Wilderness Underfoot: Steppin'out, part 2

StoryImage( ‘/Images/Story//Auto-img-116118915422921.jpg’, ‘Image provided my author’, ‘Fast-running Nanotyrannus – This dinosaur’s speed comes from its upright posture.’);
StoryImage( ‘/Images/Story//Auto-img-116118921525260.jpg’, ‘Image provided’, ”);

Life has an unyielding drive to fill every niche and take advantage of every opportunity to multiply.

It was only natural that bony animals, after thriving in the oceans, lakes and rivers for hundreds of millions of years, would finally come out of the water to inhabit new ecosystems on the land.

When they did colonize the land, these tetrapods (four-limbed animals) still looked like their fishy ancestors. They had inherited a side-to-side gait, which, under the water, had allowed them to swim and paddle. On land, this bending motion increased the reach of their strides.

Animals with a sprawling gait tend to walk with their forelimbs diagonal to their rear limbs, and their bodies bending to accompany each step. Sometimes, their bellies and tails drag along the ground. It isn’t the most efficient way to get around, but it served the first amphibians

320 million years ago, and it still works just fine for most modern amphibians and reptiles.

However, because of the way they’re built, amphibians and reptiles have always been somewhat limited in their locomotion on land. The sprawling gait is not particularly fast. And because they’re generally incapable of breathing while on the run, these creatures must pause to catch a breath. Also, their simpler hearts aren’t very efficient for delivering fuel and oxygen to muscles during physical activity. As a result, since ancient times, most amphibians and reptiles have been ambush hunters rather than pursuers.

But other tetrapods continued to evolve as new opportunities arose for those with better speed and efficiency. Faster tetrapods could catch faster prey and escape from other predators. To get faster, they needed the ability to breathe while running, to pump more blood to oxygen-starved muscles and organs, and to extend their strides. It required the evolution of more erect postures.

Moving with a sprawling gait is like doing push-ups. Much of the energy is wasted in supporting the animal’s weight. But if the legs are set beneath the body, bones can support the load so the muscles can put more energy into balance and stride. While a few reptiles did evolve stances that were slightly more upright, it was the dinosaurs, birds and mammals that took the upright stance and ran with it.

After that, great speeds were possible. The new body plan allowed running, jumping, climbing and, ultimately, flying. Animals could move on four limbs, two limbs or both. Upright posture, better lungs and efficient four-chambered hearts proved to be the right recipe for bursts of biodiversity across the world’s continents.

From the Oct. 18-24, 2006, issue

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