As if the revelations about Brontornis and all that new work on the ameghinornithids weren’t enough, 2007 also saw the publication of a long-awaited new study on the age of Titanis walleri, North America’s only phorusrhacid, and – supposedly – a species that survived until as recently as 15,000 years ago. Yes, it’s time to crack on with more terror birds, or phorusrhacids. It’s a story of ancient island-hopping, of the proposed re-evolution of clawed forelimbs, and of Raven, the giant claw-handed bird of Native American folklore…
Titanis walleri Brodkorb, 1963 is one of the largest and last of the phorusrhacids, and after its 1961 discovery near the Sante Fe River in Florida (Brodkorb 1963), additional remains referred to the species were reported from three other Floridan localities as well as from Texas. As of 2005, Titanis was known from 41 separate elements including several skull fragments, some vertebrae, assorted pectoral and forelimb bones, and femoral shafts and other hindlimb fragments, including multiple isolated phalanges (Gould & Quitmyer 2005). Some of the Sante Fe material was discovered by diving, as the fossils actually occur on the river bottom. Unfortunately none of this material is articulated, and the reconstructions of Titanis that you can see in museums (the Florida Museum of Natural History is home to a mounted skeleton: it’s shown below) and in the literature are based on more complete skeletons of other phorusrhacids. It is assumed that Titanis was a phorusrhacine phorusrhacid, so it’s often modelled on what we know of taxa like Devincenzia. However, phorusrhacine affinities are not certain for Titanis and it actually resembled mesembriornithines in tarsometatarsal morphology (Alvarenga & H÷fling 2003) [I don't know who did the excellent restoration used above; it's available widely online. Looks like one of Carl's].
Two exciting proposals
Two key recent claims have made Titanis one of the most famous and most discussed of all fossil birds. The first is that, rather than possessing stunted, redundant little wings that were effectively functionless (bar possible, and unknown, uses in display, intraspecific combat or in assisting balance), Titanis actually sported semi-prehensile, clawed hands that probably functioned in predation. The second is that Titanis survived until far more recently than all of its South American cousins, and persisted into the Pleistocene and even well into the Late Pleistocene (a possible Pleistocene phorusrhacine was reported from Uruguay by Tambussi et al. (1999), but if it is Pleistocene and not Pliocene it can only be Early Pleistocene). Let’s think about both of these proposals for a minute. If Titanis actually had clawed, mobile fingers, then most of the classic stuff written about phorusrhacid behaviour and habits (e.g., Marshall 1994) could very well be completely wrong, as rather than relying on their hindlimbs and/or massive bill to procure and process prey, this discovery would suggest that manual apprehension and manipulation played a significant role in phorusrhacid predatory behaviour. This is radical and might have made Titanis (and perhaps all phorusrhacines, or even all phorusrhacids) superficially like Cretaceous non-avian maniraptorans, like dromaeosaurs. This extremely appealing idea was not lost on some (Zimmer 1997).
Secondly, if Titanis was still extant as recently as the Late Pleistocene (perhaps as recently as 15,000 years ago), then there’s the possibility that something very interesting happened. Namely, that people saw this amazing animal…. if, that is, people were present in south-eastern North America this early (and they were: e.g., Goebel et al. 2008). A giant predatory flightless bird, over 1.5 m tall (Titanis has been down-sized in recent years), with a massive hooked bill and the ability to knock down and eviscerate even quite large mammals would probably leave quite an impression on the imagination, to say the least. Might there be any Native American folklore, mythology or artwork hinting at ancient knowledge of this giant bird? According to Jaroslav Mare?’s 1997 book Svet Tajemnřch ZvÝ?at, maybe there is, because both the Kwakiutl and Haida people had stories of Raven, a ‘monstrous giant bird with a massive hooked beak, long sturdy legs …. and a feathered body – but possessing front paws with claws, instead of wings!’ (Shuker 2003: I haven’t seen Mare?’s book).
Understandably, it’s been very tempting to suggest that the Kwakiutl and Haida people might perhaps have encountered Titanis, thereby explaining the origins of Raven. However, all is not well with this idea. The Kwakiutl (who are, wikipedia tells me, properly known as the Kwakwaka’wakw) and Haida are not from the southern US, but from British Columbia and elsewhere on the Pacific coast – not exactly in the right place to see a bird that (so far as we know) didn’t live any further north than central Florida. This all suggests to me that linking Raven with Titanis was, unfortunately, a mistake. In any case, other evidence has now come along that also destroys what is an otherwise fine hypothesis.
