To continue from the last post, where were the feathered dinosaurs? And how did paleontologists begin to reconcile that birds and dinosaurs should start to come together again in their family tree?
Throughout the 1970s and 1980s, the hypothesis of a dinosaur-bird relationship was revived in part because of re-study of the Archaeopteryx specimens, the discovery of the “raptor” known as Deinonychus, and a new approach to understanding evolutionary relationships called cladistics.
Archaeopteryx and Deinonychus are known and discussed in great detail in many sources. Suffice it to say John Ostrom, among others, began to notice striking skeletal similarities between Archaeopteryx,Deinonychus, and dinosaurs generally. It was eventually recognized that there are a number of special, shared traits that only seem to occur together in birds and dinosaurs, and especially among predatory dinosaurs and birds. I could provide a substantial list, but here are a few, selected key features:
- A fully erect stance where the shaft of the femur (thigh bone) is perpendicular to the femoral head. (Incidentally, the femoral head points inwards towards the pelvis, and this allows the femur to be held vertically.)
- The ankle is a modified mesotarsal ankle joint. What this means is that the proximal and distal ankle bones form a cylinder-like roller joint between themselves. You can see the upper part of this roller joint at the end of a chicken or turkey drumstick, and you also see it in dinosaurs.
- Predatory dinosaurs and birds have specialized, hollow bones.
- Predatory dinosaurs and birds have a three-fingered hand, and Archaeopteryx has a clawed, three-fingered hand with deep ligament pits, just like other predatory dinosaurs.
- A large majority of predatory dinosaurs are classified as tetanurans, and it has been discovered that the tetanuran predators and birds have a furcula. Despite earlier suggestions to the contrary, many dinosaurs have clavicles and furcula.
- Coelurosaurs are predatory dinosaurs with specialized wrist bones that allow the hand to swivel sideways. In other words, the hand doesn’t flex and extend, it rotates sideways towards the ulna. Guess what other group of vertebrates has this specialized wrist? Birds!
- Within coelurosaurs are the maniraptorans, the predatory dinosaurs that include Deinonychus and the now universally-knownVelociraptor. These dinosaurs have highly flexible necks, elongate forelimbs, and the ulna is bowed outwards — the only other vertebrates with these features? Birds.
These observations, while powerful on their own, really started to hit home when placed within a scientifically-testable framework called cladistics. In a nutshell, cladistics relies on special, shared traits rather than overall similarities to determine common ancestry. In extremely simplified form, cladistics attempts to do what your family tree does: group everyone together who is related by common ancestry. Yes, we all have an uncle or group of relatives we wish were not part of our family, but our shared genetic traits still show our close relationships.
Cladistic analyses of dinosaurs among the vertebrates revealed what Huxley had hypothesized all those years ago: birds were not just relatives of dinosaurs, they were a branch of the predatory dinosaur family tree! Birds were dinosaurs just like humans are mammals.
But where were the feathered dinosaurs? Until the 1990s, all paleontologists could do is point to the special, shared traits of Archaeopteryx, predatory dinosaurs, and birds and infer that maybe some dinosaurs had feathers. This ambiguity was seized on by opponents of the birds-as-dinosaurs hypothesis to again suggest all the features (and more) that we have listed here were simply due to an amazing amount of convergent evolution.
Enter the Cretaceous Chinese predatory dinosaur discoveries of the 1990s in the Liaoning Province. Unprecedented soft-tissue preservation in these fossils showed what was predicted by cladistics, Archaeopteryx, the suite of features shared between dinosaurs and birds only, and even back to Huxley’s observations: unmistakable dinosaurs with unmistakable feathers*. And not flight feathers, either. Barb-like and downy-like feathers that ran along the lengths of dinosaurs that could not have flown. These animals would have used the feathers for insulation and perhaps display, but many could not have flown. To tick off a few on the list of feathered dinosaurs discovered since the 1990s:
And in the past few years, non-predatory dinosaurs and large predatory dinosaurs with feathers have appeared. Among them:
- Dilong, a distant relative of Tyrannosaurus rex, showing feather impression along the jaw and tail.
- Tianyulong, a heterodontosaurid dinosaur in the same major group as the so-called “duck-billed” dinosaurs, sporting feathers on neck, back, and tail
- Yutyrannus, a 30-foot long predator weighing over 1 metric ton, and displaying elongate feathers along its belly, foot, and even projecting from its arms.
This many dinosaurs with feathers, some nowhere near the bird-line let alone among the predatory dinosaurs at all, leads to what we call in science robust evidence.
*Now, the reason for the asterisk — to be absolutely clear and fair, “feather” can be a rather broad term. Some of these dinosaur feathers are long, hollow barbs, and some don’t branch like modern feathers. However, Richard Prum and Jan Dyck have demonstrated through detailed studies of feather development in modern birds how feathers begin and diversify. They have “staged” feathers, meaning that he has hypothesized what the earliest types of feathers should be and so on. Interestingly enough, the variety of filamentous structures found in the many so-called feathered dinosaur fossils fit these predictions very, very well.
But perhaps you’re still not satisfied that birds are indeed dinosaurs? Okay, stay tuned …