The paleontologists didn’t understand what they’d found when they first unearthed the fossil of a primitive whale nine years ago. Philip Gingerich was thrown off by the jumble of adult and fetal-size bones. “The first thing we found [were] small teeth, then ribs going the wrong way,” Gingerich said. Later, “it was just astonishing to realize why the specimen in the field was so confusing” [National Geographic News]. The answer to the riddle, he soon realized, was that the fossil represented a pregnant female proto-whale and her unborn calf.
The 47.5 million-year-old mother represents a transitional phase in whale evolution before the behemoths had fully committed to a life in the ocean deeps, researchers say. The findings lend credence to the idea that early whales — protocetids — were amphibious animals that fed in the oceans but came ashore to sleep, mate and give birth [Nature News]. Researchers reached this conclusion because the fossilized fetus was positioned with its head near the birth canal. While all large land mammals are typically delivered headfirst, so they can breathe during their birth, all modern cetaceans are born tail first to ensure they don’t drown during delivery [Science News].
The researchers found another nearly complete fossil of the same new species nearby, in a region of central Pakistan where prehistoric waves once lapped at the shore. Gingerich’s team dubbed the whales Maiacetus inuus. Maicetus means “mother whale,” and Inuus was a Roman fertility god [Wired News]. The fossils show flipper-like legs that could have supported the proto-whales’ weight on land, but only for short distances. “They clearly were tied to shore,” Gingerich said. “They were living at the land-sea interface and going back and forth” [Reuters].
The findings, published in the journal PLoS ONE, help fill in a gap in the story of whale evolution. Researchers believe that the first whale ancestor was 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 [National Geographic News]. By 30 million years ago, they had evolved into the toothed and baleen whales that we know today.
The Loom: The Backward Whale has more on this discovery
80beats: Whales Had Legs Until 40 Million Years Ago, Fossils Show
DISCOVER: All in the Whale Family compares whales to their closest relatives, hippos