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 themajor 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 bird diversity, but bird fossils from the first few million years after the extinction are exceptionally rare. In fact, the earliest known definitive representative of Neoaves (the giant extinct penguin,Waimanu manneringi, from New Zealand) is ∼60.5 Ma (8). What happened in neoavian evolution between the K–Pg extinction event and the earliest record ofWaimanu? At ∼62.5Ma, Tsidiiyazhi exceeds Waimanu in age, and appears to be an early stem group representative of a living group of birds called mousebirds (sometimes also known as colies). Tsidiiyazhi’s age implies not only that the lineage leading to mousebirds had diverged from its closest living relatives by 62.5 Ma but also that a host of other deep divergences within the neoavian tree of life had taken place by this early time as well. If the neoavian radiation was stimulated by the mass extinction of nonavian dinosaurs as has been suggested (6, 7, 9–11), its pace must have been amazingly rapid. The evolutionary insights yielded by Tsidiiyazhi do not end there, however; this discovery also enhances our understanding of the biogeographic history of mousebirds and is part of a broader evolutionary picture. Living mousebirds comprise a small group of only six species, endemic to sub-Saharan Africa (12). Mousebirds are classified in their own taxonomic order, Coliiformes, owing to the fact that they share no particularly close affinities with other groups of living birds. Although their phylogenetic position within the broader bird tree of life was debated for decades, we now believe that they represent an early offshoot of the lineage ultimately giving rise to groups like kingfishers, woodpeckers, and hornbills (6, 7). Considering the geographic distribution of living mousebirds, it is easy to assume that the group simply arose in Africa and never left. However, the discovery of Tsidiiyazhi in the southwestern United States illustrates that the past distribution of the total group of mousebirds was likely more widespread than the current distribution of the group’s living representatives. Indeed, an impressive diversity of early stem group mousebirds is known from the Paleogene of North America and Europe (13). Mousebirds are in good company, because many living bird groups with restricted modern-day distributions, such as the seriemas of today’s South American plains, hummingbirds from the New World, and Amazonian hoatzins, have early fossil representatives known from very different regions of the world (14). Such observations plainly illustrate the value of fossils to historical biogeography: Only the direct evidence of the fossil record can definitively show us where groups of birds were