On December 8, the journal Current Biology published an article that describes a special tuft of feathers found in a piece of amber only as big as a matchbox. What makes these feathers special is that they are attached to the tail of a juvenile dinosaur!
Since the 1860s, scientists have classified birds as closely related to and descended from dinosaurs, specifically a group called theropods, which includes familiar two-legged walking (bipedal) favorites like Tyrannosaurus and Velociraptor. In 1861, a transitional fossil was discovered that linked birds to dinosaurs. Named Archaeopteryx, or “ancient wing,” the animal was a crow-sized, bipedal reptile with a short snout, many teeth, and a long tail—with a full set of feathers! Since then, other fossils that share characteristics of both birds and dinosaurs have been unearthed, and birds are now considered to be the last survivors of the much larger dinosaur group. However, until quite recently, scientists thought most non-bird dinosaurs were scaly and hairless, like modern day lizards or snakes. Starting in the mid-1990’s, that began to change, as more feathers of varying complexity were found with dinosaur fossils. Scientists have now identified so many feathered dinosaurs and early birds that there is a published field guide.
So, what’s so special about finding a feathered dinosaur tail in amber? Many feathers, ranging in appearance from primitive to modern, have been found in amber, which is fossilized sap from extinct conifers. But, this is the very first recorded amber specimen to have feathers that are attached to a part of the dinosaur.
This is important for two reasons. First, it allows researchers to know what kind of animal grew those feathers. In this paper, the researchers conclude that the tail is from a non-bird theropod dinosaur, possibly a maniraptoran, based on vertebral number and morphology. The amber specimen contains 8 ½ articulated caudal vertebrae and is only a small part of the whole tail, which is estimated to have at least 25 vertebrae. In contrast to the long flexible tails of theropod dinosaurs, both early and modern birds have fused tail vertebrae. Several anti-evolutionary organizations have recently published articles casting doubt on whether the tail is from a dinosaur (see here and here and here). However, young-earth creationist scientists Todd Wood and Marcus Ross have criticized these responses, and argue that the evidence pointing to the dinosaur identification is very compelling.
Knowing the tail is from a dinosaur helps to place the feathers into a context and timeframe that is useful for studying how feathers evolved over time. For example, the feathers found on this dinosaur tail are not the type needed for flying; this suggests that these dinosaurs used their feathers for other purposes such as regulating temperature or courting mates.
Second, amber preserves aspects of ancient tissues that other fossils do not. Non-amber fossils usually contain only the squashed impressions of stiff, robust feathers. The delicate plumes trapped in this gem are preserved in intricate three-dimensional detail. One particularly exciting feature of these feathers is that they have an arrangement of barbs and barbules (the hooks that hold feathers together and create iridescence) that have not been seen before in either modern birds or dinosaurs, yet this arrangement fits nicely between two previously predicted stages of feather evolution.
Credit: Figure 4 from Xing et. al, © 2016 Elsevier Ltd.
Earlier, I referred to Archaeopteryx as a transitional fossil, but I want to clarify what that means when thinking about evolutionary change in groups of organisms. It is easy to over-simplify, and think that evolution acts in a linear fashion, like this:
Instead, evolution is better visualized as a branching tree. As lineages evolve and diversify, they become distinct from one another, which is represented by new branches. Not only does the tree branch but, over time, it is also pruned as species or entire lineages (e.g. all Tyrannosaurs) go extinct. The dinosaur tail in amber represents only a single point on a branch that is now extinct.
Remarkably, this is not the first discovery of feathers in amber published by this research team in 2016. In June, the team described two tiny partially feathered wings that belonged to an enantiornithine bird. The feathers are more modern in structure than those in the dinosaur tail but both specimens are from the same region of Myanmar. These samples are all about the same age, which suggests that non-bird feathery dinosaurs lived side-by-side with early birds with modern feathers, just as a branching-tree model of evolutionary history predicts.
Discoveries like these amber-preserved feathers provide new insights into the evolutionary history of theropod dinosaurs and early birds, as well as an inkling of their coloration. They also demonstrate the power of evolutionary theory to make accurate predictions about what the fossil evidence will show. But even as these discoveries confirm the evolutionary link between birds and dinosaurs, they continue to reshape our assumptions about dinosaur appearance. If the vibrant birds we see today are living dinosaurs, why did we ever assume the extinct ones were dull-looking?