Eremopezus

Careful study of the remains suggests that their apparent similarity to ratites is misleading. They actually combine a number of traits not found in any known ratite lineage, and in particularly not in the ostriches and elephant birds (the only ratites biogeographically close to Eremopezus). Moreover, the prehistoric bird had several peculiar traits not at all found in ratites, but present in certain neognaths: its toes were widely divergent and could flex through a wide range of positions, while strong tendons gave the bird a firm grip. In the associated anatomical details, E. eocaenus resembled the secretarybird (Sagittarius serpentarius) and the shoebill (Balaeniceps rex), two rather singular African endemics.[4] Nonetheless, more recent studies seem to group it among "aepyornithid-like" taxa.[9]

Adult secretarybird (Sagittarius serpentarius)

The secretarybird is a bird of prey that can fly well, but prefers to walk around on its long legs, especially when foraging. It uses its flexible toes to grab prey—large arthropods and small to mid-sized terrestrial vertebrate. It throws the prey around and kicks it forcefully, smashing it to death or breaking its spine. The shoebill, meanwhile, for lack of a better theory was long considered a large aberrant stork relative in the Ciconiiformes. This eventually proved incorrect. While B. rex is indeed a member of the same group of "higher waterbirds" as are storks and herons, it is closely related to pelicans and thus came to be included in the Pelecaniformes—which are not monophyletic as described in old times, however, and whose small core group, for simplicity, is included in the Ciconiiformes by some recent authors. Insofar, the shoebill is today recognized as the formerly missing link uniting pelicans and storks. Like the pelicans, it uses its massive bill and throat sac to catch large fish and similar aquatic vertebrates, but unlike them it is a wading, not a swimming bird, and correspondingly has long legs like a stork. It clambers through reed beds in search of good fishing spots, and uses its flexible toes to firmly hold on to such uncertain substrate as heaps of wind-blown vegetation at the edge of the open water or roots and logs of trees.[10]

But this does not mean that Eremopezus was a close relative of either secretarybird or shoebill. Even though modern taxonomy generally tries to avoid using monotypic families as much as possible, its placement in a distinct family Eremopezidae may well be warranted, as its closest relatives remain completely obscure.

For the birds of prey, a Paleocene (perhaps even Late Cretaceous) origin seems most likely; the first fossils of the present-day lineages (and some entirely extinct forms) are abundant in Eocene strata of the Holarctic, particularly Europe. The falcons seem to have rapidly spread throughout the Americas, where the plesiomorphic caracaras are found. The other diurnal raptors, family Accipitridae—by some considered more distinct, as core group of an order Accipitriformes –, apparently diversified eastwards into Asia and eventually into Africa.[4]

As regards the secretarybird, the genus Pelargopappus occurred at about the same time as E. eocaenus in today's France, separated from the site of Faiyum by more than 1,500 km[11] of the shrinking Tethys Sea. The Maghreb was closer to Europe however, not more than today's Mediterranean where it is widest, and numerous bird lineages are known to have occurred in Africa as well as in Europe during the Eocene. Pelargopappus seems to be an ancient secretarybird, not an ancestor of the living species but not far removed from the last common ancestor either. Its remains, and how they differ from secretarybirds, give an idea how the Sagittariidae of that time looked like. So even though Pelargopappus has not yet been directly compared to the African fossil in a cladistic analysis, the pronounced differences between Eremopezus and Sagittarius and the biogeography of Pelargopappus suggest that Eremopezus was not especially close to the secretarybird's lineage. Thus, even if it belongs with the Falconiformes (or Accipitriformes), it would probably remain in its distinct family.[12]

Shoebill (Balaeniceps rex). Though stork-like in habitus, the bill's "nail" betrays its relationship to the pelicans

The same is true if it were placed in the Pelecaniformes. For as it seems, Goliathia andrewsi, an ancestral shoebill that was slightly larger and presumably far less apomorphic than the living species, lived at about the same time and in the same region as E. eocaenus. It is only known from a single ulna, which proves however that it was far less in bulk than Eremopezus and certainly able to fly well.[13]