No prehensile clawed hands for you, Titanis
The idea that Titanis had unusual hands was proposed by Robert Chandler who described new wing material (a partial humerus and a carpometacarpus) discovered around the banks of the Sante Fe. Chandler (1994) proposed that the strong, robust humerus of Titanis suggested that its wings were powerful organs, and not weak, vestigial ones. He also argued that Titanis had stiff wrists which resulted in the hands being held in line with the forearms (as is the case in, for example, penguins), and – most importantly – that a large ball joint on the carpometacarpus indicated the presence of a flexible thumb equipped with a claw. Reconstructions based on Chandler’s work (Zimmer 1997) also equip the phorusrhacid hand with a large claw on the end of digit II. It is these observations which resulted in the idea that Titanis had prehensile, clawed hands… but, things don’t quite follow here [as you can see from the accompanying life restoration, some people unfortunately took the 'prehensile hands' thing all too literally!].
Titanis did apparently have a relatively rigid wrist. However, the ball joint of the Titanis carpometacarpus is actually nothing special as the same sort of joint is present in seriemas (Gould & Quitmyer 2005). It does not indicate the presence of a large flexible thumb with a big claw, it merely shows that the alular digit was mobile (of course, this digit is mobile in all birds, but they don’t all have a convex articular facet on the carpometacarpus – it is usually flat)*. The alular digit of Titanis is unknown, so any ideas about big claws are entirely speculative. Similarly, the idea that digit II sported a big claw is also totally speculative as the phalanges of the hand are totally unknown! Given that claws on digits I and II are actually not uncommon in birds, it’s still possible that phorusrhacids had them. However, there’s no reason to think that they were hypertrophied, nor is there evidence that they were large or had a special function [image below, from Alveranga & H÷fling (2003), shows assorted phorusrhacid and other bird carpometacarpi. Titanis, with its large, ball-like articular facet, is A, Cariama is F].
So, alas, the idea that phorusrhacids were unique among birds in having prehensile, clawed forelimbs has fallen by the wayside and was never based on good evidence. Does the apparently robust humerus suggest any special function for the wing? We don’t know, but what data we have shows that the wings of Titanis were not large or unusual in any other way (Gould & Quitmyer 2005). As with so many cool ideas, I will admit that many of us (myself included) promoted the whole ‘prehensile hand’ thing mainly because it was just so awesome.
* Incidentally, the alular digit almost certainly corresponds with digit I (the pollex) – see Vargas & Fallon (2005) – despite arguments from some corners stating that it must represent digit II.
Was Titanis really alive in the Late Pleistocene?
The idea that people encountered Titanis rests on the assumption that Titanis really was contemporaneous with people: in other words that it made it into the Late Pleistocene. Did Titanis really manage to hang on this late? We need to look at a few things before answering this.
Given that all phorusrhacids are either Antarctican or South American (undescribed Eocene fragments from North America and Europe notwithstanding), Titanis (or an unknown ancestor) must have travelled northwards to get to Texas and Florida. It’s conventionally been thought that this could only have happened following the formation of the Panamanian isthmus during the Pliocene (4.5-3.5 million years ago, or thereabouts). Ergo, Titanis fossils must be no older than Early Pliocene, and perhaps much younger.
In Florida, the type material of Titanis from near the Sante Fe River was found associated with Late Pleistocene birds and mammals, including the pampathere Holmesina septentrionalis, the sloth Megalonyx jeffersonii, bison, mammoths, dire wolves and lions (MacFadden et al. 2007), and it was on the basis of this associated fauna that a Late Pleistocene age was first proposed (Brodkorb 1963). However, the same locality also yields an assemblage of Late Pliocene mammals that include the pampathere H. floridanus, the sloth M. leptostomus, the hyaena Chasmaporthetes, borophagines, llamas, pronghorns and three-toed horses. So is the Sante Fe Titanis material Late Pliocene or Late Pleistocene in age?
Baskin (1995) reported phorusrhacid material from south Texas. Although consisting of but a single pedal phalanx [shown here: from Jon Baskin's site], its strong similarity to the holotype pedal phalanx of T. walleri led him to refer the Texan specimen to that species. Occurring alongside this new bone were both Early Pliocene horses and Late Pleistocene mammoths, bison and other fossils (the fossils appear to have been mixed after the Pliocene elements were washed in from a different locality 15-25 km away). So was the Titanis fossil Early Pliocene or Late Pleistocene? Baskin (1995) concluded that the Titanis bone couldn’t go together with the Pliocene fossils, because their Early Pliocene age meant that they pre-dated the formation of the Panamanian isthmus. It must, he concluded, therefore be Late Pleistocene like the mammoths and bison from the site. He also noted that the Titanis bone was more similar in colour to the Pleistocene bones than to the Pliocene ones.
Clearly, any conclusions about the age of Titanis, based on the associated fauna, are, well, inconclusive. To resolve this controversy, MacFadden et al. (2007) used rare earth element (REE) analysis to date the Titanis bones. REEs occur in low abundance in living animals, but during diagenesis are rapidly taken up and concentrated. Different REE concentrations have proved to be reliable signatures of specific local conditions and, among collections of bones that come from mixed sources, REE concentrations can therefore be used to show which bones originated from the same geological environment.
