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Friday, 30 August 2019

We need to talk about teratorns

Very awesome take on the teratornithid Teratornis merriami by Charles Knight. Like virtually all illustrations of teratorns, the implication of this image is that Teratornis is a scavenger, arriving to steal parts of this American camel (Camelops hesternus) from noble Smilodon. But how accurate is this widely portrayed view? Image © AMNH, borrowed from Gizmodo.
Teratorns (formally known as Teratornithidae) are a group of large to gigantic raptorial birds that roamed the Americas for much of the Neogene, only becoming extinct about 11,000 years ago. On account of their large size, carnivorous habits and association with charismatic mega mammals, they are some of the most famous of all fossil avians. The most widely known teratornithid is surely Argentavis magnificens, a Miocene Argentinian species often regarded as the largest flying bird of all time, but our most complete picture of their anatomy stems from Teratornis merriami,, a 3.5 m wingspan taxon from the Pleistocene La Brea Tar Pits. Exactly where teratornithids fit into avian evolution is not entirely resolved but they likely have close affinities with New World vultures, Cathartidae (Mayr 2009). Traditionally, this would have made teratorns relatives of storks and herons, but recent shifts in avian phylogenetics have seen cathartids reclassified as Accipitriformes, a large raptor group only excluding falcons and owls. This being the case, Teratornithidae should be regarded as Accipitriformes as well.

Teratornithids occupy an unusual status in popular palaeontological culture. They are legitimately popular animals, but fundamental aspects of their palaeobiology are poorly known to non-specialists. Indeed, it’s accurate to say that the two things most people ‘know’ about teratornithids - 1) that they were enormous, 7-8 m wingspan giants and 2) that they lived as condor-like scavengers - conflict with modern interpretations of their palaeobiology. There's clearly a need to bring folks up to speed on what science actually thinks about these awesome but frequently mischaracterized fossil birds, and that's what I aim to do here.

1) No, teratornithids were not the largest flying birds (at least, in terms of wingspan)

Teratornithids were large animals which routinely attained sizes beyond those of living fliers. Even the moderately-sized Teratornis merriami likely massed around 14 kg (Chatterjee et al. 2007), a figure comparable to masses of the largest modern flying birds, and upper size estimates for Argentavis are staggering: up to 6-8 m wingspans and body masses of 65-120 kg (Palmqvist and Vizcaino 2003; Chatterjee et al. 2007). If these estimates are correct, Argentavis was the largest flying bird we know of by a comfortable margin. The only birds rivalling it, the pelagornithids, have comparable 6-7 m wingspans but only 16-40 kg body masses (Mayr and Rubilar-Rogers 2010; Ksepka 2014). These predictions of Argentavis size have shaped our understanding of its flight and ecology. Generally assuming a 70-80 kg mass and 6-7 wingspans, several authors have suggested that Argentavis could only launch under favourable conditions and relied on strong winds for soaring flight (e.g. using downward slopes and headwinds - Campbell and Tonni 1981; Vizcaíno and Fariña 1999; Chatterjee et al. 2007). Vizcaíno and Fariña (1999) presented calculations of Argentavis energetics, range and ecology based to these size estimates and concluded that only a scavenging lifestyle could sustain such enormous birds.

It’s paramount to ask, therefore, how reliable these mass and wingspan predictions are. Argentavis, after all, is only known from a few limb bones, some shoulder material and a lower jaw, and this means we're extrapolating data from other birds to get our size estimates. We should probably tackle this question in two parts.

Like many large extinct fliers, we don't have a great skeletal inventory for Argentavis magnificens. This means that any size predictions of this species are just that - predictions - and only as reliable as the assumptions they're based on. For those very large Argentavis wingspan estimates, that's a critical point. Argentavis skeletal from Chatterjee et al. (2007), white bones are known elements. I'm pretty sure that this figure is an (uncredited) mash-up of teratornithid skeletals by Greg Paul (2002).
Firstly, those 7-8 m wingspan estimates are definitely looking shaky. It’s difficult to know what the skeletal wingspan of Argentavis was as we do not have any complete wing bones but, using a projected humeral length of 57 cm, Mayr and Rubilar-Rogers (2010) regressed a surprisingly small wing spread of just 366 cm. This is way under the 4-5 m skeletal spans measured for some Pelagornis species and an immediate red flag for those 7-8 m Argentavis wingspan estimates. To attain such sizes Argentavis would need primary feathers reaching unprecedented lengths of 1.5-2 m (Chatterjee et al. 2007), entirely unlikely proportions given that primary feathers actually scale negatively to wingspan (in other words, big birds actually have proportionally small flight feathers - Kspeka 2014). 7-8 m wingspans are thus extremely unlikely for Argentavis skeleton and lower values - around 6 m - are more realistic total wingspans.

