A 48-million-year-old horselike fossil pulled from the rich Messel Pit site near Frankfurt is yielding insights into the evolution of mammalian reproduction. A fetal specimen described in a new study and circled in the photograph at right is the earliest and best-preserved mammalian fetus.

“The question was, is it possible that the placenta was preserved?” said Jens Lorenz Franzen, a retired paleontologist affiliated with the Senckenberg Museum in Frankfurt and one of the study’s authors.

In looking at the fossilized soft tissue, he and his colleagues discovered a preserved placenta, uterus and one broad uterine ligament — evidence, Dr. Franzen said, that the ancient mare had a reproductive system very similar to that of a modern horse.

This means a sophisticated reproductive system was already present in mammals, he said.

The bones of the fetus are also intact except for the skull, which is crushed. The findings appear in the journal PLOS One.