In 2000, a team of scientists from the Senckenberg Research Institute in Germany discovered a fossil of an ancient horse at the Messel Pit, one of the most productive fossil sites in the world.

What makes the find stand out from other fossils discovered at the site is that it was the fossilized remains of an adult ancient horse called Eurohippus messelensis and her unborn foal.

In their study presented at the 2014 annual meeting of the Society of Vertebrate Paleontology in Berlin on Thursday, Nov. 6, Jens Lorenz Franzen, from the Senckenberg Research Institute, and colleagues said that the skeleton of the ancient mare is paired with an intact and similarly well-preserved fetus.

"Completely preserved skeletons of fossil horses are rare," said University of Florida curator of vertebrate paleontology Bruce MacFadden. "Usually, they're fragmented and the bones are all dissociated. If you find a skeleton with a preserved foal inside, that indicates exceptional preservation, which is normally not found in the fossil record."

The pregnant horse did not die because of childbirth, the researchers have determined. While the fetus was nearly at term at the time of the pair's death, it was still facing upside down and has not yet rotated to a birth position.

The researchers are not certain on the cause of death but the pregnant horse and her unborn baby are believed to have died because of toxic volcanic gases from a lake 47 million years ago. Like many of the ancient animals discovered at Messel, the pregnant mare may have died from suffocation when Lake Messel released poisonous carbon dioxide gas due to volcanic activity.

The discovery is giving scientists new insights on the reproduction of the now extinct species of horse. Although the ancient horse, which was about the size of a fox terrier, is way smaller than modern day horses, the researchers found reproductive similarities between them, something that came as a surprise given their differences in size and anatomy.

X-ray analysis showed the horse's broad ligament, which connects its uterus to the backbone and support the unborn foal. The X-ray also revealed traces of the mare's crumpled outer uterine wall, which still exists in present day horses.

The researchers have also found that while ancient horses were smaller and toed (they had four toes on their front feet and three on the rear feet), the animal's basic biological structure has not significantly changed over the past 46 million years.

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