Birds Guide

Teratorns were very large birds of prey who lived in North and South America from Miocene to Pleistocene. They were somewhat close to modern condors and as such, they are more closely related to storks rather than Accipitridae which includes most other diurnal predatory birds, including Old World vultures; however, Rhys (1980) put the family Teratornithidae in the order Accipitriformes. They include some of the largest known flying birds. So far, four species have been identified:

• Teratornis merriami (Miller, 1909). This is by far the best-known species. Over a hundred specimens have been found, mostly from La Brea Tar Pits. It stood about 75 cm (29.5 in) tall with estimated wingspan of perhaps 3.5 to 3.8 metres (11.5 to 12.5 ft), and weighed about 15 kg (33 lbs); making it slightly bigger than extant condors. It became extinct at the end of Pleistocene, some 10 000 years ago. Teratornis is Greek for "monster bird".

• Aiolornis incredibilis (Howard, 1952), previously known as Teratornis incredibilis. This species is fairly poorly known, finds from Nevada and California include several wing bones and part of the beak. They show remarkable similarity with merriami but are uniformly about 40% larger: this would translate to wingspan of about 5 metres (16.5 ft) for incredibilis. The finds are dated from Pliocene to late Pleistocene which is considerable chronological spread, and thus it is uncertain whether they actually represent the same species.

• Cathartornis gracilis (Miller, 1910). This species is known only from a couple of leg bones found from La Brea Ranch. Compared to T. merriami, remains are slightly shorter and clearly more slender, indicating more gracile body build.

• Argentavis magnificens (Campbell & Tonni, 1980). A partial skeleton of this enormous teratorn was found from La Pampa, Argentina. It is the oldest known teratorn, dating to late Miocene, about 6 to 8 million years ago, and one of the very few teratorn finds in South America. Initial discovery included portions of the skull, incomplete humerus and several other wing bones. Even conservative estimates put its wingspan at 6 meters and up (some 20 ft), and it may have been as much as 8 metres (26 ft). Weight of the bird was estimated to have been around 80 kg (176 lbs). Estimated weight and wing area rival those of the largest pterosaurs.

Another form, "Teratornis" olsoni, was described from the Pleistocene of Cuba, but its exact affinities are not completely resolved; it might not be a teratorn at all. There are also undescribed fossils from southwestern Ecuador, but apart from these forms, teratorns were restricted to North America (Campbell & Tonni, 1983).

Description and ecology

Despite their size, there is little doubt that even the largest teratorns could fly. Visible marks of the attachments of contour feathers can be seen on Argentavis wing bones. This defies some earlier theories that modern birds like condors, swans, and bustards represent the ultimate size limit for flying birds. Wing loading of Argentavis was relatively low for its size, a bit more than a turkey (Campbell & Tonni, 1983), and if there were any significant wind present, the bird could probably get airborne merely by spreading its wings, just like modern Albatrosses. It is noteworthy that South America during Miocene probably featured strong and steady westerly winds, as the Andes were still forming and not yet very high.

T. merriami was small enough to take off with a simple jump and a few flaps. The fingerbones are mostly fused as in all birds, but the former index finger has partially evolved into a wide shelf at least in T. merriami, and as condors have a similar adaptation, probably other species, too. Wing length estimates vary considerably but more likely than not were on the upper end of the range, because this bone structure serves to bear the load of the massive primaries

Traditionally, teratorns have been described as large scavengers, very much like oversized condors, owing to considerable similarity with condors. However, the long beaks and wide gapes of teratorns are more like the beaks of eagles and other actively predatory birds, rather than vultures. Most likely teratorns swallowed their prey whole; Argentavis could technically swallow up to hare-sized animals in a single piece. Although they undoubtely engaged in opportunistic scavenging, they seem to have been active predators most of the time (Campbell & Tonni, 1983). It is noteworthy that teratorns have relatively longer and stouter legs than Old World vultures, thus it seems possible that teratorns would stalk their prey on the ground, and take off only to fly to another feeding ground, or their nests; especially Cathartornis seems well-adapted for such a lifestyle. Argentavis may have been an exception, as its sheer bulk would have made it a less effective hunter, but better adapted to taking over other predators' kills. As teratorns were not habitual scavengers, they most likely had completely feathered heads, unlike vultures.

As with other large birds, a clutch probably had only one or two eggs; the young would be cared for for more than half a year, and take several years to reach maturity, probably up to a dozen in Argentavis (Palqvist & Vizcaíno, 2003).

References

• Campbell, Kenneth E. Jr. & Tonni, E. P. (1983): Size and locomotion in teratorns. Auk 100(2): 390-403 PDF fulltext

• Miller, Love H. (1909): Teratornis, a new avian genus from Rancho La Brea. University of California Publications, Bulletin of the Department of Geology 5: 305-317.

• Palmqvist, Paul & Vizcaíno, Sergio F. (2003): Ecological and reproductive constraints of body size in the gigantic Argentavis magnificens (Aves, Theratornithidae) from the Miocene of Argentina. Ameginiana 40(3): 379-385. PDF fulltext

• Rhys, David (1980): Argentavis magnificens: World's Largest Flying Bird. Origins 7(2): 87-88. HTML abstract