You're not a proto-phorusrhacid, but you're still a cariamaen, and that's alright with me (ode to the Ameghinornithidae)

You'll recall me saying recently that 2007 was a good year for publications on phorusrhacids, aka terror birds. And as I discussed in the previous post, one of the most interesting contentions made about phorusrhacids last year was that one of the most remarkable members of the group (super-robust Brontornis from the Miocene) is actually not a phorusrhacid at all. Here we look at recent work on a group of birds that, while initially suggested to be part of the phorusrhacid radiation, now, also, seem not to be. They are the ameghinornithids...

Originally named as a phorusrhacid 'subfamily' when first recognised by Cécile Mourer-Chauviré in 1981, Ameghinornithidae first included but a single species, Ameghinornis minor (originally Strigogyps minor Gaillard, 1939) from the Upper Eocene-Lower Oligocene Phosphorites du Quercy (its exact discovery site is unknown). It's known from a humerus, two coracoids and two carpometacarpi, though whether these elements really belong to the same animal is now doubtful (read on). A second taxon, Aenigmavis sapea Peters, 1987 from the Middle Eocene Grube Messel site was later added to the group and is known from a near-complete specimen and a referred foot (its specific name honours SAPE, the Society of Avian Palaeontology and Evolution, founded in 1985). To my astonishment, I couldn't find a picture of this specimen (Forschungsinstitut Senckenberg SMF-ME 1818) anywhere on the web, nor do I have a pdf I can steal it from, so in the end I resorted to tracing and scanning a drawing. It's not good, but it could be worse, and it's shown above. The animal it depicts would have had a total length of about 50 cm.

During the 1980s and 90s, these flightless or poorly-flighted, grouse-sized birds were imagined as 'proto-phorusrhacids', and their presence in Eocene-Oligocene Europe led to numerous speculations as to the possibility of faunal interchange between Europe and South America (e.g, Peters & Storch 1993). The presence in Eocene Europe of the alleged xenarthran Eurotamandua [skeleton and life restoration shown in adjacent image] and of sebecosuchians (on which go here) provided additional support for this supposed link (for a discussion of all this please see the ver 1 post here).

However, phorusrhacid affinities for the ameghinornithids were doubted by Alvarenga & Höfling (2003) who pointed to differences in the proportions of the two groups and to different hindlimb morphologies. Mayr (2005) agreed, noting that ameghinornithids lack the proportionally large skull, deep lower jaw, reduced wing, narrow-shafted, strut-like coracoid, extremely narrow pelvis, strongly reduced hallux and other characters present in the terror birds. He concluded that ameghinornithids are, however, still members of Cariamae, the clade thought to include seriemas and the extinct phorusrhacids, bathornithids and idiornithids (for more on cariamaens and what they are see Giant hoatzins of doom on ver 1).

Incidentally, if you've ever looked at anything in the palaeornithological literature you'll be familiar with bathornithids as much as you are with phorusrhacids, for Neocathartes grallator from the Eocene of Wyoming, the long-legged 'terrestrial vulture' depicted consistently in all the older literature, is not a vulture at all, but actually a bathornithid ['traditional' restoration shown here]. Time for another famous Storrs Olson quote: 'The reconstruction published with the original description of Neocathartes has often been reprinted and has now made the 'terrestrial vulture' an integral part of the lore of avian paleontology. Well, forget it. Neocathartes is just our old friend Bathornis in another guise' (Olson 1985, p. 150). I've just noticed that wikipedia's entry on Neocathartes uses exactly the same quote.

Anyway, if ameghinornithids are part of Cariamae, and if they lack the derived characters of Phorusrhacidae, could they still however be 'proto-phorusrhacids': closer to Phorusrhacidae than to any other group? The answer is probably no, because ameghinornithids don't just lack the derived characters that characterise phorusrhacids; they also lack the characters that unite phorusrhacids with idiornithids and seriemas (Mayr 2007), in particular the block-like hypotarsus that lacks crests and sulci. By inference they are therefore down at the base of Cariamae.

In a taxonomic revision of the ameghinornithids, Mayr (2005) argued that Ameghinornis and Aenigmavis were both synonymous with Strigogyps Gaillard, 1908: Aenigmavis sapea and S. dubius share detailed tibiotarsal characters and hence should be considered congeneric (S. dubius is only known from a tibiotarsus*), while Ameghinornis minor shares a derived humeral morphology with Aenigmavis sapea, and hence should be regarded as congeneric with Ameghinornis, and hence with Strigogyps too (in these birds, the proximal end of the humerus is very small, the bicipital and deltopectoral crests are reduced, and the humeral head is obliquely oriented relative to the shaft) [if you have a particularly good memory this might sound familiar: I covered it previously on ver 1 here].

* Which was lost when Munich was bombed during WWII. Casts remain however.

Mayr (2005) further argued that, given that the Ameghinornis minor humerus (now Strigogyps minor) and S. dubius tibiotarsus both come from Quercy and both represent Strigogyps, they should provisionally be considered as conspecific, so Ameghinornis not only fails to win recognition as a distinct genus, it is (according to Mayr) not even a distinct species. Interestingly, however, the coracoids and carpometacarpi referred to Ameghinornis minor by Mourer-Chauviré (1981) differ from the same elements known from another Strigogyps specimen (a wing from Messel that has the same humeral characters listed above), and hence probably aren't from an ameghinornithid at all: they might be from a large, flightless idiornithid like Propelargus (Mayr 2005) [adjacent image, from Alvarenga & Höfling 2003, shows phorusrhacid carpometacarpi as well as (E) a carpometacarpus referred to Ameghinornis minor, (F) a seriema carpometacarpus, and (G) a hoatzin carpometacarpus. The 'Ameghinornis minor' carpometacarpus probably doesn't belong to this taxon, but to an idiornithid].

