Discovery Of Earliest-Known Fossil-Bird Hatchling Gives Insight Into The Evolution Of Birds

June 6, 1997. The discovery of the earliest-known bird hatchling was announced today in the journal Science by an international team of scientists that includes Luis Chiappe, from the American Museum of Natural History, and José Sanz, from the Universidad Autonoma de Madrid. This extremely rare find provides unusual insight into the early evolution of birds and supports the idea that birds are in fact descended from dinosaurs.

Dating from the Lower Cretaceous, approximately 130 million years ago, the fossil was uncovered in the Spanish Pyrenées in the El Montsec range. The research team has determined that this bird hatchling falls between the very early Archaeopteryx (approximately 150 million years old) and the more modern fossil birds Hersperonis and Ichthyornis, both of which are approximately 85 million years old. No more than four inches long, the hatchling is believed to be a new bird species; it has been classified as an Enantiornithes (a member of a diverse group of birds that were capable of flight and that arose in the Cretaceous Period) and has not yet been scientifically named.

While discoveries during the last seven years have more than tripled the number of early bird species known to science, the understanding of the evolution of the modern bird skull has not advanced significantly since the first complete fossil bird skull was found a century ago. One of the most important aspects of this new finding is the unusually high level of preservation of the head and neck, which provides a wealth of new information about the skull anatomy of early birds, and, in particular, sheds insight into the skull of Archaeopteryx, the earliest bird ever discovered.

The primitive skull of this new hatchling contains teeth, which early birds shared with their reptile antecedents, but modern birds have since lost. In addition, openings surrounding the hatchling's braincase are larger than those of modern birds, indicating that its skull muscles were very different from modern birds and were, like many primitive birds, almost fully reptilian. To accommodate this musculature, the hatchling's brain would have been significantly smaller than that of modern birds.

In addition to the well-preserved skull, the neck and portions of the wings, shoulders, and sternum of this tiny hatchling have been preserved, as well as evidence of feathers. The shoulder and wing reveal advanced anatomical features, suggesting that this bird had a much better mastery of flight than Archaeopteryx. This also reinforces the idea that the development of flight took precedence over any other anatomical system in early bird evolution, and that birds retained a skull very similar to that of a meat-eating dinosaur even after they evolved a sophisticated flight capacity.

The bones of the hatchling reveal yet another interesting characteristic: the neck and wing bones, and the area near where the jaw attaches to the skull, all display clusters of tiny holes, very similar to the pattern found in the nestlings of modern birds. This cluster is a tell-tale sign that "like modern-day birds" this early hatchling grew steadily and rapidly while it was young. However it is very likely that it would have grown more slowly were it to have reached adulthood, as previous research on the growth rings of early birds (similar to those in trees) has indicated that, unlike adult, modern birds, primitive birds may have matured slowly and experienced seasonal growth cycles.

The combination of both advanced and primitive physical characteristics exhibited by the new hatchling fossil provides a window onto the evolutionary transition between theropods and birds and provides further evidence for the position that modern birds are in fact short-tailed, feathered descendants of meat-eating dinosaurs.