Whales' slow transition from land to sea is documented in other fossils, but this is the most complete to fill a gap during this time period.
"This is a big discovery because it tells us about life history, or the way early whales lived their lives, [which is something] that is difficult to learn from fossils," Gingerich said.
The most famous other seafaring animals to be found fossilized with a complete fetus were ichthyosaurs, a reptile group that lived roughly 245 to 100 million years ago.
"Not since have we seen fossils of marine-dwelling vertebrates that tell us so much about the biology of evolving an ocean dwelling way of life from a terrestrial ancestor," said Louis Jacobs, a vertebrate paleontologist at Southern Methodist University in Texas.
"It is a missing link of the most informative sort," Jacobs added.
"Charles Darwin would delight."
February marks the 200th anniversary of Darwin's birth and the150th anniversary of publication of the Origin of Species.
From Land to Sea
For the last 30 years, Gingerich has scrubbed the sands of Pakistan and Egypt—where an ancient ocean once hit the shore—for fossils that help piece together whale evolution.
Still, much remains unknown about the trajectory from land to sea.
What paleontologists do know about the first whale ancestor is that it was originally a furry, four-legged omnivore that evolved into a range of amphibious species nearly 50 million years ago, and then into fully aquatic species around 45 million years ago.
Whales eventually lost the connection between their backbone and hind legs, then gradually lost the hind legs and vestigial bones completely.
The new adult fossil—8 feet (2.6 meters) long—has four legs, with the hind two still connected to the backbone. The fetus has well developed teeth, indicating that it was prepared to fend for itself soon after birth.
"The completeness of the limbs and limb bones make it possible for the first time, I believe, to analyze the limb function and ability of the animal to move in and out of the water," said Duke University paleontologist Elwyn Simons.
"These complete limbs are almost exactly intermediate between a seagoing creature and a land animal."
Before they shed their hind legs, land-dwelling whale ancestors are believed to have scavenged dead fish that washed up along the shore, eventually sliding into the ocean after recognizing feeding opportunities there, Gingerich said.
"Seals and sea lions did the same thing," he added. "But they seemingly stopped part way, whereas whales kept going.
The new study appears this month in the online journal PLoS One.