But all things considered, very little can be said about the affiliations of Eremopezus. As ratites are nowadays presumed to have evolved from flying paleognaths similar to tinamous after the Cretaceous—see also Palaeotis—the Egyptian fossil may be a distinct lineage of ratites after all, and is sometimes allied with the Paleocene Remiornis from France. And indeed, as noted by the first scientists that studied it, its known bones—in particular the distal tarsometatarsus—are more similar to those of ratites in general shape and in some details than to other birds. The details (such as lack of the supratendinal bridge) may however be simply plesiomorphies, while the overall resemblance might be the result of convergent evolution—for the autapomorphies of its toes are starkly unlike those found in any ratite. Thus, if it was a ratite, it probably was the most unusual ratite known to date (while the hardly better-known Remiornis may very well have been a rather conventional proto-ostrich).[6]

As far as can be told however, ratites did never evolve gripping feet like seen in the fossils, and neither did they fare very successfully in wetland habitat as represented in the Jebel Qatrani Formation. The latter holds true for Galliformes, which are ancient neognaths of the fowl clade (Galloanserae). Anseriformes, the other living member of this group, include the typical waterfowl such as ducks, and many of them independently evolved flexible toes, giving them the ability to perch despite having only a vestigial hallux (see also Cairinini). The Anseriformes also contain the gigantic flightless mihirungs (Dromornithidae) of Australia (which were, at first, also believed ratites). It is not inconceivable that E. eocaenus was a more aquatic African equivalent of these.[14]

Thus, until more fossils are found, all that can be said is that Eremopezus is unlikely to have been a galliform or a member of the "near passerines" group (in the strict sense—perching birds and their immediate allies); these latter orders are generally smallish, live in trees, and probably evolved even later than the birds of prey. But beyond that, it cannot even be ruled out with reasonable certainty that the initial assessment—however lacking by the standards of today's scientific method—was correct and that E. eocaenus was indeed a highly apomorphic paleognath.[6]

Adult male American rhea (Rhea americana). E. eocaenus probably had legs longer than but about as thick as this bird.

Its tibiotarsus length is not precisely known; if it had the proportions found in elephant birds and moa, it would have been 80–90 cm long. But there is no real reason to assume that Eremopezus, living in wetlands rich in predators, was similar to these bulky lumbering insular ratites. The tibiotarsus of DPC 20919 preserves most of the bone and is as long as the complete tarsometatarsus already (34 cm); an overall tibiotarsus length of c. 40–50 cm, as found in the small slow-moving elephant bird Mullerornis agilis and the nimble emu, is a reasonable estimate.[15]

The tarsometatarsus had a small hypotarsus with a single simple crest and devoid of foramina for tendons. The ankle joint consists of a sizable midward and an even larger outward groove. The conspicuous ridge on the backside of the tarsometatarsus is formed by the third toe's midfoot bones, which are set to the plantar side in respect to their neighbors, with which they are fused. This is particularly noticeable in the lower part of the bone, where a corresponding groove is present on the dorsal side. This smooths out proximad, with the proximal half of the bone—which is not known from an unflattened specimen to date—probably oval or triangular (with the point backwards) in cross section.[4]

The dwarf cassowary (Casuarius bennetti) is one of the few ratites with toes as splayed as (but still less mobile than) E. eocaenus

As regards the length and stoutness of the lower leg and the foot, E. eocaenus would probably have been similar to a good-sized American rhea (Rhea americana) in measurements. Its toes, however, differed vastly in shape and posture from those of this ratite (and most other birds). Above the tibiotarsal trochleae of the African bird, the bone was considerably wider than the ankle, among ratites being matched by some moa and the dwarf cassowary (Casuarius bennetti). Thus, towards its toes the midfoot bone must have flared outwards, resulting in an overall wide-footed appearance. It is notable that while elephant birds have toes that are only a little more flaring than those of rheas, those of fast-running long-legged birds (ratites and others, like seriemas and some Charadriiformes) generally tend to bunch together in a narrower angle, with the most extreme situation found in the ostrich. This suggests that wherever Eremopezus did walk, it indeed walked or trotted and ran only in dire need—if it could run quickly at all, which is by no means certain.[15]

