HINDLIMB OF A GIANT TERRESTRIAL BIRD FROM THE UPPER EOCENE, FAYUM, EGYPT by
D. T. RASMUSSEN, E. L. SIMONS, F. HERTEL
and A .
ABSTRACT. Eremopezus eocaenus Andrews, 1904 is a giant groundbird from upper Eocene deposits of the Fayum, Egypt, which has hitherto been known from non-diagnostic fragmentary material. New fossils collected from quarry L-41 of the Jebel Qatrani Formation include two well-preserved distal tarsometatarsi and an associated whole tarsometatarsus and distal tibiotarsus that allow a more precise evaluation of the phylogenetic position and tarsal function of Eremopezus. Unlike most ratites, the distal tarsometatarsus has a patent distal foramen and a slight hallucal digit. The trochlea for digits II and IV are only slightly reduced in size, are splayed to the right, and the heads lack deep grooving. These features resemble the condition seen in Balaeniceps and Sagittarius, suggesting active use of the toes in grasping or manipulation, rather than the condition in graviports and cursors, which have reduced medial and lateral trochleae often with distinct grooving of the heads. The limb is relatively long and gracile, another difference from graviports. There is no compelling evidence to link Eremopezus to any known ratite lineage, to the Phorusrhacoidea, or to the extinct predatory birds of the Eocene (e.g. Diatryma, Gastornis). We suggest that Eremopezus represents an endemic African group that independently attained large size and ¯ightlessness. KEY WORDS:
Eremopezus eocaenus, tarsometatarsus, African groundbird, ratite, Egypt, upper Eocene, Jebel Quatrani
T H E presence of a large, ratite-sized bird in the lower Tertiary of Africa has been known for nearly a century (Andrews 1904, 1906), but its precise relationships, age, and palaeobiology have remained unknown because of the few, relatively uninformative fossil fragments that were found and the lack of detailed information about stratigraphy and the age of the deposits (Andrews 1906; Lambrecht 1929, 1933; Rasmussen et al. 1987). Based on the three available non-diagnostic materials, it was hypothesized that two genera of large Fayum birds were present, and that they might be related to the extinct elephant birds of Madagascar (Aepyornithidae). Recent collecting in the Fayum region of Egypt directed by E. L. Simons has yielded new specimens that provide enough information to test several hypotheses about the phylogenetic af®liation of the bird and to determine its age. Extremely large, ¯ightless groundbirds are today restricted to the few ratite groups of the southern landmasses: the rheas (Rheidae) of South America, the ostrich (Struthionidae) of Africa, the emu (Dromiceidae) and cassowaries (Casuariidae) of Australia and New Guinea; and two recently extinct groups: the New Zealand moas (Dinornithidae and Anomalopterygidae) and the Malagasy aepyornithids. Whether or not these diverse birds represent a ratite clade, or several convergent developments of large size and ¯ightlessness, remains a perennial debate in avian evolution (e.g. de Beer 1956; Bock 1963; Gingerich 1967; Cracraft 1974; Feduccia 1980, 1996; Olson 1985). An early fossil record of any of these ratite groups would be a valuable contribution to understanding their evolutionary histories, but to date, only a few fragments of apparent ratites are known as fossils as old as the Paleogene (Olson 1985; Patterson and Rich 1987; Martin 1992; Feduccia 1996). Most of the few possible ratites that are known are impossible to tie con®dently to speci®c modern ratite lineages. Ostriches are one exception to this rule, having Eocene relatives known from the northern continents (Houde and Haubold 1987; Feduccia 1996). Apart from ostriches, one of the few proposed phylogenetic links between an early Tertiary fossil bird and a recent ratite lies between the fragmentary remains of the Fayum bird and the elephant birds of Madagascar (Lambrecht 1929, 1933; Brodkorb 1963). An informative fossil record of well-preserved, large, terrestrial, ¯ightless birds does exist from the [Palaeontology, Vol. 44, Part 2, 2001, pp. 325±337]
q The Palaeontological Association
PALAEONTOLOGY, VOLUME 44
