Oldest pregnant horselike fossil found

48-million-year-old skeleton shows modern horse reproductive system developed earlier than thought

horse fetus fossil

IN FOAL  The oldest known skeleton of a pregnant Eurohippus messelensis mare holds an almost fully grown fetus (circled). The petite mare weighed around 5 kilograms, some 100 times less than horses of today. 

Franzen et al/PLOS ONE 2015, Sven Tränkner/Senckenberg Forschungsinstitut Frankfurt (photo)

A confluence of unlikely events led to the exquisite preservation of the oldest fossil of a primitive pregnant horse ever discovered.

Some 48 million years ago, an equoid mare (Eurohippus messelensis) and her almost fully gestated fetus fell into an ancient lake in the area of Grube Messel near Frankfurt, Germany. The lake had just the right mix of bacteria to preserve even soft tissue from the mare’s uterus and placenta. She lay at the bottom of the lake — uneaten by predators — until discovered in 2000.

The unusually detailed fossil, which dates from the Eocene epoch, provides a window into the evolution of modern placental reproductive systems, researchers report October 7 in PLOS ONE.

“This is the oldest known specimen of a pregnant mammal ever found,” says study coauthor Christine Aurich, a reproductive veterinarian at the University of Veterinary Medicine in Vienna, Austria. “Usually the fetus is not well preserved, but here, it is, and most of the mare’s bones were intact.”

The fossil’s reproductive tract looks much like that found in horses today, the researchers say. The placenta is only loosely attached to the uterine wall, allowing a mare to stand and move soon after birth without the risk of excess bleeding.

The discovery shows that horses had evolved this feature earlier than previously thought, says Aurich. The fossil’s reproductive anatomy is so advanced that researchers believe it would have taken a long time to develop. That means it could have arisen during the Mesozoic era, as early as 250 million years ago, Aurich says. “We didn’t know it went back that far,” she says. Researchers had earlier found a 44-million-year-old fossil of a pregnant horse, but the reproductive tract was not as well preserved, says Aurich, so it was difficult to draw firm conclusions.

The similarities in uterine anatomy and fetal development between the fossil and modern horses is “remarkable,” says Ken Rose, a vertebrate paleontologist at Johns Hopkins University not involved in the study. The correlation is particularly striking given the differences in other features, such as teeth and hooves, between primitive and modern horses, he says.

Primitive horses had more toes and smaller teeth compared with modern horses. Evolutionary changes to hooves and larger teeth date back to the Eocene period, between 56 and 34 million years ago. It’s unclear why horses developed their reproductive anatomy much earlier, Aurich says. But giving birth is a risky business especially for prey animals like horses, she says, so being able to get up and run away soon after delivery gives them a clear advantage.

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