As I started to write this, a rather ridiculously meaty creature (Meleagris gallopavo to be precise) had been stuffed and was being slowly roasted. Eventually, it was consumed by a family of apes of genus Homo. Stripped of flesh, the skeletal remains of this creature marked it as a maniraptoran coelurosaur of clade Aves – the sole line of Dinosauria to persist after the Cretaceous-Palaeogene extinction event. Literally every example of this line is more closely related to the mighty Tyrannosaurus rex than Triceratops, Diplodocus, or Stegosaurus were to either.
The fossil record is notoriously stingy, particularly in regards to soft tissues. Decomposition and scavengers tend to take a toll on any remains before they can be buried, let alone mineralised. The Solnhofen limestone in Bavaria is a special exception which has yielded a very detailed snapshot of the late Jurassic. These strata were laid down at the edge of the Tethys Sea, where dropping sea levels created highly saline lagoons with pockets at the bottom which were devoid of oxygen. In these deeper reaches, the environment would have been hostile even to the sorts of decomposing bacteria which typically obliterate soft tissues. Such preservation conditions are known as lagerstätte.
[Tweet “Did you roast a dinosaur relative for Christmas?”]
In 1861, a quarry in the Solhofen limestone yielded a remarkably intact and well-preserved specimen about the size of a crow, dubbed Archaeopteryx. It possessed clearly preserved asymmetrical flight feathers, a furcula (wishbone) and wings formed from an extension of the second finger akin to modern birds (and as opposed to pterosaurs where the fourth finger is extended). It also possessed a very reptilian mouth full of sharp teeth, clawed fingers, and a long tail It was evidently represented an intermediary between reptile and bird, but it was not entirely apparent just where it fit in.
It would be about 100 years later before palaeontologists would have a sufficiently developed fossil record to note the anatomical similarities between Archaeopteryx and small theropod dinosaurs, particularly those of family Dromaeosauridae, such as Velociraptor mongoliensis and Deinonychus antirrhopus. In particular, it shared, it shared the hyper-extensible “killing claw” upon the second toe. The stiffness of its tail and its three-fingered forelimbs were also notably similar. By this time, there were also many examples of small theropod specimens with their furculae intact. This was also approximately when palaeontologists began to entertain hypotheses of warm-blooded dinosaurs, with Dromaeosauridae being touted as a prime example. Thus, it was it was intriguing but not necessarily unexpected when evidence of feathers on other dinosaurs eventually began to be discovered starting in the 1990s.
New dinosaurs with preserved feathers:
This list makes no claims to be exhaustive (and will inevitably get less and less exhaustive as it ages, and merely illustrates some significant, representative examples.
Sinosauropteryx prima: discovered in 1998, this compsognathid dinosaur from China had preserved, filamentous proto-feathers. These feathers were apparently hollow and and much more coarse than mammalian hairs. Length: ~1.0 metres. Dated: ~125-120 million years ago (Early Cretaceous).
Microraptor gui: a specimen this small dromaeosaurid discovered in China in 2003 bore preserved flight feathers on both its fore and hind limbs, suggesting biplane flight. As the feathers on its hind legs would have impeded its ability to run, it has been suggested that it was a tree-dweller. It may have flown using both limbs as flight surfaces, enabling high manueverability in close quarters. Length: 0.4 – 0.8 metres. Dated: ~125-120 million years ago (Early Cretaceous).
Dilong paradoxicus:described in 2004, Dilong is thought to represent either a basal tyrannosauroid or pre-tyrannosauroid coelurosaur. Found in China, it bore filamentous proto-feathers similar to Sinosauroptheryx. Conservatively, it occupies an extremely primitive position on the tyrannosauroid line. Length: estimated ~2.0 metres. Dated: ~125 million years ago (Early Cretaceous).
Yutyrannus huali: a much larger tyrannosauroid from China, described in 2012. Yutyrannus specimens show signatures of downy and/or filamentous feathers. It is currently the largest known feathered dinosaur. Length: 9.0 metres. Dated: ~125 million years ago (Early Cretaceous).
Ornithomimus edmonicus: though a long-known species, feathers were not known on Ornithomimus until recently. Starting in 1995, several specimens were discovered in North America with traces of well-developed feathers on the forelimbs. In 2015, a specimen was discovered in Alberta’s Dinosaur Park formation which showed adult Ornithomimus to be heavily feathered, with plumage on its tail as well as its arms. It is the first known feathered ornithomimid. Length: 3.8 meters. Dated: ~76 – 65 million years ago (Late Cretaceous).
