The reproductive
system of horses has barely changed at all over the last 48 million years,
according to a new study which analyzed the fossil of a fetus belonging to what
is the oldest known horse ancestor.
The discovery of
the fetus — and its mother — was made at the Grube Messel, a UNESCO World
Heritage Site located near Frankfurt, Germany. The bones were first found in
2000, but scientists have only now concluded their study of the fetus. The
resulting paper was published on Thursday in the open-access science journal
PLOS ONE.
The horse-like
creature was classified as belonging to Eurohippus messelensis, a now-extinct
genus of equids—the taxonomic family to which horses, donkeys and zebras
belong. The horse is thought to date back to the Eocene Epoch, which lasted
from about 56 million to 34 million years ago.
The fetus, which
scientists believe to be the oldest of its kind, measures 4.9 inches in length,
with almost all of its bones present (apart from the skull, which seems to have
been crushed). The researchers believe that the mare died shortly before it
could give birth, but that animal's death was unrelated to childbearing.
Significantly,
soft tissue from both the mother and fetus has been preserved. The researchers
believe they have tissue from the placenta — that supplied the fetus with
nutrients from the mother's uterus — as well as from uterine ligament that
connected the uterus to the mother's vertebrae and the pelvis.
The discovery
suggests that the reproductive system in the ancient horse-like animals was
very similar to that of modern descendents, according to Jens Lorenz Franzen, a
paleontologist at the Senckenberg Natural History Museum in Frankfurt and the
paper's lead author. On the other hand, says Franzen, a lot else has changed in
the last 48 million years. For example, horses of the Eocene Epoch would have
had four hooves on each forelimb and three on each hindlimb, making a total of
14 hooves. Now, horses only have four hooves.
"You can
imagine the amount of evolution which took place...during those 48 million
years but the [reproductive] system, in so far as we know it now, was already
present at that time," says Franzen. He says the reproductive system may have
developed during the Paleocene Epoch—between 66 million and 56 million years
ago—or even during the time of the dinosaurs in the Mesozoic Era, which lasted
from about 252 million to 66 million years ago.
The original
discovery was made by a team from the Senckenberg Museum. According to UNESCO,
the Grube Messel is the world's most important fossil site for understanding
the Paleogene Period, which includes the Eocene Epoch and stretches from 66 to
23 million years ago. The Paleogene is
also the period during which the first modern mammals emerged. Most mammals are
placental, meaning that they bear live offspring that are nourished prior to
birth in the mother's uterus via the placenta. Some 4,000 species are placental
mammals, including horses, dogs and humans.