October 13, 2016
Life reconstruction of Vegavis iaai based on a new specimen recovered in Antarctica.
Credit: Gabriel L. Lio -for the Museo de Ciencias Naturales, Bernardino Rivadavia, Argentina.
Birds sing. Dinosaurs didn't.
Scientists know this after the recent discovery of the oldest known vocal organ of a bird, found in an Antarctic fossil of an ancestor of ducks and geese that lived more than 66 million years ago during the last days of dinosaurs. No dinosaur of that era had it.
The discovery of the Cretaceous-era vocal organ, called a syrinx, and its apparent absence in nonavian dinosaur fossils of the same age, indicates the organ may have originated late in the evolution of modern birds, and that other dinosaurs may not have been able to make noises similar to the bird calls heard today, according to findings published in Nature Wednesday (Oct. 12) by an international team of paleontologists that included Horn Professor Sankar Chatterjee, curator of paleontology at the Museum of Texas Tech University.
Acoustic communication in sonic vertebrates involves both the generation and detection of signals. Those who make sounds can discriminate those auditory cues in their cochlear duct of the inner ear. In terrestrial vertebrates, vocalizations are produced by specialized organs such as the larynx in amphibians, reptiles and mammals or the syrinx in birds that vibrate air as it moves from the lungs through the respiratory tubes. Vocalizations are used to communicate across as well as within species.
"This finding helps explain why no such organ has been preserved in a nonbird dinosaur or crocodile relative," said Julie Clarke, a paleontologist at The University of Texas at Austin Jackson School of Geosciences, who discovered the fossil syrinx and led the analysis. "This is another important step to figuring out what dinosaurs sound like as well as giving us insight into the evolution of birds."
The syrinx is made of stiff cartilage rings supporting soft tissues that vibrate to produce the complex songs and calls of modern birds. Cartilage does not fossilize as well as hard tissues such as bone. But the high mineral content in the syrinx's rings sometimes allows for fossilization. All other known examples of fossilized syrinxes occur in birds that lived well after nonavian dinosaurs were extinct.
The team found the syrinx in a fossil of Vegavis iaai, a foot-propelled bird that lived during the end of the Cretaceous period (about 65 million years ago). The specimen in which the syrinx was found was discovered on Antarctica's Vega Island in 1992 by a team from the Argentic Antarctic Institute, who gave it to Chatterjee for collaborative work. Chatterjee has built a reputation among the paleontological community; he and several students have been to Antarctica several times and amassed a large collection of fossils from Antarctica.
This specimen was preserved in hard sandstone, which concealed some of the features, and technology at that time did not allow for the kind of examination needed.
Since UT has a high-resolution CT scanning facility, Chatterjee reached out to Clarke, and in 2013, more than two decades after the discovery of the fossil, Clarke noticed the Vegavis fossil included a syrinx. For two years after that discovery the team searched the dinosaur fossil records for other examples of a syrinx, but so far has found none.
"We all know birds can sing or can make specific calls, but the origin of vocalization remains unexplored in birds," Chatterjee said. "For the first 100 million years of bird evolution, there is no direct evidence of vocalization preserved in the fossil record. Vegavis, the ancestor of modern birds, is the oldest known fossil that shows unequivocal skeletal evidence of a syrinx, implying these ancient birds communicated with other members of the same species."
The asymmetrical shape of the syrinx indicates the extinct species could have made honking noises via two sound sources in the right and left parts of the organ. The researchers also scanned syrinxes of other birds to compare with the Vegavis syrinx. This included 12 syrinxes from living birds and the next oldest fossilized syrinx, which had not yet been studied.
Other collaborators include Franz Goller, a physiologist at the University of Utah; Zhiheng Li, a professor at the Chinese Academy of Sciences; Tobias Riede, a professor at Midwestern University in Arizona; and Fernando Novas and Federico Agnolin of the Museo Argentino de Ciencias Naturales Bernardino Rivadavia.
The study was funded by the Gordon and Betty Moore Foundation, the National Science Foundation Office of Polar Programs and the Agencia Nacional do Promoción Cientifica y Technológica.
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