Oldest fetus found in 48-mln-year-old horse-like fossil

Despite the fetus’ skull having been crushed, the researchers were able to reconstruct its original position using most of its bones present at the site.

The horse’s, which is termed Eurohippus messelensis, uterus is very similar to the uterus of horses today but on the whole they were quite unlike the modern horses.

Jens Lorenz Franzen from Senckenberg Research Institute Frankfurt and colleagues evaluated the bones and anatomy and used scanning electronic microscopy (SEM) and high-resolution micro-x-ray to describe the approximately 12.5 cm foetus. Their report was published on Thursday in the open-access science journal PLOS ONE. The researchers believe that the mare died shortly before it could give birth, but that the death of the mother was unlikely related to childbearing. 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. The ancient horse in question was about the size of a fox terrier.

Significantly, soft tissue in the mother and fetus has been preserved, which the researchers believe is the placental tissue that supplied the fetus with nutrients from the mother’s uterus, as well as a broad uterine ligament that connected the uterus to the vertebrae and the pelvis. The observable details correspond largely with living mares, which lead the authors to posit that the reproductive system was already highly developed during the Paleocene, and possibly even earlier.

Grube Messel is a former shale quarry that is famous for its complete vertebrate skeletons. Now that they’ve properly examined them, however, they say they date back to 48 million years ago. This dark spot was consistent with other soft body tissue preserved in specimens from the same dig site. Surprisingly, scientists found the fetus as well as a few of the tissues from the animal’s placenta and uterus in good condition.

As a result, the bacteria produced carbon dioxide, which in turn precipitated iron ions present in the lake water. Consequently, Messel fossils preserve these remains not directly, but as images.

The Senckenberg Research Institute in Frankfurt and Naturhistorisches Museum in Basel, Switzerland, teamed up for the joint study, according to a press release. Aurich is with the University of Veterinary Medicine in Vienna.

Franzen said that the Eurohippus messelensis specimen is the oldest and best preserved primitive equoid fetus they have found.

The full study can be read here.