An early web-footed diving bird that lived about 110 million years ago in China hints that all living birds might have had aquatic ancestors. The newly unearthed fossil also fills a key gap in the avian evolutionary record.
Gansus yumenensis was a foot-propelled diver like modern loons or grebes and about 25 centimetres long, says Matt Lamanna of the Carnegie Museum of Natural History in Pittsburgh, US. Although Gansus was not a direct ancestor of modern birds, it was the closest to modern birds yet found from the early Cretaceous.
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The discovery offers new insight into the origins of modern birds, which remain a puzzle because fossils of their ancestors are scarce. Although many fossil birds are known from 125-million-year-old deposits in China鈥檚 Liaoning province, the most common belonged to a now-extinct group called enantiornithines.
Gansus was much closer to modern birds as the oldest known member of the Ornithurae, a group that includes both modern birds and toothed marine birds that lived about 85 million years ago.
Muscle attachments
Gansus was known only from a foot discovered in Changma, 2000 kilometres west of Liaoning, until Hai-lu You of the Institute of Geology in Beijing returned to the site in 2002. You, Lamanna, and Jerry Harris of Dixie State College in St George, Utah, US, have now uncovered some 40 new fossils and describe five of them in the journal Science (vol 312, p 1640).
The fossils preserve impressions of a webbed foot and of flight feathers. Their fossil bones also preserve muscle attachments, which are like those found on modern foot-propelled swimmers.
鈥淵ou wouldn鈥檛 have expected something to be so modern so far back into the Cretaceous,鈥� Harris told 快猫短视频. But the structures are not as well developed as in modern loons or grebes, indicating Gansus might not have been as strong a swimmer.
They identified the closest relatives of Gansus as two well-known toothed marine birds which lived about 30 million years later, the flightless diver Hesperornis and the tern-like Ichthyornis. That suggests that modern land birds may have had aquatic ancestors.
Note of caution
鈥淭he more fossils are found, the more it seems that these early relatives of modern birds lived in an aquatic realm,鈥� says co-author Luis Chiappe of the Natural History Museum of Los Angeles County, US. In contrast, most early enantiornithines were perching birds.
鈥淚鈥檓 excited to see the material,鈥� says Julia Clarke of North Carolina State University, US. But with fossils from that period being rare, she warns: 鈥淲e can鈥檛 make strong ecological conclusions from a single new taxon.鈥� And she notes Gansus 鈥渋s not particularly close to living birds鈥�, being separated by three or four divergences on the evolutionary tree.
Harris believes the Changma deposits could yield lots more new birds 鈥� so far the team has only studied a region about 100 square metres and 1 metre deep.
View a sample video of the Science Channel鈥檚 (mov file). The programme airs in the US on Monday 19 June at 2100 EDT.
