Since the time the English anatomist Richard Owen described Archaeopteryx as the “by-fossil-remains-oldest-known feathered Vertebrate” in 1863, the curious creature has been widely regarded as the earliest known bird. Lately, though, the status of the iconic animal has been up for debate. Earlier this summer, one team of paleontologists proposed that Archaeopteryx was not a bird but actually a feather-covered, non-avian dinosaur more closely related to genera like Microraptor and Troodon. Now a different team of paleontologists has published a paper in Biology Letters that says Archaeopteryx was an early bird after all.

The ongoing back and forth over Archaeopteryx reminds me of the old Looney Tunes bit where Bugs Bunny and Daffy Duck keep going back and forth over which hunting season it is. “Duck season.” “Wabbit season!” “Duck season” “WABBIT SEASON!” In the same way, the argument over Archaeopteryx could seemingly go on indefinitely. The reasons why have everything to do with how both science and evolution work.

The study of prehistoric life, like any other science, is not restricted to the slow and steady accumulation of facts. Facts are most certainly acquired through studies in the field and lab alike, but to tell us anything significant about dinosaurs, these facts must be understood according to theories and hypotheses. An exasperated Charles Darwin conveyed this truth eloquently in an 1861 letter he wrote to colleague Henry Fawcett:

About thirty years ago there was much talk that geologists ought only to observe and not theorise; and I well remember some one saying that at this rate a man might as well go into a gravel-pit and count the pebbles and describe the colours. How odd it is that anyone should not see that all observation must be for or against some view of it is to be of any service!

Facts, theories and hypotheses are all necessary and interacting parts of the scientific process. As new discoveries are made and ideas are tested, the context by which we understand what dinosaurs were and how they lived changes. This is to be expected—there are always more questions and mysteries about dinosaurs than readily available answers. In the case of Archaeopteryx, we know this feather-covered dinosaur lived on a group of roughly 150-million-year-old islands that would eventually become southeastern Germany. Whether or not Archaeopteryx belonged to that successful lineage of feathered dinosaurs called birds, though, is something that depends on other feathered dinosaur discoveries and the techniques used to test ideas about relationships among animals.

Teasing out relationships among prehistoric animals is a comparative science. The key is finding traits that are shared in some organisms due to common ancestry but are absent in others. This can be a tricky process. Due to a shared way of life, for example, unrelated organisms may have developed superficially similar traits through a phenomenon called convergent evolution. Paleontologists must carefully choose the traits being compared, and the discovery of additional dinosaurs adds more grist to the comparative mill.

Archaeopteryx is actually a perfect example of how new discoveries can change our perception of relationships. When the first skeleton was discovered in 1861, nothing quite like it had been found. Archaeopteryx seemed to stand by itself as the first bird. Over a century later, though, the discovery of dinosaurs such as Deinonychus, an updated understanding of dinosaurs and the eventual discovery of many, many feathered dinosaurs illustrated that Archaeopteryx exhibited a number of transitional features that illustrated how the first birds evolved directly from feathered dinosaurs.

The trouble is that Archaeopteryx appears to be so close to the emergence of the very first birds. At the moment, Archaeopteryx is most often regarded as being an archaic member of the group called the Avialae, which contains all birds (Aves) and forms more closely related to them than to other dinosaurs. What this means is that, as our understanding of what a bird actually is changes, the position of Archaeopteryx might shift. The animal might have been one of the earliest birds within the avialian group, or Archaeopteryx might have been just outside the bird group among non-avian dinosaurs. This is simply how science works and is a wonderful—if frustrating—demonstration of the fact of evolution.

Birds did not simply pop out of nowhere. The earliest avians went through a long period of transformation, and the continuum between feathered, non-avian dinosaurs and the first birds, which paleontologists are now filling in, demonstrates the beauty of major evolutionary change. The debate over the position of Archaeopteryx is happening now precisely because of all the evidence for this evolutionary change that has been accumulated in the past two decades. No matter what Archaeopteryx turns out to be, the creature will remain important to both the historical development of our ideas about evolution and the actual, prehistoric transition from non-avian to avian dinosaurs.

For more on changing perspectives on long-known dinosaurs, see this week’s post on the fate of the horned dinosaur Torosaurus.

References:

Lee, M., & Worthy, T. (2011). Likelihood reinstates Archaeopteryx as a primitive bird Biology Letters DOI: 10.1098/rsbl.2011.0884

Xu, X.; You, H.; Du, K.; Han, F. (2011). An Archaeopteryx-like theropod from China and the origin of Avialae Nature, 475, 465-470 DOI: 10.1038/nature10288