Forty-seven million years ago, a pregnant mare drank from a lake that may have poisoned her with its deadly volcanic gases.
Now, her remarkably well-preserved body has been recovered by palaeontologists working in the Messel Pit, a former coal, in Darmstadt, Germany.
The fossilized remains of the mare and her unborn foal are providing scientists with new insights into these ancient creatures.
They reveal that, despite great differences in their size and shape, ancient horses had very similar reproduction to modern horses.
These ancient creatures, known as Eurohippus messelensism had four toes on each forefoot and three toes on each the hind foot, and it was about the size of a modern fox terrier.
‘Almost all of the bones of the foetus are still articulated in their original position. Only the skull is crushed,â€™ said Dr Jens Lorenz Franzen of the Senckenberg Research Institute, lead author of the study.
The specimen was discovered by a team from the Senckenberg Research Institute nearly 15 years ago, but its extent was not fully appreciated until it was studied using micro X-ray.
The micro X-ray analysis revealed a structure known as the broad ligament that connects the uterus to the backbone and helps support the developing foal.
Remnants of the wrinkled outer uterine wall became visible after the specimen was prepared, a feature shared between Eurohippus and modern horses.
The specimen was well-preserved due to the oil shales at Grube Messel, which have long been known for their intricate fossils.
These oil shales formed at the bottom of ancient Lake Messel and preserve the remains of mammals, birds, and other animals that were living near what is now Darmstadt, Germany about 47 million years ago.
No oxygen was present at the bottom of the lake when the dead animals sank down and finally became embedded in the muddy sediments.
There, anaerobic bacteria immediately began to decompose skin, muscles, and other soft tissues. As a result, the bacteria produced carbon dioxide, which in turn precipitated iron ions present in the lake water.
As a result, the organ was not fossilised directly, but is visible as a dark shadow left by bacteria that consumed the tissue and then were fossilised.
The size of the fetus and the presence of fully developed milk teeth indicate that it was close to term when it and its mother died.
But its position in the uterus indicates that the two did not die during the birthing process. The fetus was upside down rather than right side up, and its front legs were not yet extended as they should be just before birth.
This is only the second example of a fossil where the placenta can be identified, researchers claim.