs are proportionally smaller and its stride less than in condors.[9] Thus, Teratornis may have primarily inhabited cliff terrain, where it could take off and soar through the air easily. [7]
The T. merriami was similar to condors, although an analysis of the functional morphology of its skull, namely its larger bill and ability to spread its mandibles and swallow its prey whole, suggests that it was an active and carnivorous predator rather than a scavenger. In fact, some studies of skull morphology indicated that the T.merriami were incapable of tearing pieces of flesh off of carcasses in the manner of condors. Alternative viewpoints note that many old world vultures possess large bills, and a longer bill is a common feature among scavenging raptors, as this allow them to probe deeper into large carcasses - larger than those fed upon by active-hunting raptors. Other anatomical features, such as the relatively small and sideward facing orbits and the low skull, are also consistent with a scavenging lifestyle.[10][11] For the T.merriami, small prey such as frogs, lizards, young birds, and small mammals were swallowed whole, while carrion would have been fed on in a manner similar to that of condors or vultures.[3]