Stidham replies

— I have presented a hypothesis for the identification of a Late Cretaceous fossil as the oldest known parrot1. The specimen lacks the characters distributed more widely in non-avian maniraptoriforms, such as abundant teeth and unfused dentaries. It has many characters1, including the absence of an internal pillar of bone supporting a midline ridge, that are not present in oviraptoroids2. To assign the specimen to a non-avian clade requires a less parsimonious hypothesis of character evolution. The K-shaped neuro-vascular canal pattern character1, mapped onto various phylogenetic hypotheses of the relationships of crown group parrots3,5, is primitive for that clade. Like most characters, the K-shaped neurovascular canal pattern exhibits homoplasy and variation within natural populations. The complete absence of this character from some extant parrots seems to be the result of secondary loss due to the relative shortening of the jaw symphysis in some parrots ( Neophema, for example). However, this character, as figured and described1, is not found outside crown-group parrots, although it superficially resembles the state in extant cathartid vultures. Although individual characters seen in the fossil can occur in other taxa, the combination of characters seen in the fossil is not present outside crown-group parrots, and Dyke and Mayr have not demonstrated what clade, other than parrots, has this combination of characters.

It seems less than defensible to propose that we cannot have Cretaceous parrots because the oldest well-preserved fossils known so far are Eocene6. Previously proposed sister groups to parrots (reviewed in ref. 7), and the known fossil record of these sister taxa8,9, show that the parrot lineage should have been present at least 5 to 10 million years before the (middle Eocene) Messel and London Clay parrots6. This is the same amount of missing fossil record required by both my hypothesis of a latest Cretaceous parrot and Mayr and Daniels' suggestion6 (made during their study of the Eocene parrots) of a Cretaceous origin of parrots.

The other known Cretaceous neornithines (listed earlier1: in contradiction with Dyke and Mayr, the New Jersey fossil birds are Cretaceous in age10) placed in various phylogenetic hypotheses of the ordinal level relationships of neognaths, including parrots7,11, show that parrots and most other neognath ordinal level clades are constrained to have diverged from other orders of modern birds in the Cretaceous or early Palaeocene.

If non-crown-group parrots are present in the Eocene6, then the sister group to those taxa (the stem leading to the crown group or the crown group itself, possibly with a modern-looking jaw) must have been present by the middle Eocene as well. The identification of the Cretaceous jaw as a parrot is subject to test and refutation, like any hypothesis. However, the accepted methods of the field, not statements about gaps in our current knowledge and preconceived notions of character evolution, must be used to falsify hypotheses and generate alternatives.

Dyke et al