Heyday 



Horses 



by Bruce J. MacFadden 



North America is the ancestral home of 

 horses, and many fossil sites across the 

 continent contain abundant remains of an- 

 cient members of the family. During the 

 past fifteen years, my colleagues and I 

 have excavated fossil horses at Thomas 

 Farm, a site in Florida that some eighteen 

 million years ago was a sinkhole and per- 

 fect trap for animal remains. In addition to 

 such long-vanished creatures as extinct 

 rhinoceroses and bear-dogs, we have un- 

 earthed thousands of teeth and bones of 

 fossil horses. 



Three different kinds of fossil horses are 

 found at Thomas Farm, but by far the most 

 common we have encountered is Parahip- 

 pus — "side-toed horse," so named be- 

 cause of the toes flanking either side of the 

 central digit. About the size of a small 

 white-tailed deer or pronghom, Parahip- 

 pus probably lived in small bands, or 

 harems (as do many modem horses in the 

 wild), consisting of a dominant male, sev- 

 eral females, and juveniles. It may have 

 inhabited both woodlands and grasslands 

 and fed on leaves from trees and shrubs, as 

 well as on grasses. Thus, in its social struc- 

 ture, habitats, and diet, this early horse 

 combined characteristics of primitive, an- 

 cestral horses with more modem traits. 

 With its hoof — and toes — in two worlds, 

 Parahippus stood on the verge of the great 

 heyday of horses during the Miocene. 



At any given fossil locality in North 

 America from about fifty-five to twenty- 

 five million years ago, we usually can find 

 two to four species of horses that presum- 

 ably lived side by side. Thereafter, from 

 about twenty to ten million years ago, 

 horses evolved rapidly and adapted to var- 

 ious environments and ways of life. Horse 

 diversity increased so dramatically that at 

 some fossil sites from fifteen million years 



ago as many as a dozen species can be 

 found. Today, the world's horses (and their 

 relatives the zebras, asses, and onagers) 

 are reduced to the single genus Eqmis, 

 whose wild members live only in parts of 

 Asia and Africa. All are powerful runners 

 and feed predominantly on grass. Such 

 uniformity contrasts starkly with fossil 

 horses, a group with a rich fifty-five-mil- 

 lion-year-old history, represented by some 

 three dozen extinct genera. 



The coexistence of so many species of 

 similar ancestry and general adaptive 

 traits in the same ecosystems suggests 

 that, as in modern-day communities, 

 horses divided up the niches and resources 

 available to them. Before about twenty 

 million years ago, most horses were pre- 

 dominantly browsers, feeding on leaves of 

 trees, bushes, and low-lying shrubs and 

 supplementing this diet with whatever soft 

 vegetation formed the local ground cover. 

 This appears to have been the main feed- 

 ing strategy not only of primitive horses 

 but also of most other herbivorous mam- 

 mals of the time. With rapid diversifica- 

 tion, however, this feeding strategy 

 changed. We know this because about 

 twenty million years ago, fossil horse 

 teeth changed dramatically. 



Today, for example, many browsers, 

 such as giraffes and camels in Africa, have 

 relatively short-crowned teeth; in contrast, 

 all grazers, or mammals whose diet con- 

 sists predominantly of grasses, have tall 

 teeth. These high-crowned teeth evolved 

 to crop and process grasses, an adaptation 

 that has immediate and long-term "costs." 

 Grasses contain abrasive compounds 

 called phytoliths (microscopic, elongated 

 structures with the same chemical compo- 

 sition as glass, SiOj). When grazers eat 

 grasses, they acquire nutrients, but the 

 phytoliths cause much more wear on the 

 grinding teeth than do grit-free leaves and 

 the softer vegetation favored by browsers. 



During the Miocene, several groups of 

 horses throughout the Northern Hemi- 

 sphere evolved high-crowned teeth at 

 about the same time. Based on our knowl- 

 edge of modem herbivores and the nature 

 of grasses, the acquisition of these new, 

 taller teeth suggests that Miocene horses 

 were becoming predominantly grazers. 

 Recent chemical analyses of Miocene 

 horse teeth indicate that during that time 

 most grasses were primitive, photosynthe- 

 sizing carbon in the manner of trees and 

 shrubs rather than the way modem tem- 

 perate and tropical grasses do. Thus, coex- 

 isting horse species of the Miocene not 

 only divided up the available browse more 



narrowly but also began to exploit the 

 grasslands and savannas that were becom- 

 ing more widespread. 



The addition of grass to ancient horses' 

 diets, and the possibility of grazing rather 

 than browsing as a way of life, are re- 

 flected in horse biology. Fossil horses have 

 been thought to exemplify Cope's Rule (in 

 which an increase in body size over time 

 results in descendant species being larger, 

 on average, than their ancestors). The dog- 

 sized eohippus (the dawn horse more 

 properly known as Hymcotherium), the 

 smallest and oldest member of the horse 

 family, lived at the far end of the spectrum, 

 fifty-five million years ago. The large 

 modem-day Eqiius species is at the other 

 end, with a gradual continuum of horses of 

 increasing size between the extremes. Re- 

 cent work has shown this evolutionary pat- 

 tem to be grossly oversimplified, if not in- 

 correct, for fossil horses. In the first half of 

 their evolutionary history, horses changed 

 very little in body size. Then, during the 

 Miocene, they diversified rapidly to in- 

 clude large species and even a few dwarf 

 lineages. Browsing and grazing species of 

 this period ran the size gamut. Thus, 

 Miocene horses appear to have minimized 

 competition for the available food and 

 space by occupying slightly different 

 niches. 



During the Miocene, a major adaptive 

 shift in horse locomotion occuned. In gen- 

 eral, fossil horse limbs lengthened, and 

 side toes were reduced and ultimately lost. 

 While the evolutionary advantages of hav- 

 ing a foot with one rather than three toes 

 are not clear, the classic interpretation for 

 limb elongation holds that it allowed 

 horses to better escape fast-mnning preda- 

 tors. Another factor may also have favored 

 longer limbs. The late Miocene, from 

 about ten to five million years ago, was a 

 time of great climate shifts and increased 

 seasonality — more defined dry versus 

 rainy and warm versus cold cycles. The 

 ability to travel longer distances may have 

 enabled horses to migrate hundreds of 

 miles to take advantage of local plant 

 foods available at certain times of the year, 

 as do zebras and wildebeests in Africa 

 today. About five million years ago, horses 

 also evolved functional locking mecha- 

 nisms of the forelimbs and hind limbs, al- 

 lowing them to stand for long periods 

 without great fatigue (modem horses can 

 stand for about eighteen to twenty hours a 

 day). These physiological changes en- 

 dowed some groups of Miocene horses 

 with great speed and stamina. 



While behavior does not fossUize, body 



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