The wing bones of various large birds, as illustrated by Campbell and Tonni (1983). That's the incomplete Argentavis humerus at top, a critical bone for estimating its wingspan. Alas, it's not 100% complete and we have to estimate how much was missing, especially from the proximal region.
And this brings us to our second point: although 6 m has been the generally accepted wingspan estimate in teratorn studies, it might actually be the upper bound for Argentavis size, not an average or middling value. Kspeka (2014) modelled Argentavis wingspans using several different means of predicting feather length and found 6.07 m as his top value, with all other equations suggesting wing spreads of 5.09-5.7 m (Kspeka 2014). If accurate, these predictions suggest Argentavis might have been more likely to hit wingspans of 5-5.5 m than 6 m. This definitely takes Argentavis out of the running for having the largest wingspan of any bird. Wingspan estimates of big Pelagornis are 6-7 m and, in being based on much more complete material, we can be confident that we aren't overestimating their proportions (Mayr and Rubilar-Rogers 2010; Ksepka 2014).

The actual winner of the Grand Cenozoic Wingspan-off, Pelagornis (species shown here is P. chilensis, other species may have been a little larger). Of course, giant pterosaurs look at this competition to reach 6-7 m wingspans with rolled eyes and a bemused smile.
These revisions set our 70-80 kg Argentavis mass estimates in a new and interesting light. The mass of Argentavis was calculated using measurements of the hindlimb (specifically, the circumference of the tibiotarsus) which correlate well with total mass in living birds (Campbell and Tonni 1983). 70-80 kg sounds tenable for a 7-8 m span bird, but is very heavy for a 5-6 m one, especially if it’s meant to be a soaring species. According to my own mass/wingspan regressions*, 25-40 kg is a more likely mass for a bird this size. Might this suggest that Argentavis was either a heavyset bird with proportionally small wings (and maybe more swan-like than raptor-like in flight?), or might its legs be sending a skewed signal on account of being proportionally robust? There may be something to the latter idea (see below) but, in any case, this conflicting data is something we could investigate using other means of predicting mass, ideally those which estimate body volume instead of relying on scaling equations. Regressing mass from linear measurements can be useful but is also easily thrown when fossil animals are outside the size range or body shapes of living animals. There are already Argentavis skeletal reconstructions out there (e.g. Paul 2002, Chatterjee et al. 2007) waiting for this approach, and the results would shed light on which of those mass values (if either) is more likely.

*Based on 90 bird species with values taken from various literature.

2) Teratorns probably weren't giant Neogene vultures

This conflicting mass data has bearing on the other widely known ‘fact’ of teratornithid palaeobiology: that they were vulture-like scavengers. This idea is hugely influential in teratorn palaeoart where they are unwaveringly restored with condor- or vulture-like integuments and colours. It may be surprising to learn that, while vulture-like lifestyles are not without support (e.g. Palmqvist and Vizcaíno 2003; Fox-Dobbs et al. 2006), since the 1980s most studies of teratornithid functional morphology have suggested they were actually poorly suited to scavenging, and were instead active predators. Much of this research focuses on the best-known teratorn, Teratornis, but there's little reason to think that what's said for this taxon does not apply to the group as a whole.

It’s true that, at first glance, teratornithids seem like ideal scavengers. After all, flight studies and their phylogenetic affinities suggest that teratornithids were exceptional soarers, using updrafts to travel vast distances across American mountain ranges and open plains (e.g. Campbell and Tonni 1983; Vizcaíno and Fariña 1999; Chatterjee et al. 2007). Their upper jaws terminate in a hook that seems suited to ripping into carcasses and, as likely relatives of cathartids (and once considered relatives of storks), habitual scavenging would seem to be in their blood. The recovery of many Teratornis bones from the La Brea Tar Pits, to which they were presumably attracted by the promise of dead or dying animal flesh, is the cherry atop this particular palaeoecological cake.