early Tertiary, but most of these fossils represent a kind of bird entirely different from the ratites: massiveheaded, thick-beaked forms that have been interpreted by some as terrestrial predators. These include the family Gastornithidae containing late Paleocene and early Eocene Diatryma, Gastornis and Zhongyuanus, from North America, Europe and Asia, respectively (Lemoine 1881; Matthew and Granger 1917; Troxell 1931; Fisher 1978; Hou 1980; Witmer and Rose 1991; Andors 1992; Martin 1992; Buffetaut 1997a, b) and the extinct phorusrhacids, a diverse group best known from the Oligocene to Pliocene of South America, but also represented by early forms in the Eocene of Europe (Mourer-Chauvire 1983; Peters 1987) and late survivors in the Pliocene of North America (Tambussi 1999). Andors (1992) questioned that Diatryma was predatory, but a detailed biomechanical analysis of the jaw by Witmer and Rose (1991) suggests that the traditional interpretation of carnivory is correct. No comparable early Tertiary ground predator has been reported from Africa. However, the Paleogene fossil record of Africa is regrettably poor, and the apparent absence of many groups is surely a palaeontological artefact. Much smaller, volant analogs to the extinct giant ground predators include the secretarybird (Sagittarius serpentarius, Sagittariidae) of Africa and the seriemas (Cariamidae) of South America. Until now, the large bird from the Fayum has been known only by material that is inadequate for making reliable phylogenetic and functional assessments. Initially, Andrews (1904, 1906) described the distal end of a tibiotarsus and a single phalanx. Later Lambrecht (1929, 1933) described a fragment of the distal shaft of a tarsometatarsus lacking trochleae. Lambrecht (1933) believed the tibiotarsus was not aepyornithid based on the lack of deep epicondylar pits, whereas he concluded that the tarsometatarsal shaft was aepyornithid based on the presence of a prominent plantar ridge or keel on the shaft. Thus, two genera were named: Andrew's original Eremopezus for the non-aepyornithid distal tibiotarsus, and Lambrecht's Stromeria for the aepyornithid-like tarsometatarsal fragment. Subsequent researchers have been skeptical that two different species of giant groundbirds were present in the same time and place in Egypt, especially given that the relative sizes of the fragments of tibiotarsus and tarsometatarsus are compatible with being from a single species (Moustafa 1974; Rasmussen et al. 1987). Also, researchers have questioned whether a sole character, the plantar ridge of the tarsometatarsal shaft, serves as credible evidence of af®nities to aepyornithids (Rasmussen et al. 1987). Despite these concerns, the idea that aepyornithids inhabited northern Africa contributed to the rationale for allocating several large fossil eggshell samples from North Africa, Namibia, Turkey and the Canary Islands to the family Aepyornithidae (Lambrecht 1933; Sauer and Rothe 1972; Sauer 1976; Senut et al. 1995). The purpose of this paper is to describe a whole tarsometatarsus, an associated distal tibiotarsus, and two additional distal tarsometatarsi with intact trochleae that have been recovered from quarry L-41 in the Jebel Qatrani Formation. The key questions to be addressed are: (1) Is the material best assigned to the ratites or to a predatory group? (2) If the bird is a ratite, is there speci®c evidence for a phylogenetic link to elephant birds? (3) If the bird is related to predatory taxa, is there evidence for a tie to known ground predators from the northern continents or South America? (4) What is the precise age of the Fayum bird and what does this imply biogeographically? GEOLOGY
Egypt's early Tertiary continental deposits are located on the northern escarpment of the Fayum Depression, about 100 km south-west of Cairo and 50 km north-west of the city of El Faiyum, capital of Fayum province. The deposits consist of a primarily marine formation of Eocene age, the Qasr el Sagha Formation (Gingerich 1992), and an overlying primarily ¯uvial formation of Eocene±Oligocene age, the Jebel Qatrani Formation (Bown and Kraus 1988). The Jebel Qatrani Formation consists mainly of sandstones and mudstones deposited as river bars and overbank sediment, often showing alterations due to paleosol formation. Conglomerates and limestones are also present. Sedimentological analyses indicate that the formation accumulated as part of an aggrading system of river channels ¯owing primarily westward into a basin south of the Tethys Sea shoreline. The age of the Jebel Qatrani Formation has been assessed by the comparative study of the fossil mammalian fauna (Rasmussen et al. 1992), correlations of a palaeomagnetic column (Kappelman et al.
