Graduate Student Illuminates Evolution of Bird Locomotion

Dan Jung of The Daily Californian staff contributed to this report.

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Inspired by the giant Tyrannosaurus rex exhibit in the Valley Life Sciences Building, John Hutchinson decided to study how the dinosaur moved.

Although his research originally focused on the gait and posture of T. rex, Hutchinson later broadened his focus to include earlier reptiles and birds.

"Initially, I thought I would only be interested in T. rex, but (studying the dinosaur) led me to much broader questions about the evolution of locomotion, how bipedalism evolved (in birds), how different muscle groups evolved and whether their changes occurred very early on or occurred in a step-wise pattern," Hutchinson said.

Hutchinson studied the evolution of thigh musculature in ancestral reptiles and early birds to determine how locomotion evolved in the line.

By studying the muscles of both the living relatives of ancient reptiles and early birds and by examining their fossil remains, Hutchinson worked to reconstruct the muscle changes that made bird locomotion possible.

"I want to resolve how birds became the way they are today," Hutchinson said. "Learning about how the functions of leg muscles changed on the lineage leading to birds will give clues about how large-scale reorganizations of locomotion evolve."

This work can support one of three hypotheses about archosaur leg bone and muscle evolution. The archosaur lineage includes dinosaurs, crocodiles and birds, which are a part of the reptile line.

The first hypothesis states that leg bone and muscle features could have appeared very early in archosaur evolution. Another theory states that no locomotive features appeared until the arrival of the first birds in the late Jurassic period, about 150 million years ago.

The final possibility is that the traits evolved in a step-wise fashion, slowly creating the unique leg bone and muscle features present in living birds.

"My work totally and unambiguously supports that everything fell into place slowly, which is what we would expect, and is often the way evolution happens," Hutchinson said.

Hutchinson focuses on the theropod line of reptiles, which contains the two-legged meat-eating dinosaurs, but focuses on a branch of the theropod group which led to birds.

His findings help to illuminate how locomotion and bipedalism evolved in the theropod lineage.

"My work is important because we understand few specifics of how locomotion evolves in any lineage," Hutchinson said. "One of the most dramatic evolutionary changes in terrestrial locomotion was the evolution of bipedalism in birds."

Studying the stance and locomotion of T. rex is a good way to elucidate some of the intricacies of bipedalism.

"T. rex is an extremely large biped. It tells us a little about how size interacts with bipedalism, sort of like how biomechanics are limiting joint orientations in larger sizes," Hutchinson said.

In order to reconstruct the way T. rex moved, scientists study the changes that took place during the evolution of bird locomotion.

Knowing how an animal stands at rest requires knowledge of its bone structure and of associated soft tissues like muscles and ligaments.

In order for locomotion to change dramatically, both the skeletal and muscular systems of the animal must evolve.

Hutchinson assumed that bone structure changes correspond directly to muscle attachment because the bone responds to mechanical stresses the muscle places on it.

"The muscle, in a way, causes the bone to develop in a certain form because of the way it attaches and pulls when contracting," added Hutchinson.

Because of their molecular make-up, muscles are not preserved in fossils. Bone remains can offer insight into their structure because the muscles leave scars and bumps. These markings can be analyzed to determine aspects of the animal's musculature.

But many muscles do not leave scarring, so scientists shed light on the locomotion of extinct animals by studying the musculature of their living relatives.

To understand the stance and gait of an extinct animal, Hutchinson infers information by studying a living relative and tracing the changes in leg muscles and bones back in time.

Among the dozens of animals Hutchinson studies are alligators, birds, lizards, turtles and the tuatara, a spiny reptile.

"The tuatara was a critical thing to study because it is the living cousin to all snakes and lizards and has lots of primitive traits," Hutchinson said.

He compared the bone structures of living animals to fossils of extinct animals and then tried to reconstruct the way locomotion evolved.

"When I found a feature on a (fossil) bone, I would ask what could it correspond to in a living animal," Hutchinson said. "I would go back and do more dissections to make sure that the feature had a similar structure."

Hutchinson searched for bone and muscle features that vary within the groups. He considered muscle attachments present in many reptile groups to be primitive and characters unique to certain groups to be derived.

Major changes along archosaur evolution and relationships between the groups became clear when Hutchinson mapped out the primitive and derived characteristics.

A broader picture of the animal's skeletal and muscular anatomy can provide insight into how the animal is able to move.

Hutchinson also uses computer models and principles of physics and biomechanics to answer questions about posture and locomotion in T. rex and other dinosaurs.

He uses a musculoskeletal modeling computer program to experimentally manipulate T. rex' muscle size, attachment and range of bone motion.

Different researchers think that the T. rex was capable of exhibiting a wide range of motions and postures - from the straight-legged posture of an elephant to the crouched position of birds.

Muscle scars have not provided sufficient evidence to determine the exact posture of the dinosaur. But Hutchinson can determine the stances impossible for the animal to exhibit by calculating the minimum force the muscles needed to exert on the body to create balance and maintain posture.

"The question is what is off-limits, (not) what (T. rex) really did," Hutchinson said. "The best way to answer these questions (is) to falsify hypotheses and to narrow down (the possibilities.)"


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