Over the years palaeontologists like Philip Gingerich have done a great deal to unravel the fossil history of whales. There are now a range of ancient specimens that show us the changes that occurred as whales adapted to an aquatic lifestyle. Now Gingerich & his team (2009) have described a new archaeocete (ancient whale) that, while found in marine sediments, almost certainly gave birth on land.
How did they know this whale probably gave birth on land? Maiacetus (a name that means 'good mother' whale) had relatively large, robust limbs, suggesting that it spent at least some time on land, although its long fingers & toes indicate that it would not have travelled very far on land. And of the two skeletons described in Gingerich et al.'s paper, one was female - and she was carrying an almost full-term foetus in a position that meant it would have been born head first. This is what we see in modern land mammals, but not in whales - because if whale calves were born head-first there's a good chance they'd drown (remember that the birth process takes time!) There would have been very strong selection pressure against the genetic lineage of a female proto-whale who gave birth head-first!
Head versus tail presentation of near- and full-term calves in a domestic cow and harbour porpoise. (A-C show how the calf's position changes before & during birth. Note that the cow's calf is born head-first while the porpoise calf's birth is tail-first.) doi:10.131/journal.pone.0004633.g014
Adult female & fetal skeletons of the protocetid Maiacetus inuus. Skull of the female is coloured beige with brown teeth; her postcranial skeleton is covered red; the fetal skeleton is coloured blue with red teeth. doi:10.1371/journal.pone.0004366.g002
It could be that the 'foetal' skeleton actually represents a smaller Maiacetus that was eaten by the larger female. However, the research team decided this was unlikely as there was no tooth damage to the skull - damage you'd expect, given that Maiacetus had molars adapted for slicing & shearing bones and flesh. The baby had its own teeth already present in the jaw, and it was the state of development of those teeth that helped the scientists to decide that the foetus was almost full-term, & that it was probably precocial - capable of moving around & following its mother shortly after birth.
As Carl Zimmer has said in his blog, "Maiacetus breaks the rules of whales, even as it shows how those rules were made."
P.D.Gingerich, M.ul-Haq, W.von Koenigswalk, W.J.Sanders, B.H.Smith & I.S.Zalmout (2009) New protocetid whale from the Middle Eocene of Pakistan: birth on land, precocial development, and sexual dimorphism. PLoS One 4(2): e4366