RASMUSSEN ET AL.: EOCENE TERRESTRIAL BIRD
1992), radiometric dating of overlying basalts (Fleagle et al. 1986; Kappelman et al. 1992), and by identi®cation of unconformities correlating with regional and global changes in sea level (Bown and Kraus 1988). These lines of evidence suggest that the lower sequence (sensu Bown and Kraus 1988) of the Jebel Qatrani Formation is late Eocene in age. The Eocene/Oligocene boundary lies near the middle of the formation, perhaps at a major unconformity associated with a marker bed called the `barite sandstone' (Kappelman et al. 1992; Rasmussen et al. 1992). The upper sequence of the formation is early Oligocene (Fleagle et al. 1986; Kappelman et al. 1992). A late Eocene age for the lower part of the formation has been questioned on the basis of sedimentation rates and location of unconformities (Gingerich 1993), but more recently collected mammalian taxa from the lower sequence (the primates Aframonius and Anchomomys) represent groups that are exclusively Eocene in Europe and Asia (Simons et al. 1995; Simons 1997). Quarry L-41, the source of the new giant bird fossils, is the stratigraphically lowest of the major terrestrial vertebrate sites in the Fayum; palaeomagnetic data suggest an age of 35.6±35.9 Ma for the site (Kappelman et al. 1992). Quarry L-41 is one of the most remarkable Eocene localities in the world. It consists of ®ne mudstones and claystones with abundant evaporites and dense concentrations of vertebrate bone, mainly ®sh and mammal (Bown and Kraus 1988; Rasmussen and Simons 1991). Brownish plant traces are evident in the grey-green mudstone. The L-41 mammalian fauna is the most abundant and diverse for the entire African Paleogene (Simons and Rasmussen 1990; Rasmussen et al. 1992; Gagnon 1997). Unlike most Fayum quarries, relatively delicate elements (such as small primate crania) are sometimes preserved at L-41, but because of post-depositional processes the materials are crushed, which causes in situ distortion and fragmentation of elements. The mammalian fauna is dominated by hyracoids (Rasmussen and Simons 1991) followed by rodents, primates, creodonts, macroscelideans, anthracotheres, and others (Gagnon 1997). Many of the Fayum mammals represent taxa endemic to Africa at the time, while others had close relatives in contemporary Europe and Asia. The only bird described from L-41 is a species of Palaeoephippiorhynchus, an early stork (Miller et al. 1997), but a variety of other waterbirds have been recovered at L-41 and are being studied. Fossil birds from other stratigraphical levels in the Jebel Qatrani Formation have been described by Andrews (1906), Lambrecht (1929, 1933), Rasmussen et al. (1987) and Miller et al. (1997). The avian fauna of the formation consists primarily of non-passeriform waterbirds, including herons (Ardeidae), jacanas (Jacanidae), rails (Rallidae), storks (Ciconiidae), shoe-billed storks (Balaenicipitidae), and cormorants (Phalacrocoracidae) along with several other groups, such as touracos (Musophagidae), ®sh eagles (Accipitridae), and ospreys (Pandionidae), among other taxa (Rasmussen et al. 1987). The fossil avifauna contributes to palaeoenvironmental reconstructions that draw also on sedimentology, mammals, invertebrates and plants, which together indicate that the Jebel Qatrani Formation was deposited in lowland, tropical swamp forests, marshes, and meandering rivers bordered by forests in a seasonal, probably monsoonal climate (Bown 1982; Bown et al. 1982; Wing and Tiffney 1982; Olson and Rasmussen 1987; Bown and Kraus 1988). SYSTEMATIC PALAEONTOLOGY
