Extant Psittaciformes are zygodactyl birds and are well characterized by a derived beak morphology, which enables them to get access to their prime food resource, the substantial content of hard-shelled seeds and nuts. As in other zygodactyl birds, the tarsometatarsal trochlea for the fourth toe bears a large accessory trochlea for the retroverted fourth toe.
Fossil Psittaciformes have been known for a long time from the Neogene of the Northern Hemisphere, and the specimens are morphologically very similar to extant parrots (Mhkovsky 1998b; Mayr and Göhlich 2004). Only in the last two decades, however, Paleogene stem group representatives were identified, all of which stem from European fossil sites. No representatives of crown group Psittaciformes are known from Paleogene fossil deposits. On the basis of a single humerus, Waterhouse et al. (2008) reported a presumptive psittaciform bird from the early Eocene of the Danish Fur Formation. This species, which was named Mopsitta tanta, is clearly distinguished from crown group Psittaciformes, which have a stouter humerus with a more elongated dorsal tubercle and ventral condyle (contra Waterhouse et al. 2008). Apart from vaguely similar overall proportions, which can be quite misleading in the case of Paleogene birds, I can see no reasons for an assignment of the Danish fossil to the Psittaciformes.
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