The preserved fetus identified in an ancient fossilized horse unearthed 15 years ago in Germany has been described this week in detail by researchers in a scientific journal.
The specimen is estimated to be 48 million years old.
Scientists used scanning electronic microscopy and high-resolution micro x-ray analysis on the specimen of the ancient horse, Eurohippus messelensis, to learn as much as they could about the fetus.
Some details of their findings were announced late last year. A scientific paper on the find was published this week in the open-access journal, PLOS ONE.
The specimen was discovered at the Messel pit, near Frankfurt, in 2000.
Jens Lorenz Franzen, Christine Aurich and Jörg Habersetzer reported that the fetus was about 12.5cm long. It appeared to be well-preserved, with almost all bones present and connected, except for the skull, which appeared to have been crushed.
The well-preserved condition of the fossil allowed the researchers to reconstruct the original appearance and position of the fetus.
The size of the fetus and the presence of fully developed milk teeth indicate that it was close to term when it and its mother died.
Nevertheless, its position in the uterus indicates that the two did not die during the birthing process. 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 authors also found preserved soft tissue, like the uteroplacenta and one broad uterine ligament, which may represent the earliest fossil record of the uterine system of a placental mammal.
The authors said the evidence pointed to the reproductive system already being highly developed during the Paleocene, and possibly even earlier.
The research shows that reproduction in early horses was very similar to that of modern horses, despite great differences in size and structure. The ancient horse in question was about the size of a fox terrier.
The placenta in this specimen is only the second one that has been described for a fossil placental mammal.
The oil shales at Grube Messel where the specimen was found have long been known for their marvelous fossils. These shales formed at the bottom of ancient Lake Messel and preserve the remains of mammals, birds, and other animals that were living near what is now Darmstadt, Germany about 47 million years ago, during the Eocene epoch.
No oxygen was present at the bottom of the lake when the dead animals sank down and finally became embedded in the muddy sediments. There, anaerobic bacteria immediately began to decompose skin, muscles, and other soft tissues.
As a result, the bacteria produced carbon dioxide, which in turn precipitated iron ions present in the lake water.
In this way, the bacteria petrified themselves, developing only a very thin bacterial lawn depicting the soft tissue as black shadow. Consequently, Messel fossils preserve these remains not directly, but as images.
Lorenz Franzen and Habersetzer are with the Senckenberg Research Institute. Aurich is with the University of Veterinary Medicine in Vienna.
Franzen JL, Aurich C, Habersetzer J (2015) Description of a Well Preserved Fetus of the European Eocene Equoid Eurohippus messelensis. PLoS ONE 10(10): e0137985. doi:10.1371/journal.pone.0137985
The full study can be read here.