Order INDETERMINATE EREMOPEZIDAE Rothschild, 1911 Genus
Type species. Eremopezus eocaenus Andrews, 1904. Holotype. Distal end of a left tibiotarsus, The Natural History Museum (BMNH) A843. Referred specimens. Duke University Primate Center (DPC) 20919, an entire right tarsometatarsus and associated right tibiotarsus consisting of the distal end and much of the shaft; DPC 5555, distal end of a left tarsometatarsus, preserving all distal trochleae and about half the shaft, collected by Prithijit Chatrath; DPC 18309, distal end of a left tarsometatarsus preserving the three trochleae but none of the shaft, collected by Verne Simons; a tarsometatarsal shaft
PALAEONTOLOGY, VOLUME 44
near the distal end, in Munich (Lambrecht 1929, 1933). The new DPC specimens are perfectly compatible in size with the tibiotarsus analyzed by Andrews (1906) and the tarsometatarsus shaft fragment studied by Lambrecht (1929, 1933). The width of the distal articular end of A843 is 48 mm, whereas DPC 20191 is about 55 mm; anteropostero depth of A843 is 35 mm while that of DPC 20191 is 38 mm. Lambrecht's tarsometatarsus was characterized by a medial plantar ridge on the shaft, also evident in DPC 5555 and 20191. The three tarsometatarsal specimens are of a size compatible with the holoype tibiotarsus (Rasmussen et al. 1987). Synonymy. Stromeria fajumensis Lambrecht, 1929, tarsometatarsal shaft minus trochleae; S. fayumensis Lambrecht, 1933. (Specimen number 1914 I 53, Bayerische Stadtssammlung fuÈr PalaÈontologie und Historische Geologie).
Emended diagnosis. Large-bodied ¯ightless bird with the following attributes of the tarsometatarsus: a scar for the hallucal metatarsal reduced in size; a patent distal foramen in adults; a dorso-plantarly ¯attened, mediolaterally splayed distal end of the shaft; a plantar longitudinal ridge or keel on the central portion of the shaft; medial and lateral trochleae reduced relative to the central one; variably slight or absent grooving of the medial trochlea. The distal end of the tibiotarsus lacks a supratendinal bridge. Horizon and age. The new DPC specimens are from quarry L-41, lower sequence, Jebel Qatrani Formation, Fayum, Egypt. The age of the site is considered to be late Eocene, about 35.6±35.9 Ma. Less precise locality data are available for the specimens collected early in the century. The holotype is from what Andrews (1904, 1906) correctly called the upper Eocene, but which most later authors considered to be lower Oligocene (e.g. Lambrecht 1933; Rasmussen et al. 1987). Andrews' specimen came from the vicinity of quarries A, B and C; these are younger than L-41 but older than the Eocene/Oligocene boundary as delimited by the unconformity at the barite sandstone. Although Lambrecht's specimen was cited as being from the `lower Oligocene', it came from the same horizon as Andrews' tibiotarsus. Therefore, all specimens are from the lower sequence, and are late Eocene in age. Extensive collections of thousands of vertebrates from the upper sequence (lower Oligocene) have not yielded a comparably large bird.
