The Hoatzin

They were all over the path and on the field beside it. There must have been about 50 of them scampering about, twittering to each other and occasionally bending forwards to peck at some insect or seed. They were Japanese pheasants, slightly smaller and much duller than our own magnificent pheasants, which we regard as indigenous but were actually imported by the Romans from their natural range to the east of the Black Sea. I ran closer and they scattered and ran away, behaving not so much like birds but like little dinosaurs. It reminded me of a scene from ‘Jurassic Park’. Flying is a last resort for pheasants; with their small wings and large bodies they can only make it to the next copse or area of scrub. Other ground nesting birds, the American Road Runner, which is actually a type of Cuckoo, the Stone Curlew, and the Great Bustard, also seem to resemble dinosaurs more than they do birds.

So have birds evolved from dinosaurs? Our notion of dinosaurs, large lumpen grey-green reptiles that roamed through swamps or terrifying blood thirsty monsters as large as a double decker bus would seem to make that highly unlikely.  Nevertheless, the discovery of the Archaeopteryx is Southern Germany in 1861 shortly after the publication of The Origin of Species, seemed to settle the question beyond any doubt. This strange creature, which was about the size of a pigeon, but had a skeleton like a lizard and wings with flight feathers, was heralded as the missing link between dinosaurs and birds. Only a few specimens were discovered, but deep in Amazonia there is a strange bird that sort-of resembles Archaeopteryx. This is the Hoatzin. I spotted one once while travelling up Rio Negro beyond Manaus in a small motor boat. It was about the size of large pheasant, with a bald face, big maroon eye, punk-like spiky crest and the most striking rufous red wing and tail feathers which it displayed frequently. The Hoatzin is the sole member of the family, Opisthocomidae, which is thought to have split off from the evolutionary trajectory of other birds after the extinction of the dinosaurs. This weird chimera eats fruits and leaves, which it ferments in its chambered crop like a cow. For this reason, it is also called ‘the stink bird’. Hoatzins are too heavy to fly far, preferring to spend most of their time eating and calling noisily to each other. The chicks have one other curious atavistic feature, claws on the first two digits of their developing wings, which, together with the claws on their feet, help them climb trees. So that was it: the evolution of birds from dinosaurs rested on the Archaeopteryx, a strange bird living in the Amazon basin that resembles the  Archaeopteryx, and few other strange creatures, such as the Hesperornithes, large seabirds that resembled divers but had teeth, and the flightless ratites (Ostriches, Emus, Rheas, Cassowaries, Kiwis and Moas).

Nevertheless, for more than a hundred years, the fossil trail from dinosaurs to birds ran cold. No new feathered reptiles were discovered. Palaeontologists muttered about freak mutations, but nothing seemed to make much sense. Then twenty years ago, in the sediments of prehistoric lake beds in Liaoning Province, North Eastern China, local farmers started finding strange fossils.  These were identified as dinosaurs but they all had feathers. In many cases these were just hollow quills with tufts that probably served to insulate them from the cold, but some had flight feathers and wings, which were probably used for display before they evolved for flight. Under normal circumstances, the soft tissues of fossils, including feathers, would decay and be eaten by insects, but the Liaoning lake beds were covered by a layer of volcanic ash, which preserved the structure of fossils in great detail. The fossil beds are so extensive and the specimens so numerous that new discoveries are being made at a rate of one a fortnight.

Palaeontologists now have a complete fossil record of the evolution of birds from Therapod dinosaurs, depicting the development of every change: reduction in size, a light, air-filled skeleton, flight feathers, a beak like structure, the loss of teeth and a unique system of air sacs for breathing. Microscopic examination of the feathers has even revealed melanosomes, little packages of colour, so it is likely that like birds, these small theropods were multicoloured and probably used their plumage for display.

Dinosaurs were the dominant species on Earth for 200 million years between the Triassic and Cretaceous epochs and over that time achieved a remarkable diversity. They were extinguished when a six mile wide asteroid plunged into what is now the Gulf of Mexico, 65 million years ago, but they did not die out completely. The survivors evolved into birds and crocodiles as well as a host of reptiles. But why was it that birds survived while most other groups died out? Was it their small size? After all, it was the shrews, that survived to diversify into mammals. Could they have escaped the destruction of their habitat by flying? Did their covering of feathers allow them to survive the prolonged volcanic winter at would have followed the asteroid impact?

The recent discovery of these spectacular dinosaur fossils in Liaoning overwhelmingly supports the hypothesis that birds are descended from theropod dinosaurs and is probably the best-documented major evolutionary transitions in life history.  As ever, these new discoveries raise lots of new questions, but what an exciting time to be a palaeontologist. The irony is: this exciting discovery comes just as we are on the brink of another species extinction.