47 Million Years Ago, Horses Were About the Size of a Labrador Dog

Window into the 47 million year old ecosystem of the Geiseltal fossil locality with the small-sized horse-ancestor Propalaeotherium on the left, the ancient tapir Lophiodon in the middle, and a young terrestrial crocodile Bergisuchus in the background.
Credit: Márton Szabó

Researchers open a window onto ancient mammal evolution using fossils from Germany.

The former coalfield of Geiseltal in Saxony-Anhalt has yielded large numbers of exceptionally preserved fossil animals, giving paleontologists a unique window into the evolution of mammals 47 million years ago. A team led by the University of Tübingen and the Martin Luther University Halle-Wittenberg (MLU) has shown that the body size of two species of mammals developed in opposite directions. The study was published in Scientific Reports.

47 million years ago — the middle Eocene — the Earth was much warmer and the area of Geiseltal was a swampy subtropical forest whose inhabitants included ancestors of the horse, ancient tapirs, large terrestrial crocodiles, as well as giant tortoises, lizards, and ground-dwelling birds. So rich are the Geiseltal finds that they give researchers an unprecedented high-resolution picture of evolutionary dynamics at the population level.

A team led by Dr. Márton Rabi from the University of Tübingen and the Martin Luther University Halle-Wittenberg (MLU) has shown that the body size of two species of mammals developed in opposite directions. The study, published in Scientific Reports, was carried out with Simon Ring and Professor Hervé Bocherens at the Senckenberg Centre for Human Evolution and Palaeoenvironment and the University of Tübingen in cooperation with Dr. Oliver Wings from the MLU.

“We were initially interested in the evolution of the ancient horses, which were about the size of a Labrador dog. These animals are particularly abundant in the Geiseltal fossil record,” Rabi says. Researchers initially believed they had several species of early horse. “However, we found that here, there was only one species, whose body size shrank significantly with time,” Rabi explains. The team wanted to test whether this body size shift was climate-induced, since past global warming caused body-size reduction in ancient mammals.

Carbon and oxygen isotope studies on fossil teeth provided the scientists with information about the local middle Eocene climate. “They indicate a humid tropical climate. However, we didn’t find any evidence for climatic changes in Geiseltal over the period investigated,” says Bocherens. To further test the data, the team sought to discover whether the dwarfing process was unique to the horses. For comparison, they examined the evolution of the tapir ancestor called Lophiodon. “We had reason to question the Geiseltal’s constant-climate data; so we expected that other mammals would show the same body-size trends as the horses,” Simon Ring explains. In a surprising result, the tapirs — also a single species — revealed the opposite trend. They grew larger instead of shrinking. While the ancestors of the horse shrank from an average body weight of 39 kilograms to around 26 kilograms over about a million years, the tapirs increased from 124 kilograms to an average body weight of 223 kilograms.

Differing survival strategies

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