Basilosaurus: The Tale of King Basil

A Reconstruction of how Basilosaurus (and the one and only King Basil) would have looked!
Image Credit: Dmitry Bogdanov,

35 Million years ago during the Late Eocene period, in a shallow sea that would one day become the Sahara Desert, a king ruled the waters. His name was Basil, he was 20 years old and one of the largest of his kind. He has reached this age in large part because of the excellent early parental care of his mother, a key trait of mammals like him. He was taught how to navigate, to hunt and his mother used every ounce of her might to protect him from danger. Now he is in his prime, has fathered many offspring and has his pick of food that sustains his enormous appetite. Life is good for the king and there is nothing that can threaten him……for now.

Basil is a Basilosaurus, a name meaning “King Lizard” in Latin. However Basil and his kind are far from lizards! They are part of what was at the time a relatively new group of sea faring animals; the whales. Two species of Basilosaurus have been discovered. The first is Basilosaurus cetoides, which ranged from the coast of what is now Alabama and New Mexico in the USA to Egypt. The second is Basilosaurus isis, which swam in waters covering much of Morocco and Egypt. Combining the localities of these two species we can see that Basilosaurus had a wide geographical distribution, stretching across almost half the globe. Along this distribution both Basilosaurus species are thought to have resided in coastal and shallow water regions rather than the deep ocean; a slightly unusual habitat considering their huge size. Furthermore both Basilosaurus species lived in an ocean which no longer exists! This ocean was known as the Tethys, a once mighty watery expanse that linked the Indian and Atlantic Ocean. During the Late Eocene period it covered where much of Arabia, North Africa and the Mediterranean Sea area are now. The Tethys would eventually disappear around 23 million years ago but the coastal margins of this strange, primordial ocean was a place where Basilosaurus (and many other marine fauna) called home.

The fossilised skull of Basilosaurus isis (top) and Basilosaurus cetoides (bottom). Note the slight differences in teeth and skull shape between the two species.
Image Credit: Ghedoghedo, Ninjatacoshell,

The first Basilosaurus fossils were found in the 1830s. Rather curiously, when they were first unearthed, the large vertebrae were used as furniture! After this the fossils were initially studied by Richard Harlan, a Philadelphia based Naturalist, before being passed on to Sir Richard Owen, the famous British Naturalist who’s best known for coining the term “dinosaur” and for founding the Natural History Museum in London. Harlan at first thought that these bones belonged to a giant marine reptile, partly due to the animal’s unusually long body (like an actual sea serpent!) and superficially reptile-like skull. This is why it was given the name of Basilosaurus, or “King Lizard”. However later studies revealed Basilosaurus’ characteristic mammalian features. For example it was found to have heterodont dentition, which means they have teeth of various shapes each with their own function (in this case sharp incisors and flattened serrated molars). In contrast reptiles have homodont dentition, meaning their teeth are all the same shape and have the same function. Once he realised the mistake Sir Richard Owen suggested that this animal’s name should be changed to Zeuglodon; which means “Yoke Teeth” on account of their distinct shape. However this would have violated the golden rule of scientifically naming an organism; “The first name that is given is the one that is always used (unless the name is already in use or the new organism turns out to actually be one we’ve previously discovered and named)”. Therefore the name Basilosaurus stayed, resulting in the rather bizarre situation where a whale (which is a mammal) is called a lizard (which is a reptile). A great example of how palaeontology is full of weird names that sometimes don’t make sense!

A serpentine shape isn’t the only feature that differentiated Basilosaurus from modern whales. It also possessed small, stumpy and external hind limbs. While modern whales also possess hind limbs they are internal, hidden beneath the large layers of fat, muscle and skin. These tiny legs are remnants from a time (roughly 20 million years or so before Basilosaurus) when the ancestors of whales were small, four-legged hoofed mammals that were first dipping their toes back into water. Whereas these ancestors used their hind limbs for walking, Basilosaurus’ were way too small and stumpy for such a “feet” (sorry, couldn’t resist the pun). Instead it’s thought that the hind limbs were used to help lock and intertwine the long bodies of two Basilosaurus’ together during mating. Basilosaurus also lacked some characteristic anatomy that modern whales possess. This included a “melon” in their heads; a mass of tissue which modern whales use for echolocation. Furthermore Basilosaurus’ blowhole, which it used to take breaths at the surface, was positioned further forward (between the snout and top of its head) than in modern whales. As Basilosaurus preferred shallow water environments it is also thought that they didn’t dive to great depths (like Sperm Whales and Cuvier’s beaked whales do).

