More on phorusrhacids: the biggest, the fastest, the mostest out-of-placest

In the previous post we looked briefly at phorusrhacid diversity, stopping on the way to look at the discovery and naming of that ‘well known’ species Phorusrhacos longissimus from the Miocene of Argentina. All of this has been inspired by Chiappe & Bertelli’s (2006) description of the immense new specimen BAR 3877-11, an unnamed Miocene phorusrhacine phorusrhacid that represents one of the biggest members of the group: its skull is 71 cm long and the live animal probably stood 3 m tall (life restoration at left).

By comparing BAR 3877-11 with phorusrhacids known from fairly complete skeletons, we can estimate that it was about 10% bigger than the previously largest known phorusrhacids. But its markedly slender tarsometatarsus indicates that it was gracile, and thus almost certainly not as heavy as the far more robust giant aepyornithids (aka elephant birds, restricted to Madagascar bar a few dubious reports from continental Africa and elsewhere) and dromornithids (aka mihirungs, an Australian group argued to be giant waterfowl). These slender legs suggest that BAR 3877-11 was a fast-moving, cursorial predator, and Chiappe & Bertelli state that ‘the long-established correlation between their corpulence and reduced cursorial agility needs to be re-evaluated’. In other words, they imply that previous studies have associated giant size with ponderous locomotion.

This is somewhat misleading however, in that the only reason that some giant phorusrhacids have been thought of as relatively slow-moving is that they belong to that particular robust-limbed subgroup, the Brontornithinae. The biggest brontornithine, Brontornis burmeisteri from the Miocene of Argentina, was also arguably the biggest phorusrhacid prior to the discovery of BAR 3877-11, but its leg bones are immensely wide and stocky for their length, and its tarsometatarsi are between 50 and 60% the length of its tibiotarsi. These features suggest that it was a walking bird, not a runner, and it is on the basis of this that some authors have interpreted brontornithines as scavengers. You’ll know from discussions about tyrannosaurs that a pure scavenging lifestyle is highly unlikely for any flightless tetrapod (for energetic reasons). Sure, they probably did scavenge (you can imagine them trying to scare teratorns, or a group of hyaena-like borhyeanids, away from a carcass), but they probably foraged for live prey of various kinds as well.

Having mentioned teratorns, while they are now known from the Upper Oligocene/Lower Miocene of South America (Olson & Alvarenga 2002), and thus were contemporaneous with brontornithines, the immense teratorn Argentavis is only known from the Late Miocene, and brontornithines are unknown from this time. So, sorry, you shouldn’t imagine Brontornis scrapping with Argentavis. The oldest teratorn, Taubatornis campbelli, is actually from the same unit – the Tremambé Formation – as the brontornithine Physornis brasiliensis. Both lived alongside New World vultures, flamingos, screamers and caviomorph rodents [the adjacent photo, of the brontornithine Paraphysornis, is borrowed from Cais de Gaia's phorusrhacid blog post].

Getting back to Chiappe & Bertelli’s claims about phorusrhacid running speed, highly relevant is a recent study specifically devoted to this issue. Based on limb proportions and limb bone strength, Blanco & Jones (2005) estimated the running speed of the mesembriornithine Mesembriornis, the patagornithine Patagornis, and a giant phorusrhacine specimen from the Pliocene or Pleistocene of Uruguay. This latter bird (significant in being the youngest phorusrhacid from South America) was one of the largest members of the group, with an estimated height of c. 2.5 m. It might be a species of Devincenzia. Patagornis and the giant phorusrhacine were predicted to have running speeds of 14 metres per second (about 50 km/h) while Mesembriornis had a ridiculous predicted running speed of 27 metres per second (about 97 km/h). For comparison, emus run at 14 metres per second, an ostrich reaches perhaps 17 metres per second (about 60 km/h) and cheetahs are reported to reach or exceed 27 metres per second.

Unsurprisingly, Blanco & Jones (2005) doubted if their predictions were accurate and they wondered if the unusual bone strength of some phorusrhacids – Mesembriornis in particular – might be related to something other than running speed. Could it be something to do with kicking? Based on the forces needed to break bones, they showed that Mesembriornis would be able to produce a force of over 2000 Newtons with its kick: strong enough to fracture bones. Thus it’s possible that some phorusrhacids used kicking as a way of breaking open bones to feed on marrow (how do the brontornithines, with their super-robust limb bones and inferred scavenging habits, fit into this?). It’s also possible that the birds used this kicking power to stun or kill prey, and here you will of course be thinking of the Secretary bird Sagittarius serpentarius, a cursorial raptor (superficially similar to a seriema) that kills or stuns snakes and other terrestrial prey with repeated kicks.

The foot claws of some phorusrhacids also support the idea that they used their feet in maiming or killing as the claws are laterally compressed, curved and sharp-tipped. That’s not necessarily the normal morphology for predatory birds, as many cursorial birds (and non-avian theropods) actually have rather blunt, stout foot claws. Tonni & Tambussi (1988) described the foot morphology of the Miocene psilopterine Psilopterus and showed that its foot claws were nearly identical to those of the living seriema Cariama cristata.