MORPHOLOGY General description. The tarsometatarsus has three trochleae, with the central one (digit III) largest, and the two others of about equal size (Text-®gs 1±3). The central trochlea extends farthest distally, followed by the lateral trochlea (digit IV) and then the medial trochlea (digit II). The medial and lateral trochleae are de¯ected only slightly in a palmar direction relative to the central trochlea. The three trochlea are somewhat splayed, rather than aligned in parallel. The distal part of the shaft at the bases of the trochleae is very broad from side to side, but unusually thin dorsopalmarly. Some but not all of the dorsopalmar compression could be due to post-depositional crushing. The medial and lateral trochleae of DPC 5555 show only slight grooving, while the central trochlea is deeply grooved. On DPC 18309 the medial trochlea is almost ¯at mediolaterally, not grooved at all. In DPC 20191 the medial trochlea appears to be intermediate. The posteromedial edge of the medial trochlea and the posterolateral edge of the lateral trochlea are extended backwards into slightly hooked processes. Deep ligamental pits occur on the sides of the medial and lateral trochleae. A distinct distal foramen is present. There is a single point of entry on the dorsal surface. All three DPC specimens show that there was one exit distally in the intertrochlear notch, presumably for the adductor tendon of digit IV, and a second exit on the palmar surface, for vessels and nerves. The distal foramen is well separated from the intertrochlear notch. The intertrochlear notch between digits III and IV is actually shallower than that between digits II and III. Interpretation of a possible hallucal digit (digit I) is problematic due to specimen damage and possible variability. DPC 20191 does not show compelling evidence of a hallucal scar but the bone is crushed and shattered at the key spot. On the medial side of the palmar surface of the shaft of DPC 5555, there is a slight depression bounded by small ridges that may represent the scar for the hallucal metatarsal (Text-®g. 1). Damage to the region make the details of shape and size uncertain. The scar appears to be a shallow, almond-shaped depression placed at the typical location for this feature in birds. The scar is relatively slight in size; among living large carinates it is closer to that of a pelican than an accipitrid eagle. The lateral border of the scar is particularly distinct; a ®ne, crisp crest separating the ¯at plantar surface from the rougher, concave scar. Distally, a thin line runs from the scar down the palmar surface of the medial trochlea; this could be the lateral border of the abductor digiti II muscle. Proximally, an extension of the thin crest runs up to and along the palmar ridge, possibly representing the seam between intrinsic foot muscles lying medially (the extensor hallucis longus) and laterally (adductors and abductors of the toes) to the line. There are no comparable crests
RASMUSSEN ET AL.: EOCENE TERRESTRIAL BIRD
1. Fossil tarsometatarsus fragments of Eremopezus eocaenus from quarry L-41, Fayum, Egypt. DPC 5555 preserves the distal end in good condition and much of the shaft (A, posterior view; B, anterior view). Close-up photographs of this specimen in posterior (D), lateral (E), and anterior views (F) show the morphology of the trochleae and other distal structures. Another specimen, DPC 18309, also preserves the trochleae (C, anterior view). Scale bars represent 60 mm.
or lines on the lateral trochlea or lateral part of the shaft. We conclude that DPC 5555 suggests the presence of a hallucal digit, but the evidence is not conclusive. At best, a reduced digit was present. While the distal end of the shaft is extremely ¯attened in a dorsopalmar plane, the central part of the shaft is deep, largely as a result of the palmar ridge. Crushing of the dorsal surface makes reconstruction of the exact cross-sectional shape imprecise, but the midpoint shaft would have been ovoid or triangular rather than ¯attened. The prominent ridge running up the middle of the palmar surface of the shaft was noted by Lambrecht (1933). The shaft is relatively long and slender compared to the much shorter, robust tarsometatarsi of graviportal birds (e.g. larger moas, aepyornithids, Diatryma) and in this respect it resembles extant cursorial ratites (Table 1) but with very different distal structure (Text-®g. 4). The proximal end of the tarsometatarsus (DPC 20191) shows two deep cotyla for articulation with the tibiotarsal condyles, with the lateral one being deeper than the medial. The hypotarsus is damaged by crushing, but it appears to be a relatively reduced, gracile structure. There appears to be one simple hypotarsal crest positioned centrally; tendinal foramina are apparently absent. The distal tibiotarsus (DPC 20191) matches precisely with the holotype of E. eocaenus (Andrews 1906). As reported by Andrews (1906), there is no supratendinal bridge, but instead a crisp ridge for anchoring a supratendinal ligamentous sling.
