BERLIN: The former oil shale mining site of Messel, near Frankfurt, Germany, is well known for its spectacular fossils of organisms that lived between 47 million and 48 million years ago, during the Eocene epoch. But a fossil of the early horse species Eurohippus messelensis, described at this year’s Society of Vertebrate Paleontology meeting in Berlin, stands out even in that illustrious company.

The tiny specimen—full grown, Eurohippus was about the size of a modern fox terrier--preserves a mare and her unborn foal (circled in the image above) in exquisite detail, with many of the bones in anatomical position. Also visible are parts of the uterus, including the placenta and the so-called broad ligament that attaches the uterus to the mare’s lumbar vertebrae and helps support the fetus. The soft tissue is not preserved directly, but as images formed by the petrification of bacteria that replaced the soft tissue when the animals died.

Comparing the fossil to the known phases of fetal development and birth in modern horses, Jens Lorenz Franzen of the Senckenberg Research Institute and his colleagues determined that the mare did not die during birth. The fetus was nearly at term when the pair died, but it was still facing upside down rather than having rotated into the right side up birth position.

The exact cause of death of the mare and foal is unknown. But like many of the animals at Messel, they may well have perished from asphyxiation when ancient Lake Messel belched up a cloud of noxious carbon dioxide gas, as it did from time to time as a result of volcanic activity.