1. The Unlikely Evolution of Swimming Pigs
A Whale Story

2. Early discoveries
Basilosaurus found in Alabama and Arkansas

3. Darwin speculated
In North America the black bear was seen by [the explorer Samuel] Hearne swimming for hours with widely open mouth, thus catching, like a whale, insects in the water. Even in so extreme a case as this, if the supply of insects were constant, and if better adapted competitors did not already exist in the country, I can see no difficulty in a race of bears being rendered, by natural selection, more and more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale

4. T. H. Huxley
Squalodon suggested that whales evolved from a carnivorous terrestrial mammal and shark-like teeth were adaptations to a fish-eating diet. Thus, whales were descended from an early wolf or seal.

5. Sir Wm. Henry Fowler 1883
Because seals use their hind legs rather than their tails, whales likely descended from mammals with broad swimming tails rather than seals.
Suggested an unknown artiodactyl (even-toed ungulate) as an ancestor based on some vestigial organs (complex stomach, and other soft tissue relationships).

6. The artiodactyl ankle
Artiodactyls are unique in having a so-called double pulley astragulus (one of the ankle bones). This was also found in early cetaceans.
FIGURE 1. Astragali of: a-c, ?pakicetid cetacean shown in dorsal (a), plantar (b) and medial (c) view (H-GSP 97227, Locality 300); d, mesonychian (plantar view, Disaccus europaeus, MNHN Br 21 L); e, ?ambulocetid cetacean (lateral view, H-GSP 97113, Locality 9205, distal part missing); and f, artiodactyl (lateral view, Sus scrofa).
Thewissen et al Whale ankles and evolutionary relationships. Nature. 395: 452.

7. Eocene 50 MYA

8. Much work by:
Philip D. Gingerich (University of Michigan)
Hans Thewissen (Northeastern Ohio Universities Colleges of Medicine and Pharmacy)

9. Indohyus
Thewissen discovered Indohyus the fossil dated to 60 MYA in Kashmir region of India
Characters identify it with the even-toed ungulates
Semi-aquatic (marine?)

10. Pakicetus Earliest known Archaeocete
52 MYA in Pakistan discovered by Gingerich, 1983
Teeth-more carnivorous (piscivorous?)
Ear not well-developed for hearing underwater
Skull long and narrow

11. Ambulocetus natans
Thewissen in 1992 found Ambulocetus in sediments of 49 MYA, 120 meters higher than Pakicetus
Though it was more aquatic than Pakicetus, it likely could walk because the femur is very strong
In life style it may have been similar to seals or crocodilians
The articulations of the vertebral column suggest that the body and tail undulated up and down while it was swimming
The skull was long

12. Rodhocetus
Discovered in Pakistan from sediments dated MYA Stromer 1908
Lumbar and sacral vertebrae articulated to allow for more flexibility to aid in swimming
The femur was much shorter than that of Ambulocetus making terrestrial locomotion very difficult
Skull elongated with small eyes; nostrils and ears further back on the head

13. Basilosaurus
Discovered in 1832 (southern USA) and later in Egypt. Dated to 35-41MYA.
It was up to 15 meters long
Basilosaurus was completely aquatic with vestigial pelvis and hind limbs, which were completely useless for terrestrial locomotion
They almost certainly had tail flukes because of the body length and small hind legs
The nostrils had moved to the top of the head

14. Dorudon Found by Gingerich (1994) in rock dated to about 40MYA
Like Basilosaurus, Dorudon was completely aquatic with tail flukes and vestigial hind limbs flukes.
Smaller than Basilosaurus, Dorudon was 4-5 meters long)

15. Changes to an Aquatic Lifestyle
Changes to the ear to improve hearing under water
Movement of the nostrils (blow hole) to the top of the head
Reduction of hind limbs to vestigial structures

16. Auditory bulla

17. Mesonychids originally thought to be ancestral group
Mesonychids originally thought to be ancestral group. Now, hippopotamus accepted as the sister group to the cetaceans

18. Molecular evolution tracks macroevolutionary transitions in Cetacea.
From McGowen et al TREE. 29: doi:  /j.tree )

19. Cladogram generated from genomic, morphological, and paleontological data for crown Cetacea
Geisler et al BMC Evolutionary Biology. 11:112

20. Cetacea within Artiodactylomorpha
Strict consensus of 20 minimum length trees for the equally-weighted parsimony analysis of the combined data set (57,269 steps). The contents of 12 taxonomic groups, including the total clades Cetaceamorpha and Cetancodontamorpha are delimited by different colored boxes (‘Hippo’ = Hippopotamidamorpha). Lineages that connect extant taxa in the tree are represented by thick gray branches, and wholly extinct lineages are shown as thin black branches. Estimates of branch support scores are above internodes; given the complexity of the data set, these should be interpreted as maximum estimates. Illustrations are by C. Buell and L. Betti-Nash.
Michelle Spaulding, Maureen A. O'Leary, John Gatesy - Spaulding M, O'Leary MA, Gatesy J (2009) Relationships of Cetacea (Artiodactyla) Among Mammals: Increased Taxon Sampling Alters Interpretations of Key Fossils and Character Evolution. PLoS ONE 4(9): e7062. doi: /journal.pone