*(cetaceans = whales & dolphins; sirenians = manatees & dugongs; pinnipeds = seals, walruses & sea lion).
Modern whales can be divided into two groups, odontocetes and mysticetes. Odontocetes are characterized for having teeth and using echolocation; mysticetes are characterized for having baleen instead of teeth (there are other adaptations that I won’t discuss now). Some examples of odontocetes are orcas and bottlenose dolphins; mysticetes include blue whales and right whales.
Based on molecular evidence, whales evolved from artiodactyls – a group that includes pigs, hippopotamus, camels, cows, lambs, etc – (Graur & Higgins, 1994; Shimamura et al., 1997), whereas, for a long time, morphological studies used to indicate a close relationship with mesonychids – a group of extinct terrestrial carnivores – (Luo & Gingerich, 1999). In part, the reason for this disagreement about the origin of whales was that the oldest fossils of cetaceans consisted of forms that were already completely adapted to a life in the water, or were only known from crania. This has already been resolved.
In 2001, two groups of paleontologist published papers where they described primitive cetaceans, including parts of the postcranium that corroborated an artiodactyl relationship (Gingerich et al., 2001; Thewissen et al., 2001). Gingerich and his team found remains of Artiocetus clavis and Rhodocetus balochistanensis, whereas the Thewissen team described Ichthyolestes pinfoldi and Pakicetus attocki (illustration above of Pakicetus by Carl Buell, taken from the Thewissen Lab webpage); the remains included one of the ankle bones, the astragalus, which was key to determine that cetaceans evolved from artiodactyls. All these fossils were found Middle Eocene (49-41 million years ago) deposits. More recently, Thewissen et al. (2007) describe postcranial material of the primitive artiodactyl, Indohyus, and show that it was an animal with aquatic adaptations, providing additional evidence about the origin of cetaceans. (Go here for a magnificent reconstruction of Indohyus).
The closest living relatives of manatees and dugongs are elephants, this relationships is supported by both, molecular and morphological evidence (Seiffert, 2007; Tabuce et al. 2007). Sirenians originated in northern Africa about 54 million years ago, where they last shared a common ancestor with proboscideans (elephants). Interestingly, the most primitive sirenian fossils have been found in Jamaica, which demonstrate that, very early, they seem to have been well adapted for life in an aquatic environment. For a long time, the most primitive sirenian known was Prorastomus sirenoides found in Jamaica in deposits that are between 51-49 million years old (Owen, 1855; Savage et al., 1994). Unfortunately, the postcranium was and it is still mostly unknown.
Another sirenian from Jamaica, found in slightly younger deposits – 49-45 million years old – was described by Domning (2001). The remains of this new sirenian, named Pezosiren portelli (illustration above from Domning, 2001), include cranial and postcranial material. The postcranial material include fore and hind limbs, pelvis and most of the vertebral column; all together, these indicate that Pezosiren was able to support its own weight on land, but at the same time, it had aquatic adaptations such as pachyosteosclerotic (enlarged & dense) ribs and in the cranium, retracted external nares (Domning, 2001). The combination of characters imply that Pezosiren spent time, both, in and out of the water.
Morphological and molecular studies show that pinnipeds belong to a group of mammals called arctoids (Deméré et al., 2003), that, along with pinnipeds, includes bears (ursids), weasels (mustelids), raccoons (procyonids) and skunks (mephitids), among others. Nonetheless, the origin of pinnipeds from one of these arctoids is not clear, and different studies place pinnipeds as originating from a common ancestor with mustelids or with ursids (Deméré et al., 2003). Other experts in the field support a multiple origin for pinnipeds, with seals sharing a common ancestor with mustelids and sea lion and walruses with ursids (Uhen, 2007 and references therein). Anyways, whatever is the group from which pinnipeds originated – this can only be resolved by finding more fossils – it is well known that these originate from a terrestrial ancestor. Interestingly, pinniped fossils that show a transitional morphology had not been found until recently.
The discovery of Puijila darwini in lacustrine sediments deposited between 23-21 million years ago, gives us an idea about the morphology of the earliest pinnipeds (Rybczynski et al., 2009). Although fossils of yet even older pinnipeds, such as Enaliarctos tedfordi and E. barnesi, have been found in rocks that date between 28.5-23.8 million years in Oregon (Deméré et al., 2003), these already show full adaptations to a life in the marine realm like those observed in modern taxa; this means that pinnipeds must have evolved previous to that date. So, even if Puijila (illustration above from Rybczynski et al., 2009) comes from younger deposits, its importance rests in that morphologically it is the most primitive known pinnipeds, providing evidence about the evolutionary steps that were taken in the transition from land to sea in this group of mammals.
Also, here is the Spanish version of this post.
Deméré, T. A., A. Berta & P. J. Adams. 2003. Pinnipedomorph evolutionary biogeography. Bulletin of the American Museum of Natural History 279:32-76.
Domning, D. P. 2001. The earliest known fully quadrupedal sirenian. Nature 413:625-627.
Gingerich, P. D., M. ul Haq, I. S. Zalmout, I. H. Khan & M. S. Malkani. 2001. Origin of whales from early artiodactyls: hands and feet of Eocene Protocetidae from Pakistan. Science 293:2239-2242.
Graur, D. & D. G. Higgins. 1994. Molecular evidence for the inclusion of cetaceans within the order Artiodactyla. Molecular Biology and Evolution 11(3):357-364.
Luo, Z. & P. D. Gingerich. 1999. Terrestrial Mesonychia to aquatic Cetacea: transformation of the basicranium and evolution of hearing in whales. University of Michigan Papers on Paleontology 31:1-98.
Owen, R. 1855. On the fossil skull of a mammal (Prorastomus sirenoides, Owen), from the island of Jamaica. Quarterly Journal of the Geological Society of London 11:541-543.
Rybczynski, N., M. R. Dawson & R. H. Tedford. 2009. A semi-aquatic Arctic mammalian carnivore from the Miocene epoch and origin of Pinnipedia. Nature 458:1021-1024.
Savage, R. J. G., D. P. Domning & J. G. M. Thewissen. 1994. Fossil Sirenia of the West Atlantic and Caribbean region. V. Prorastomus sirenoides Owen, 1855. Journal of Vertebrate Paleontology 14(3):427-449.
Seiffert, E. R. 2007. A new estimate of afrotherian phylogeny based on simultaneous analysis of genomic, morphological, and fossil evidence. BMC Ecolutionary Biology 7:224 Open access
Shimamura, M., H. Yasue, K. Ohshima, H. Abe, H. Kato, T. Kishiro, M. Goto, I. Munechika & N. Okada. 1997. Molecular evidence from retroposons that whales form a clade within even-toed ungulates. Nature 388:666-670.
Tabuce, R., L. Marivaux, M. Adaci, M. Bensalah, J.-L. Hartenberger, M. Mahboubi, F. Mebrouk, P. Tafforeau & J.-J. Jaeger. 2007. Early Tertiary mammals from North Africa reinforce the molecular Afrotheria clade. Proceedings of the Royal Society B 274:1159-1166.
Thewissen, J. G. M., E. M. Williams, L. J. Roe & S. T. Hussain. 2001. Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls. Nature 413:277-281.
Thewissen, J. G. M., L. N. Cooper, M. T. Clementz, S. Bajpai & B. N. Tiwari. 2007. Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature 450:1190-1195.
Uhen, M. D. 2007. Evolution of marine mammals: back to the sea after 300 million years. Anatomical Record 290:514-522.