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JUAN CARLOS LEAL and Jos茅 Bonaparte were lost in a thick, thorny acacia forest in north-west Argentina when Leal stumbled on the bones. The big ones turned out to be from a new long-necked dinosaur species. But the 60 small bones they found alongside were puzzling. They were far too small to be from dinosaurs.

Intrigued, Bonaparte flew to London to show them to a fossil expert at the Natural History Museum. Cyril Walker immediately recognised them as a rare find 鈥� the remains of ancient birds. By this point in the 1970s, some palaeontologists suspected that all birds had evolved from flying dinosaurs, but the idea was not yet mainstream.

Looking closely, Walker discovered that the fossil shoulders and feet had grown quite differently to those of modern birds. A key ball-and-socket joint in the shoulder was reversed. This was a whole new avian category, not just a new species. In a short paper published in 1981, he named the fossils Enantiornis leali: Leal鈥檚 opposite bird.

Now we know that Enantiornis wasn鈥檛 an evolutionary oddity. Millions of years ago, the skies were full of such creatures. Then, some 65 million years ago, an asteroid hit Earth and opposite birds were relegated to the history books along with the vast majority of dinosaurs. The only survivors gave rise to every bird alive today.

Therein lies the mystery. How did modern birds escape total annihilation? Despite looking remarkably similar to their opposite cousins, they must have had some mysterious features that allowed them to survive the devastating impact. By looking at the subtle differences between modern birds and their extinct opposite cousins, palaeontologists are trying to piece together what made Tweety such a remarkable survivor.

Opposite birds split from modern birds between 150 and 130 million years ago, and for more than 60 million years, both types of flying beasts shared the skies. Opposite birds were just as diverse and numerous as the ancestors of modern birds 鈥� at one time, they were even more diverse. Two decades after Leal and his colleagues whacked their way through the Argentinian bush, Chinese farmers led another team of palaeontologists to a bed of fossils in China. It was stuffed full of ancient winged things, the most varied by far were opposite birds. Some 50 to 60 species have now been found on every continent except Antarctica.

When birds had teeth

The most noticeable differences between the two groups were their sizes and habitats. Opposite birds were much smaller, with adults ranging from the size of small songbirds to that of vultures. They lived in forests, unlike the ancestors of modern birds, which hung about near shorelines. So far, however, it is not clear that these differences would have doomed opposite birds to extinction.

Other differences were subtle. Yet the group鈥檚 complete demise could not have been random, says Gareth Dyke of the University of Debrecen in Hungary. They must not have been doing something the survivors were doing, otherwise we would still have opposite birds 鈥� or rather their descendants 鈥� flitting around our forests today.

The differences in their bones, the very feature that led Walker to define Enantiornis as a whole new class of bird, would not have been visible when covered in muscles and feathers. There is also no evidence that they would have affected flight, says Dyke. Recent studies show that both groups had strong flight muscles and small feathered thumb-like flaps called alulae to precisely control their trajectory. Dyke says this suggests the two types of birds flew in very similar ways.

The wings of some opposite birds did have one unusual feature 鈥� little claws peeping out from under the feathers 鈥� but it is likely that at least some ancestors of modern birds had these as well. Today, they are still seen on the juveniles of the ground-dwelling hoatzin, a large, tufted rust-coloured bird that lives in South America. The claws help young birds to climb trees.

What of their diets? Both types of birds evolved from toothed dinosaurs and for a time, both groups kept their teeth. Some of the older opposite bird fossils from China have impressive choppers (see picture, above left). But then so did the ancestors of some modern birds, such as the fish-eating Yanornis. By the time of the asteroid impact, many species in both groups had evolved toothless beaks.

Why? A beak is only an advantage if you are a herbivore, says Jingmai O鈥機onnor at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. Carnivores just rip and gulp prey for a big hit of energy, and teeth do that well. But plant-eaters need to grind through their food鈥檚 tough fibres. Some, like horses, have banks of heavy grinding teeth for this, but early flyers would never have got off the ground with these. Instead, they evolved gizzards and light beaks.

Derek Larson at the University of Toronto and his colleagues wondered why the asteroid killed toothed birds that were closely related to modern birds and small toothed dinosaurs, but not beaked birds. On the basis of the diets of living birds, they say it is likely that their ancestors had beaks and gizzards and ate seeds. That could have been an advantage after an asteroid impact. Hard shells would have protected the seeds鈥� nutritious cargo, turning them into durable reserves that hungry animals could tap for decades, while ecosystems recovered. It is a nice theory, but one that鈥檚 impossible to test without direct evidence of what the different birds ate.

The hollow bones of both groups offer another clue. They contain air sacs that boost lung capacity, and the sacs of the ancestors of modern birds were larger. Breathing would have been hard as wildfires swept the planet after the impact, so it鈥檚 possible that modern birds had an advantage there.

Or perhaps all it took was a bit of parental love. In 2004, Zhonghe Zhou and Fucheng Zhang, also at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, revealed that they had discovered the fossilised embryo of an opposite bird, still curled up inside its eggshell. The remarkably well-preserved fossil has an almost fully developed skeleton, wings and even tail feathers. In July last year, partial wings belonging to two very young opposite birds were discovered preserved in a 99 million year old piece of amber.

Together, these finds suggest that newly hatched opposite birds were much more mature than modern chicks. They came out of the egg fully feathered, ready to run and perhaps to fly. As a result, it is unlikely they got much parental care. Parents probably brooded their eggs, but once the chicks hatched, they fended for themselves.

鈥淧erhaps all it took for modern birds to survive was a bit of parental love鈥�

Where the eggs hatched is also odd. We know that adult opposite birds mostly lived in forests and had feet made for perching on branches, but fossil nests show they began their lives in eggs partly buried in soil. The ground-dwelling bush turkeys, or megapodes, of Australia are a modern example of this. Their chicks dig their way out of the ground after hatching, a bit like turtles. Megapodes are also the only living birds with superprecocial young, says David Varricchio of Montana State University. 鈥淸They] are completely independent of their parents. They hatch and run off on their own.鈥�

That may have made life particularly challenging on post-impact Earth. Gerald Mayr of the Senckenberg Research Institute in Frankfurt, Germany, says unprotected young would have been more vulnerable to predation. Given that many species of dinosaurs that were wiped out after the asteroid also had superprecocial young, could a bit of parental care have made all the difference for the ancestors of modern birds?

At the end of the day, each of these factors may have been only a minor benefit for Tweety鈥檚 ancestors, says O鈥機onnor. But when it comes to survival, every little bit helps.

This article appeared in print under the headline 鈥淔lipping the birds鈥