Article by: J. D. Dixon
Edited by: Jack Wood, Lewis Haller, and Harry T. Jones
Cetacea is an infraorder of vertebrate animals. The modern members of the group are split into two suborders. The Odontoceti suborder consists of the species with teeth (for example, dolphins, porpoises, and narwhals). The Mysticeti suborder contains the species that filter-feed (for example, the gray whale, blue whale, and bowhead whale). All modern cetaceans are aquatic, so are adapted for this lifestyle by having limited-to-no hair or no hind limbs, having flippers and highly streamlined bodies. But how did these incredible creatures come to be? They didn’t just appear one day in our seas. There was an expansive transition between the terrestrial and aquatic environments over millions of years, but we can’t run through every genus of ancient whale. This article will instead be a whistle-stop tour through some of the most interesting species in the cetacean fossil record, aiming to piece together the land-to-sea transition of these remarkable animals.
The evolution of cetaceans is puzzling, but it is thought to have started in India or Pakistan during the Early and Middle Eocene. Around 54 million years ago (Ma), archaeocete whales (a suborder of prehistoric whales) diverged from a terrestrial ancestor. This ancestor was an artiodactyl, the hoofed animals that support their body weight on two of their five toes. Gradually, over millions of years, they evolved into the aquatic organisms we see today.
Firstly, there is the family Pakicetidae. This consists of three genera, all of which are known from the Kuldana Formation in Pakistan. The genus Pakicetus is the largest, and is considered to be one of the earliest examples of Cetacea. Despite this, it looks nothing like the whales and dolphins we know today. Pakicetus was a four-legged mammal, with a long snout, and teeth remnant of a carnivorous lifestyle. If you imagined the body of a modern tapir with the head of a wolf, you’d be pretty close. The genus is aged at around 56 – 47.8 Ma and was discovered in the 1980s. From looks alone, it might be hard to see how this is an ancestor of modern whales, but the secret is in the ear. The inner ear structure of this animal is highly unique to cetaceans, and so can only be shared by ancestors of the cetaceans and all of their descendants. Therefore, it is an early member of the order.
Ambulocetidae is another family of early cetaceans. They are aged at around 47.8 – 41.3 Ma, and would have likely lived in a coastal environment. Members of this family, such as Ambulocetus natans, were robust animals that would have looked more like a modern crocodile than any current whale. However, unlike crocodiles, or even Pakicetus, they had long back legs and paddle-like hands and feet for swimming more effectively, enabling a more aquatic lifestyle.
Protocetidae is an even more derived family of cetacean. The genus Rodhocetus is the most well-known of this group. The two species Rodhocetus kasrani and Rodhocetus balochistanensis age from the Lutetian (~ 47 – 46.5 Ma) of Pakistan. They are hypothesised as having been distributed across the southern coast of the Tethys Ocean. The genus members are thought to have used their feet to power swift locomotion through the surface waters while they used their tail as a rudder. On land they were less elegant and probably moved across terrain more like a modern seal rather than their terrestrially-capable ancestors.
Basilosauridae is the next stop in this journey, as they possess even more adaptations to the aquatic environment. This includes shortened neck vertebrae, similarly lengthened thoracic and lumbar vertebrae, tiny hind limbs, and a flattened wrist and lower forearm. The front limbs are more adapted to swimming than in any previous families. Dorudon is a genus of basilosaurid whale, with nostrils at the top of its head and stiff flippers for front limbs. The tail is even thought to have exhibited the split end common to modern whales.
Basilosaurus is a larger member of the family. It may sound like a ferocious dinosaur, but rest assured, it is a cetacean. This genus was named in the 19th century as “king lizard” due to its serpent-like features, however it was reclassified by Richard Owen as an early whale. The two species, Basilosaurus cetoides and Basilosaurus isis, have whale-like bodies, but their skulls look remarkably like a cross between a dolphin and an alligator. The hind limbs of this genus were very small, too small to support the animal on land but also too small to have assisted in swimming. They may have actually played a part in facilitating reproduction, but the jury is still out on this. Both genera age from the late middle and late Eocene, about 41.3 – 33.9 Ma. Basilosaurus would have been an apex predator, preying on large fish or even Dorudon. On the other hand, Dorudon is believed to have dined on cephalopods, molluscs, crustaceans and fish.
Around 34 Ma the extinction of numerous serpent-like or limbed cetaceans occurred, and the two modern cetacean suborders (Odontoceti and Mysticeti) emerged from the archaeocete whales. These diversified into a variety of environments, and now there are around 89 living species swimming globally. Whales face a number of threats, most due to human activity, so we must all try to do our part in protecting these strange mammals while admiring their exceptional history. For more information about cetaceans, conservation efforts, and to find out how to help, follow this link: https://www.worldwildlife.org/species/whale.
