Ancient American Horses
Horses were introduced into the Americas during the 16th Century by the Spanish Conquistadors. Escapees from the Spanish stocks thrived and quickly established large wild populations and the absence of native herds these "noble animals" before the colonization of the New World and the horse's success thereafter were seen by some Europeans as one more bit of evidence that the American fauna was inferior to that of the Old World (1).
The Old World pedigree of the horse was first challenged by the discovery of fossil teeth in South America which had been collected by a young Charles Darwin and examined by Richard Owen in 1840 (2). There had been scattered reports of fossil horse teeth from the United States, but these were either dismissed or ignored by the European scientific community.
In 1848, Joseph Leidy settled the question of ancient horses in North American by publishing a systematic examination of Pleistocene (Ice Age) horse fossils from the collections at the Academy of Natural Science of Philadelphia and elsewhere (3). He concluded that there were at least two North American horses species and that both were different from Old World species (4).Top view of four molars of Equus occidentalis. This image is a detail from a lithograph in Extinct Fauna of the Western Territories (1873).
The implication of ancient horses in the Americas was significant. After all, if wild horses were present during the Pleistocene (Ice Age) and thrived once they were re-introduced by the Spaniards, why were they missing when the Spaniards arrived? In 1848 Leidy wrote:
"For it is very remarkable that the genus Equus should have so entirely passed away from the vast pastures of the western world, in after ages to be replaced by a foreign species to which the country has proved so well adapted: and it is impossible, in the present state of our knowledge, to conceive what could have been the circumstances which have been so universally destructive to the genus upon one continent, and so partial in its influence upon the other." (5).
In later years Leidy had the opportunity to examine and describe other fossil horses. Some were the same or closely related to the Pleistocene species he wrote about in 1848. Others were much older. The most notable of these were the Pliocene horses from the Niobrara River area of Nebraska and the Miocene horses from the White River Badlands of South Dakota. By the end of his career Leidy had described and named several new species of horses belonging to Equus, Anchippus, Anchitherium, Hipparion, Hippodon, Merychippus, Parahippus and Protohippus (6).Top view of the molar teeth of Protohippus perditus (top) and Merychippus mirabilis (bottom). These images are details from a lithograph published in On the Extinct Mammalian Fauna of Dakota and Nebraska (1869).
Leidy provided detailed descriptions of these fossil horses and compared the anatomies of the different species. He noted patterns of increased crown height and structural complexity in horses teeth. Although most of the material at his disposal were teeth or jaw fragments, he was also aware that some ancient horses such as Hipparion had three toes rather than one. However, Leidy —ever reluctant to theorize— never published on the evolutionary relationships of horses.
That task would be acomplished by O. C. Marsh a few years after Leidy had abondoned paleontology. Marsh was guilty of oversimplification, (7) but his sequence of transitional fossils was a stunning achievement instrumental to the acceptance of Evolution among scientists and the public.
The honors assocated with the elucidation of horse evolution would rightly go to Marsh, but it was also Leidy who alerted the world to the riches in fossil horses to be found in North America. In his 1869 monograph, On the Extinct Mammalia of Dakota and Nebraska Leidy took delight in noting that all of the eight genera of fossil horses then known to science could be found in North America while only three were known from the Old World. In addition he commented:
"The Solidungulates [horses and their close relatives] are better represented than in the recent fauna of any part of the world, and indeed the Pliocene deposits of the Niobrara would appear to indicate that the North American Continent was at one time emphatically the country of Horses."
- Florida State Museum's Fossil Horse Cybermuseum: www.flmnh.ufl.edu/natsci/vertpaleo/fhc/firstCM.htm
- Kathleen Hunt's web page on Horse Evolution:
- The idea of the inferiority of the New World's fauna was shared by many Europeans, including some scientists. The formalized by the great French scientist de Buffon (1707-1788) during the 1760s. He theorized that the lack of superior animals such as the horse, lion or elephant in the New World was caused by its harsher climate. Buffon de-emphasized the theory later in his long and productive career, but many Europeans continued to perceive an inferiority in the American fauna; a notion readily reinforced by comparisions with the Eurasian and African faunae. Fossil discoveries by Leidy, O.C. Marsh, E.D. Cope, among others, thoroughly discredited any notion of Old World monopoly on impressive animals. [go back]
- Charles Darwin collected the fossil horse teeth during his voyage on the Beagle. He forwarded the fossils to Richard Owen, the great British paleontologist and anatomist who first coined the word "Dinosauria". Owens concluded that the South American horse was distinct from the Old World species and named it Equus curvidens. [go back]
- On the fossil horses of America. Proceedings of the Academy of Natural Sciences of Philadelphia 3(11): 262-266. [go back]
- Leidy identified two species. The first was the same species named by Richard Owen, Equus curvidens. The second was a new species which Leidy first named Equus americanus. Ten years later, Leidy discovered that the name "americanus" had already been used for another South American fossil horse, so he renamed his species Equus complicatus. [go back]
- Leidy wondered why the horse become extinct in the Americas when its successful reintroduction by the Spaniards demonstrated that the New World was a favorable environment. Darwin asked the same question (in nearly the same way) 11 years later. Today's scientists still grapple with this question. Most consider climatic change, over-hunting by newly arrived humans or a combination of the two as the most plausible explanations for the extinction of the American horse (and other large mammals) at the end of the Pleistocene. [go back]
Equus (EH-kwis) = horse
Anchihippus (ANK-ee-HIP-us) = near or close to horse
Anchitherium (ANK-ee-THEER-ee-um) = near or close to beast
Hipparion (hih-PAIR-ee-on) = pony
Hippodon (HIP-oh-don) = horse tooth
Merychippus (MARE-ee-HIP-us) = ruminant-like horse
Parahippus (PAR-ah-HIP-us) = near horse
Protohippus (PRO-toe-HIP-us) = first horse
- O. C. Marsh, with considerable encouragement from Thomas Huxley, presented the story of horse evolution as a simple, linear progression from a small forest mammal with generalized teeth and multi-toed feet to the large and fleet horse with single-toed feet and teeth specialized for feeding in the open grassland.
This scenario reinforced the widespread perception of progressive evolution. Although it was first proposed as a demonstration of Darwin's Theory of Natural Selection, this linear progression later prompted some notable paleontologists, including Henry Fairfield Osborn and William B. Scott, to propose orthogenesis (innately directed evolution) as more plausible than Darwin's natural selection in explaining evolution. It wasn't until the middle of the 20th century, George Gaylord Simpson demonstrated a more complicated and considerably less progressive history of horse evolution, that Darwinian natural selection became the central mechanism for evolution. [go back]