Modern birds grow amazingly fast. After hatching, many species grow to adult size in a matter of days to weeks. But a new study published in the journal PLoS One suggests that birds did not always exhibit the same rapid rate of growth. By looking at chips of bone taken from the legs of some of the earliest birds and their close dinosaur relatives, paleontologist Gregory Erickson and colleagues found that when it came to growing up, early birds like Archaeopteryx were a lot more like dinosaurs than their living relatives.
In order to study how Archaeopteryx and other early birds (such as Jeholornis and Sapeornis) grew, paleontologists had to move beyond gross anatomy and look at the microscopic structure of the fossilized bone sampled from the legs of the selected specimens. Different growth rates are represented by the presence of patterning of different kinds of bone, and what the scientists expected to find was rings of bone filled with holes for blood vessels that represent rapid growth. Instead they found bone tissue that was not well-supplied by blood vessels and was more similar to that of slow-growing animals, like living reptiles.
This presented something of a paradox. Larger dinosaurs that were closely related to birds, but not actually birds, had bone tissue indicative of rapid growth—yet the earliest birds did not. Why should this be? The scientists proposed that it may be a matter of size.
The larger the animal that was studied, the more their bones seemed to indicate fast growth. The small dinosaur Mahakala, by contrast, exhibited bone types more similar to that seen in the early birds. This suggested that growth patterns were tied to size and that the earliest birds had inherited their relatively slow growth rate from their small dinosaur ancestors. Indeed, while presently recognized as the earliest bird, Archaeopteryx had far more in common with its dinosaur forebears than modern birds, leading the authors of the paper to conclude, "Archaeopteryx was simply a feathered and presumably volant dinosaur."