A sticky end for one of the horses in the beak of Gastornis

The fossil was found at the Messel open-pit mine in Germany, where more than 70 specimens of ancient horses have now been unearthed.

Details of the discovery are revealed in the forthcoming BBC TV series Walking With Beasts.

Dr Stephan Schaal, of the Senckenberg Research Institute, Germany, told BBC News Online: "The find of the adult horse includes the best preserved foetus we have ever discovered in Messel.

"Compared with other Messel horses, it has impressive preservation of the complete jaws with its teeth."

Click here to see the fossil

The forest dwelling horses come from a time, 49 million years ago, when tropical forests stretched right to the poles.

The largest mammals were about the size of a pig, and giant stalking birds, Gastornis, took the role of top predators.

The horses may have been spotted, like this modern-day one

Two species of the tiny forest-dwelling horses, Propalaeotherium, are known from fossil evidence at Eocene sites in Germany.

The smallest was the size of a fox terrier, and the largest about the size of a German shepherd dog.

The stomach contents of most specimens show that they ate foliage but one was full of fruit - a grape similar to that used to make wine.

Scientists believe the mammals browsed on whatever they could, including fallen fruit when it was available.

The fossil also sheds light on how these ancient horses raised their young. All of the 10 fossils of pregnant mares found at Messel were carrying one foal.

Running with the herd

This is evidence, says palaeontologist Dr Jens Franzen, that even primitive horses had an evolutionary strategy of raising one or two offspring.

"That would point to some kind of special care of the offspring and would indicate that there was a herd involved in joint care," he told BBC News Online.

The horses had just begun to diverge away from the group of odd toed mammals, or perissodactyls, that were the common ancestors of living horses, rhinos and tapirs.

They had five toes, which over the course of evolution, fused into the one hoof found in modern-day horses.