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TABLE 1. Tarsometatarsal dimensions (mm) of Eremopezus and a selection of other birdsa. Species
Eremopezus eocaenus DPC 20191 340 DPC 5555 ± Rea americana ÐÐ 335 ÐÐ 347 ÐÐ 319 Struthio camelus ÐÐ 439 ÐÐ 448 ÐÐ 441 Casuarius bennetti ÐÐ 259 ÐÐ 239 Casuarius sp. ÐÐ 256 Dinornis sp. ÐÐ 210 Sagittarius serpentarius ÐÐ 287 a
Proximal Width/ Distal Width
43 45 40
39 39 41
0´13 0´13 0´13
71 70 71
59 54 56
0´16 0´16 0´16
Specimens housed in the University of California at Los Angeles (UCLA), Department of Biology.
Comparisons with extant neognathous birds. Eremopezus is not particulary close in morphology to any extant bird. Interesting comparisons can be made to the largest extant waterbirds and to large, terrestrial predators, even though these are still considerably smaller than Eremopezus and differ from it in many details. The shape of the distal tarsometatarsus is most closely approximated in the shoebilled stork (Balaeniceps rex, Balaenicipitidae) and in the secretarybird (Sagittarius serpentarius, Sagittariidae). Like Eremopezus, the shoebill has a markedly compressed distal end of the tarsometatarsal shaft, with splayed trochleae of about equal size and without much posterior de¯ection. The secretarybird also has notable splay, and is similar to Eremopezus in the shape and proportions of the trochleae. Both the secretarybird and the shoebill use their feet for active manipulation; the secretarybird captures snakes and other prey with its toes, while the shoebill grasps its way along tangled mats of ¯oating vegetation. Comparison with giant ground predators. The foot of the large North American genus, Diatryma, is very robustly built (Shufeldt 1915; Sinclair 1928). The shaft is relatively short and thick, and the central trochlea is relatively massive. The trochleae are splayed, but the medial and lateral ones are more reduced in size than those of Eremopezus, especially the medial one. The heads of the medial and lateral trochleae are nearly ¯at in Diatryma, rather than grooved, probably re¯ecting pro®cient rotation, abduction and adduction of the toes that originate on them. Both Diatryma and Gastornis have patent supratendinal bridges of the tibiotarsus. The tarsometatarsus of Gastornis is similar to that of Diatryma but the central trochlea is less enlarged; the medial and lateral trochleae are somewhat splayed and their heads are notably grooved (Martin 1992). In phorusrhacids the medial and lateral trochlea are reduced more than in the Fayum fossil (Matthew and Granger 1917). The tarsometatarsus of Eremopezus is not specialized for cursoriality to the extent seen in phorusrhacids. Comparison with ratites. Eremopezus is notable for the presence of a distal foramen and a possible scar for the hallux. Both of these features are unusual among extant ratites, but moas of the family Anomalopterygidae
RASMUSSEN ET AL.: EOCENE TERRESTRIAL BIRD
2. Partial tibiotarsus (left, DPC 20919) and associated complete tarsometatarsus (right) from quarry L-41 showing the relatively gracile limb proportions of Eremopezus. Scale bar represents 60 mm.
do have both (Anderson 1989). The ÂŻattened, splayed distal end of the tarsometatarsus seen in Eremopezus is also unusual among ratites. Some ratites are much more compressed side to side with size reduction of the medial and lateral trochleae. Moas and elephant birds have the most splayed appearance among ratites, but not to the extent of the secretarybird and the shoebill stork. Ostriches and rheas are most morphologically different from Eremopezus among the ratites, with their reduction of digits (two in
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TEXT-FIG. 3. Three drawings of the distal end of the type specimen of Eremopezus eocaenus Andrews, 1906 (top) from the original publication compared to the distal tibiotarsus of DPC 20919 (bottom), in anterior, lateral, and posterior views, showing comparable proportions and morphology, despite signi速cant crushing to the DPC specimen. The DPC specimen is associated with a tarsometatarsus, which allows con速dent synonymy to be con速rmed for Eremopezus eocaenus and Stromeria fajumensis (see text).
