Actually, given that they’ve been extinct for quite some time now, 2007 wasn’t really a particularly good year for either terror birds, aka phorusrhacids, or for mega-ducks (on which read on), but it was a pretty good year in terms of the new material that was published on them. Phorusrhacids were covered a few times back on Tet Zoo ver 1 in 2006 (go here and here): we looked at their phylogeny, taxonomy, anatomy and palaeobiology, and at the then-new discovery of BAR 3877-11, a gigantic Patagonian skull, 716 mm long, and most similar to the skull of Devincenzia pozzi (Chiappe & Bertelli 2006). What has happened since then? Well, quite a lot…
Among phorusrhacid taxa, we looked previously at Alvarenga & Höfling’s (2003) division of the group into Psilopterinae (small and gracile; Palaeocene to Pliocene), Mesembriornithinae (mid-sized, shallow-skulled, gracile-legged; Miocene to Pliocene), Patagornithinae (mid-sized; Oligocene to Pliocene), Brontornithinae (gigantic, robust; Oligocene to Miocene), and Phorusrhacinae (mostly large, gracile-legged; Miocene to Pleistocene). Most of the groups are hardly ever written about in the literature and getting information on them has long been tricky. This was always a permanent frustration of mine, especially when some of them – in particular the brontornithines – are so amazing. Brontornithines, best known for Brontornis burmeisteri from the Miocene of Argentina, possessed incredibly stocky legs and have been imagined as avian hyaenas, bone-crunching scavengers that didn’t really run fast but were specialised for walking. There’s more discussion of them in the ver 1 post here.
Anyway, the big news is that something very strange happened to Brontornis in 2007: it was, shock horror, argued not to be a phorusrhacid at all. While it’s often been noted that Brontornis – the most robustly built and probably heaviest of phorusrhacids – would have superficially recalled gastornithids and dromornithids, recent proposals that these two groups are not gruiforms or palaeognaths as long supposed but are in fact (apparently) anseriforms led Agnolin (2007) to look anew at the detailed morphology of Brontornis. It turns out that Brontornis differs from all phorusrhacids in several respects: its hypotarsus is different in shape from that of all other phorusrhacids, its pedal phalanges (foot claw bones) are sub-triangular in cross-section (rather than sub-circular), and (based on the shape of its lower jaw) its rostrum appears to have relatively short and wide (rather than long and narrow). But… so what, maybe it was just some super-graviportal, broad-billed phorusrhacid that modified a few of the characters otherwise considered typical for the group?
Alas, there’s more, for not only does Brontornis differ from phorusrhacids in these ‘key’ features, it also shares a list of derived characters with….. anseriforms. Waterfowl. In Brontornis‘s quadrate, there are two distinct ventral condyles, the medial condyle is strongly expanded mediolaterally, the orbital process is very deep dorsoventrally as it emerges from the quadrate shaft, and the pterygoid process projects dorsally rather than just horizontally. In the mandible the dentary symphysis is relatively short (comprising less than a third the total length of the jaw) and the mandibular rami strongly diverge posteriorly [Brontornis mandible shown above, from Alvarenga & Höfling 2003]. In the tibiotarsus, the distal intercondylar notch is wide and shallow (rather than narrow and deep) and the medial condyle is elongated anteroposteriorly, and the intercotylar prominence on the proximal end of the tarsometatarsus is wide and tall (rather than narrow, or low). In these characters, Brontornis not only differs from phorusrhacids, it exhibits, time and time again, the derived condition present in members of Galloanserae, and specifically in Anseriformes (Agnolin 2007) [image below is a hilarious bit of old art I found on the web. Brontornis is here pitted against... against.. well, who knows what the hell that thing is. It has a hadrosaur's head].
While I find it easy to think that the shapes of unguals and the widths of skulls are phylogenetically quite plastic, trifling little details of quadratic condyles and of the articular surfaces between limb bones seem less easy to dispel, so (ironically perhaps) they carry more weight. In saying this, of course, I’m reminded of the contention that braincase characters are less prone to selection and hence carry a stronger phylogenetic signal than other parts of the skeleton: a contention which, as we saw briefly in one of the SVPCA 2007 articles, is probably erroneous (Rauhut 2007). I think theory would have it that all characters are equally afflicted by homoplasy and reversal, but in reality we often can’t help but think that some features really do carry more weight than others.