Image References
[1] Balaenoptera musculus, the blue whale, the largest of all cetaceans. Click Here.
[2] Pakicetus sp. by Lucas Lima.
[3] Basilosaurus sp. by Lucas Lima.
Information References and Further Sources
[1] American Museum of Natural History. (2013). ‘Pakicetus: The First Whale’. Accessed 23rd October 2019. Click Here.
[2] Benton, M. J. (2014). Vertebrate Palaeontology. 4th ed. Oxford: Wiley-Blackwell. pp. 372-375.
[3] Connor, S. (Unknown). ‘The anatomy of a whale’, BBC Earth. Accessed 27th July 2020. Click Here.
[4] Cooper, L. N., Thewissen, J. G. M., and Hussain, S. T. (2009). ‘New middle Eocene archaeocetes (Cetacea: Mammalia) from the Kuldana Formation of northern Pakistan’, Journal of Vertebrate Paleontology, 29 (4), pp. 1289-1299. Accessed 23rd October 2019. Click Here.
[5] Fahlke, J. M., Bastl, K. A. Semprebon, G. M., and Gingerich, P. D. (2013). ‘Paleoecology of archaeocete whales throughout the Eocene: Dietary adaptations revealed by microwear analysis’, Palaeogeography, Palaeoclimatology, Palaeoecology, 386, pp. 690-701. Accessed 27th July 2020. Click Here.
[6] 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), pp. 429-454. Accessed 27th July 2020. Click Here.
[7] Gingerich, P. D., ul Haq, M., Zalmout, I. S., Khan, I. H., and Malkani, M. S. (2001). ‘Origin of Whales from Early Artiodactyls: Hands and Feet of Eocene Protocetidae from Pakistan’, Science, 293 (5538), pp. 2239-2242. Accessed 27th July 2020. Click Here.
[8] Gingerich, P. D., Raza, S. M., Arif, M., Anwar, M., and Zhou, X. (1994). ‘New Whale from the Eocene of Pakistan and the origin of cetacean swimming’, Nature, 368 (6474), pp. 844-847. Accessed 27th July 2020. Click Here.
[9] Gingerich, P. D., and Russell, D. E. (1981). ‘Pakicetus inachus, a new archaeocete (Mammalia, Cetacea) from the early-middle Eocene Kuldana Formation of Kohat (Pakistan)’, Contributions From The Museum of Paleontology: The University of Michigan, 25 (11), pp. 235-246. Accessed 23rd October 2019. Click Here.
[10] Gingerich, P. D., Smith, B. H., and Simons, E. L. (1990). ‘Hind Limbs of Eocene Basilosaurus: Evidence of Feet in Whales’, Science, 249 (4965), pp. 154-157. Accessed 30th July 2020. Click Here.
[11] Madar, S. I., Thewissen, J. G. M., and Hussain, S. T. (2002). ‘Additional holotype remains of Ambulocetus natans (Cetacea, Ambulocetidae), and their implications for locomotion in early whales’, Journal of Vertebrate Paleontology, 22 (2), pp. 405-422. Accessed 23rd October 2019. Click Here.
[12] Mead, J. G., and Brownell Jr, R. L. (1993). ‘Order Cetacea’, in Wilson, D. E., and Reeder, D. M (3rd ed.) Mammal Species of the World: A Taxonomic and Geographic Reference. The Johns Hopkins University Press. pp. 723-743. Accessed 23rd October 2019. Click Here.
[13] Milinkovitch, M. C., Bérubé, M., and Palsbøll, P. J. (1998). ‘Cetaceans are highly derived artiodactyls’, The Emergence of Whales, in Advances in Vertebrate Paleobiology, 1, pp. 113-131. Accessed 23rd October 2019. Click Here.
[14] Paleobiology Database. Accessed 23rd October 2019. Click Here.
[15] Snively, E., Fahlke, J. M., and Welsh, R. C. (2015). ‘Bone-Breaking Bite Force of Basilosaurus isis (Mammalia, Cetacea) from the Late Eocene of Egypt Estimated by Finite Element Analysis’, PLOS ONE, 10 (2), e0118380. Accessed 24th October 2019. Click Here.
[16] Thewissen, J. G. M., Williams, E. M., Roe, L. J., and Hussain, S. T. (2001). ‘Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls’, Nature, 413 (6853), pp. 277-281. Accessed 23rd October 2019. Click Here.
[17] Voss, M., Antar, M. S. M., Zalmout, I. S., and Gingerich, P. D. (2019). ‘Stomach contents of the archaeocete Basilosaurus isis: Apex predator in oceans of the late Eocene’, PLOS ONE, 14 (1), e0209021. Accessed 24th October 2019. Click Here.