ostriches, three in rheas) lack of dorsoplantar compression, and absence of splaying (as well as absence of a hallux and a distal foramen). Emus and cassowaries also show much greater central trochlea dominance than seen in Eremopezus with considerable reduction of the other trochleae. Some elephant birds and moas are heavily built, graviportal birds, but some members are lighter and more gracile. Among elephant birds, Mullerornis betsilei is similar in size and proportions to Eremopezus, but with greater reduction of medial and lateral trochlea and with very broad intertrochlear notches. Remiornis is an apparent ratite from the Paleocene of Europe that Martin (1992) considered close to Eremopezus and aepyornithids, but its tarsometatarsus is very unlike that of Eremopezus: the medial and lateral trochleae are greatly reduced while the central one is massive. Remiornis does retain a patent distal foramen. Functional morphology. Eremopezus is interpreted to have been an active bird of medium to light build. In size, it is slightly larger than cassowaries and rheas. It lacked the heavy, robust graviportal proportions of the heaviest bird taxa, Aepyornis, Dinornis and Diatryma. At the same time, it was no cursorial specialist
RASMUSSEN ET AL.: EOCENE TERRESTRIAL BIRD
TEXT-FIG. 4. Comparisons of the distal tarsometatarsi of several large terrestrial groundbirds. A, Eremopezus, DPC 5555; B, Diatryma (Troxell 1931); C, Mullerornis (Lambrecht 1933); D, Casuarius (UCLA specimen, see Table 1); E, Titanis (Brodkorb 1963); F, Dinornis (Anderson 1989). Not to same scale (see original publications). All specimens
have been reproduced or reversed to be seen as the left side.
like Rhea, Struthio or Cariama, as judged from the lack of side-to-side compression of the bone and lack of signi®cant reduction of the medial and lateral trochleae. Ratites with relatively unreduced medial and lateral trochleae, and with a fair amount of splay in the trochlear arrangements, nevertheless have relatively shorter, more robust tarsometatarsi than those from the Fayum. The lack of notable central trochlear dominance, the trochlear splay, and the low-relief grooving of the medial and lateral trochlear heads suggest second and fourth digits that are more mobile and prehensile than those of graviports or cursors. In this respect, Eremopezus resembles secretarybirds and shoebill storks among extant birds, and to a lesser extent, Diatryma. The lack of grooving on the medial trochlea may be functionally signi®cant. Raptorial birds that use the medial and lateral digits for grasping and handling prey items often have relatively smooth trochlear heads (e.g. owls), as do some of the cuculiform birds that use the feet to climb and grasp in vegetation (hoatzins). This presumably allows the digits to rotate, abduct and adduct on the trochlear heads, rather than just ¯ex and extend. Eremopezus is characterized by heavy grooving of the largest third digit, suggesting that it was mainly involved in ¯exion and extension, but the splay and relative smoothness of the medial and lateral trochleae suggests a manipulative role besides passive weight support or cursorial propulsion. Functionally, the best analogy may be to a giant ¯ightless secretarybird. DISCUSSION
Features of the tarsometatarsus that can be used to distinguish ratites from ground predators are few and subtle. Matthew and Granger (1917, p. 322) found that the fragmentary hindlimb remains of Diatryma
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available to them were not different in obvious ways from those of ratites: `Nor does it seem likely that a complete knowledge of the hind limb bones [of Diatryma] would have materially altered [Cope's and Shufeldt's] concept of its form and near af®nity to the ratites.' Based on insuf®cient material, Lambrecht and other previous researchers concluded or assumed that the giant Fayum birds were ratites. Lambrecht (1929, 1933) believed that the distal foramen was absent from his Fayum specimen, and that the plantar crest represented a `primitive' manifestation of MIII, therefore representing a primitive bird, a ratite. Both of these observations were incorrect: a typical avian distal foramen is present, and the plantar crest is a derived architectural feature probably relating to the biomechanics of movement or weight support in Eremopezus. If Eremopezus is related to ratites, it is a very primitive taxon retaining the distal foramen and probably a small hallux. Of course, the presence of both of these features in anomalopterygid moas is blunt evidence that an `ancestral ratite', if such a thing did exist, had these characters as