In a story today by BBC Science Reporter Matt McGrath, paleontologists are debating the long-held view of Archaeopteryx lithographica as the “first bird.” At issue is the discovery of new birdlike fossils, notably Xiaotingia from China, that were very similar anatomically to Archaeopteryx but – and this is the part that interests me most – predate it in the fossil record. Xiaotingia is older than Archaeopteryx, dating back 155 million years ago to Archaeo‘s 150. Now this really is getting interesting.
I’ve written and lectured in the past about the degree to which we should consider the many feathered dinosaur fossils discovered in China as ancestral to modern birds. Despite the birdiness of those fossils, they were discovered in strata younger than Archaeopteryx, and were less well developed for powered flight. If Archaeopteryx is the oldest and most birdlike fossil, then the real ancestor of birds must be something much older still, say something from the early Jurassic.
Xiaotingia (and Epidexipteryx) are older than Archaeopteryx. Epidexipteryx does not show evidence of being able to fly, but it did possess a birdlike trait more derived than Archaeopteryx – reduced caudal vertebrae supporting long feathers for a tail. Xiaotingia was very Archaeo-like, but both it an Archaeopteryx seem to show the beginning development of the famous “retractile” claw on the hind feet that we see in Deinonychus and its relatives, suggesting that these two forms might be heavily feathered dinosaurs, and not ancestral to birds at all.
So what does that mean? Well, maybe Epidexipteryx is a more important figure in the ancestry of birds than generally assumed. I suspect, however, that we’ll continue to push the envelope deeper into the Jurassic to find the elusive avian ancestor. In other words, Archaeopteryx has more accurately been considered for 150 years to have been the earliest bird known, not the earliest bird. The fact that we are less confident today that it even should be considered a bird just further illustrates what we know about evolution: it does not progress in an orderly fashion, and our phylogenetic trees are really just estimates of dense phylogenetic bushes in which identifying the actual main stem is always difficult. Efforts to identify “the first __________ ” are also hampered by the fact that our arbitrary classification systems for labeling something a “dinosaur” or a “bird” or a “mammal” or anything else are insufficient to capture the phenotypic variability in the innumerable “missing links” that exist between our categories of biosystematic classification. At some point, a dinosaur brooded a clutch of eggs in which at least one of its offspring was a bird. That’s the ancestor we’re looking for . . .