What did the data reveal? That both the Floridan and Texan Titanis material is identical in REE composition to the Pliocene fossils from the same sites, with the Texan material being Early Pliocene and hence about 5 million years old, and the Floridan material being Late Pliocene and hence about 2.2-1.8 million years old (MacFadden et al. 2007) [I first learnt of this while writing the ver 1 phorusrhacids article here, and it explains the 'cough cough' used in the first line]. So, there endeth the idea that Titanis made it all the way to the Late Pleistocene, or even into the Pleistocene at all… at least, so far as we can tell based on present data, of course. But if – as the REE data shows – Titanis was in North America 5 million years ago, and hence prior to the completion of the Panamanian isthmus (which occurred c. 4.5-3.5 million years ago), how on earth did it get there? [replica Titanis skull shown below. As discussed previously, reconstructions like this probably make the rostrum too deep].
The answer seems to be that faunal interchange between North and South America was more complex, and more drawn out, than conventionally thought. Exchange didn’t only occur after the land-bridge was complete, but occurred prior to this during the very earliest Pliocene and even Late Miocene. We know that some sloths made the crossing prior to the formation of the land-bridge, for example (previously discussed on Tet Zoo here), as did certain mastodonts, tapirs and raccoons, and it now seems that phorusrhacids did too. These animals presumably island-hopped, but they might also have done a lot of swimming in order to complete the crossing. Notably, sloths and proboscideans are reasonably good long-distance swimmers… were phorusrhacids too? Large extant flightless birds like cassowaries are reported to be strong swimmers.
Phorusrhacids at Tet Zoo: there and back, again and again
With the ending of this article, Tetrapod Zoology is now (probably) the largest repository of publicly-available phorusrhacid information, with the exception of Alveranga & H÷fling’s open-access review of the group (available for free here). And of course, once Phorusrhacid Bibliography is up and running, even more information on these birds will be out there. In order to have all the Tet Zoo phorusrhacid articles linked to in one place (partly for my own convenience), here is a list…
— Terror birds (Oct’ 27th 2006: general introduction to phorusrhacid diversity and classification)
— More on phorusrhacids: the biggest, the fastest, the mostest out-of-placest (Nov’ 1st 2006: on brontornithines and phorusrhacid palaeobiology in general)
— Giant hoatzins of doom (Nov’ 3rd 2006: on South American landbird hypothesis)
— Goodbye my giant predatory, cursorial, flightless hoatzin (Nov’ 12 2006: on affinites of Cariamae)
— 2007: a good year for terror birds and mega-ducks (June 10th 2008: on Brontornis and Kelenken)
— You’re not a proto-phorusrhacid, but you’re still a cariamaen, and that’s alright with me (ode to the Ameghinornithidae) (June 13th 2008: on ameghinornithids)
Finally, thanks to Tommy Tyrberg for sending the Gould & Quitmyer (2005) article: I couldn’t get it via inter-library loan. What’s next? Well…
Refs – –
Alveranga, H. M. F. & H÷fling, E. 2003. Systematic revision of the Phorusrhacidae (Aves: Ralliformes). PapÚis Avulsos de Zoologia, Museu de Zoologia da Universidade de SŃo Paulo 43, 55-91.
Baskin, J. A. 1995. The giant flightless bird Titanis walleri (Aves: Phorusrhacidae) from the Pleistocene coastal plain of south Texas. Journal of Vertebrate Paleontology 15, 842-844.
Brodkorb, P. 1963. A giant flightless bird from the Pleistocene of Florida. The Auk 80, 111-115.
Chandler, R. M. 1994. The wing of Titanis walleri (Aves: Phorusrhacidae) from the Late Blancan of Florida. Bulletin of the Florida Museum of Natural History, Biological Sciences 36, 175-180.
Goebel, T., Waters, M. R. & O’Rouke, D. H. 2008. The Late Pleistocene dispersal of modern humans in the Americas. Science 319, 1497-1502.
Gould, G. C. & Quitmyer, I. R. 2005. Titanis walleri: bones of contention. Bulletin of the Florida Museum of Natural History 45, 201-229.
MacFadden, B. J., Labs-Hochstein, J., Hulbert, R. C. & Baskin, J. A. 2007. Revised age of the late Neogene terror bird (Titanis) in North America during the Great American Interchange. Geology 35, 123-126.
Marshall, L. G. 1994. The terror birds of South America. Scientific American 270 (2), 64-69.
Shuker, K. P. N. 2003. The Beasts That Hide From Man. Paraview Press, New York.
Tambussi, C., Ubila, M. & Perea, D. 1999. The youngest large carnassial bird (Phorusrhacidae, Phorusrhacinae) from South America (Pliocene-Early Pleistocene of Uruguay). Journal of Vertebrate Paleontology 19, 404-406.
Vargas, A. O. & Fallon, J. F. 2005. The digits of the wing of birds are 1, 2, and 3. A review. Journal of Experimental Zoology (Mol Dev Evol) 304B, 206-219.
Zimmer, C. 1997. Terror take two. Discover 18 (6), 68-74.