well. Because the loss of the hallux is associated with an evolutionary history of terrestriality (Raikow 1985), and loss of the distal foramen is neotenic, both conditions would be expected to occur independently in `ratite' lineages no matter what their phylogenetic af®nities. Therefore, the only way to conclude positively that Eremopezus is not near the base of one or another ratite lineages is to accept the untenable position that a ratite clade can be de®ned based on parsimonious assessments of the distribution of morphological characters in extant forms (untenable because of indisputable parallelism among ratites). The opposite view, that Eremopezus is indeed a ratite, has no compelling morphological basis either. Each of the three southern continents (excluding Antarctica) has large ¯ightless groundbirds. In addition, each of the two largest Gondwana-derived islands had endemic large, ¯ightless groundbirds until humans arrived and eradicated them (Madagascar, New Zealand). Madagascar separated from Africa as part of an Indo-Antarctica-Madagascar landmass before 120 Ma (Sampson et al. 1998). Whereas the large groundbirds of four of the ®ve landmasses are apparently endemic, the ostrich currently occupying Africa is a Miocene immigrant from Asia (Mourer-Chauvire et al. 1996). This leaves Africa as the only large southern landmass without a known endemic large-bodied groundbird. Furthermore, each of the three northern continents generated Diatryma-like ground predators, as well as early relatives of phorusrhacids, ostriches, and Remiornis, during the early Tertiary. We propose that Eremopezus, rather than being the African relative of a bird known elsewhere, represents instead an endemic African group with only ancient relationships to other known bird groups. Whether or not the endemic ¯ightless Gondwana birds share a Cretaceous cosmopolitan ancestor is a point requiring further fossil evidence (Kraus et al. 1997). The Fayum is full of vertebrate taxa apparently endemic to continental Africa, including proboscideans, hyracoids, embrithopods, ptolomaiids, phiomyids, macroscelideans, balaenicipitids, xenerodiopids, and musophagids (Cooke 1972; Rasmussen et al. 1987, 1992). Certain volant bird groups widespread later in the Tertiary may also have had their origins in Africa, such as ardeids and jacanids (Rasmussen et al. 1987). The famed `splendid isolation' of South America is only slightly more marked than that of Paleogene Africa. Lambrecht (1929) considered the presence of large ¯ightless birds on Madagascar and Africa to be compelling circumstantial evidence of a phylogenetic relationship between the two, but just the opposite is actually more logical. Madagascar split from Africa at a very early date (Jurassic) and the presence of a giant ¯ightless bird on Africa during the early Tertiary therefore makes it a less likely ancestor for aepyornithids than would be a volant species. If the aepyornithid ancestor ¯ew to Madagascar, it was no close kin of Eremopezus. On the other hand, if there really was a very ancient common stock cladistically linking a `ratite clade', and the ancestor of aepyornithids actually walked to Madagascar, then the common ancestor occurred so anciently (more than 100 My before the age of the Fayum deposits) that we should hardly expect Eremopezus to show off morphological evidence of its af®nities in just two known leg bones. We can ®nd no satisfactory morphological evidence to link Eremopezus to Aepyornithidae, or to any of the disparate ratites. We favour the view that Eremopezus simply cannot be linked con®dently to any other known bird group no matter how many sister taxa would be de®ned by default in parsimony cladistic analyses that are devoid of a null hypothesis. In the case of Eremopezus, the mosaic of morphological features in combination with non-morphological factors suggest that Eremopezus was an endemic African group of giant, mobile-footed birds that should not be forced into a clade with any other known bird taxa without further fossil evidence.
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Department of Anthropology Washington University St. Louis, MO 63130, USA e-mail email@example.com ELWYN L. SIMONS
Department of Biological Anthropology and Anatomy Duke University Durham, NC 27710, USA e-mail firstname.lastname@example.org FRITZ HERTEL
Department of Zoology University of California Los Angeles, CA 94024, USA e-mail email@example.com AMY E. JUDD
Typescript received 22 July 1999 Revised typescript received 11 August 2000
Department of Anthropology University of Missouri-Columbia Columbia, MO 65211, USA e-mail firstname.lastname@example.org