Gotta feel sorry for the male of the genus--it's forever stuck with a female moniker, which is the reason I'm not a huge fan of the name Maiacetus. I understand why Gingerich, et al. (2009) named it that--one specimen is a pregnant female with a near-term fetus preserved--but when you think about it, any mammalian fossil could potentially be preserved in such a state. Heck, Maiasaura was named on the idea that the mother duckbills must have taken care of their babies after hatching. If anything, Maiasaura is based on circumstantial evidence, which makes its name even more inappropriate. But what if they had found a pregnant brontothere, would they name it Maiatherium? If it had been a pregnant hominid, would they call it Maiapithecus? These are tough questions. Gingerich, et al. basically gave their whale a redundant name. It's a "good mother whale." Well, name me a whale that isn't.
Anyway, what makes Maiacetus so awesome is that it tells us a lot about archaeocetid whales. This is the group that includes Ambulocetus, Rodhocetus, and Dorudon. They all have hindlimbs, but the later members, like Dorudon and Basilosaurus, have vestigal hindlimbs and pelves which are not connected to the spinal column. Earlier members, like Rodhocetus and Maiacetus, have fully functioning hindlimbs with large pelves that connect to the spinal column.
This picture, which is helpfully labeled, is taken from Not Exactly Rocket Science. But it shows quite clearly the female (who died on her back) and the baby (in blue). The baby's teeth are colored brown, and show that adult molars were already present in the baby. That means that the baby is near-term and it is highly precocial, like modern whales. That is, the baby can pretty well take care of itself right out of the womb. The alternative is altricial, which would be more like baby primates. But wait--there's more! The near-term fetus is facing the birth canal, suggesting babies were pushed out head-first. That would mean a land-lubbing birth for the mother.
This has interesting implications. First, you should know that modern whales of course give birth at sea. Because of that, their young are born tail-first. This allows the baby to breathe while being pushed out of the mother. It also places the baby roughly parallel to the mother, so when it starts swimming (right away), it is right next to mommy, which is advantageous for a lot of reasons. Anyway, giving birth at sea probably has a lot to do with being able to come up on land or not. Later members of the Archaeoceti, like Dorudon and Basilosaurus, are both too bulky and lack the hindlimbs necessary to come ashore, so they must have given birth at sea. But just a few million years earlier, Maiacetus was giving birth on land. So the transition must have happened pretty quickly. And that transition was probably easier because of the precocial infants!
Aside from the pregnant female, the authors describe a nearly complete male skeleton. It is 12% larger than the female and its canines are 20% longer. Gingerich, et al. suggest that such a size gap is comparable to modern marine mammals that do not engage in fierce territorial and harem mating systems. However, this also suggests that Maiacetus and its relatives were not gathering in large groups as modern harem-based marine mammals do (like elephant seals). This may mean that the environments where early whales "grew up" lacked such stable structures.
So aside from the terrible name, Maiacetus is a pretty awesome animal. It tells us a lot about archaeocete paleobiology, and our picture of early whale evolution becomes that much more complete!