A field guide to mesozoic birds and other winged dinosaurs

Avians
Avians, “(modern) birds”, represent the bird ‘crown group’, descendants
of the most recent common ancestor of all birds alive today. This
ancestor seems to have existed as early as 130 million years ago, if fragmentary
remains such as those classified as Gallornis are indeed avians.
All known avians lack teeth, and instead have extensive, often fully-fused
beaks. It is likely that teeth were lost completely in a predecessor to the
common ancestor of all modern bird groups; however, it is also possible
that two or more avian lineages lost teeth independently of one another.
Numerous species of avians have been identified from the Mesozoic,
mainly from the latest Cretaceous, but are based on remains too fragmentary
to reconstruct their life appearance with any degree of confidence
(see Appendix A). Due to this poor fossil record, there has historically been
a debate regarding the timing of modern bird diversification. However, the
likely presence of many modern bird “orders” in the Mesozoic suggests that
avians had begun to diversify into many of their modern forms by the time
of the K-Pg extinction event that ended the Mesozoic era.
Based on tentative interpretations of the fossil evidence, it is likely
that members of the following modern bird groups existed before the K-Pg
boundary: Charadriiformes (wading shorebirds like Cimolopteryx), Anseriformes
(including Vegavis, above), Galliformes (including Austinornis), Palaeognathae
(ratites and allies represented by Limenavis and possibly some
early lithornithids), and Pelecaniforms (including Torotix and some cormorant-like
birds). Grebes and rail-like birds (possibly ancestral to Cenozoic
“terror birds”) have also been reported from the latest Cretaceous. At least
one species or lineage representing each of these groups survived the K-Pg
extinction and diversified into all the remaining groups of modern birds
in the Cenozoic Era. In the case of the anseriformes, at least two species or
lineages survived the mass extinction (presbyornithids and the ancestors of
modern ducks and geese represented by Vegavis).
In addition to these avian lineages, it is possible that one or more
groups of non-avian birds also survived across the K-Pg boundary. One
species from the Paleogene, Qinornis paleocenica, is known from fossils
which show unfused bones in the tarsus, despite the interpretation of the
specimen as an adult, a trait known only in non-avian birds. This species
may represent a lineage of toothed carinate birds that survived for several
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