Anyway, I think Agnolin made quite a strong case, and at the moment it seems that Brontornis really does look more like an anseriform than a phorusrhacid. It was a mega-duck. This is particularly neat because, as I mentioned earlier, both gastornithids (the group that includes Gastornis, the probable senior synonym (Buffetaut 1997) of Diatryma) and dromornithids (the Australian mihirungs) are now regarded as anseriforms too, so we now have three distinct groups of large to gigantic, robust-limbed, deep-skulled, flightless birds at the base of Anseriformes (though note that stronger evidence demonstrating the inclusion of the mihirungs within anseriforms wouldn’t go amiss.. and note also that not all workers are happy extending Anseriformes to include these additional groups. Rather, they regard gastornithids and dromornithids as close to Anseriformes, but not necessarily within it (e.g., Livezey & Zusi 2007)). Agnolin (2007) actually found gastornithids (represented by Diatryma) to be the most basal anseriform clade, with Brontornis and a dromornithid + Anseres clade being united by the presence of a short dentary symphysis and a dorsally directed pterygoid process on the quadrate (also of interest is that, within Anseres, Agnolin found magpie-geese and screamers to be sister-taxa). Incidentally, 2007 also saw the description of the oldest gastornithid material yet reported: a large, massively stout coracoid from the Middle Palaeocene of Walbeck, Germany (Mayr 2007). One more thing: crown-group anseriforms (like the anatoid Vegavis iaai) are now known from the Upper Cretaceous (Clarke et al. 2005), so if these phylogenetic proposals are valid, gastornithids, Brontornis and dromornithids all have ghost-lineages extending back into the Cretaceous [anseriform phylogeny as recovered by Agnolin (2007) shown below].
If gastornithids, Brontornis and dromornithids really are all down at the base of Anseriformes, what does this tell us about the early evolution of the clade? It would be tremendously cool if ducks, geese and swans descended from mostly large-bodied, deep-skulled carnivores*: another reason to heap on anseriforms more accolade than they already deserve. I’ve long held the contention that these are probaby the most amazing of birds… as I’ve said before, we’re talking about a group that exhibits ‘the evolution of carpal spurs and knobs, extreme pugnacity and territoriality, nest parasitism, creching behaviour, parental carrying of young both in the water and (!) in the air, monogamous pair-bonding, underwater copulation and the (?)reinvention of the penis, major sexual variation in tracheal structure, grass-eating and 20-minute gut carrying time, niche partitioning according to intestine size, carrion feeding on Subantarctic islands, the evolution of fern-eating, island giantism, island dwarfism, crepuscularity, serrated bill margins, filter feeding with buccal lamellae, deep-diving, species where males are flightless but females flighted, coevolution of browsing forms with spiky lobelioideaens, repeated increases and decreases in body size during phylogeny, the annual transportation of TONNES of sand… and, pant pant pant, quacking’.
Plus, don’t forget that some workers now contend that another group of awesome and famous fossil birds – the pelagornithids, pseudodontorns, odontopterygiforms or bony-toothed birds – are, though not part of Anseriformes, very closely related to them (Bourdon 2005). Bourdon proposed that anseriforms and pelagornithids should be united in a new clade dubbed Odontoanserae. I still find this hard to accept to be honest… it just seems so, so… wrong [I wanted to accompany this part of the article with a good pelagornithid illustration. To my horror, I wasn't able to find any online. Harrumph, what kind of a world are we living in?]. Anyway, moving on…
* Exactly what gastornithids and dromornithids did for a living has been controversial. We’ll avoid that issue for now but if you want to know now see Witmer & Rose (1991), Murray & Megirian (1998) and Wroe (1999a, b).
If Brontornis isn’t a phorusrhacid, what does this mean for Physornis and Paraphysornis, the other ‘brontornithines’? Well, these taxa do not exhibit the anseriform characters that Brontornis does, and do exhibit all the phorusrhacid characters that Brontornis doesn’t. So they still are phorusrhacids, it’s just that Brontornis has been removed from among them. In view of this, the phorusrhacid ‘subfamily’ Brontornithinae needs renaming, so Agnolin (2007) formally renamed it Physornithinae, with Physornis fortis Ameghino, 1895 as the type species.
The monster Kelenken
Given that my phorusrhacid articles of 2006 were mostly inspired by the initial description of the immense skull of BAR 3877-11 (Chiappe & Bertelli 2006), it’s fitting that 2007 saw the proper description and formal naming of the taxon that this specimen represents: it’s Kelenken guillermoi Bertelli et al., 2007 [skull shown here]. Named after a ‘fearsome spirit of the Tehuelche tribe … represented as [a] giant bird of prey’ (Bertelli et al., 2007, p. 410), Kelenken has a particularly long rostrum and notably deep jugal bar, and its tarsometatarsus (which, together with a partial toe bone and some indeterminate fragments, is the only other part of the skeleton currently known) also exhibits several unique characters. It appears to be a phorusrhacine: its long, narrow rostrum ends in a wickedly sharp premaxillary hook, and its slender tarsometatarsus indicates that it was moderately gracile.