What’s interesting about this is that the second toe claw in Cariama is slightly enlarged relative to those of digits III and IV (as you can see from the very poor accompanying photos). Seriemas reportedly use the claw to aid in tree-climbing, and I’d like to know if they use it in attacking or killing prey. A great many other birds, including raptors and many passerines, have similarly enlarged claws on digit II however. I’ve previously been guilty of comparing this ‘enlarged’ claw with the raised sickle-claw seen in dromaeosaurids and other Cretaceous theropods, but what we have in phorusrhacids and seriemas clearly isn’t as elaborate, so there’s no indication that they used it to slash open the bellies of prey or anything like that (and here I’ll avoid the debate about the function of sickle-claws*). I doubt if even small phorusrhacids climbed trees, so presumably they used the claw in dispatching or manipulating prey.

* A recent study has claimed that sickle-claws could not function as slashing or stabbing weapons, but were perhaps used instead as climbing crampons, enabling dromaeosaurids to climb the bodies of their prey. I feel that there are major flaws in this study and that its conclusions are erroneous (see comments in Naish 2006).

While conventionally regarded as South American birds, there have been occasional reports of phorusrhacids from elsewhere. Two supposed European members of the group, Ameghinornis minor from Eocene-Oligocene France and Aenigmavis sapea from Eocene Germany, were identified in the 1980s (actually, Ameghinornis minor was first described [as Strigogyps minor] in 1839, but its new name and proposed affinity to phorusrhacids weren’t published until 1987). Both were weakly flighted or flightless birds about the size of a partridge.

Given that a few other Eocene European tetrapods have been suggested to be particularly closely related to South American taxa (namely the ratite Palaeotis, the peradectine opossums and the supposed anteater Eurotamandua), Ameghinornis and Aenigmavis were thought to perhaps indicate that phorusrhacids had originated in Europe and later spread (via Africa) to South America (Peters & Storch 1993). However, reanalysis has shown that both names are best regarded as junior synonyms of Strigogyps, and furthermore that Strigogyps differs significantly from phorusrhacids in lacking the derived characters that unite the members of this group (Mayr 2005). We’re not actually sure what Stigogyps is (though its tarsometatarsus is similar in some details to that of a trumpeter), but its re-evaluation strikes phorusrhacids off the list of European fossil taxa. Incidentally, there are unpublished Palaeocene and/or Eocene fragments from England and North America that, inspired by the 1987 identification of Aenigmavis, have also been suggested to be phorusrhacids. They await evaluation but, like Strigogyps, it is doubtful if they really have anything to do with Phorusrhacidae.

I should point out that the other European Eocene forms previously regarded as being of South American affinity have also been reinterpreted. Palaeotis, a small ratite argued by some to be a stem rhea, has more recently been found to be outside of the clade that includes rheas, ostriches, cassowaries and emus. Peradectine opossums may or may not be of South American origin: however, by the Eocene they occurred in North America and Europe and they later occurred in Asia and Africa. They do not seem to provide special evidence for a faunal link between South America and Europe. Finally, the supposed anteater Eurotamandua seems not to be an anteater, nor even a xenarthran, and as such there is nothing South American about it [adjacent image shows, at top, skeleton and life restoration of Eurotamandua, with the Eocene pangolin Eomanis at bottom. Taken from here].

What is almost certainly a non-American phorusrhacid was reported in 1987… from the Eocene of Antarctica (Case et al. 1987). Known only from the anterior part of the premaxillae, the specimen must have belonged to a reasonably large bird, but not much more than that is known about it. Older phorusrhacids are known from the Palaeocene of South America, so the specimen does not demonstrate that phorusrhacids originated in Antarctica: rather, it probably shows that they were common to both continents prior to their separation in the Oligocene.

An interesting parallel is provided by the fossil record of sloths, as while long regarded as of South American origin, the oldest sloth is a Middle Eocene fossil from Seymour Island (Antarctica). From time to time people make the point that some really interesting, major events in tetrapod history must have occurred in ancient Antarctica – if only it wasn’t for that damned ice sheet. Luckily, we’re doing all we can to get rid of it (that’s meant to be ironic). Anyway, we might speculate that Antarctica was home to numerous phorusrhacid lineages prior to its glaciation, but we’ll likely never know about them.

More to come…

Refs - -

Blanco, R. E. & Jones, W. W. 2005. Terror birds on the run: a mechanical model to estimate its maximum running speed. Proceedings of the Royal Society of London B 272, 1769-1773.

Case, J. A., Woodburne, M. O. & Chaney, D. S. 1987. A gigantic phororhacoid(?) [sic] bird from Antarctica. Journal of Paleontology 61, 1280-1284.

Chiappe, L. M. & Bertelli, S. 2006. Skull morphology of giant terror birds. Nature 443, 929.

Mayr, G. 2005. “Old World phorusrhacids” (Aves, Phorusrhacidae): a new look at Strigogyps (“Aenigmavis”) sapea (Peters 1987). PaleoBios 25, 11-16.

Naish, D. 2006. The Carnivorous Dinosaurs [review]. The Palaeontology Newsletter 62, 122-126 [free pdf available here].

Olson, S. L. & Alvarenga, H. M. F. 2002. A new genus of small teratorn from the Middle Tertiary of the Taubaté Basin, Brazil (Aves: Teratornithidae). Proceedings of the Biological Society of Washington 115, 701-705.

Peters, D. S. & Storch, G. 1993. South American relationships of Messel birds and mammals. Kaupia 3, 263-269.

Tonni, E. P. & Tambussi, C. P. 1988. Un nuevo Psilopterinae (Aves: Ralliformes) del Mioceno tardio de la Provincia de Buenos Aires, Republica Argentina. Ameghiniana 25, 155-160.