A New Understanding in Bird Evolution
A new discovery by researchers from the University of Kansas and China pushes back by millions of years proof that birds’ digestive systems have ancient origins. The investigators found fossil evidence of a crop — the muscular pocket in the esophagus that most modern birds use to store and soften seeds — in two avian species from the Early Cretaceous, the most recent period of the Mesozoic Era, about 130 million years ago.
Their discovery is to be published in a forthcoming edition of the influential journal Proceedings of the National Academy of the Sciences.
“We think that perhaps the development of a gizzard and a crop are specialization for eating seeds, and it was eating seeds that may have been one of the great motivations for birds to lose their teeth,” said Larry Martin, professor and senior curator at the KU Biodiversity Institute. “It shows that seed eating was an important driving force in the early diversification and radiation of modern-type birds.”
The two species showing evidence of crops, Sapeornis and Hongshanornis, were located in the collection of the Tianya Museum of Nature in Shandong Province, China. Fossils of both species contained preserved seeds in the anatomical location of the crop in modern birds. Additionally, some specimens show a soft tissue structure that closely matches the outline of a crop in birds today.
According to Martin, the crop is an important clue to how birds evolved from the Mesozoic era, when the vast majority possessed teeth, to modern bird species that lack teeth.
“These animals that we’ve found that have crops and gizzards are also among the few Mesozoic birds that show a loss of teeth,” the KU researcher said. “So we think that development of a crop is related.”
Martin co-authored the paper with Xiaoting Zheng of Linyi University and the Tianya Museum of Nature, China; Zhonghe Zhou of the Chinese Academy of Sciences in Beijing, China; and KU colleagues David Burnham and Desui Miao.
The two species found to have crops go far back in the evolution of birds, showing birds to be specialized eaters from close to the beginning of their development.
“These birds in question are around 130 million years old,” Marin said. “This is very early in bird evolution, about 10 million years after what was thought to be the first bird, Archaeopteryx.”
Of the two species found to have crops, Martin said that one belonged to a long-extinct evolutionary side-branch, while the other was a relative of modern birds.
“Sapeornis was a pretty fair-sized bird, about the size of an ordinary chicken,” said Martin. “It belongs to a group of basal birds that are related to, but actually separate from, the line that leads to modern birds. The other bird that we have, Hongshanornis, is a very early example of the group to which all modern birds belong. It’s essentially a modern bird, but an awfully old one — one of the oldest modern birds.”
The finding is the latest accomplishment in the long relationship between paleontologists from KU and China, where government support and large quarrying efforts have led to a boom in fossil findings.
“We’ve been working in China in the Early Cretaceous since the beginning of research in that area,” Martin said. “KU has in fact been the central institution for much of the research that’s been done there, especially on fossil birds for the Early Cretaceous. This is the oldest avian fauna that we can study in detail, and it’s produced thousands of complete skeletons, often with feathers, stomach contents and internal organs.”