It is also notable that the attachment point of the tibiotarsal trochleae was flattened in Eremopezus, leaving little room for a bulky pad where the toes joined. In cursorial birds, this area is typically wide, to provide space for a large cushioning pad where the toes join, which improves balance during walking or running. The structure in E. eocaenus, by contrast, must have allowed a much better toe-flexing ability. The shallow grooves on the trochleae allowed for considerable sideways mobility of the toes; if the single phalanx bone known is assigned correctly to this species (it is the best match in size by far), it is very wide at the proximal end and tapers noticeably distad. The third (middle) toe was the most robust and largest. The inner toe was about as large as the outer one, and attached noticeably more proximal on the tibiotarsus. There is a single foramen on the dorsal side of the tarsometatarsus, with a plantar exit hole between the third and fourth metacarpal's distal ends (presumably for the outer toe's adductor tendon) and another (presumably for nerves and blood vessels) on the plantar surface of the tarsometatarsus. The outer sides of the outer trochleae have a hook at their backward end, which together with deep pits for ligaments nearby must have served to hold strong tendons. Its hallux was vestigial, as is typical for birds that move about on foot a lot; it even cannot be ruled out that the hallux was entirely missing (as in ratites).[4]

Though its habitus remains unknown, E. eocaenus was probably roughly as tall and heavy as this emu (Dromaius novaehollandiae)

E. eocaenus was somewhat ponderous, at about the body size of Mullerornis betsilei. As this was one of the heavy-boned elephant birds, Eremopezus was probably lighter than it, especially if it retained the air-filled bones of flying ancestors. From the ground to the pelvis, it stood probably as high as a large rhea or small emu—slightly less than one meter in all probability –, though the robust bones of the prehistoric bird appear to have supported a larger bulk than those of the fleet-footed ratites. Altogether, even if it was not particularly long-necked Eremopezus must have stood about as high as an average adult human. It weighed probably less than 50 kg—slightly more than a large emu—and more than 10 kg—slightly more than the largest living storks (which reach over 150 cm in height) and the shoebill.[15]

Adult kori bustard (Ardeotis kori). Perhaps, E. eocaenus was only as heavy as this bird

The general limit for weight in flying birds is at about 25 kg, the exceptional Argentavis magnificens with its size and weight of a good-sized adult human notwithstanding as it evolved under unusual ecological conditions. Large males of the kori bustard (Ardeotis kori) and great bustard (Otis tarda) are perfectly capable of flight, while weighing in excess of 20 kg. A wing loading of c. 25 kg/m² is the known limit for bird flight under normal circumstances; thus, a case could be made for Eremopezus to have been capable of flight. Paleontologists do not consider this likely however, due to the stoutness of the bones, which in such a degree is only known from birds that were certainly flightless. The secretarybird's tarsometatarsus, for example, is only some 20% shorter but more than 60% slimmer than that of E. eocaenus. Thus, its wings and arm bones were probably reduced, a process known to have taken as little as 10,000 years in some island rails.

As birds become flightless, their feathers soon lose the barbule hooks that keep them in shape, becoming more hair-like; when flight performance is of no significance anymore, this can improve the insulating properties of the plumage. Nothing is known about the plumage color of Eremopezus; it was presumably not very gaudily colored as it had to avoid apex predators, but little else can be inferred. Perhaps most likely it had white, black or grey feathers with at least some eumelanin but little carotenoids and phaeomelanins, as usual among the "higher waterbirds" in general, and specifically those that inhabit similar habitat.

If it was a terrestrial animal, it might have been fairly short-necked; if it inhabited wetlands, its neck was probably not short and quite possibly rather long. The bill of Eremopezus was of unknown shape, but must in some way have been adapted to its feeding. Even if not a predator, being as it seems a member of the Neoaves its not-too-distant ancestors probably fed on animals. If it fed on terrestrial prey, it might have had the hooked bill of a bird of prey, but this is very specialized. Among the "higher waterbirds" that variously consume aquatic and/or terrestrial large invertebrates and small vertebrates, a spear-like pointed bill seems to have proven most apt for the task. The Pelecaniformes in particular have a distinct hook or "nail" on the billtip, which is even present in the otherwise plesiomorphic bill of the hammerkop (Scopus umbretta). If E. eocaenus was a pelecaniform, it probably also possessed this "nail", but apart from that its bill might just as well have evolved autapomorphies as bizarre as those of the pelicans.