The phylogenetic affinities of few avian taxa are as enigmatic as those of Cladornis pachypus Ameghino, 1895. The holotype and only fossil record of this large species is an incomplete tarsometatarsus from the late Oligocene (Deseadan) of the
Argentinian part of Patagonia. Ameghino (1895) identified C. pachypus as a penguin-like aquatic bird, but Simpson (1946) rejected sphenisciform affinities of Cladornis. Olson (1985, p. 193) remarked that the bone "appears to be from some sort of very large and extremely weird land bird." The tarsometatarsus of C. pachypus, of which a photo is published here for the first time (Fig. 16.17), is very flat dorsoventrally and appears to have been quite short; although the proximal end is not preserved, the proportions of the shaft indicate that probably not more than about one quarter of the bone is missing. Judging from the presence of a very large articulation facet for the first metatarsal, C. pachypus had a well-developed hallux; the distal vascular foramen is very small. The rims of the trochlea for the third toe are separated by a marked furrow, and the trochlea for the fourth toe bears a plantarly directed flange, which led Olson (1985, p. 193) to assume that C. pachy-pus may have been "tending toward being zygodactyl." Placement of Cladornis in the present chapter is nevertheless largely by default. Although the taxon is highly distinctive, its phylogenetic affinities cannot be resolved without additional fossil material
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