Basilosaurus

As the first known prehistoric whale and the earliest known fully aquatic species, Basilosaurus (Class Mammalia, Order Artiodactyla, Infraorder Cetacea, Family Basilosauridae) holds an important place in the study of whale evolution. It lived in the late Eocene Period, 34–40 million years ago, throughout the southern United States (including what is now Arkansas) and the Middle East from Egypt to Pakistan.

The first fossils of Basilosaurus, consisting of a vertebra and tooth, were found by Judge Henry Bry in Ouachita Parish, Louisiana, along the Ouachita River in 1832; he sent them to Dr. Richard Harlan at the Philadelphia Museum. Studying them along with more fossils sent to him by Judge Creagh from Alabama, Dr. Harlan misinterpreted the tooth as a shell fragment and identified the vertebra as coming from an aquatic reptile. In 1834, he named it Basilosaurus, “King Lizard.” However, Sir Richard Owen in England identified the fossils as belonging to a whale in 1839. Owen named it Zeuglodon cetoides, but the zoological rule of precedence gave priority to the original name, which became Basilosaurus cetoides.

In 1938, another vertebra was discovered in Arkansas by Katherine V. W. Palmer along Crow Creek near Crowley’s Ridge close to Forrest City (St. Francis County). Palmer was researching fossil mollusks of the southeastern United States along with G. D. Harris and E. L. Palmer. The vertebra, now housed in the Paleontological Research Institute in Ithaca, New York, is the only formally published specimen of Basilosaurus in Arkansas (although it should be noted that most fossils found in the state are never formally reported).

Basilosaurus cetoides was the apex predator of its time, reaching lengths of 15–20 meters (49–66 feet). The most recent study estimates that its thin, serpentine body weighed approximately seventeen tons. Basilosaurus isis, a species found in Egypt, was smaller than B. cetoides; its bite force was estimated to have been from 16,400 Newtons (3,700 pounds) to more than 20,000 Newtons (4,500 pounds), the largest bite force of any mammal. Canine-like teeth lined the front part of the jaws, and blade-like molars with large serrations occupied the rear, which may have allowed the animals to chew their prey, unlike modern whales. They did not possess the mass of forehead tissue known as a melon that modern cetaceans use for echolocation. They were active hunters, known to have preyed upon the young of other basilosaurids, particularly Dorudon, as well as several large fish species, including sharks.

Basilosaurus appears to have been fully aquatic, as indicated by its small pelvis and vestigial hind limbs. These limbs were far too weak to function for movement, but it has been suggested that they served as copulatory guides.

It lived in warm, shallow marine environments, although the possibility that Basilosaurus was not entirely marine, like the bull shark, cannot be discounted. The Gulf of Mexico covered what is now southeastern Arkansas 50 million years ago (mya), but by 35 mya, the ocean had receded to southern Louisiana due to cooling temperatures and the expansion of the ice caps. Basilosaurus went extinct at the beginning of the Eocene-Oligocene extinction event 33.9 mya ago, most likely due to rapid global cooling, possibly exacerbated by one or more meteor impacts, particularly the Popigai impact, which occurred, according to the most recent study, 33.9 mya, slightly later than the most commonly cited age of ~35 mya, and the Chesapeake Bay impact, dated to 35.3 mya.

For additional information:
Fahlke, Julia M. “Bite Marks Revisited—Evidence for Middle-to-Late Eocene Basilosaurus isis Predation on Dorudon atrox (both Cetacea, Basilosauridae).” Palaeontologia Electronica 15, no. 3 (2012): 32A. Online at https://palaeo-electronica.org/content/pdfs/341.pdf (accessed October 29, 2026).

Gingerich, Philip D. “Paleobiological Perspectives on Mesonychia, Archaeoceti, and the Origin of Whales” In The Emergence of Whales: Evolutionary Patterns in the Origin of Cetacea, edited by J. G. M. Thewissen. New York: Springer, 1998.

Gingerich, Philip D., B. Holly Smith, and Elwyn L. Simons. “Hind Limbs of Eocene Basilosaurus: Evidence of Feet in Whales.” Science 249 (July 13, 1990): 154–157.

Harlan, Richard “Notice of Fossil Bones Found in the Tertiary Formation of the State of Louisiana.” Transactions of the American Philosophical Society 4 (1834): 397–403. Online at https://archive.org/details/sim_transactions-of-the-american-philosophical-society_1834_4_contents (accessed October 29, 2026).

Lukashov, A. A., and V. L. Masaitis. “Popigai Impact-Explosive Crater.” Doklady Earth Sciences 525, no. 63 (2025). https://doi.org/10.1134/S1028334X25609824 (accessed October 29, 2026).

Owen, R. “Observations on the Basilosaurus of Dr. Harlan (Zeuglodon cetoides, Owen).” Transactions of the Geological Society of London 6 (1841): 69–79. Online at https://archive.org/details/transactionsof261841geol/page/68/mode/2up (accessed October 29, 2026).

Palmer, Katherine V. W. “Basilosaurus in Arkansas: Geological Notes.” AAPG Bulletin 23, no 8 (1939): 1228–1229.

Paul, Gregory S., and Asier Larramendi. “Further Trimming Down the Marine Heavyweights: Perucetus colossus Did Not Come Close to, Much Less Exceed, the Tonnage of Blue Whales, and the Latter Are Not Ultra-Sized Either.” Palaeontologia Electronica 28, no. 1 (2025): a6.

Scotese, Christopher R. “An Atlas of Phanerozoic Paleogeographic Maps: The Seas Come In and the Seas Go Out.” Annual Review Earth and Planetary Sciences 49 (2021): 679–728. https://doi.org/10.1146/annurev-earth-081320-064052 (accessed October 29, 2026).

Snively, Eric, Julia M. Fahlke, and Robert C. Welsh. “Bone-Breaking Bite Force of Basilosaurus isis (Mammalia, Cetacea) from the Late Eocene of Egypt Estimated by Finite Element Analysis.” PLoS One 10, no. 2 (2015): e0118380. https://doi.org/10.1371/journal.pone.0118380 (accessed October 29, 2026).

Voss Manja, Mohammed Sameh M. Antar, Iyad S. Zalmout, and Philip D. Gingerich. “Stomach Contents of the Archaeocete Basilosaurus isis: Apex Predator in Oceans of the Late Eocene.” PLoS One 14, no. 1 (2019): e0209021. https://doi.org/10.1371/journal.pone.0209021 (accessed October 29, 2026).

Joseph Daniel
Little Rock, Arkansas