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Archosaur skull changes (juveniles on the left, adults on the right). While there was a significant amount of change between the juvenile and adult skulls of alligators (top) and the non-avian dinosaur Coelophysis (middle), there was little change between the juvenile and adult skulls of early birds such as Archaeopteryx (bottom) and their closest dinosaur relatives. From Bhullar et al., 2012.

Birds are dinosaurs. That much is certain. That deep connection, bolstered by fossil finds and theoretical frameworks, has made dinosaurs seem more bird-like than ever expected. From feathers to nesting behavior, many aspects of avian natural history are now known to have originated among non-avian dinosaurs.

But non-avian dinosaurs were not just like birds in every respect. The way many dinosaurs grew is vastly different from the way avian hatchlings mature. Take Triceratops, for example. Even if we ignore the controversial “Toroceratops” hypothesis—which suggests drastic skull transformation late in the life of the horned titan—the reconstructed growth trajectory for this dinosaur involves major skull changes. The horns of young Triceratops curved backward but reoriented as the animal grew to point forward. And the little ornaments around the fringe of the dinosaur’s frill, called the epiossifications, transformed from high, pointy spikes to flattened structures.

Granted, Triceratops was about as far from being an ancestor of birds as is possible while still being a dinosaur. But major transformations seem to have been the norm among dinosaurs, from Tyrannosaurus to Diplodocus to Edmontosaurus. Juvenile dinosaurs had significantly different skull shapes from adults of the same species, and in some cases, it seems that juvenile dinosaurs were occupying different habitats or consuming different food than more mature individuals. (This would be a prehistoric example of what ecologists call niche partitioning.)

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.

What could trigger this kind of change? That’s difficult to say. Paedomorphosis is a result of modifications to growth, a developmental phenomenon regulated by particular genes. Bhullar and collaborators suggest that something happened to truncate the development of eumaniraptoran dinosaurs, which included the ancestors of birds. Quirks of development caused these dinosaurs to mature in a juvenile form. And while birds continued this trend in their evolution, its first glimmerings can be traced back to their non-avian ancestors. Non-avian dinosaurs are the key to understanding how birds came to be.

Reference:

Bhullar, B., Marugán-Lobón, J., Racimo, F., Bever, G., Rowe, T., Norell, M., & Abzhanov, A. (2012). Birds have paedomorphic dinosaur skulls Nature DOI: 10.1038/nature11146