If you've found any or all of this a bit difficult to follow, I assure you you're not alone. I reread Mayr's 2005 paper about five times and had to draw a little flow-chart before I was on top of all this.

Anyway, here's where we come to the 2007 stuff again, because Peters (2007) - in a review of current knowledge of the ameghinornithids - contested Mayr's synonymisations, arguing that Ameghinornis and Aenigmavis are actually different enough from Strigogyps for all to be considered distinct. However, Mayr restated his arguments in another 2007 paper (Mayr 2007). He did more than that however. We've looked at hornbills a couple of times on Tet Zoo, and one of the things we'd all like to see is a better fossil record for this group. Long, long ago I mentioned Geiseloceros robustus Lambrecht, 1935, a Middle Eocene bird from Geiseltal in Germany originally regarded as a New World vulture but later classified as a hornbill. There has been widespread suspicion since that Geiseloceros is in fact not a hornbill but, as is the case with so many enigmatic fossils, saying what something isn't is often all too easy, but the question then remains as to what it is.

The good news is that Mayr (2007) was able to resolve this. Geiseloceros is yet another Palaeogene bird very similar to Strigogyps (as evidenced by that distinctive humeral morphology again), and it thereby becomes Strigogyps robustus. Consisting of forelimb and pectoral bones, the specimen was found adjacent to another specimen also identified by Lambrecht as a New World vulture: the holotype of Eocathartes robustus (you'd think Lambrecht could have been a bit more imaginative on the specific names). It consists of hindlimb and pelvic elements, and you might not be surprised to learn that some workers have suggested that both 'taxa' represent the different halves of the same individual. Comparison with other Strigogyps specimens (like the Messel S. sapea holotype) confirms this (Mayr 2007).

So it would seem that we have another ameghinornithid to add to the list. What can we now say about what these birds looked like, and how they lived? Unlike phorusrhacids, ameghinornithids did not have proportionally large skulls, and overall they appear to have been proportioned like certain gamebirds, or like trumpeters (aka psophiids). Ameghinornithid lower arm bones are proportionally short and, as we've seen, the crests on their humeri are comparatively small. These features indicate that they were flightless or near-flightless: if capable of flight, it was probably only weak gliding.

The tarsometatarsus is also proportionally short and the hallux was large, so they don't appear to have been cursorial (unlike phorusrhacids), and the proportionally long femur (it's much longer than the tarsometatarsus in S. sapea) led Alvarenga & Höfling (2003) to compare these birds with perching or climbing forms like parrots, pigeons and guans and curassows. Their foot anatomy suggests some climbing or perching ability of a sort similar to that present in guans and chachalacas, but note that I say this based on cursory comparison and am not aware of any detailed work comparing the phalangeal proportions of these birds. Mayr (2007) preferred the idea of terrestrial habits, and noted that the 'Eocathartes' pelvis resembles that of Cariama [image above shows Rufous-vented chachalaca Ortalis ruficauda, from wikipedia. Similar to an ameghinornithid?].

All in all, it looks like ameghinornithids might have lived like guans, curassows and other tropical galliforms, but it's also been noted that they're similar in some respects (like tarsometatarsal morphology) to the trumpeters [Grey-backed trumpeter Psophia crepitans shown here, from wikipedia]. Guans, chachalacas and trumpeters are all omnivorous generalists that eat plants and small animals, and at the moment I think it's reasonable to imagine this sort of lifestyle for the ameghinornithids. Peters (2007) wrote that ameghinornithid foot claws are 'powerful' and 'raptor-like'. To me, they don't look as slender and curved as do the foot claws of many raptors, but again this observation is, well, just an observation and proper work on claw curvature and so on remains to be done. In fact a detailed comparative analysis of these birds is still lacking, and I've only scratched the surface here. There don't appear to be any good ameghinornithid life restorations kicking around - there's something on wikipedia but I can't say I think very highly of it.

Yet again, what was meant to be a brief aside of a few hundred words has grown into a full article. This means that I still haven't finished the phorusrhacid stuff - more soon...

Refs - -

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.

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

- . 2007. Synonymy and actual affinities of the putative Middle Eocene "New World vulture" Eocathartes Lambrecht, 1935 and "hornbill" Geiseloceros Lambrecht, 1935 (Aves, Ameghinornithidae). Paläontologische Zeitschrift 81, 457-462.

Mourer-Chauviré, C. 1981. Première indication de la présence de Phorusrhacidés, famille d'oiseaux géants d'Amérique du Sud, dans le Tertaire européen: Ameghinornis nov. gen. (Aves, Ralliformes) des Phosphorites du Quercy, France. Géobios 14, 637-647.

Olson, S. L. 1985. The fossil record of birds. In Avian Biology, Volume III, pp. 79-238.

Peters, D. S. 2007. The fossil family Ameghinornithidae (Mourer-Chauviré 1981): a short synopsis. Journal of Ornithology 148, 25-28.

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