For example, scientists from the University of Bristol recently showed that the evolutionary relationships between different dinosaurs arecontinuously changing in the light of new fossils. It’s a bit of an etch-a-sketch science—no sooner are family trees drawn before they’re re-drawn again. Even well-known transitions are prone to big shake-ups.

Consider the origin of birds. There is now overwhelming evidence thatbirds evolved from small predatory dinosaurs. Hundreds of stunning fossils illustrate the transitions from dino-fuzz to flight feathers and fromgrasping arms to flapping wings. The avialans (all birds, living and extinct) fit within a group of dinosaurs called the Paraves, which also includesdromaeosaurids (sickle-clawed predators like Velociraptor andDeinonychus); and troodontids (large-brained predators like Troodon).

But which of these creatures were the first birds, and which specific group of paravians were their closest relatives? That’s still the subject of heavy debate.

The famous Archaeopteryx, with its winged arms, clawed hands, toothed jaws, and long bony tail, was one of the first fossils to suggest a link between birds and other dinosaurs. Since its discovery in 1861, it has been widely heralded as one of the earliest birds (avialans). But two years ago, Chinese palaeontologist Xing Xu cast its pivotal position into doubt.