Horse evolution, new study

This video says about itself:

7 August 2015

The “evolution of the horse” occurred over a period of 50 million years, transforming the small, dog-sized, forest-dwelling “Eohippus” into the modern horse. Paleozoologists have been able to piece together a more complete outline of the evolutionary lineage of the modern horse than of any other animal.

The horse belongs to the order Perissodactyla, the members of which all share hooved feet and an odd number of toes on each foot, as well as mobile upper lips and a similar tooth structure. This means that horses share a common ancestry with tapirs and rhinoceroses. The perissodactyls arose in the late Paleocene, less than 10 million years after the Cretaceous–Paleogene extinction event. This group of animals appears to have been originally specialized for life in tropical forests, but whereas tapirs and, to some extent, rhinoceroses, retained their jungle specializations, modern horses are adapted to life on drier land, in the much harsher climatic conditions of the steppes. Other species of “Equus” are adapted to a variety of intermediate conditions.

The early ancestors of the modern horse walked on several spread-out toes, an accommodation to life spent walking on the soft, moist grounds of primeval forests. As grass species began to appear and flourish, the equids’ diets shifted from foliage to grasses, leading to larger and more durable teeth. At the same time, as the steppes began to appear, the horse’s predecessors needed to be capable of greater speeds to outrun predators. This was attained through the lengthening of limbs and the lifting of some toes from the ground in such a way that the weight of the body was gradually placed on one of the longest toes, the third.

From Science News:

Horse evolution bucks evolutionary theory

Speciation events not accompanied by big changes in teeth and body size

BY RACHEL EHRENBERG, 2:00PM, FEBRUARY 9, 2017

A cautionary tale in evolutionary theory is coming straight from the horse’s mouth. When ancient horses diversified into new species, those bursts of evolution weren’t accompanied by drastic changes to horse teeth, as scientists have long thought.

A new evolutionary tree of horses reveals three periods when several new species emerged, scientists report in the Feb. 10 Science. The researchers found that changes in teeth morphology and body size didn’t change very much during these periods of rapid speciation.

“This knocks traditional notions that rapid diversification of new species comes with morphological diversification as well,” says paleontologist Bruce MacFadden of the University of Florida in Gainesville. “This is a very sophisticated and important paper.”

The emergence of several new species in a relatively short time is often accompanied by the evolution of special new traits. Classic notions of evolution say that these traits — such as longer teeth with extensive enamel — are adaptive, enabling an animal to succeed in a particular environment. In horses, the evolution of such teeth might permit a shift from browsing on leafy, shrubby trees to grazing on grasses in open spaces with windblown dust and grit.

“You can’t live on a grassland as a grazer and have short teeth,” says MacFadden, an expert in horse evolution. “You’ll wear your teeth down and that’s not a recipe for success as a species.”

Similarly, a big change in body size can indicate a move to a new environment. Animals that live in forests tend to be smaller and more solitary than the larger herd animals that live in open grasslands.

Paleontologist Juan Cantalapiedra and colleagues compiled decades of previous work to create an evolutionary tree of 138 horse species (seven of which exist today), spanning roughly 18 million years. The tree reveals three major branchings of new species: a North American burst between 15 million and 18 million years ago, and two bursts coinciding with dispersals into Eurasia about 11 million and 4.5 million years ago.

The researchers expected to see evidence of an “adaptive radiation,” major changes in teeth and body size that allowed the new horse species to succeed. But rates of body size evolution didn’t differ much in sections of the family tree with low and high speciation rates. And rates of change in tooth characteristics were actually lower in sections of the tree with fast speciation rates, the team reports.

“It’s very tempting to see some change in body size, for example, and say, ‘Oh, that’s adaptive radiation,’” says Cantalapiedra, of the Leibniz Institute for Evolution and Biodiversity Science at the Museum für Naturkunde in Berlin. “But that’s not what we see.”

Cantalapiedra and his collaborators speculate that during the periods of rapid speciation, the environment was so expansive and productive that there just wasn’t a lot of competition to drive the evolution of adaptive traits. Perhaps, for example, North American grasslands were so rich and dense that there was enough energy for various species to evolve without having to develop traits that gave them an edge.

That scenario might be special to horses, says MacFadden, but it might not. Similarly, classic adaptive radiation scenarios might be true in many cases, but as this work shows, not always.