Dark Banquet - Bill Schutt [13]
Even after establishing a link between vampires and vampyres, questions remain about the real-life creatures—questions that have puzzled and intrigued those of us who study them: How did blood feeding evolve in vampire bats? And why (among twenty thousand species of terrestrial vertebrates) is obligate vampirism confined to only three, closely related New World bats?
First of all, as far as the origin of vampire bats is concerned, the fossil record (so important in detailing the life histories of many prehistoric creatures) is no help here. Although there are several species of fossil vampire bats (including a supersized version, the wonderfully named Desmodus draculae), these bats are clearly vampires, not transitional forms that might shed light on their previous feeding habits. Paleontologists get all tingly at the very mention of transitional forms. But to better understand them, let’s leave vampire bats for a minute to examine what is arguably the most famous of these transitions—one that beautifully illustrates the evolutionary changes that led to the modern horse.
Using the combined results of both classical and modern studies, vertebrate paleontologists have been able to correlate gradual changes in the skull, teeth, and limbs of horse ancestors with environmental changes that took place on the North American continent starting some fifty million years ago (during the early Eocene epoch). One of the groups that evolved to fill the niches left open by the dinosaurs was a rather diverse assemblage of mammals called the Perissodactyla (odd-toed ungulates).*20 Within this group, which also included the ancestors of rhinos and tapirs, was Hyracotherium, a fox-sized creature that inhabited the extensive forests that covered much of the region. With short legs and eyes set in back of a short snout, Hyracotherium was well adapted for a life spent hiding in the underbrush and browsing on soft, leafy plants and fruit.
Starting around twenty-five million years ago (as indicated by clues such as changes in fossil plant species and their seeds), the climate in North America gradually became drier. Forests dwindled and grasslands spread. Some of the small browsing mammals went extinct (as did many other forest types), but others survived, mainly because they evolved adaptations for coping with their new environments. For example, higher crowned (i.e., longer) teeth enabled these mammals to deal with the constant wear and tear of eating the tough, silica-laden†21 grass that had replaced the soft leaves and shoots popular with forest diners.
With less plant cover in which to hide, longer limbs became important for moving quickly over open ground. Basically there are only two ways to augment running speed: by increasing stride frequency and by increasing stride length. Longer limbs contributed to the latter since each time the limb moves forward during a stride, more ground is covered. As the limbs lengthened, toes that were once on the ground either disappeared or remained as vestiges, like the splint bones found in the front legs of the modern horse, Equus cabalis.*22
Protohorse skulls became longer as well, with the eyes set farther back from the mouth. Longer snouts (rostrums) allowed these creatures to graze while simultaneously watching for predators.
In addition to looking more and more like modern horses, these ungulates became extremely diverse—with up to fifteen North American species living at the same time (around ten million years ago). For whatever reasons, though, by roughly five million years ago, only the modern horse remained, spreading into Asia and Europe across a land bridge that spanned what is now the Bering Strait (separating Russia from Alaska). By about thirteen thousand years ago, climate changes, humans, or perhaps, as hypothesized by American Museum of Natural History Curator of Mammalogy Ross MacPhee, a rabieslike hyperdisease drove many large North American mammals to extinction.*23