In 1861 a 150 million year old fossil was found in Langenaltheim, Germany, that would spark the debate on the evolution of birds. The specimen was bought by the Natural History Museum, London, under the instruction of Richard Owen for £700 - a minor fortune considering the time. Owen recognised the importance of the specimen, as it was half way between a bird and a reptile. Darwin's theory of evolution was still young though so the fossil's importance was not immediately recognised. This find though would prove to be critically important in discovering the origin of birds. This specimen was of course Archaeopteryx lithographica.

Archaeopteryx

Archaeopteryx lithographica means 'ancient wing from the printing stone', referring to the limestone it was found on where it was used for printing. On examining the specimen and others found at later dates, it was discovered that Archaeopteryx had several reptilian and avian features which are given below.

The Reptilian Features of Archaeopteryx

• The front of the skull tapers to form a snout with small replaceable teeth, much like a crocodile
• Archaeopteryx has relatively numerous ribs (12 pairs)
• The digits of the fore-arm are not fused and have claws
• The pelvis is fairly small
• The tail is long and has 20 vertebrae (birds have a reduced tail and fused vertebrae)
• The cerebellum is small and the hemispheres smooth.

The Avian Features of Archaeopteryx

• Archaeopteryx had feathers, including the asymmetrical feathers needed for powered flight as in modern-day birds
• The clavicles (collar bones) are fused into a fercula (wishbone), a feature also of some dinosaurs
• The pelvis is pointed backwards
• The first digit of the foot was opposable
• Archaeopteryx was bipedal, though this was also seen in some dinosaurs.

Reptiles or Dinosaurs?

So, if Archaeopteryx shared reptilian and avian features, as well as several characteristics of the dinosaurs, what had it evolved from? Thomas Huxley, in the 1860s, was the first to propose that birds had evolved from dinosaurs but he gained few supporters. At the time, dinosaurs were still thought to be over-sized reptiles that had died out once they reached the limit of their evolution. The debate began again when John Ostrom found the fossil of a two-legged carnivorous dinosaur in Montana, 1964. This specimen, which he named Deinonychus (meaning terrible claw) showed indications of being capable of bursts of speed and was probably warm-blooded, opening the case again for bird's evolution from dinosaurs. Ostrom published his work in the 1970s and this led to the discovery of about 200 features that modern birds share with theropod (meat-eating) dinosaurs.

While John Ostrom¹ led the case for bird's evolution from theropod dinosaurs, Alan Feduccia has been the main campaigner against this idea. His main point is to do with fingers. Birds and theropod dinosaurs alike have three digits on each hand, but are these three digits are not the same three. Birds have digits 2, 3 and 4, theropods had 1, 2 and 3. The other main part of his debate is based on his belief that birds learned to fly as a result of gliding (i.e. from the trees down). Ostrom believes birds gained flight from the ground up (i.e. as a result of evolution to jump higher to catch more insects). This is to do with the evolution of feathers; several theropods had feather-like structures, probably for insulation or display. Ostrom believed that these later evolved into feathers capable of sustaining flight. Feduccia however believes that this idea is silly, as feathers are expensive to form using large amounts of energy, so why would a creature evolve them for any purpose other than flight?

Ostrom's theory however was by far the most popular, and has found the greatest amount of support. There simply is too much evidence for birds having to have evolved from dinosaurs for the link to be missed. Brooding for example is an essential feature of modern birds to enable them to stay in the air all year round, and this was also practised by theropod dinosaurs. Recent fossil findings have been the most conclusive, and have also lead to a good understanding of how the wing, and therefore flight, first evolved.

Sinosauropteryx was a ground-running, three fingered dinosaur. It had filaments on its body that probably evolved for insulation, and could well be the origins of feathers.

Velociraptor was a predatory theropod dinosaur about the size of a turkey and was originally found in Mongolia. Its wrist could swivel - an essential feature for powered flight.

Unenlargia, found in Patagonia was eight foot long and could move its arms up and down in a similar way to how a surfer can move their arms to help keep their balance. This action is necessary for flapping flight.

Caudipteryx is one of the recent fossils from China. It was a fast runner and was covered in feathers, though these were symmetrical and therefore useless for powered flight.

Protarchaeopteryx, another fossil from China is very similar to Archaeopteryx, except that it had symmetrical feathers and so could not fly (though it could probably glide, at least for short distances).

Archaeopteryx as previously mentioned had asymmetrical feathers giving them the aerodynamic properties necessary for powered flight.

Eoalulavis, found in Spain, is the first known bird with an alula. This is a tuft of feathers attached to the thumb that help guide airflow over the aerofoil and so enable good maneuverability and control and low flying speeds.

Corvus, the crow, represents modern birds with a shortened tailbone and a large wing surface allowing full flight ability.

But are birds dinosaurs?

The fossil evidence indicates that birds evolved from dinosaurs, namely theropod dinosaurs, but are modern birds today actually dinosaurs? This point is in debate and probably will never be satisfactoraly answered. After all, at what point does a dinosaur become a bird? Evolution is an on-going process, and there's no line on which one side are the dinosaurs, the other birds. They are only called birds and dinosuars in the first place because humans like to categories things. Birds share particular features with each other that means well grop them all together and call them 'birds'. We don't know enough about dinosaurs to know how many of these features they also shared, so cannot say whether birds are dinosaurs or not. Ostrom however firmly believed that there are indeed dinosaurs in our back garden.