The earliest whales may not have lost their links to land
See a gallery of the evolution of whales and how reconstructions are created
The first whales gave birth on land, suggests a unique 47 million-year-old fossil of a pregnant female. The discovery provides the first concrete proof to a long-standing theory that the ancestors of whales lived something of a double life, moving back and forth between land and sea.
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鈥淭hese were speculations. I never thought we鈥檇 get evidence for or against,鈥� says , a palaeontologist at the University of Michigan in Ann Arbor, whose team found the fossilised mother and calf in north-eastern Pakistan.
As if that wasn鈥檛 enough, Gingerich鈥檚 team also unearthed a nearly complete fossil 鈥� 鈥渞ight down to the end of the tail,鈥� he says 鈥� belonging to a male of the same species, Maicatetus inuus.
Together, the fossils shed light on the earliest stages of whale evolution, when the descendants of hoofed deer-like animals took to the sea.
Head first
The first whales, known as protocetids, did not fully embrace their new ocean habitat. They probably stuck close to shore and made occasional trips back to the land, Gingerich says.
鈥淭hey certainly weren鈥檛 walking,鈥� he says. 鈥淭hey were more like sea lions, which can move more than you would think from their morphology.鈥�
Also like sea lions, Maicatetus gave birth on land. The fossilised calf鈥檚 head faces outward from the uterus, which is how all land mammals give birth. Whales and dolphins, on the other hand, deliver their calves tail first 鈥� possibly so they don鈥檛 drown before the end of labour.
The fossilised fetus also has developing molar teeth, an indication that it was well-developed when it was born. 鈥淎nything that is born with molars 鈥� is born ready to live in open environments鈥�, and perhaps able to evade predators, Gingerich says.
He adds that he has no idea what killed the ancestral whales.
Unique find
, a vertebrate palaeontologist at the University of Alabama in Tuscaloosa is excited by the new fossil. 鈥淚鈥檝e searched through every piece of literature on fossil whales on the planet. I鈥檝e never seen a fetus inside a mother,鈥� he says.
It is now important to find fossils that fill in the final details of whales鈥� slow march to the sea, Uhen says. 鈥淚 hope we find more of this sort of thing.鈥�
Journal reference: (DOI: 10.1371/journal.pone.0004366)