Teratornithid skull material, so much as it is known, contrasts markedly with that of scavenging birds. The overall construction is more consistent with raptors that take live prey and, some details (the distended palate) are albatross-like, perhaps an adaption for prey restraint. From Campbell and Tonni (1981).
There are several compelling reasons to reject the scavenging hypothesis, however. To start with, Campbell and Tonni (1981, 1983) and Hertel (1995) noted numerous differences in skull structure between Teratornis and scavenging birds and linked these to foraging mechanics. Raptor skull shape is strongly influenced by dietary preferences (Hertel 1995) so the distinction between vulturine and teratornithid skulls is not to be shrugged off. Vulture skulls have low, narrow, largely inflexible and strongly hooked rostra which work essentially like meat hooks: they latch into chunks of flesh and pull them from carcasses using strong neck movements. They’re also mechanically weak against all but vertical forces, this probably reflecting the immobile nature of vulture foodstuffs and, for some species, limited options for lateral head motion when throngs of vultures feed at one carcass (Hertel 1995). Campbell and Tonni (1981) report that Teratornis, in contrast, has a highly flexible and broad skull with a deep, dorsoventrally parallel rostrum. Though possessing a well developed rostral hook, its size and association with a robust and straight jaw better matches raptorial grabbing aids than a vulturine 'meat hook', and thus seems ill-suited to tearing flesh (Hertel 1995). While scavenging is not precluded by this configuration, the bulky but loosely built teratorn skull does not match predictions of skull morphology for a habitual scavenger.

A better morphological match for teratorn skulls are birds which dine on living prey, such as large eagles and - more surprisingly - albatross (Campbell and Tonni 1981; Hertel 1995; Paul 2002). Like albatross, Teratornis has a low-slung palate which nestles neatly between the rami of the lower jaws when the mouth closes. This configuration grips prey by pinching it between the lateral surface of the palate and the inner margin of the mandible. Intriguingly, Hertel (1995) also found a maritime connection with Teratornis skulls, noting some skull proportions uniquely matching those of piscivorous raptors. Combined with the albatross-like jaw structure, we might wonder if aquatic prey was a routine part of teratorn diets (an idea also suggested by Paul 2002). I’m not fully convinced of this because the biometric signal of piscivorous raptor skulls is not strongly separated from those with less specialised diets (data in Hertel 1995), teratornithid skeletons lack features we’d expect of habitual waders or fishers and - perhaps most tellingly - Teratornis bone chemistry indicates a diet of terrestrial animals (Fox-Dobbs et al. 2006). These skull features are nevertheless evidence of teratorns being live-prey carnivores, not scavengers. Their strongly kinetic skulls - which included a loosely jointed mandible, and a jaw joint that expanded their gape 10% when the mouth is opened - implies a great ability to swallow prey whole at the expense of capabilities to tear it apart (Campbell and Tonni 1981). We might thus summarise their skull morphology as being suited to grabbing, holding and swallowing small animals.

The pelves of teratornithids weren't like those of other raptors, but more akin to those of birds which spend a lot of time walking around. From Campbell and Tonni (1983).
Working out what sort of prey teratorns preferred is aided by examining their skeletons. We can immediately rule out the use of their feet in capturing and restraining prey, these lacking the long, robust talons and indicators of a powerful grip characteristic to many birds of prey (Campbell and Tonni 1983). Teratornithids also have pelves which differ markedly from raptors that use their legs in prey capture. Eagles, falcons and similarly adapted birds have strongly bent posterior pelvic regions which optimise the orientation of their hindlimb musculature for powerful leg action. Teratornithids, in contrast, have relatively straight pelves that recall those of storks and other birds adapted for walking more than those of their raptorial cousins. Combined with their non-raptorial feet, we can probably rule out teratornithids gripping and carrying prey with their legs (Campbell and Tonni 1983), but can assume that they'd be much more comfortable striding around the ground than most other raptors.

Put together, these hindlimb features, the functional signature of their skulls and terrestrially-derived bone chemistry has seen many authors agree that teratornithids must have been caracara-like ground predators of smallish prey (e.g. Campbell and Tonni 1981, 1983; Vizcaíno and Fariña 1999; Paul 2002; Chatterjee et al. 2007). Their stork-like pelves would have facilitated more efficient walking than those of other raptors and, without large claws to imbue their locomotion, their strong feet and long legs would be ideal for sustained bouts of terrestrial activity. This ecology might play into their mismatched leg and wingspan proportions because strong legs have clear advantages for terrestrially hunting birds. Perhaps teratornithids used their legs for occasional powerful ground activity, such as stamping or standing on prey (suggested by Campbell and Tonni 1981), providing bursts of speed or digging for hiding animals? Strong legs could have also facilitated rapid landing if prey was spotted from the air, or allowed for explosive launches to avoid danger. Teratornithids may have been large, but some species lived alongside even larger predatory mammals. Their leg proportions were not suited to fast running (Campbell and Tonni 1983) and rapid escape to the air was surely necessary on occasion. Around 80-90% of avian launch power stems from their hindlimbs so, if teratornithids wanted to get airborne rapidly, having substantial, strain-resistant leg bones would be a good start. I'm curious to know what the launch prospects of Argentavis are if we factor extremely robust hindlimbs at the lower body masses proposed above: could these birds perhaps launch from a standing start?