Mounted skeleton of Basilosaurus isis from the Museum d’Histoire Naturelle in Nantes, France
Image Credit: Asmoth,,_Nantes_History_Museum_01.jpg

Basilosaurus was a giant of its time; measuring up to 20 metres long and weighing up to 20 tons. This meant that it was the undisputed top predator of the Late Eocene seas. The size of Basilosaurus varied between the two species; with B.cetoides measuring between 17-20 metres while B.Isis was between 15-18 metres. As you can see from this range in lengths there was also size variation between members of the same Basilosaurus species. One reason for this is that they exhibited sexual dimorphism. We know this because on average male Basilosaurus vertebrae measure 20% longer than in females of the same age. In terms of its lifestyle a modern day comparison to Basilosaurus might be the Orca (aka “The Killer Whale”). Both are large predatory whales that are top predators in their environments. Like Orcas, the diet of Basilosaurus consisted of fish (e.g. Pycnodus) and other marine mammals. One particular marine mammal that was on the menu was Dorudon atrox, a 5 metre long prehistoric whale which actually belonged to the same family as Basilosaurus; “The Basilosauridae” (guess what animal the family was named after). Fossils discovered in the last two decades have shown a direct predator-prey relationship between these two whales. Skulls of Dorudon have been found with bite marks that perfectly match the size and shape of Basilosaurus teeth. Furthermore these marks form a pattern on the Dorudon skull that indicate that the bite was across the head, a tactic that’s often used by predators to quickly immobilise and cripple their prey. In addition approximately 50% of these skulls were deduced to be of young Dorudon. This was due to the presence of deciduous (i.e. baby) teeth, while accompanying vertebrate had open growth plates which is the part of the bone that grows, before hardening when animal matures. This suggests that Basilosaurus targeted Dorudon “nursery groups” that consisted of young Dorudon and a few adults. It undoubtedly would have been a waking nightmare for the young Dorudon to see a large and hungry Basilosaurus burst into their nursery. But from the Basilosaurus’ perspective hunting the more vulnerable prey (e.g. young, old, injured and/or sick) gives them a greater chance of getting a vital meal, one that could prevent them from starving, and is a tactic employed by every predator from Lions, to Eagles, to Orcas.

King Basil eventually reached the end of his road as the years of swimming, hunting and fighting finally caught up to him. His latest wounds, sustained from a fight with a younger and faster male, are this time going to be fatal. As blood loss starts to take its toll Basil takes one last breath at the surface, before his body eventually gives out. The king has been dethroned. The younger male has long since swum off to start his own reign, but the line of succession will eventually end 33 million years ago. A drop in sea levels and a changing climate would push all Basilosaurus to extinction. As for Basil his body eventually sinks to the bottom of the sea and over time is picked clean by scavengers, buried by ocean sediment, and undergoes the fossilisation process that will turn his bones into hard rock. 35 million years later his bones will eventually resurface as another Basilosaurus fossil, with an intrepid palaeontologist meticulously dusting the sand of the Sahara Desert away to expose his remains. His final resting place is Wadi Al Hitan, a fossil site located 150km South-West of Cairo in Egypt. This place is also known as the “Valley of the Whales”. This is fitting, as just like the Egyptian pharaohs buried in the “Valley of the Kings”, Basil was a monarch among his own kind.

References/Further Reading

Fahlke 2012 paper that further examined the bite marks on Dorudon skulls, reinforcing the hypothesis that they match the teeth of Basilosaurus and that Basilosaurus actively hunted Dorudon

Fahlke, J. M. (2012). Bite marks revisited—evidence for middle-to-late Eocene Basilosaurus isis predation on Dorudon atrox (both Cetacea, Basilosauridae). Palaeontologia Electronica, 15(3), 32A.

Voss et. al. 2019 paper describing the preserved stomach contents of a Basilosaurus isis fossil

Voss M, Antar MSM, Zalmout IS, Gingerich PD (2019) Stomach contents of the archaeocete Basilosaurus isis: Apex predator in oceans of the late Eocene. PLoS ONE 14(1): e0209021.

Uhen 2004 paper that first described the bite marks on Dorudon skulls and suggested that they were due to Basilosaurus predation

Uhen MD. Form, function, and anatomy of Dorudon atrox (Mammalia, Cetacea): an archaeocete from the middle to late Eocene of Egypt. University of Michigan Papers on Paleontology. 2004; 34: 1–222.

Fossilworks database on Basilosaurus species, synonyms and papers related to it

Fossilworks, “Basilosaurus Harlan 1834 (whale)”, Fossilworks,

The New York Institute of Technology College of Osteopathic Medicine (try saying that five times fast!) page describing Basilosaurus (written by Robert Boessenecker and Jonathan Geisler)

Boessenecker, Robert, Geisler, Jonathan, “Basilosaurus spp.”, College of Osteopathic Medicine, The New York Institute of Technology,

A Comparative Anatomy website page, from the University of the Cumberlands, giving an overview of Heterodont and Homodont Dentition

Comparative Anatomy, “The Teeth of Vertebrate Animals”, Comparative Anatomy,

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