The reproductive system of horses has barely changed at all over the last 48 million years, according to a new study which analyzed the oldest-known fossil of a fetus belonging to a distant horse ancestor.
The discovery of the fetus and its mother was made at the Grube Messel, a UNESCO World Heritage Site located near Frankfurt, Germany. The mare was first discovered in 2000 but scientists have only now concluded their study of the fetus. Their report was published on Thursday in the open-access science journal PLOS ONE.
The horse-like creature was classified as belonging to Eurohippus messelensis, a now-extinct genus of equids—the taxonomic family to which horses, donkeys and zebras belong. The horse is thought to date back to the Eocene Epoch, which lasted from about 56 million to 34 million years ago.
The fetus, which scientists believe to be the oldest of its kind, measures around 12 centimeters (4.9 inches) in length, with almost all of its bones present apart from the skull, that seems to have been crushed. 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.
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 discovery suggests that the reproductive system in horses was one of the earliest features to develop into its modern form, according to Jens Lorenz Franzen, a paleontologist at the Senckenberg Natural History Museum in Frankfurt and the paper's lead author.
Franzen says that horses of the Eocene Epoch would have had four hooves on each forelimb and three on each hindlimb, making a total of 14 hooves. In the 48 million-year-period since then, horses have evolved so much that they now only have four hooves, whereas the reproductive system remains largely unchanged.
"You can imagine the amount of evolution which took place...during those 48 million years but the [reproductive] system, in so far as we know it now, was already present at that time," says Franzen. He suggests the reproductive system may have developed during the Paleocene Epoch—between 66 million and 56 million years ago—or even during the time of the dinosaurs in the Mesozoic Era, which lasted from about 252 million to 66 million years ago.
The original discovery was made by a team from the Senckenberg Museum. According to UNESCO, the Grube Messel is the world's most important fossil site for understanding the Paleogene Period—which includes the Eocene Epoch and stretches from 66 to 23 million years ago—that is also the period when the first modern mammals emerged. Most mammals are placental, meaning that they bear live offspring that are nourished prior to birth in the mother's uterus via the placenta. Some 4,000 species are placental mammals, including horses, dogs and humans.
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