Wednesday, May 14, 2008

Teratorns

I had previously mentioned teratorns all the way back here, noting that family Teratornithidae is grouped with the New World vultures (Cathartidae) - possibly in the order Cathartiformes. The "possibly" is partially due to the fact that traditional grouping in Falconiformes or Ciconiiformes has been challenged and cathartids may require their own order. It should be mentioned that some workers feel that they may be more closely related to storks or Pelicaniformes, but unfortunately there seems to be a lack of available information. Welcome to the world of teratorns!

Teratornis merriami was a Pleistocene teratorn of North America that recently had isotopes from its collagen analyzed by Fox-Dobbs et al. 2006. 2H isotope levels possibly suggested a marine diet in one specimen, but 13C and 15N isotope levels suggest teratorns fed in a terrestrial c3 environment (i.e. the non-arid plants). The 15N isotope levels suggested a mixed diet of browsers and grazers, and the authors suggested the teratorn was a scavenger with wide dietary flexibility due to its size. The authors appear to have assumed that teratorns were obligate scavengers by their large numbers in the tar pits, but of course carnivorous birds will engage in facultative scavenging. If teratorns were large enough to require a broad diet, then perhaps they exploited the tar pits in great numbers despite usually preying on something else. I say preying because two morphological studies cited by Fox-Dobbs et al. strongly suggest just that.

Campbell and Toni 1983 mention a 1981 study by them (which I can't access) documenting the plane of rotation of the quadrates, maxillary rostrum and mandible structures and concluded that the birds swallowed prey whole and could not tear pieces off a carcass like a vulture. Hertel 1995 used a number of indices to classify avian skulls into ecomorphs; scavenging species were well separated from other ecomorphs with fairly high confidence (caracaras were somewhat hard to determine) and were marked by such distinctive features as smaller orbits, greater occipital distance, a deep and narrow ramus, large foramen magnum angle and other features related to twisting food off from a carcass and eating it quickly (and apparently less need for acute vision). Teratorns did not show these vulture-like features and in many features it was classified as a piscivore - except for mandibular and maxillary indices (which apparently classified it as a scavenger and mammalivore/generalist, respectively). Hertel speculates that it could have fed from fish from the surface (the feet were not strongly raptorial), but the isotopes from Fox-Dobbs et al. suggest this was either rare or not the case. I think that the mandibular and maxillary features interpreted by Campbell and Toni as being from a predator and the other features resembling a piscivore (and the facultative scavenging) indicate that this was either a generalized species or occupied an ecomorph with no modern counterparts. Fox-Dobbs et al.'s implication that teratorns went extinct directly because of the loss of megafauna is thus probably not correct. Even if it was fairly generalized, it was still fairly large (12.5-15 kg or 27-33 lbs - Campbell and Toni 1983), presumably had fairly large home ranges and was probably more prone to extinction.

Campbell and Toni 1983 further noted that T. merriami* has always been portrayed as a super-condor (this is still true 25 years later) despite the fact that we now know that the skull indices indicated a much different niche. And since when do birds in different families look identical down to the coloration? The post-cranial skeleton apparently shows a mosaic of cathartid, ciconiid and unique features; the flight was apparently condor-like, the pelvis was stork-like and indicated that sustained walking was possible and the stout legs indicated that running was not likely. I'm guessing that this teratorn could have hunted mammals, reptiles and amphibians on the ground and possibly fish in freshwater, although the short legs would have been a hindrance. However it "earned a living", relatives with similar bauplans existed for millions of years.

*Some material from Cuba may indicate that there is another species in the genus, or that there is yet another genus of teratorn (Olson and Alvarenga 2002). This article mentions a "Teratornis" olsoni. Nothing has been published to my knowledge.


The Incredible teratorn (Aiolornis incredibilis) was one of three teratorn species that lived in North America in the late Pleistocene (from the early Pliocene) and was primarily differentiated from T. merriami by its size. Where T. merriami had a 3.5 to 4 m (11-13') wingspan, A. incredibilis was more along the lines of 5-5.5 m (16.5 to 18') and it presumably weighed around 36 kg (80 lbs) or more. Campbell et al. 1999 examined new and old specimens and determined there were enough characteristics to establish a new genus (it was originally in Teratornis) and suggested that Pliocene specimens may actually belong to other species and/or genera. Clearly a lot remains to be discovered and written about this species, Campbell et al. thought that it had distinctive flight capabilities but didn't (probably couldn't) expand upon that. The beak was deeper than Teratornis, although without much skull material nothing about potential ecomorphology has been written. I doubt it was a "super condor" as alleged by some, and presumably it shared the same walking abilities (since Argentavis did...) and specialized on some other food source. It seems dubious that three large generalists could have co-existed in the North American southwest in the late Pleistocene.

As I've been hinting at, the third species in question is Cathartornis gracilis which is very rarely discussed (it is known from two tarsometatarsi). Campbell et al. 1999 discuss it briefly; it is comparable in size to Teratornis but more gracile, although Campbell et al are not convinced it belongs in its own genus. Olson and Alvarenga 2002 mention that more material has apparently been found and Campbell now thinks it is worthy of genus-level distinction. Like the Cuban material, I haven't heard of any publications.

