A well-preserved fetus and soft tissues were found within 48-million-year-old fossils of a very small horse-like species, a new study confirms. This sheds light on the evolution of modern-day mammalian births.

The fossils were originally discovered at the Messel pit near Frankfurt, Germany. The findings were presented at last year's Society of Vertebrate Paleontology annual meeting, but the new study confirms that the fossils represent the earliest-known uterine system of such an animal, according to Discovery News.

"The fetus is the earliest and best-preserved fossil specimen of its kind," researchers from the Senckenberg Research Institute Frankfurt wrote in their study. 

Researchers believe the fossil species, Eurohippus messelensis, died shortly before giving birth. It was about the size of a modern fox terrier. The fetus appears to be well-preserved, with almost all its bones present and connected, except for the skull, which appears to have been crushed, according to the study. After reconstructing the original appearance and position of the fetus, researchers believe the horse's death is unrelated to its pregnancy.

However, researchers noted that Lake Messel, present during the horse's lifetime, occasionally released toxic carbon dioxide gas as a result of volcanic activity. Exposure to these gases could have killed the mother, who may have been heading towards the river when preparing to deliver.

The researchers determined that the soft tissues found were uteroplacenta tissues and one broad uterine ligament. This may represent the earliest fossil record of the uterine system of a placental mammal, the researchers noted in their study.  

Therefore, researchers concluded that the reproductive system of today's horses was already highly developed at this early time in the Paleocene Era, and could have possible evolved sooner. Since humans are also placental mammals, this study shed light on how far back this form of birth goes.

Their findings were recently published in the journal PLOS ONE.

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