Some fossils are just too cool. And slightly tragic. A 48 million year old fossil from the Messel beds of Germany fits this category well, preserving a mare with her unborn offspring. Beyond macabre fascination, however, what do these fossils tell us? A detailed description of the fetus, published just yesterday in PLOS ONE, explains. [Full disclosure: I was the handling editor for this paper.]
The Messel beds preserve remarkable fossils of everything from beetles to birds. The organisms (some perhaps killed by volcanic gases) accumulated in an oxygen-free lake bed, resulting in gorgeous preservation. Among the fossils are a good number of early “horses”, representing off-shoots of the origin of the group that spans zebras and My Little Ponies. The Messel “horses” are more similar to the latter than the former, scarcely a foot or two tall at the shoulder and sporting several toes on each foot instead of hooves. They get scare quotes because on the evolutionary tree, they fall just outside of the modern horse family Equidae. However, our Messel critters (in this case, falling under the genus Eurohippus) are still more closely related to today’s horses than to anything else.
The newly described fossil comprises a mother Eurohippus with a number of fetal bones in the abdominal region. Based on the development of the bones in the fetus, and through comparison with pregnancies in modern mammals, the fossil fetus was probably pretty close to birth. This is not the only fetal horse specimen from this age in Germany, as noted by the authors (there are at least seven other specimens), but it is among the best preserved. Some of the soft tissues of the uterus and associated structures are still visible, probably captured in time by bacterial colonies that colonized the tissues after death. The fossil purportedly houses the oldest known uterus in the fossil record, and shows numerous anatomical similarities with those seen in modern horses.
Beyond their “wow” factor, fossils such as this pregnant mare help scientists to track the evolution of reproduction within mammals. Ignoring size (the ancient “horse” was much smaller than the modern horse), not much has changed in horse pregnancies over the past 48 million years. The basic anatomy is the same, and the presumed orientation of the fetus prior to birth is the same. Tiny ancient Eurohippus even had single births, versus the litters seen in other groups such as cats and dogs. That’s some remarkable consistency!
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