Whales are remarkable in the number of transitional fossils that they demonstrate. Based on molecular evidence, whales descended from hoofed mammals, with their closest living relatives being the hippos. This makes sense in that both whales and hippos share numerous features, such as dense limb bones (reduced buoyancy to help stay underwater), they lack sebaceous glands (the glands that make most mammals’ hair oily), have reduced hair, have a semi-aquatic/aquatic lifestyle, and nurse their young underwater. Based on this hypothetical common ancestor, whales next have a series of transitional fossils that document changes in their body plan from a probable semi-aquatic ancestor to a fully aquatic one.
A figure from McGowen et al.  documents this transition:
Indohyus 55.8-40.4 million years ago*
The first transitional fossil the authors describe is Indohyus, a small, herbivorous (plant-eating) mammal that appeared about 55.8 million years ago. This strange creature possessed dense ear bones which likely facilitated underwater hearing. Sound does not travel underwater the same way it does through air, so modifications for hearing underwater are consistent with a transition from land to water-based ecosystems. Its otherwise typical terrestrial features show that it’s still a long way away from whales.
Pakicetus 55.8-40.4 million years ago
The next transitional fossil shows evidence of abandoning an herbivorous diet in favor of a carnivorous/piscivorous (i.e., fish-based) diet. It also has a more robust tail than Indohyus, which may have facilitated better swimming.
Ambulocetus 48.6-40.4 million years ago
Ambulocetus appears more recently in the fossil record than Pakicetus and Indohyus, and it’s not difficult to tell that it looks better adapted for an aquatic lifestyle than those two species. A new feature that Ambulocetus shows is evidence for a fat pad in its jaw, likely to aid in underwater hearing.
Remingtonocetus 48.6-40.4 million years ago
Ambulocetus and Remingtonocetus together show two important changes in whale history. The first is the loss of the vomeronasal organ. The vomeronasal organ is a sensory structure used to detect chemical signals, particularly pheromones. If you’ve ever seen a cat do this:
it’s using its vomeronasal organ. Underwater, chemical signaling is not quite as useful, so the loss of the vomeronasal organ suggests a further commitment to living in the water. Additionally, Ambulocetus and Remingtonocetus show evidence of a move to saltwater, the medium that the majority of whales live in today.
Rodhocetus 48.6-40.4 million years ago
Artiocetus 48.6-40.4 million years ago
Protocetus 48.6-40.4 million years ago
These three species, plus Georgiacetus and Dorudon (see below) document another crucial transition in the whale body plan. First off, their external nares (=nostrils) have moved more posteriorly than the earlier whales and hoofed mammals. If you’ve never thought about it before, where are the nostrils on a dolphin or humpback whale? It’s their blowhole(s)! These fossils show the beginning of that transition from forward facing nostrils to nostrils on the top of their head.
Another critical transition is the novel trait of tail flukes, the characteristic horizontal tail fin found in whales which aids in aquatic propulsion.
Georgiacetus 48.6-37.2 million years ago
Your pelvis is a structure that connects your legs to your spine, allowing you to walk or run while maximizing stability. The same is true for animals with four legs. Georgiacetus, however, detached its pelvis from its spine. This makes sense for an animal that is aquatic and no longer needs to support its body weight on four legs. Interestingly, we see the opposite transition in fossil fishes that began leaving water for land.
Dorudon 40.4-33.9 million years ago
Basilosaurus 40.4-33.9 million years ago
Dorudon and Basilosaurus, early whales that appeared later than the eight fossil species I’ve described so far, are the most whale-y looking species at this point. An obvious feature is the major reduction in the hind limbs into tiny vestiges. This is indicative of a fully-aquatic lifestyle, as opposed to an amphibious habit like that found in seals and sea lions.
They differ from modern whales however, in that they have greater elbow mobility and they still possess hind limbs.
This is in contrast to modern whales which have retained a vestigial pelvis and occasionally a femur but no lower limb elements, further evidence of a gradual loss in hindlimb structure and function.
These fossils very neatly document the transition in the body plan of a terrestrial mammal to the ancestor of modern whales. In my next two posts, I’ll highlight the transitional fossils that display the transition from this hypothetical common ancestor to the two lineages of modern whales: toothed and baleen whales.
*all fossil ranges based on the paleobiology database
Questions for creationists
Is it simply a coincidence that there are fossils that seem to document a transition between a terrestrial mammal and whales? Since many of them could swim, shouldn’t these animals have survived Noah’s flood? Why would a whale need a tiny, hindlimb, such as those found in Basilosaurus and Dorudon?