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Despite the fact that birds represent the only surviving dinosaurian lineage, though, their growth pattern is different. Instead of going through a period of protracted change, as with non-avian dinosaurs, the skulls of young birds are anatomically almost identical to those of adults. And birds take a much faster, more direct route to maturation—many bird species grow to adult size in a year or less. As a new Nature study by Bhart-Anjan Bhullar and collaborators suggests, this feature of bird life can be traced back to ancient transformations that effectively locked bird skulls into a permanent juvenile anatomy.Bhullar and co-authors used a technique called geometric morphometrics to survey the degree of skull change among birds, various non-avian theropods, the archaic archosaur Euparkeria and the modern American alligator. By tracking landmarks on the skulls in virtual models, the researchers were able to quantify how much the skulls of particular creatures changed. As expected, most non-avian dinosaurs retained the ancestral growth pattern—juvenile skulls were significantly different from adult skulls, regardless of how big those dinosaurs were.The dinosaurs most closely related to birds showed a different pattern. The eumaniraptoran dinosaurs—the group that contains the sickle-clawed, feathery deinonychosaurs as well as birds—had skulls that looked more juvenile in form, and there was less change in shape between youngsters and adults. A juvenile skull form was undergoing little modification through maturity. Biologists know this as paedomorphosis, when descendent species resemble the juvenile stages of their ancestors.