COMMENTARY Big-time insights from a tiny bird fossil COMMENTARY Daniel J. Fielda,1 Birds are among the most diverse and widely distrib- exceeds Waimanu in age, and appears to be an early uted groups of vertebrate animals. There are well over stem group representative of a living group of birds 10,000 recognized species alive today, occupying called mousebirds (sometimes also known as colies). virtually every subaerial ecosystem (1). The amazing Tsidiiyazhi’s age implies not only that the lineage breadth of extant bird diversity is manifested in dizzy- leading to mousebirds had diverged from its closest ing varieties of forms, colors, and lifestyles, ranging living relatives by 62.5 Ma but also that a host of other from iridescent, hovering, nectar-feeding humming- deep divergences within the neoavian tree of life had birds to nocturnal, flightless, worm-eating kiwis. How, taken place by this early time as well. If the neoavian when, and why has this spectacular diversity arisen? radiation was stimulated by the mass extinction of The only direct evidence informing such questions can nonavian dinosaurs as has been suggested (6, 7, 9–11), be obtained from the fossil record of the modern bird its pace must have been amazingly rapid. radiation, but the early fossil record of modern birds is The evolutionary insights yielded by Tsidiiyazhi do exceedingly sparse. In PNAS, Ksepka et al. (2) help to not end there, however; this discovery also enhances improve our understanding of this pivotal interval of bird our understanding of the biogeographic history of evolutionary history by reporting the discovery of a new mousebirds and is part of a broader evolutionary pic- fossil bird filling an important temporal gap. ture. Living mousebirds comprise a small group of The fossil, Tsidiiyazhi abini (derived from the Na- only six species, endemic to sub-Saharan Africa (12). vajo Din ´eBizaad language for “little morning bird”)is Mousebirds are classified in their own taxonomic or- indeed little, because the specimen was collected der, Coliiformes, owing to the fact that they share no within a 25-cm × 25-cm grid from fossil beds in the particularly close affinities with other groups of living San Juan basin of New Mexico. In fact, Tsidiiyazhi’s birds. Although their phylogenetic position within the broad evolutionary implications are far from obvious broader bird tree of life was debated for decades, we from a casual glance at its broken and incomplete skel- now believe that they represent an early offshoot of eton. However, thanks to careful and detailed anatomi- the lineage ultimately giving rise to groups like king- cal work, Ksepka et al. (2) demonstrate that this tiny fossil fishers, woodpeckers, and hornbills (6, 7). Considering bird punches well above its weight in helping to eluci- the geographic distribution of living mousebirds, it is date the nature and timing of the modern bird radiation. easy to assume that the group simply arose in Africa Attempts to correlate the geological time scale with and never left. However, the discovery of Tsidiiyazhi in important events early in modern bird evolutionary the southwestern United States illustrates that the past history are often controversial (3–5). Still, recent studies distribution of the total group of mousebirds was likely integrating the fossil record and molecular clock tech- more widespread than the current distribution of the niques suggest an extremely rapid radiation of the major group’s living representatives. Indeed, an impressive avian subclade Neoaves in the aftermath of the Creta- diversity of early stem group mousebirds is known ceous–Paleogene (K–Pg) extinction that wiped out the from the Paleogene of North America and Europe (13). nonavian dinosaurs, 66 million years ago (Ma) (6, 7). Mousebirds are in good company, because many living Today, Neoaves comprises over 90% of living bird bird groups with restricted modern-day distributions, diversity, but bird fossils from the first few million years such as the seriemas of today’s South American plains, after the extinction are exceptionally rare. In fact, the hummingbirds from the New World, and Amazonian earliest known definitive representative of Neoaves hoatzins, have early fossil representatives known from (the giant extinct penguin, Waimanu manneringi, from very different regions of the world (14). Such observa- New Zealand) is ∼60.5 Ma (8). What happened in neoa- tions plainly illustrate the value of fossils to historical bio- vian evolution between the K–Pg extinction event and geography: Only the direct evidence of the fossil record the earliest record of Waimanu?At∼62.5 Ma, Tsidiiyazhi can definitively show us where groups of birds were aMilner Centre for Evolution, Department of Biology and Biochemistry, University of Bath, Bath BA2 7AY, United Kingdom Author contributions: D.J.F. wrote the paper. The author declares no conflict of interest. See companion article 10.1073/pnas.1700188114. 