Changes

Whales are estimated to have diverged from the lineage leading to modern ungulates around 55-60 million years ago<ref>http://www.pnas.org/content/95/26/15464.short</ref>.  Fossils of an animal, called ''Pakicetus'', with unambiguous ancestral whale-like traits are found in strata dated to ~50-53 million years ago.  Pakicetus was a land-dwelling carnivore, somewhat similar in size and appearance to a modern wolf (more illustratively, imagine a carnivorous dog-sized goat).

Whales are estimated to have diverged from the lineage leading to modern ungulates around 55-60 million years ago<ref>http://www.pnas.org/content/95/26/15464.short</ref>.  Fossils of an animal, called ''Pakicetus'', with unambiguous ancestral whale-like traits are found in strata dated to ~50-53 million years ago.  Pakicetus was a land-dwelling carnivore, somewhat similar in size and appearance to a modern wolf (more illustratively, imagine a carnivorous dog-sized goat).

Moving into younger strata, the fossil record provides a detailed stepwise transition from ''Pakicetus'', to ''Ambulocetus'' (a family of animals very similar to ''Pakicetus'', but more adapted to a semi-aquatic lifestyle), to ''Remingtonocetus''(clearly well-adapted to a mostly aquatic lifestyle), to ''Protocetus'' (the earliest whale fossils found outside of the area around western India), to the ancestral divergence of modern whale families.

Moving into younger strata, the fossil record provides a detailed stepwise transition from ''Pakicetus'', to ''Ambulocetus'' (a family of animals very similar to ''Pakicetus'', but more adapted to a semi-aquatic lifestyle), to ''Remingtonocetus'' (clearly well-adapted to a mostly aquatic lifestyle), to ''Protocetus'' (the earliest whale fossils found outside of the area around western India), to the ancestral divergence of modern whale families.

These fossils also show the stepwise emergence of complex traits, most notably the cetacean-specific inner-ear adaptations required for acute underwater hearing and echolocation <ref>http://www.nature.com/nature/journal/v361/n6411/abs/361444a0.html</ref><ref>http://www.ncbi.nlm.nih.gov/pubmed/3692104</ref>.

These fossils also show the stepwise emergence of complex traits, most notably the cetacean-specific inner-ear adaptations required for acute underwater hearing and echolocation <ref>http://www.nature.com/nature/journal/v361/n6411/abs/361444a0.html</ref><ref>http://www.ncbi.nlm.nih.gov/pubmed/3692104</ref>.