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Wishbone helps in the puzzle of early evoluti -

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Robyn Williams: Our PhD this week, Roger Close from Monash, is linking the past with the present, using wishbones.

Roger Close: Have you ever wondered how the aerial finesse of a swallow or the effortless soaring abilities of a wedge tailed eagle could possibly have arisen in species descended from earthbound dinosaurs? I have been investigating the biomechanics of the furcular, more commonly known as the wishbone.

Studying the form and function of this bone in living birds has allowed me to shed light on how their prehistoric relatives may have evolved into the myriad flight style niches birds occupy today.

The origin of modern birds is one of the most striking functional transformations in the history of life, and they owe their spectacular success to the evolution of flight. Exactly how theropod dinosaurs first evolved flight has been hotly debated for many years, and we are still a fair way from reaching a consensus.

My particular interests though lie in the functional changes to birds' anatomy that took place after the origin of powered flight, because although they could definitely take to the air, the very earliest dino-birds would have been put to shame by today's sophisticated fliers.

Now is a great time to review this evolutionary transition. For over a century after the Jurassic dino-bird Archaeopteryx came to light in the 1860s, discoveries of Mesozoic fossil birds, those dating to the age of the dinosaurs 250 million to 65 million years ago, were scant. In the last couple of decades though, fossils of all sorts of very early birds have been discovered at an ever increasing rate, most notably from China. Truly modern birds probably appeared during the late Cretaceous, around about 85 million years ago. But it wasn't until after the end Cretaceous extinction 65 million years ago that they really took off in number and diversity.

Mesozoic skies were dominated by more primitive groups, many of those birds still had teeth, and some even had long bony tails. So, how well could these prehistoric birds fly? Researchers have already examined a variety of characteristics that throw light on their flight capabilities, such as the proportions of their wing bones, their shoulder joint anatomy and the bending strengths of the flight feathers.

My own work focuses on the wishbone or fused clavicles, which as you probably know sits in the chest region in modern birds. Although today the wishbone is unique to birds, it first appeared in early non-flying dinosaurs. In modern birds though it's essential for flight. It anchors key flight muscles and braces the shoulder joint as a bird flaps its wings. In many species, each wing beat causes it to flex and recoil like a spring, helping to pump air through the bird's unique flow-through lungs.

Its shape in living birds is extremely variable. If you're a carnivore you may have noticed the difference between the Y-shaped wishbone of a turkey and its U-shaped counterpart in a goose. In some flightless birds like emus however, the wishbone has been almost completely lost. Earlier studies have hinted that these differences in shape were linked to flight mode, such as soaring, flapping or diving.

This, together with the fact that the wishbone is commonly preserved in the fossil record, makes it potentially very useful for investigating the flight capabilities of extinct species of birds. In a recent paper in the open access journal PLOS ONE, I presented a detailed study of wishbone shape in modern birds. My results show that wishbone shape is clearly correlated with flight style.

For example, on the basis of wishbone shape, soarers, including some very small species like swifts, can be easily distinguished from so-called bounding species, like sparrows, which only flap in short bursts. This link between form and function has allowed me to make inferences about the flight styles of Mesozoic birds by comparing them with modern species.

Intriguingly, while some of the more anatomically modern early birds overlap with living flappers, the more primitive Mesozoic species have unique shapes largely unseen in birds today. One possibility is that Mesozoic birds simply flew differently to modern species. Alternatively though, they may have evolved different anatomical solutions to accomplish similar feats of aerial locomotion. Perhaps their flight muscles were arranged differently to those of modern birds.

The second part of my research investigates the structural performance of the wishbone. For this I am using a computer modelling technique borrowed from engineering called finite element analysis. This tool is becoming increasingly popular in biomechanical investigations of extinct organisms. Until now it has been assumed that the shape of the wishbone varies with flight style because of its role as an anchoring point for the flight muscles. However, the wishbone is unusual in being one of the few bones that can deform substantially during normal operation, something you will have realised if you've ever used a wishbone to actually make a wish.

Most bones are adapted to be as stiff and unyielding as possible. Not so the wishbone. During the downstroke its arms can flex enough to add an extra 50% to its original width. We think this is an adaptation that helps to meet the intense metabolic demands of flight. Air sacs connected to the lungs are located between the arms of the wishbone, and the cyclical flexing during flapping pumps air through them like bellows.

This phenomenon has only been studied directly in a few modern species. In certain groups however, the wishbone is clearly too stiff to flex very much. My question then is, might the correlation between wishbone shape and flight mode be the result of differences in bending resistance related to respiratory physiology? Shape differences do seem to have a big impact on a wishbone's flexibility.

Although I am based at Monash University, my work has benefited enormously from links with international institutions. I was lucky to spend over a year at the University of Bristol where one of my PhD supervisors is based. She is a pioneer in the application of finite element analysis to studying dinosaurs. This allowed me to learn new cutting-edge research techniques and develop collaborations with foreign researchers. This really has given me the best of both worlds and allowed me to bring back knowledge gained overseas to my colleagues in Australia.

Robyn Williams: Roger Close. Even while doing a PhD he has published and travelled the world. Next week's PhD studies gravity in space.