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Survival of the Littlest

© Peter Schouten from Feathered Dinosaurs: The Origin of Birds

Jinfengopteryx, a feathered bird-like dinosaur. © Peter Schouten from Feathered Dinosaurs: The Origin of Birds

By Michael Lee

Birds co-existed with their dinosaur ancestors for nearly 100 million years, but eventually outlived them. Two new studies have revealed why.

The transformation of lumbering ground-dwelling dinosaurs into agile flying birds may have seemed fanciful in the 19th century, but it now represents a poster child for evolution. The dinosaurian ancestry of birds had been suspected ever since Archaeopteryx was unearthed from a slate quarry in Germany more than 150 years ago. This famous transitional fossil had wings and feathers like a bird, but retained a long reptilian tail, toothy jaws, and legs with sickle-shaped claws like those of carnivorous dinosaurs.

Spectacular discoveries of bird-like dinosaurs in the past few years have cemented the ancestory of birds within dinosaurs. More and more avian traits have been found in bipedal theropod dinosaurs, such as wishbones, long flexible arms, large brains and light hollow bones. We now also know that relatives of Tyrannosaurus rex and Velociraptor were adorned with feathers. Many of the most revealing fossils are from northern China, where a Pompeii-style eruption more than 120 million years ago covered these dinosaurs in fine volcanic ash, preserving their feathers with such detail that even their colours can be reconstructed.

We are now very certain that birds are a subgroup of dinosaurs, just like humans are a subgroup of mammals. However, some important aspects of the dinosaur–bird transition remain elusive.

Birds first appeared more than 150 million years ago – the age of Archaeopteryx – but non-avian dinosaurs went extinct much later, about 66 million years ago, when the famous meteorite struck. Thus, birds co-existed with their dinosaurian ancestors for more than half their evolutionary history in much the same way as humans today co-exist with other mammals. During their long overlap, birds increased in terms of numbers of species and anatomical diversity, both in absolute terms and relative to other dinosaurs.

Two new studies now reveal why birds diversified faster and survived longer than other dinosaurs: they were more evolutionarily flexible and innovative. Both studies looked in detail at the family tree of dinosaurs, including birds, and focused on what was happening in the lineage of dinosaurs destined to become birds.

This lineage, called the “bird stem lineage”, is the series of successive branches leading from the very base of the dinosaur tree all the way to living birds. A good analogy would be taking a real tree, identifying a “special” bunch of leaves, and tracing the single path that leads from the trunk all the way to this bunch. On an evolutionary tree, the bunch of leaves would be birds, and the path would be the bird stem lineage. It turns out the bird stem lineage was special in two ways, which helped it outdiversify and ultimately outlive other dinosaur lineages.

One study published in PLOS Biology by Roger Benson of Oxford University looked at how body size changed across the entire dinosaur tree. Non-avian dinosaurs have historically been stereotyped as massive lumbering beasts, but many were no bigger than large mammals (e.g. horses), and a few were chicken-sized. Given the size variation across different dinosaurs, the results were in some ways predictable: body size increased on many branches but decreased on many other branches. For instance, the long-necked quadrupedal herbivores (sauropods) became extremely large, but later an island form (Euposaurus) reversed this trend and became a “dwarf giant”.

But there was one striking pattern: the bird stem lineage underwent rapid and abrupt changes in body size more frequently than other dinosaur lineages. These changes in body size would have permitted rapid exploitation of new niches. Thus, the dinosaur ancestors of birds seemed to be unusually evolvable in body size.

These conclusions were amplified by a recent study published in Science. My colleagues and I focused on the evolutionary tree of bipedal, meat-eating dinosaurs (theropods), and again looked at the bird stem lineage. We noticed that evolutionary innovations appeared more frequently in this lineage than in other dinosaurs: complex feathers, bigger brains, longer arms and light skeletons. In fact, the proto-birds were evolving approximately four times as fast as other dinosaurs.

We also noticed that body size was continually decreasing along the bird stem lineage. This occurred on more than a dozen consecutive branches spanning about 50 million years.

Across the entire dinosaur tree in general, body size increases and decreases were randomly distributed across branches, so the probability of size decreasing so many times in a continuous row by chance alone is miniscule. Rather, this was a long-term adaptive trend, likely driven by natural selection. Among dinosaurs, only the ancestors of birds kept pushing the lower limits of size, exploring life with successively smaller and smaller bodies.

The rapid evolution and sustained size reduction that characterised the ancestors of birds might have gone hand-in-hand. Smaller size would have allowed exploration of new ecological niches that were off-limits to their larger cousins, such as arboreal or aerial habits, which in turn would have selected for novel anatomical adaptations.

Smaller size is also less restrictive anatomically. For instance, pillar-like legs are not necessary in small animals. Birds hold their thigh bones horizontally, walking like Cossack dancers. (The vertical “legs” we see on a chicken are entirely shin-bones and ankle bones.) This was probably an adaptation to shift their feet forwards to compensate for the forward displacement of their centre of gravity resulting from their big chest muscles and reduced tails. However, a horizontal femur (with horizontal thigh muscles) is inefficient for weight support, and probably could only evolve in small animals.

Thus, the evolution of small body size perhaps permitted proto-birds to evolve and explore this novel body form, again facilitating rapid evolutionary innovation. Ultimately, this novel posture and associated body proportions, only possible at small body size, proved very useful for flight.

Diminutive bodies might even have fostered the elaboration of feathers, as small animals lose heat rapidly due to their high surface-area-to-volume ratio, and would benefit from improved insulation.

When the massive meteorite struck Mexico 66 million years ago and plunged the world into a sudden and extended winter, the fast-evolving bird lineage had more evolutionary novelties than other dinosaurs, some of which proved essential for survival. Flight gave them the ability to move long distances across inhospitable terrain in search of pockets of remaining suitable habitat; dense feathers helped them regulate their body heat as the temperature plummeted; and their small body size might have enabled them to exploit food resources that were unavailable to their larger dinosaurian kin, such as insects in detritus.

Thus, the most rapidly evolving dinosaur lineage survived the impact, and ultimately evolved into today's birds.

The ability to rapidly adapt and respond to change is a key to long-term survival in spheres as diverse as nature, business, sport and industry. With the recent passage of our southern winter, a timely example concerns the influenza virus. Of the many and varied flu strains circulating last winter, the fastest-evolving strains are more likely to outwit our immune systems and to persist and cause problems next year. In the natural world, everything is trying to out-evolve everything else: if prey evolves better armour and more effective camouflage, predators are compelled to evolve sharper teeth and better eyesight – or starve.

Evolutionary biologist Leigh Van Valen used a metaphor from Alice in Wonderland to call this the “Red Queen effect”: if everyone and everything is moving, you need to run just to stay in the same relative place. Dinosaurs are no exception to this rule, and it is no surprise that the dinosaur lineage that was the best evolver – the bird stem lineage – prevailed.

Michael Lee is a senior research scientist at the South Australian Museum and the University of Adelaide.