Big-time insights from a tiny bird fossil

Birds are among the most diverse and widely distributed groups of vertebrate animals. There are well over 10,000 recognized species alive today, occupying virtually every subaerial ecosystem (1). The amazing breadth of extant bird diversity is manifested in dizzying varieties of forms, colors, and lifestyles, ranging from iridescent, hovering, nectar-feeding hummingbirds to nocturnal, flightless, worm-eating kiwis. How, when, and why has this spectacular diversity arisen? The only direct evidence informing such questions can be obtained from the fossil record of the modern bird radiation, but the early fossil record of modern birds is exceedingly sparse. In PNAS, Ksepka et al. (2) help to improve our understanding of this pivotal interval of bird evolutionary history by reporting the discovery of a new fossil bird filling an important temporal gap. The fossil, Tsidiiyazhi abini (derived from the Navajo Diné Bizaad language for “little morning bird”) is indeed little, because the specimen was collected within a 25-cm × 25-cm grid from fossil beds in the San Juan basin of New Mexico. In fact, Tsidiiyazhi’s broad evolutionary implications are far from obvious from a casual glance at its broken and incomplete skeleton. However, thanks to careful and detailed anatomical work, Ksepka et al. (2) demonstrate that this tiny fossil bird punches well above its weight in helping to elucidate the nature and timing of the modern bird radiation. Attempts to correlate the geological time scale with important events early in modern bird evolutionary history are often controversial (3⇓–5). Still, recent studies integrating the fossil record and molecular clock techniques suggest an extremely rapid radiation of the major avian subclade Neoaves in the aftermath of the Cretaceous–Paleogene (K–Pg) extinction that wiped out the nonavian dinosaurs, 66 million years ago (Ma) (6, 7). Today, Neoaves comprises over 90% of living …