Archaeopteryx, arguably the oldest-known bird, has an important place in Earth's evolution, breaking away from other dinosaurs to begin the one lineage that survived the great extinction. It has a similarly significant place in evolutionary history as marking a bridge between birds and what went before helped convince many doubters of Darwin's "dangerous" idea to accept it.

Despite the astonishing preservation of some Archeopteryx fossils – so good the early ones were considered hoaxes – only 11 are known. A description of the most recent in PeerJ reveals the oldest of the set, dating to 152 million years ago and therefore arguably forming the oldest specimen of what we consider to be a bird. Almost the entire skeleton has been preserved, but the bones have been flattened and fractured.

All Archaeopteryx fossils have been found in what is now the Altmühltal valley, northern Bavaria, then a subtropical mix of lagoons and islands extending from the Mediterranean. "Specimens of Archaeopteryx are now known from three distinct rock units, which together cover a period of approximately 1 million years," said Professor Oliver Rauhut, of Germany's Ludwig-Maximilians-Universitaet München, in a statement.

Through this period we can trace a clear evolutionary path from specimens that more clearly resemble other dinosaurs of the day, to ones with more bird-like features that emerged later. Nevertheless, all combine teeth and dinosaur-like tails with modern-looking feathers and wings. The specimen Rauhut has helped describe has distinctive features to its skull shape and vertebrae not seen in other Archaeopteryx examples. 

Despite the name meaning “first wing” or “first feather”, the plumage of Archaeopteryx was too advanced to have been a new development of the era. Other species, overwhelmingly from China, have been proposed as better candidates for the first birds. All remain disputed due to the difficulty of defining the essential features of a bird.

Irrespective of the merits of this debate, the discovery of new relatives has caused palaeontologists to ponder what is and what isn't an Archaeopteryx. When it was found, the specimen described in PeerJ was considered the 12th member of the genus. However, last year Rauhut co-authored a BMC Evolutionary Biology paper concluding that the first discovered specimen of Archaeopteryx more closely resembles the Chinese birds of the era, and renaming it Ostromia. The PeerJ paper casts doubt over whether one of the other specimens belongs to the genus but considers it too incomplete for a definitive answer.

Whether the remaining specimens of Archaeopteryx are from a single species with great internal variability, like dogs, or a genus of several species, remains uncertain. Rauhut suggested they may have been “a Jurassic analog of Darwin's finches”, diversifying into multiple species to adapt to the differing conditions of the islands on which they lived.