In other words, horse evolution was never as straightforward as Marsh's smooth fossil sequence suggested.
Kathleen Hunt, a biologist at the University of Washington in Seattle, said the modern-day horse is "merely one twig on a once flourishing bush of equine species. We only have the illusion of straight-line evolution because Equus is the only twig that survived."
- Remains Show Ancient Horses Were Hunted for Their Meat
- In Crocodile Evolution, the Bite Came Before the Body
- Evolution's "High Priest" Returns With New "Tale"
- Evolution and Religion Can Coexist, Scientists Say
- "Evolving to Eat Mush": How Meat Changed Our Bodies
- Fins to Limbs: New Fossil Gives Evolution Insight
MacFadden, the study author, agrees that horse evolution was, in fact, a pretty messy affair—a jumble of evolutionary processes such as random genetic variation and natural selection.
"Any changes in morphology [physical form and function], such as in tooth or limb evolution, can be explained within this framework," he said. "Equine mammals are adaptable critters whose size, diet, and range depended on geography and climate."
Horse teeth, which preserve well as fossils, provide evidence to support this. For instance, when grassland habitats became far more extensive around 20 million years ago, horses with teeth that were adapted to browsing forest vegetation declined. But horses with high-crowned, or tall, teeth for grazing on grasslands flourished.
"Grasses are highly abrasive and tend to increase tooth wear," MacFadden said. "A higher-crowned tooth is better adapted to feeding on grasses." Horses living today have such teeth.
Horse Diet
Chemical analysis by MacFadden to determine animal diet and the shape of later fossil teeth, however, indicates that some species went back to eating leaves as well as grasses.
Likewise, ideas about horse body size have also changed. The popular notion that horses started off the size of small dogs and grew progressively bigger is now shown to be false.
From the tooth-fossil evidence, MacFadden found that during an explosion in horse diversity some 20 million years ago, many species got smaller as well as larger. He was able to estimate their body size because teeth are proportional to the sizes of the horses the teeth once belonged to.
These findings contradict what is known as Cope's Law, an idea based on the work of 19th-century paleontologist Edward Drinker Cope. The law states that within any group of animals there is a tendency for descendants to grow progressively larger.
"There are so many exceptions where you go from small to large and back to small again that you have to ask how many exceptions to the rule you can accept before the central concept is no longer correct," MacFadden said.
John Flynn, mammal-fossil curator at New York City's American Museum of Natural History, said MacFadden's findings are important because horses have been one of the mainstays of evolutionary studies.
And MacFadden himself says there is so much more to learn from these textbook examples of evolution.
Don't Miss a Discovery
Sign up our free newsletter. Every two weeks we'll send you our top news by e-mail (see sample).
|
SOURCES AND RELATED WEB SITES
|
