The Encyclopedia of Marine Mammals takes us to some interesting places and this article is no exception: Cetacean Prenatal Development. Okay I was being sarcastic in this instance, a bit of a dry topic if you ask me. But there were a few pretty interesting facts. For instance, whales, dolphins and porpoises evolved from a four-legged land mammal, the Mesonychid, 45 million years ago. Yet, all these years later there are still skeleton remnants of this ancient past during embryonic development.
“Both forelimb and hindlimb buds are present as paddle-shaped projections,” but then the forelimbs eventually develop into pectoral fins, while the hindlimbs develop more specialized cells only to be completely reabsorbed as the embryo moves into the fetus stage of development. What a bizarre process! Going through all that hindlimb effort without any practical results.
When whales and dolphins are born there are still a few bones remaining that serve no modern purpose: “a skeletal remnant of the femur embedded into the lateral body wall, and a rudimentary pelvis that is not attached to the vertebrae.” It’s funny because in the drawings of whale skeletons everything looks pretty normal the skull, ribcage, spine, etc. Then below the entire skeleton is a tiny random bone not connected to anything and that is the rudimentary pelvis.
Later in the article it becomes clear that not having a true pelvis like land mammals is a good thing. This allows whales and dolphins to deliver their huge calves relatively smoothly since calves don’t have to pass through a “bony constriction” at birth. This may be one of the reasons many toothed whales enter the world tail first, head first just isn’t physically necessary. And there you have it, as fellow mammals whales and dolphins have many similarities and many differences associated with living in a water world.