In addition to preserved traces of feathers, sufficient evidence exists to confidently imply feathers upon other species. A specimen Velociraptor mongoliensis, for instance, was discovered in 2007 to possess preserved quill knobs upon the ulna of its forelimbs consistent with those typically found in species possessing flight feathers.
Note that I have also depicted Tyrannosaurus as the only coelurosaurian example on my chart without significant plumage, as current data leaves the question quite ambiguous. Certainly, there is plenty of evidence to suggest that feathers are widespread among Coelurosauria, but Tyrannosauridae may be an exception. Well-preserved fossilised skin casts of tyrannosaurids which lack feathers exist for anatomical regions where Yutyrannus clearly possessed them, so it may be that for reasons of climate and/or body size Tyrannosauridae lost feathers through evolution, much in the way that elephants and rhinoceros have evolved hairless hides despite being evolved from smaller, furry mammals. Or, it may simply be the case that the feathers degraded faster than the scaly hide.
Evolution of modern birds:
I have positioned Archaeopteryx as a branch rather than a true ancestor to birds, while it is commonly perceived as a “missing link”. It could very easily not be a link to any modern bird, though a good conservative inference is that it is at very least a closely-related maniraptoran theropod. It may well represent a primitive dromaeosaurid, ancestor to such fearsome predator genera as Deinonychus and Utahraptor. Its placement is convoluted by disagreements about just where to draw the line between Aves and Maniraptora, or how to even define Aves.
For all of their radiation, they have yet to change enough to not be identifiable as dinosaurs, at least not in the sense that mammals can be differentiated from other clades of amniotes by mammary glands. They instead possess a refinement of Coelurosaurian traits:
- Hollow bones, furculae, and feathers are present throughout Coelurosauria.
- Avian wings are merely the three-fingered forelimbs of Coelurosaurians, with an elongation of digit II, eventually fused to digit III, while digit I has receded.
- Warm-bloodedness appears to have been present in many, if not most or all major groups of dinosaurs.
- The tail structure is a derived theropod tail, with the vertebrae compressed and fused.
- Similar hip structures occur in a variety of small Maniraptora, including Archaeopteryx.
- Toothless beaks appear in other feathered dinosaurs like Ornithomimus, though this is likely a case of convergent evolution rather than derivation.
If it was not from Archaeopteryx that the modern orders of birds evolved, it was from something very similar, and at a similar timeframe. Though a common conception is that modern birds evolved after the dinosaurs, all three orders of modern birds were actually well-established in the Cretaceous alongside other dinosaurs. The most recent common ancestor of all extant birds appears to have inhabited South America (then part of Gondwana along with Africa, Antarctica and Australia) about 95 million years ago, and though they do not appear to have diversified much prior to the Cretaceous-Palaeogene extinction which wiped out all other dinosaurs, they appear to have attained significant geographical diversity. Being small, mobile, and adaptable, these species were well-positioned to survive the ecological collapse, and rapidly radiated in diversity to colonise the niches abandoned by their recently extinct brethren.
This closely parallels what occurred with mammals. All major lines of modern mammals were likewise established and diversifying during the Cretaceous. Mammals also suffered major losses in the Cretaceous-Palaeogene extinction. They also underwent a rapid diversification to a great variety of body plans. They underwent cycles of dwarfism and gigantism. They expanded to colonise niches opened by the extinction.
Recently, I was standing in front of an extremely dated reconstruction of a large coelurosaur of family Tyrannosauridae named Albertosaurus sarcophagus. It looked a bit sad to me, depicted in a plodding 1950s monster movie sort of posture with its tail dragging upon the ground, rather than the nimble, balanced, fast-running posture of more modern reconstructions. As I listened to the conversations going on around me, it occurred to me that the other attendees had a hard time thinking of this specimen as “bird-like”, but no trouble in their minds associating the rat-sized mammals of the Cretaceous with the whales, elephants, and giraffes of modern time. This, I suspect, is a cultural thing – the conversations were highly reminiscent of learning about the cold-blooded terrible lizards of my youth, dotted with intermediary references to naked, Deinonychus-sized Velociraptors from Jurassic Park. Aves are mundane, and commonplace, while dinosaurs are a term reserved for exotic movie monsters which stalk the imagination.
Meanwhile, I carve a coelurosaur.