The fact that the rostrum of Kelenken is far shallower than that normally reconstructed for phorusrhacines raises the question as to how accurate traditional reconstructions of these birds have been. As we’ve seen on Tet Zoo before, the oft-figured skull of Phorusrhacos longissimus is speculative as the specimen the original sketch was based on was destroyed in the field, and figures of large phorusrhacid skulls have essentially been scaled-up versions of the skulls of smaller taxa like the patagornithines. These have very deep, and proportionally short, rostra. However, did we get it wrong in assuming that the giant taxa were similarly proportioned in cranial anatomy to smaller forms like Patagornis? Bertelli et al. (2007) argued that we did, and that we need to re-imagine the giant taxa as having had rather long, low rostra.
Given that Kelenken is presently only known from that awesome skull and from a few foot bones, we can’t make confident statements about its size. Clearly – given that the skull is over 70 cm long – this was a big bird, with a standing height of over 2 m and a mass exceeding 160 kg. This makes it huge, but not as big as the biggest elephant birds and mihirungs, some of which exceeded 300 or even 400 kg, apparently.
And that’s not all – more phorusrhacids next… Oh, and let me bring your attention to Ilja Nieuwland’s recently-initiated Phorusrhacid bibliography site. Not much there yet, but once this is up and running it’ll be awesome.
Refs – –
Agnolin, F. 2007. Brontornis burmeisteri Moreno & Mercerat, un Anseriformes (Aves) gigante del Mioceno Medio de Patagonia, Argentina. Revista del Museo Argentino de Ciencias Naturales, n.s. 9, 15-25.
Alvarenga, H. M. F. & Höfling, E. 2003. Systematic revision of the Phorusrhacidae (Aves: Ralliformes). Papéis Avulsos de Zoologia, Museu de Zoologia da Universidade de São Paulo 43, 55-91.
Bertelli, S., Chiappe, L. M. & Tambussi, C. 2007. A new phorusrhacid (Aves: Cariamae) from the Middle Miocene of Patagonia, Argentina. Journal of Vertebrate Paleontology 27, 409-419.
Bourdon, E. 2005. Osteological evidence for sister group relationships between pseudo-toothed birds (Aves: Odontopterygiformes) and waterfowls (Anseriformes). Naturwissenschaften 92, 586-591.
Buffetaut, E. 1997. New remains of the giant bird Gastornis from the Upper Paleocene of the eastern Paris Basin and the relationships between Gastornis and Diatryma. Neues Jahrbuch für Geologie und Paläontologie, Monatshefte 1997, 179-190.
Chiappe, L. M. & Bertelli, S. 2006. Skull morphology of giant terror birds. Nature 443, 929.
Clarke, J. A., Tambussi, C. P., Noriega, J. I., Erickson, G. M. & Ketcham, R. A. 2005. Definitive fossil evidence for the extant avian radiation in the Cretaceous. Nature 433, 305-308.
Livezey, B. C. & Zusi, R. L. 2007. Higher-order phylogeny of modern birds (Theropoda, Aves: Neornithes) based on comparative anatomy. II. Analysis and discussion. Zoological Journal of the Linnen Society 149, 1-95.
Mayr, G. 2007. The birds from the Paleocene fissure filling of Walbeck (Germany). Journal of Vertebrate Paleontology 27, 394-408.
Murray, P. F. & Megirian, D. 1998. The skull of dromornithid birds: anatomical evidence for their relationship to Anseriformes. Records of the South Australian Museum 31, 51-97.
Rauhut, O. W. M. 2007. The myth of the conservative character: braincase characters in theropod phylogenies. Hallesches Jahrbuch Geowissenschaften B 2, 51-54.
Witmer, L. M. & Rose, K. D. 1991. Biomechanics of the jaw apparatus of the gigantic Eocene bird Diatryma: implications for diet and mode of life. Paleobiology 17, 95-120.
Wroe, S. 1999a. The bird from hell? Nature Australia 26, 58-64.
– . 1999b. Killer kangaroos and other murderous marsupials. Scientific American 280 (5), 58-64.