Threskiornithidae Ibises

The earliest unambiguous representative of the Threskiornithidae is Rhynchaeites messelensis Wittich, 1898 from Messel, of which more than a dozen partial or complete skeletons have been found (Fig. 7.8). R. messelensis was originally assigned to the charadriiform Rostratulidae (painted snipes) by Wittich (1898) and was also misidentified as a charadriiform bird by Hoch (1980). Its threskiornithid affinities were recognized by Peters (1983), and the species was classified in the monotypic taxon Rhynchaeitinae by Mayr (2002d). Derived similarities shared by R. messelensis and extant Threskiornithidae include a long, decurved, and schizorhinal beak, and the presence of a notarium which consists of at least three fused thoracic vertebrae (Peters 1983). In many aspects of its skeleton, however, R. messelensis is very different from extant ibises. Most notably, the legs are much shorter than in crown group Threskiornithidae. The tip of the upper beak lacks openings for sensory nerves, which are characteristic for extant ibises, and the Eocene taxon may thus have been a less tactile forager (Mayr 2002d). The sternum is proportionally much larger than that of extant Threskiornithidae and morphologically

Fig. 7.8 Postcranial skeleton of Rhynchaeites messelensis from the middle Eocene of Messel (Forschungsinstitut Senckenberg, Frankfurt am Main, Germany, SMF-ME 3577). Specimen coated with ammonium chloride. (Photo by Sven Tränkner)

it is very different: whereas the caudal margin of this bone bears two pairs of rather shallow incisions in modern ibises, there is only a single pair of very deep ones in Rhynchaeites (Mayr 2002d). As in the case of the long-legged Presbyornis (whose sternum resembles that of crown group Threskiornithidae) and the short-legged Anatidae (whose sternum is more similar to that of Rhynchaeites), these differences in sternum morphology may be due to the fact that the fossil taxon had much shorter legs than extant ibises (see Sect. 6.4.3). The coracoid still exhibits a plesio-morphic, cup-like articulation facet for the scapula.

As also noted by Peters (1983), Actiornis anglicus Lydekker, 1891 from the late Eocene (MP 17; Mlikovsky 2002) of England, whose identification as an ibis has been doubted by Olson (1981), was probably correctly assigned to the Threskiornithidae by Harrison and Walker (1976a). At least a humerus referred to this species closely matches the humerus of extant ibises (Fig. 7.9) and is quite different from the humerus of the Palaelodidae (contra Olson 1981). A postcranial skeleton of a representative of the Threskiornithidae was described by Roux (2002) from the early Oligocene of Cereste in France. This unnamed species also exhibits a proportionally shorter tarsometatarsus than modern ibises. Close comparisons between this fossil and the roughly contemporaneous but slightly larger Actiornis anglicus still have to be performed.

Outside Europe, the Paleogene fossil record of the Threskiornithidae is very scanty. On the basis of a distal tibiotarsus and a referred distal ulna, Hou (1982) described Minggangia changgouensis from the late Eocene of China as member of the Threskiornithidae. These specimens also need a critical reexamination and detailed comparisons with other Paleogene taxa. Stidham et al. (2005) further reported a distal tibiotarsus of a small ibis-like bird from the middle Eocene of Myanmar. The authors themselves noted, however, that an unambiguous identification

Paleocene Primates Humerus

Fig. 7.9 a, c Referred left humerus of Actiornis anglicus Lydekker, 1891 (Threskiornithidae) from the late Eocene of Hampshire in England (Natural History Museum, London, UK, BMNH A 36792) in comparison with b, d a left humerus of the Sacred Ibis, Threskiornis aethiopicus, from the Pleistocene of Madagascar (BMNH A 1972). Images to scale. (Photos by Sven Tränkner)

Fig. 7.9 a, c Referred left humerus of Actiornis anglicus Lydekker, 1891 (Threskiornithidae) from the late Eocene of Hampshire in England (Natural History Museum, London, UK, BMNH A 36792) in comparison with b, d a left humerus of the Sacred Ibis, Threskiornis aethiopicus, from the Pleistocene of Madagascar (BMNH A 1972). Images to scale. (Photos by Sven Tränkner)

of this bone is not possible and that there is a possibility that it belongs to the Geranoididae or Eogruidae (Sect. 9.3).

Jadwiszczak et al. (2008) published a note on an ibis-like partial beak from the late Eocene of Seymour Island, but univocal identification of this specimen also has to await the discovery of more material.

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