My take on Teratornis merriami as we probably need to start picturing it: a ground-stalking hunter of small prey, such as brush rabbits (seen half-eaten on the right of the image). Teratornis is deliberately reconstructed to look more 'predatory' than vulturine here, for reasons made clear in this post. A mew gull and turkey vulture are included as a nod to the rich avian fauna that once lived alongside teratorns: it's odd to think that species we have around today once lived alongside these very large and unusual birds.
With jaws suited to eating essentially any animal they could fit in their mouths, we might imagine teratornithids as stalking across Neogene plains, water margins and mountains looking for all manner of small vertebrate prey - lizards, snakes, frogs, smaller mammals, ground birds and so on. Campbell and Tonni (1981) suggest that the 24 cm long Teratornis skull would put a 9 cm diameter limit on prey size, while the much larger Argentavis - with an estimated skull length of up to 55 cm - could have swallowed 15 cm wide prey. That means hare-sized animals, including small sloths, armadillos, and the notoungulate Paedotherium borrelloi, could have been regularly going down the throat tubes of Argentavis. Sounds like some fun concepts for palaeoart to me.

So there we have it: teratornithids, household names for many of us interested in palaeontology, may have been both smaller and ecologically very different to how we've mostly imagined them. All this said, in researching this piece I was struck by how much of our work on teratornithid size and ecology is now decades old. This doesn’t invalidate the points outlined here, but there's probably scope for bringing modern techniques to teratorn studies, both to pin down their lifestyles further as well as to explore that interesting mass/wingspan issue. Teratornithids seem like pretty awesome birds, so hopefully modern insights into their anatomy and lifestyles won’t be long coming.

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References

  • Campbell Jr, K. E., & Tonni, E. P. (1981). Preliminary observations on the paleobiology and evolution of teratorns (Aves: Teratornithidae). Journal of Vertebrate Paleontology, 1(3-4), 265-272.
  • Campbell Jr, K. E., & Tonni, E. P. (1983). Size and locomotion in teratorns (Aves: Teratornithidae). The Auk, 100(2), 390-403.
  • Chatterjee, S., Templin, R. J., Campbell, K. E. (2007). The aerodynamics of Argentavis, the world's largest flying bird from the Miocene of Argentina. Proceedings of the National Academy of Sciences, 104(30), 12398-12403.
  • Fox-Dobbs, K., Stidham, T. A., Bowen, G. J., Emslie, S. D., & Koch, P. L. (2006). Dietary controls on extinction versus survival among avian megafauna in the late Pleistocene. Geology, 34(8), 685-688.
  • Hertel, F. (1995). Ecomorphological indicators of feeding behavior in recent and fossil raptors. The Auk, 112(4), 890-903.
  • Ksepka, D. T. (2014). Flight performance of the largest volant bird. Proceedings of the National Academy of Sciences, 111(29), 10624-10629.
  • Mayr, G. (2009). Paleogene fossil birds. Springer Science & Business Media.
  • Mayr, G., & Rubilar-Rogers, D. (2010). Osteology of a new giant bony-toothed bird from the Miocene of Chile, with a revision of the taxonomy of Neogene Pelagornithidae. Journal of Vertebrate Paleontology, 30(5), 1313-1330.
  • Palmqvist, P., & Vizcaíno, S. F. (2003). Ecological and reproductive constraints of body size in the gigantic Argentavis magnificens (Aves, Teratornithidae) from the Miocene of Argentina. Ameghiniana, 40(3), 379-385.
  • Paul, G. S. (2002). Dinosaurs of the air: the evolution and loss of flight in dinosaurs and birds. JHU Press.
  • Vizcaíno, S. F., & Fariña, R. A. (1999). On the flight capabilities and distribution of the giant Miocene bird Argentavis magnificens (Teratornithidae). Lethaia, 32(4), 271-278.