The evolution of whales may be the best understood and documented examples of macroevolution. The extensive fossil record indicates that modern day whales evolved from four-legged terrestrial mammals to oceanic mammals during the Eocene.
The fossil record
Extensive fossil evidence over the last few decades on the Indian subcontinent and North Africa has revealed links to explain the transition from land to sea. The fossil record of early whales, Archaeocetes, suggest that they lived in the shallow seas 39-36 million years ago. Dorudon, an example of these primitive whales found in Egypt was clearly an aquatic tail powered swimmer like modern whales that had reduced small hind legs and feet. Mesonychids, like Pachyaena, were four-legged, hoofed terrestrial mammals that lived from 58-34 million years ago. Although they were unmistakably land mammals, Mesonychids had distinct teeth that were also found in the early whales. Until 1983 there was an enormous gap between land-living Mesonychids and the water-bound Archaeocetes. Discovered in Pakistan in 1983, Pakicetus was found in a continental red-bed deposit of sedimentary rock from 50 million years ago that had built up in a coastal river associated with land mammals. Pakicetus had four legs, but whale-like periotic ear bones, to make hearing in water more efficient. It also had teeth similar Archaeocetes, the primitive whales. With its four-legs and its ear bones adapted to hearing underwater, Pakicetus is most likely semiaquatic. In 1990 another primitive whale fossil from 37 million years ago was found in Egypt. The Basilosaurus skeleton revealed marked hind legs with feet and toes. The legs were only two feet long and useless on land, yet they were still there. In 1994 fossil remains of Rodhocetus, a four-legged hoofed early whale from 46 million years ago was uncovered in Pakistan. Rodhocetus was more recent than Pakicetus and had more features suited for life in the water. It had shorter legs, whale-like ear bones, and a nostril between its eyes and the tip of its nose. In 1994 a transitional species between the more terrestrial Pakicetus and the more aquatic Rodhocetus was discovered in Pakistan. The amphibious Ambulocetus lived 49 million years ago and had four legs with large hind feet adapted to swimming. Its forelimbs had fingers and small hooves making it capable of walking on land. Although it had four legs and was able walk, Ambulocetus, was well suited for aquatic life. It had whale like periodic ear bones, a nose adaptation that allowed it to swallow underwater, and teeth similar to early whales.
Bajpai, S., Thewissen, J. G. M., & Sahni, A. (2009). The origin and early evolution of whales: macroevolution documented on the Indian Subcontinent. Journal of Biosciences, 34(5), 673-686.
Gingerich, P.D. (2012). Evolution of Whales from Land to Sea. American Philosophical Society, 156(3), 309-323.
Gingerich, P. D. (2003). Land-to-sea transition in early whales: Evolution of Eocene Archaeoceti (Cetacea) in relation to skeletal proportions and locomotion of living semiaquatic mammals. Paleobiology, 29(3), 429-454.
Nummela, S., Thewissen, J. M., Bajpai, S., Hussain, S., & Kumar, K. (2004). Eocene evolution of whale hearing. Nature, 430(7001), 776-778.