Parargornis messelensis, gen. nov., sp. nov. exhibits a completely unexpected wing morphology, which sharply contrasts with that of recent swifts.The short and rounded wings clearly show that the new taxon was not adapted to gliding, and was certainly less aerial than recent true swifts.According to the shape of its bill, Parargornis was catching insects on the wing, although it must have had a different feeding strategy from recent swifts, and most other aerial insectivores which tend to have long and narrow wings.
The fact that fossils of Parargornis are rarer than those of the ‘Messel-swift’ Scaniacypselus (of which at least six specimens are known to me) might indicate that the former mainly caught its prey in the dense vegetation surrounding the ancient Messel lake, whereas Scaniacypselus certainly also hunted insects above the lake surface, and accordingly had a greater chance to become fossilized. This assump- tion accords with the feathering of the new taxon, because short, broad wings and a long tail are often found in birds which are adapted to agile manoeu- vrability in dense vegetation, like hawks, owls or turacos (e.g. Rayner 1988, p. 30).
Small birds with similar feathering also occur among some recent passeriform families, e.g. oven- birds (Furnariidae), antbirds (Formicariidae) and wrens (Troglodytidae). Most unusual, and not found in any recent avian taxon, however, is the combina- tion of short and broad wings with greatly abbrevi- ated humeri, as in Parargornis.
Short wings generally have a high wing loading (weight supported by unit wing area) and allow high wing beat frequencies and fast flight (Pennycuick 1985; Rayner 1988). Through the concentration of wing mass near the humeral joint, abbreviation of the humerus further leads to a decreasing inertia moment of the wing, and also favours intensive flap- ping flight (Karhu 1992). However, the high wing loading also results in high energy costs for flight (Rayner 1988) which, together with the long legs of Parargornis and the well-developed hind toe, might indicate that the Eocene genus caught its prey by sallying flights from a perch. The aspect ratio (wing span squared, divided by area) is very low in Parar- gornis (about 5.3, with an estimated wing area of 25 cm2 and a wing span of 115 mm), and its combi- nation with a presumably high wing loading is rare among recent birds but cannot be related to a specific flight technique (Rayner 1988, p. 40).
© 2003 British Ornithologists’ Union, Ibis, 145, 382–391
Like its recent relatives, Parargornis exhibits a well- developed processus internus indicis on the proximal phalanx of the major wing digit. This process leads the tendon of musculus interosseus ventralis, which flexes the distal phalanx of the major wing digit. Stegmann (1963) considered a well-developed pro- cess to be functionally related to increasing aerodynamic forces on the distal primaries. This process is usually therefore well developed in rapidly flying species with long and pointed wings (Stegmann 1963). Its presence in Parargornis is unexpected and might indicate either that the short wings of the Eocene genus evolved from typical swift-like wings with long primaries, or that it might be due to a high wing beat frequency.
The phylogenetic relationships between the Jungornithidae (sensu Karhu 1999) and other apodiform birds have not been convincingly resolved so far. Contrary to most other authors, who proceed from monophyly of the taxon (Hemiprocnidae + Apodidae), Karhu (1988, p. 87; 1992, p. 383) con- sidered Jungornithidae, Apodidae and Trochilidae to be monophyletic. He did not comment explicitly on the exact phylogenetic relationships between the three families, but – because of their absence in Argornis – ascribed the unique derived features shared by Jungornis and recent Trochilidae (most notably a distal protrusion of the caput humeri, a strongly protruding tuberculum musculi pronator superficialis, and a crest distal to the processus supracondylaris dorsalis) to parallel evolution (Karhu 1999).
However, if swifts and hummingbirds are mono- phyletic and if owlet-nightjars are their sister taxon (Mayr 2002), one might expect that the ancestor of hummingbirds had a ‘swift-like’ or ‘aegothelid’ beak also. Given the unique derived characters shared by Jungornis and recent hummingbirds, as well as the peculiar wing morphology of Parargornis, it is thus tempting to assume that the Trochilidae (which also have unusually short wings) evolved from a jungornithid-like ancestor.
A presumably derived character shared by Argornis, Parargornis, Jungornis and recent Trochilidae is the weakly pronounced ventro-proximal edge of the cotyla ventralis on the proximal end of the ulna. According to Karhu (1999), this feature facilitates rotation of the elbow joint in the spread wing. Although hummingbirds have a much narrower, semi-elliptical wing with a high aspect ratio in which the outermost primary is the longest, the wing shape of Parargornis does not rule out the possibility that it was capable of hovering, which, for shorter periods,