Strange Animals Reveal Bizarre Past of Horses: Photos
The family tree of horses just took an unexpected turn, as researchers have found horses are related to the strangest animals ever discovered.
Horses are related to a mysterious group of animals that noted British naturalist Charles Darwin called the "strangest animals ever discovered." New research in the journal Nature sheds light on the still-puzzling evolutionary past of these animals: South America's native hooved species, which lived from around 60 million years ago to 10,000 years ago. "Darwin's description of Toxodon as one of the strangest animals ever discovered was probably because of the range of animals that its skeleton had similarities to," co-author Ian Barnes of the Natural History Museum told Discovery News. Darwin and Victorian anatomist Richard Owen scratched their heads over this beast, as its remains had resemblances to rodents, pigs, elephants, hippos, whales, sloths and even armadillos. The new study found that Toxodon is actually related to mammals like horses, rather than to elephants and other species with ancient evolutionary ties to Africa.
Image: Illustration of Toxodon by Peter Schouten from the forthcoming book "Biggest, Fiercest, Strangest"
Macrauchenia
is yet another early South American hooved animal that researchers found had ties to today's horses. Since horses are also related to living tapirs and rhinos,
Macrauchenia
shares those connections as well. The researchers describe this animal as looking like a humpless camel with a snout. It lived around 7 million years ago, while Toxodon came on the scene much later at 2.5 million years ago.
Image: Illustration of Macrauchenia by Peter Schouten from the forthcoming book "Biggest, Fiercest, Strangest"
Researchers now believe that most horse evolution took place in the Americas. While the precise origin of the South American hooved animals is still unknown, scientists now suspect that the animals traveled down from North America more than 60 million years ago, probably just after the mass extinction that killed off non-avian dinosaurs and many other animals. Today's horses, tapirs and rhinos might have relatives in Europe too. Remains of
Hyracotherium vasacciensis
, for example, were unearthed in London. This four-legged species is thought to have been a palaeothere, which refers to a family of plant-eating creatures close to the bottom of the horse family tree. They lived in forested areas and feasted on soft leaves, shoots, berries and other plant materials.
Image: Skeleton of
Hyracotherium vasacciensis
at the National Museum of Natural History
The
Orohippus
resemblance to horses is more evident. This animal, also called a "mountain horse," lived about 50 million years ago. It could feast on tougher plants than
Hyracotherium
could, based on analysis of its teeth.
Image: Reconstruction of
Orohippus
Epihippus
, represented by this fossilized jaw, is thought to have descended from
Orohippus
. (If you see the term "hippus" in a word, it generally refers to members of the horse/rhino/tapir family.) As you can see from the teeth of
Epihippus
, this animal could efficiently grind its food.
Epihippus
lived about 46 to 38 million years ago.
Image: Fossilized jaw of
Epihippus
More complete remains have been found for
Mesohippus
, yet another horse ancestor that lived about 30 to 40 million years ago. It was once common in North America. Researchers hoping to learn more about such ancient animals must tease out information from the remaining fossils. For the study on the "strangest animals ever discovered," scientists from the Natural History Museum, the American Museum of Natural History and the University of York joined forces to extract and then analyze collagen from the bones of
Toxodon
and
Macrauchenia
. Could such a study be applied to other prehistoric animals, like dinosaurs? "I think it would be difficult to get the level of resolution we have in this study for dinosaurs, as they lived too long ago," Barnes said. Nevertheless, as new technologies develop, present obstacles might eventually be overcome. William Murphy, a professor in the Department of Veterinary Integrative Biosciences at Texas A&M University, told Discovery News that the new study " is very convincing, using ancient protein sequencing to answer one of the most important and outstanding questions in higher level mammal phylogeny."
Image: Skeleton of
Mesohippus
Hyracodon
, which lived 32 to 26 million years ago, had a pony-like build, revealing its evolutionary connection to horses. Its teeth were more like those of later rhinos, though. It also had a short, broad snout.
Hyracodon
inhabited open forests and other wooded regions, where it could browse on foliage and graze on grass. Its anatomy suggests that it was an incredibly fast runner. Unfortunately, this species died out without leaving any descendants, marking the end of the horse family tree branch of hornless, running rhino-resembling animals.
Image: Reconstruction of
Hyracodon
The word
Onohippidium
derives from "onos" (donkey) and "hippidium" (small horse), and that's pretty much what this animal looked like. It was an extinct genus of horse that lived in the Americas about 12,000 years ago at the end of the last Ice Age. It feasted on trees and grass.
Image: Recreation of
Onohippidium
Tapirs, such as the Malayan tapir shown here, look like pigs with trunks. In fact, they are related to horses and rhinos. Perhaps they, among all living animals on the horse family tree, best live up to Darwin's adjective of "strange." Tapirs are critical to ecosystems, though, and now require protection. All four known tapir species are endangered or threatened, primarily because of habitat loss and hunting.
Photo: Malayan tapir at the San Diego Zoo
Modern horses often have their own quirky features. Icelandic horses, for example, sport a human-like mane of hair on the top of their heads. It is there for protection and added warmth, given the often cold climate where the animals live. As for whether or not these and other living horses look much like their ancient ancestors
Toxodon
and
Macrauchenia
, Barnes said, "Not much!" He added: "Horses are closer to tapirs and rhinos than they are to
Toxodon
and
Macrauchenia
, and there's not a lot of similarity between those first animals unless you know what to look for. One of the next important jobs to do is to go back and check all of the anatomical data we have for these species and see if any patterns emerge."
Photo: Icelandic horses at Borgarnes, Iceland