Teratorns aside from Merriam's aren't discussed too frequently, with the exception of the gigantic Miocene Argentavis magnificens from South America. This was the largest flying species of bird with a span of around 7 meters (23 feet) and a mass of around 70 kg (150 lbs) (The azhdarchid pterosaur Hatzegopteryx dwarfed this) and so has received a lot of attention. Campbell and Toni 1983 stated that Argentavis was simply a larger version of the Teratornis morphotype on the basis of wing and leg bone similarities (i.e. condor-like flight and stork-like walking); apparently the 1981 paper by the authors documented features of the 55 cm+ skull which indicated it was predatory (Chatterjee et al. 2007). Palmqvist and Vizcaino 2003 determined that a falconiform the size of Argentavis would have a territory of about 542 square kilometers (~200 sq. miles) and would take about three days to patrol its territory and would eat around 5-10 kg of meat per day. The authors feel that since scavenging birds do not have defined territories and can exist at higher densities that predatory ones - and they suggest that predatory sabertoothed marsupials opened up a new niche for giant vultures (they seemed to imply that Argentavis ate bones). Most recently, Chatterjee et al. 2007 stuck with the morphological implications that this was a predatory species capable of eating rabbit-size animals whole. So why gigantism? Palmqvist and Vizcaino note that while giant species have low populations, low population density, small clutch size and long breeding cycles they are resilient against predation (maybe even engaging in kleptoparasitism) and can withstand famine.

Very seldom discussed is the earliest teratorn Taubatornis (with 6 Google hits) from the late Oligocene of Brazil that had not reached the proportions of later species (the distal width of the tibiotarsus was about 70% that of Teratornis). This fossil also demonstrates that South America is likely the place of origin for this family and that teratorns in North America (and Cuba?) were a fairly recent phenomenon. Olson and Alvarenga also mention the curious fact that teratorns and cathartids are almost always found together, which would support frequent scavenging habits (the authors are neutral).


That about ends the story for teratorns thus far, and as usual there are more questions than answers. For birds with no obvious adaptations for eating carrion, teratorns sure did hang out with vultures and get stuck in tar pits a lot. Teratorns also seem very different from living birds of prey that hunt on the ground, and exactly how a short-legged bird unable to run well hunts a sufficient number of small animals per day is beyond me. Perhaps Teratornis was somewhat like a marabou stork or adjutant (Leptoptilos sp.) in that it was a presence at kills but otherwise occupied a different niche it was morphologically adapted towards. The presence of three sympatric genera (and their extinction) certainly hints that these were not all generalists. It is increasingly clear that teratorns were not "super condors", but what exactly they were is still rather unclear. Hopefully some of the unpublished material will come to light and new discoveries will be made to clear up the basic life histories of this enigmatic group.






References:

Campbell, Kenneth E. and Tonni, Eduardo P. 1983. Size and locomotion in teratorns (Aves: Teratornithidae). The Auk 100: 390-403.

Campbell, Kenneth E. et al. 1999. A New Genus for the Incredible Teratorn. Smithsonian Contributions to Paleobiology No. 89: 169–175. Available (Huge file)

Chatterjee, Sankar et al. 2007. The aerodynamics of Argentavis, the world’s largest
flying bird from the Miocene of Argentina. PNAS. Published online (for free)

Fox-Dobbs, Kena. 2006. Dietary controls on extinction versus survival among avian
megafauna in the late Pleistocene. Geology V. 34, No. 8 pp. 685-689

Hertel, Fritz. 1995. Ecomorphological indicators of feeding behavior in Recent and fossil raptors. The Auk 112(4): 890-903

Olson, Storrs. L and Alvarenga, Herculano M. F. 2002. A new genus of small teratorn from the Middle Tertiary of Taubate Basin, Brazil (Aves: Teratornithidae). Proceedings of the Biological Society of Washington 115 (4) pp. 701-705

Palmqvist, Paul and Vizcaino, Sergio F. 2003. Ecological and reproductive constraints of body
size in the gigantic Argentavis magnificens(Aves, Teratornithidae) from the Miocene of Argentina. Ameghiniana 40 (30) pp. 379-385

2 comments:

Birdzilla said...

Its quite posible that the teratorns could stil be alive and hiding out

Cameron McCormick said...

The proposal of extant teratorns is one of the least likely in all of cryptozoology. Large birds are incredibly prominent animals, the fact that birdwatchers aren't reporting "thunderbirds" daily (or at all) speaks volumes. I'd wager that if teratorns were somehow alive they would have been one of the first animals known from the Americas!

Since objects in the sky rarely have a definite scale, reported "thunderbird" size should be taken with a grain of salt. I've seen purported "thunderbird" photos which were clearly of turkey vultures and it is plausible that other large birds (black vultures, eagles, herons, etc) are also misidentified and exaggerated. Strays to the interior of the continent (brown pelicans, frigatebirds, etc) are another explanation and (less likely) are escapees such as griffon vultures and marabou storks. The most improbable escapee is far more probable than the largest living bird slipping entirely past the radar of ornithologists (and airports!).

I've heard the "maybe they fly at night excuse" - but this doesn't really, ahem, fly as teratorns appear to be condor-like soarers which rely on thermals present only during the day. I suppose one could argue that "well, maybe they're primarily terrestrial" but the lack of sightings of "thunderbirds" on the ground is notable - and highly suspicious.