The equine reproductive system can be considered “quite optimal,” one researcher said, as recent study results show that the system has undergone surprisingly little evolution over the past 48 million years.

Detailed exploration of a fossil of an Eocene-era pregnant mare has revealed uterine and placental structures that are almost identical to modern-day horses—even though the rest of the horse’s body has evolved significantly, said Christine Aurich, DVM, PhD, of the University of Vienna and the Graf Lehndorff Institute in Neustadt, Germany.

The fossil, first unearthed in 2000 in southwest Germany, contains the oldest and most well-preserved fetus of a primitive horse ancestor, Aurich said. The fact that the fetus was in a late stage of gestation and near term contributed to its preservation because its bones had become more solid. But it was also helpful that the mare had fallen into the prehistoric Lake Messel, an ancient lake that contained high levels of bacteria which had a preservation effect on soft tissues like muscles and membranes.

German researchers recently decided to examine the well-preserved fetus, mainly to better understand the evolution of equid reproduction, Aurich said. The latest technology—including scanning electron microscopy and high-resolution micro-X ray—allowed the scientists to get new glimpses of this fetus, which is partially hidden by the mare’s pelvic bones, that couldn’t have been achieved even 15 years ago.

They found that the structure of the uterus, the position of the fetus, the placental anatomy, and the membranes surrounding the fetus all appear to be v