Posted: Aug 05, 2019 9:57 pm
by Calilasseia
The Guardian has fucked up regally over this. Not least because, if you read the paper, not all surviving non-arboreal stem clades of birds were flightless. (The full paper can be downloaded from here for verification purposes).

A typical paragraph from the paper is this one:

The earliest well-constrained neoavian fossil, Tsidiiyazhi abini, was recently described from the early Paleocene (∼62.5 Ma) of New Mexico [3]. T. abini was inferred to represent an early stem mousebird (Coliiformes), a clade exhibiting predominantly arboreal habits today. However, ancestral state reconstructions that include T. abini and other early-Cenozoic fossils suggest that hindlimb modifications for perching may have arisen independently in numerous arboreal clades of Telluraves, including Coliiformes, after the K-Pg mass extinction [3]. This evidence supports a model whereby early ground-dwelling neoavians repeatedly took to the trees relatively early in the Paleocene—potentially filling arboreal niches vacated by Cretaceous enantiornithines and stem ornithurines—following the recovery of global forests after the Chicxulub impact.


If we search for the taxon mentioned there, namely Tsidiiyazhi abini, we find this interesting paper accompanying it:

Early Paleocene Landbird Supports Rapid Phylogenetic And Morphological Diversification Of Crown Birds After The K–Pg Mass Extinction by Daniel T. Ksepka, Thomas A. Stidham, and Thomas E. Williamson, Proceedings of the National Academy of Sciences of the USA, 114(30): 8047-8052 (25th July 2017) DOI: 10.1073/pnas/1700118114 [Full paper downloadable from here]

From that other paper, we have:

Ksepka et al, 2017 wrote:Evidence is accumulating for a rapid diversification of birds following the K–Pg extinction. Recent molecular divergence dating studies suggest that birds radiated explosively during the first few million years of the Paleocene; however, fossils from this interval remain poorly represented, hindering our understanding of morphological and ecological specialization in early neoavian birds. Here we report a small fossil bird from the Nacimiento Formation of New Mexico, constrained to 62.221–62.517 Ma. This partial skeleton represents the oldest arboreal crown group bird known. Phylogenetic analyses recovered Tsidiiyazhi abini gen. et sp. nov. as a member of the Sandcoleidae, an extinct basal clade of stem mousebirds (Coliiformes). The discovery of Tsidiiyazhi pushes the minimum divergence ages of as many as nine additional major neoavian lineages into the earliest Paleocene, compressing the duration of the proposed explosive post–K–Pg radiation of modern birds into a very narrow temporal window parallel to that suggested for placental mammals. Simultaneously, Tsidiiyazhi provides evidence for the rapid morphological (and likely ecological) diversification of crown birds. Features of the foot indicate semizygodactyly (the ability to facultatively reverse the fourth pedal digit), and the arcuate arrangement of the pedal trochleae bears a striking resemblance to the conformation in owls (Strigiformes). Inclusion of fossil taxa and branch length estimates impacts ancestral state reconstructions, revealing support for the independent evolution of semizygodactyly in Coliiformes, Leptosomiformes, and Strigiformes, none of which is closely related to extant clades exhibiting full zygodactyly.


I'm not aware of any semizygodactylous ground dwelling birds alive today (unless someone better informed can tell me otherwise, of course), and the presence of this foot feature, indicating gravitation toward an arboreal niche for this early Palaeogene species (presumably in the recovering forests cited to be emerging in the first paper around 3 to 4 million years after the bolide impact), strongly suggests (along with the known wing morphology) that Tsidiiyazhi abini was perfectly capable of flight. Quite simply, non-arboreal flight-capable birds could have persisted across the K-Pg boundary, a notion implicit in the remarks of Field et al in the Current Biology paper.

Once again, the lesson is simple: read the actual papers, and be wary of newspaper renditions thereof.