…whether tested using DNA or skeletal traits…
Field et al. 2018
used DNA to produce a cladogram of extant birds to determine that basal taxa were all non-arboreal. Earlier the large reptile tree (LRT, 1121 taxa) came to the same conclusion using trait analysis and fossils (Figs. 1, 2). The only difference is the Field team determined that the crown bird radiation was post-Cretaceous. The LRT recovered a crown bird radiation in the post-Jurassic and post-Cretaceous birds were also non-arboreal (Fig. 1, 2). An earlier radiation explains the Paleocene appearance of very derived fossil penguins and the Early Cretaceous appearance of the fossil chicken, Eogranivora.
Field et al. also recovered flamingos with grebes, chickens with ducks, and many other physical trait mismatches, like those in Prum et al. 2015. Such mismatches are ignored by DNA workers.
From the Field et al. 2018 abstract:
“We suggest that ecological filtering due to the temporary loss of significant plant cover across the K-Pg boundary selected against any flying dinosaurs (Avialae) committed to arboreal ecologies, resulting in a predominantly non-arboreal post-extinction neornithine avifauna composed of total-clade Palaeognathae, Galloanserae, and terrestrial total-clade Neoaves that rapidly diversified into the broad range of avian ecologies familiar today. The explanation proposed here provides a unifying hypothesis for the K-Pg-associated mass extinction of arboreal stem birds, as well as for the post-K-Pg radiation of arboreal crown birds.”
the loss of an arboreal habitat due to world-wide fires does not explain the disappearance of the Cretaceous toothed sea birds, Ichthyornis and Hesperornis. Other explanations must be invoked.
The mechanism for crown birds invading trees
appears to have been neotony, with chick-sized adults with short legs and short necks arising from larger, long-legged, long-necked predecessors (e.g. Passer the sparrow and Opisthocomus, the hoatzin, arising from Gallus, the chicken). Notably the hatchlings of arboreal taxa are typically not precocial, hatching out a more helpless stage in their ontogeny and growing to fledgling size rapidly.
Field et al. correctly report,
“…virtually the entirety of the avian crown-group fossil record is restricted to sediments of Cenozoic age, and the earliest well-supported crown bird fossil is scarcely older than the end-Cretaceous, at approximately 67 Ma.” True. This is one of the unresolved mysteries of paleontology, only now starting to crack with discoveries like Eogranivora, the early Cretaceous chicken, and the nesting of Cretaceous toothed birds between paleognaths and neognaths (Fig. 3), something the Field analysis was not able to recover.
One might ask,
if all adult birds worldwide were killed on exposure to oven-like temperatures and subsequent lack of trees, would the buried, precocious hatchlings of mound-builders, like Megapodius (Fig. 4), create the first generation of K-T event bird survivors? If so, perhaps the tinamou ancestors of modern tinamous and ratites were likewise mound builders. Currently tinamous and ratites are not mound-builders.
Basal members of all bird clades in the LRT
appear to have survived the K-T event, based on the Paleocene presence of fossil penguins, like Waimanu (Fig. 5). Overlooked by Field et al., basal members of all the major crown bird clades in the LRT (Fig. 3) are all non-arboreal, long-legged, wading taxa (Fig. 2), that do not nest in trees.
Field DJ et al. (7 co-authors) 2018. Early Evolution of Modern Birds Structured by Global Forest Collapse at the End-Cretaceous Mass Extinction. Current Biology (advance online publication) DOI: https://doi.org/10.1016/j.cub.2018.04.062