The Unlikely Evolution of Swimming Pigs A Whale Story

Early discoveries Basilosaurus 1832-1839 found in Alabama and Arkansas

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. -1859

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.

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).

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. 1998. Whale ankles and evolutionary relationships. Nature. 395: 452.

Eocene 50 MYA

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

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?)

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

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

Rodhocetus Discovered in Pakistan from sediments dated 49- 43MYA 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

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

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)

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

Auditory bulla

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

Molecular evolution tracks macroevolutionary transitions in Cetacea. From McGowen et al. 2014. TREE. 29:336-346. doi: 10.1016/j.tree.2014.04.001)

Cladogram generated from genomic, morphological, and paleontological data for crown Cetacea Geisler et al. 2011. BMC Evolutionary Biology. 11:112

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:10.1371/journal.pone.0007062