1Email: [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1710941114 PNAS Early Edition | 1of3 Downloaded by guest on October 1, 2021 AB8.0 Fig. 1. (A) Extant speckled mousebird (Colius striatus), which is widespread across sub-Saharan Africa. (B) Simplified representation of the time-scaled interrelationships among living bird groups, following a recent phylogenomic study (6). The root of the tree is estimated at roughly 73 Ma, and the dotted circle represents the approximate position of the K–Pg mass extinction event at ∼66 Ma. The approximate age (∼62.5 Ma) and phylogenetic position of T. abini, the new fossil described by Ksepka et al. (2), is denoted by the small black arrow, and the phylogenetic position of living mousebirds (Aves: Coliidae) is represented by the large black arrow. formerly distributed, because avian biogeography is vastly more Despite the largely uniform structure of living mousebirds, the complex than can be understood on the basis of modern geo- fossil record reveals a surprising menagerie of highly divergent graphic distributions alone. stem group forms, suggesting a considerable amount of ecolog- Future efforts to refine our understanding of how modern birds ical experimentation throughout their evolutionary history. For obtained their characteristic geographic distributions will there- example, Chascacocolius cacicirostris, from the early Eocene of fore need to accommodate the presence of stem mousebirds in Germany, exhibited bizarrely enlarged posteriorly directed exten- the early Paleocene of North America (15). This kind of integrative sions of its lower jaws, thought to assist in prying into tough fruits biogeographic work may have implications for our ability to pre- (17) or probing in the ground for invertebrates (18). In contrast, the dict the future, because in determining the extent to which climatic Oligocene Oligocolius, also known from German sediments, changes throughout Cenozoic Earth history were responsible for exhibited a short, deep, superficially parrot-like beak and may driving such dramatic biogeographic changes, we may gain insight have fed on tougher food items as evinced by a jumble of large into how bird biogeography may evolve in response to our planet’s seeds found in the presumed area of its crop (19). current climatic trajectory. Why should a bird as tiny as Tsidiiyazhi make such a big pale- Of course, the new fossil discovery also provides us with ontological impact? How can the early Cenozoic fossil record of important insights into the evolution of modern mousebird modern birds be sufficiently patchy that a single bird fossil can tell biology. Living mousebirds are distinctive (Fig. 1A), because they us so much? It is true that Tsidiiyazhi represents only a single data are smallish, arboreal birds with short conical bills, very long tails, point, and future refinements of the avian evolutionary time scale, feathered crests on their heads, and specialized toes that can be models of biogeographic change, and anatomical evolution must directed backward to assist with perching (16). Ksepka et al. (2) await additional fossil discoveries. confirm the presence of this flexible foot condition in Tsidiiyazhi, However, the question of why the early Cenozoic avian fossil illustrating that this specialization evolved early in mousebird record is so poor is one that is in need of attention. Detailed evolutionary history. Surprisingly, however, they illustrate that paleontological surveys of fossil avifaunas from the latest Creta- such “semizygodactyly,” although present in several living neoa- ceous and the earliest Paleocene suggest that Mesozoic bird-like vian families, including owls and the Madagascan endemic forms, including close relatives of living birds like enantiornithines courol, likely evolved independently in these different groups. (“Mesozoic opposite birds”) and hesperornithines (toothed ma- This inference is only supported in analyses that incorporate fossil rine forms) flourished up to the very latest stages of the Mesozoic information, providing another example of the potential for fos- before falling victim to the asteroid-induced K–Pg mass extinc- sils to reveal unforeseen complexity in the evolutionary history tion (10). The extinction event likely would have decimated pop- of birds. ulation sizes among the handful of surviving bird lineages, and 2of3 | www.pnas.org/cgi/doi/10.1073/pnas.1710941114 Field Downloaded by guest on October 1, 2021 limited resources in the extinction’s aftermath may even have both their geological age and their phylogenetic position. One driven transient selection for reduced body size (3). The dual in- hopes that the discovery of this little morning bird will usher in the fluence of reduced population size and preservational factors that dawn of a new phase of fossil bird discoveries from the early conspire against the fossilization and discovery of small birds may, Paleocene that will help to clarify the earliest stages of bird life together, help to explain the conspicuous rarity of fossils document- in the Cenozoic. Our understanding of the origin of modern birds, ing the earliest stages of the neoavian radiation.