An amazing fossil was recently described in the journal Plos One by a team of scientists led by Dr. Jens Lorenz Franzen as well as her associates Christine Aurich, and Jörg Habersetzer at Senckenberg Research Institute. The fossil was originally excavated from an old oil shale quarry in southern Germany. The pit itself has an extraordinary history along with the fossils that come out of it. This quarry, known as the Grube Messel (Messel Pit), was once used for industrial reasons such as oil shale mining up until the 1970s. Later, it was to be designated as a trash collection dump, but many paleontologists and collectors fought against this, saying that the invaluable fossils would be destroyed. In 1995 the pit was turned into a World Heritage Site, and remains as such today.
This fossil pit is responsible for some amazing discoveries including many new species of horses, exceptionally preserved birds and insects and even unique details in specimens that are not seen elsewhere in the world. Many insects have been uncovered with actual pigmentation still intact. Some mammals have fur and skin outlining the skeleton. Pairs of mating tortoises have even been found. Birds have pattern detail in their feathers and horses have stomach contents preserved. And now, this pit has yet again astounded scientists with another unusual and rare specimen. This time, the horse uncovered wasn’t famous because of the level of detail still held in its anatomy, but because the horse was pregnant, and still carried the fetus.
How do fossils preserve so marvelously here? It all has to do with where the Messel Pit was millions of years ago. During the Eocene (around 54.8 million – 33.7 million years ago), when the Eurohippus lived, this area of Europe was south of its currently location. This meant that much of the land we see today used to be heavily tropical. Where the Messel Pit stood was a massive lake. The lake contained a very low concentration of oxygen, which led to the death of many animals that happened upon it. What this also meant was that any animals which died for one reason or another, and then sank to the bottom of the lake, were not preyed upon by scavengers. These bodies were instead consumed or broken down by anaerobic (functioning without air) bacteria that could live at the bottom of such a deadly lake. These bacteria gradually replaced the bones and bodies of the animals and then they themselves became petrified over time, leaving behind an image of the animals that once lived in Europe. Because these animals were preserved like this, and not by “traditional” fossilization, it left more impressions of soft tissue than most other fossil sites.
The pregnant species that was found was a Eurohippus messelensis, which means it is a European horse ancestor from the Messel Pit. This small equine was about the size of a modern beagle-dog, being over a foot tall but generally under two at the shoulder. Besides size, many other features distinguish it from modern horses and ungulates (hooved-mammals) we see today. For instance, this equine had four toes on each forefoot and three toes on each hindfoot, as compared to the modern horse which has only one. The modern horse has essentially gotten rid of its other toes through evolution, although it has vestigial structures in it legs give hints of its evolutionary past.
This specimen was originally excavated in 2000, more than 15 years ago, but research was hindered by the lack of technology sophisticated enough to fully examine the equine, and it was shelved away for later processing. When the electron scanning microscope was made available to paleontologists, they jumped on the opportunity to revisit this fossil.
One might ask why this particular example of a pregnant female Eurohippus is more important than any other. In total, more than a dozen specimens of impregnated extinct equines have been uncovered, all with varying stages of growth in the fetus. In this particular example, the female was much further along in pregnancy than most of the available specimens. The only body part that seems to be missing from the foal is the skull itself. The mother is not only exceptionally well preserved herself, but so is the fetus, which remains almost entirely articulated (all bones found in the correct anatomical position).
Other unique features of this fossil and fetus are the well preserved soft tissue membranes of the uterus. With micro X-rays, paleontologists were able to distinguish the uterine canal from the rest of the mother and determine the age of the foal. This foal’s bones were not yet ossified (turned from cartilage to bone) but were close enough to suggest that the foal was close to being born. The foal’s feet were not yet extended but its head was in the same position as that of modern horses before birth. This animal and this species represents the earliest known evidence of a placental mammal, with this specimen being the best preserved to date.
Fossils like this are what paleontologists and evolutionary biologists live for. These discoveries are so few and far between yet they give us so much new information. Being able to see anatomically where the fetus sits in the Eurohippus can let us compare the same position in modern horses. The astonishing similarities between the two tell us that the way these animals grew young in utero and the way that they gave birth was extremely similar to the method modern horses use today. The methods were conserved over the span of more than 40 million years. It is truly astonishing to think that in nearly the same time that it took theropod dinosaurs to evolve into birds, horses remained almost identical to their long ago ancestors. It really goes to show that natural or sexual selection will not change an organism if the form it holds is the best suited for its environment.
For a quick summary check out this video made by Geobeats News!