48-million-year-old horse-like fetus found inside preserved uterus

An ancient find yields a new first: Inside of an equoid fossil found in Germany in 2000, scientists have uncovered not only the second fossil uterus ever found, but the oldest as well.

As reported in PLOS One, the fossil of this tiny pregnant horse relative (Eurohippus messelensis) was discovered in the Messel Pit, an UNESCO World Heritage Site that has provided us with unique information about mammalian evolution, thanks to the quality of fossils discovered there.

This fossil is no exception. The fetus—which is inside the white oval in the image above—is particularly well-preserved; nearly all the bones are present and connected, although the skull was crushed.

Fossils of pregnant horse-like species themselves are incredibly rare, and when taken into consideration with the fact that fewer than 2% of local finds involving fossilized mammals yielded more than a fragment of a single bone, this find is extraordinary.

A research team has now evaluated the bones and anatomy of the fossil using scanning electronic microscopy (SEM) and high-resolution micro-x-ray. Thanks to the preservation of the animals, the researchers have been able to reconstruct the original appearance and position of the fetus. Using this, they were able to estimate that the mare died late in her pregnancy, but before she could give birth. The mare then fell into a lake, where she and her fetus were preserved.

Lake is key player to this extraordinary find

The lake is the key player as to how the rarer bits of soft tissue became preserved. As the horse was decomposing inside the lake, bacteria feasting on the soft tissues released CO2. This CO2 reacted with iron in the water, forming Fe-carbonate. This mineral then coated the bacteria and hardened, forming a mat that matched the lines of the soft tissue.

The researchers discovered parts of this mat around the fetus, including preserved parts of the uteroplacenta and a uterine ligament. From what the researchers have seen, this uterine system within these 48-million-year-old horse relatives actually corresponds largely with those of living mares—meaning the reproductive system that horses have today was already highly developed at least as of 48 million years ago, and was greatly conserved until today.

—–

Feature Image: Franzen et al. 2015