An Ultimate Guide to Basilosaurus: The King Lizard

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NameBarosaurusDietCarnivorous (piscivorous)
Name MeaningThe King LizardHeight2 meters (6.56 feet)
PronunciationBah-sil-oh-saw-rusLength15–20 meters (49–66 feet)
EraCenozoic – Paleogene PeriodWeight5.8 tons (11,600 lbs)
ClassificationMammalia, Artiodactyla, & BasilosauridaeLocationNorth America, Africa and Asia

Basilosaurus Pictures

Computer generated image of Basilosaurus
Computer generated image of Basilosaurus | SEBASTIAN KAULITZKI via GettyImages

The Basilosaurus

Gage Beasley Prehistoric's Basilosaurus Concept
Gage Beasley Prehistoric’s Basilosaurus Concept

Basilosaurus was a large sea creature that lived during the Eocene Epoch approximately 41 to 34 million years ago. 

First described in 1834, this giant marine animal got the name Basilosaurus, which means “king lizard,” because it was initially thought to be a reptile. 

Basilosaurus was later identified as an ancient whale. 

It grew to a length of about 60 to 70 feet (18–20 meters) and was arguably the largest animal from the Paleogene Period.

Basilosaurus was a top predator of the prehistoric Tethys Sea where it lived. 

Being the first proto-whale, Basilosaurus a lot of characteristics with its modern day counterpart
Being the first proto-whale, Basilosaurus a lot of characteristics with its modern day counterpart | Lythronax via Walking With Wikis

It preyed on different marine animals, including fish, sharks, and marine mammals. 

Basilosaurus was the first prehistoric whale known to science. 

Most of what is currently known about this giant whale is based on fossils discovered in Alabama. 

However, fossils belonging to a second Basilosaurus species have been found in other locations around the world, including Africa and Asia. 

In this article, we’ll detail some of the most interesting facts about the Basilosaurus

Gage Beasley's Prehistoric Shirt Collection
Gage Beasley’s Prehistoric Shirt Collection
Gage Beasley's Prehistoric Plush Collection
Gage Beasley’s Prehistoric Plush Collection

Physical Characteristics

Although it is considered a prehistoric ancestor of cetaceans (modern whales and dolphins), Basilosaurus looked considerably different from them. 

It had an extremely long but slender body with a narrow snout lined with numerous teeth of different shapes. 

The overall body shape of the Basilosaurus is more similar to that of prehistoric marine reptiles

This is why it was initially misidentified as a reptile. 

Basilosaurus was one of the largest marine animals of its day. 

The largest species in the genus grew to an impressive size of about 17 to 20 meters (56–66 feet) long and weighed over 5.8 tons. 

Gage Beasley Prehistoric's Basilosaurus Size Comparison Chart
Gage Beasley Prehistoric’s Basilosaurus Size Comparison Chart

This makes it one of the largest known animals alive after the dinosaurs went extinct at the end of the Mesozoic. 

Like most marine mammals, Basilosaurus had a blowhole in its skull. 

But this whale’s blowhole was positioned as close to the front of its head as those of its living relatives.  

One of the most distinctive features of this Basilosaurus was its vestigial external hindlimbs. 

The limbs, which are missing in modern marine mammals, had distinct knees and toes. 

It was about 35 centimeters long (14 inches) and had only three digits. 

But the joints in the limbs were fused, so they offered limited mobility compared to the more primitive whale species. 

Habitat and Distribution

Although the Basilosaurus had rudimentary hindlimbs, it did not use them for any sort of terrestrial locomotion. 

This, and the fact that it had an eel-like body, suggests that the whale was fully aquatic. 

Basilosaurus lived in shallow marine environments, often in coastal areas or near continental shelves. 

Unlike modern whales, the vertebrae of the Basilosaurus were not made of solid bone.

Instead, they were filled with fluid, an indication that this ancient whale wasn’t a deep-sea species. 

The hollow bones would have crumpled under intense pressure in the deep sea. 

This suggests that the Basilosaurus spent most of its life close to the water’s surface.

Basilosaurus near to the surface
Basilosaurus near to the surface | Thedark85 via Walking With Wikis

Fossils of this giant whale have been discovered in locations that were once submerged under the Tethys Sea

This was an ancient sea that covered parts of the present-day southeastern United States, North Africa, and parts of the Middle East.

Basilosaurus lived during the Late Eocene Epoch. 

During this period, the Earth’s climate was relatively warm, with global temperatures generally higher than current conditions. 

This epoch is one of the warmest intervals of the Cenozoic Era.

This warm climate likely contributed to the expansion of subtropical and tropical regions, creating the shallow marine environments where the Basilosaurus lived. 

A group of Basilosaurus on shallow waters
A group of Basilosaurus on shallow waters | Andrew R Janson via Wikipedia CC0

The Tethys Sea was one of the most notable features of the Eocene Epoch. 

The region around the Tethys Sea was rich in marine life, providing an abundance of food sources for creatures like Basilosaurus.

Behavior and Diet

Basilosaurus had a long eel-like body and also moved with undulating up and down motions similar to an eel. 

It had a small fluke-like tail that supported vertical motion. 

Basilosaurus' form of location is similar to that of an eel
Basilosaurus’ form of location is similar to that of an eel | Robongio via ARK: Survival Evolved Wiki

This would make the Basilosaurus the oldest known whale genus with evidence of tail fluke. 

Experts think Basilosaurus was a solitary animal, which means it did not live in social groups or pods like some of its present-day relatives. 

One piece of evidence for this is the relatively small brain of this giant whale in contrast to its massive size. 

This small brain-to-body ratio suggests that the giant whale wasn’t capable of complex social behavior. 

Basilosaurus was a carnivore whose diet included fish, other marine mammals, and possibly invertebrates such as squids. 

This giant whale had reptilian features, including a skull that was superficially similar to that of crocodilians. 

The skull also had large jaw muscles and a fearsome dentition that included canine-shaped incisors and flattened, serrated molars towards the back of its mouth. 

Basilosaurus' jaw muscles suggests that it had a strong bite force
Basilosaurus’ jaw muscles suggests that it had a strong bite force | Snively E et al. via Wikipedia CC BY-SA 2.5

This complex dentition suggests that the Basilosaurus was an active predator. 

Unlike modern marine predators that mostly swallow large chunks of food whole, the dentition of the Basilosaurus suggests that it was capable of chewing its food before swallowing. 

Basilosaurus was an active predator that would have relied on a combination of stealth and speed to capture prey. 

Being a solitary hunter, Basilosaurus may have pursued prey quickly through the water before attacking. 

Once caught, the sharp teeth of this giant whale could tear into flesh and deliver a fatal bite to the prey’s head before subsequently piecing it apart.  

Life Cycle

The lifecycle of the Basilosaurus is comparable to that of modern whales and dolphins. 

It was most likely viviparous, meaning it gave birth to live offspring instead of laying eggs. 

A Basilosaurus with its young
A Basilosaurus with its young | Vaderxl via Walking With Wikis

There’s very little knowledge of the mating behavior of this prehistoric whale. 

Experts believe the vestigial hind limbs of the Basilosaurus were probably useful for holding the female during mating. 

After mating, female Basilosaurus individuals carried their developing fetuses inside their bodies until fully developed.

Since they were fully aquatic, births likely took place in the water. 

The calf most likely could swim and breathe almost immediately after birth. 

Calves would have relied on their mothers for nourishment and protection, just as modern whales do. 

The exact pattern of growth, growth stages, and how long the Basilosaurus lived isn’t clear from the fossil record. 

Basilosaurus might have the same life cycle as its modern day counterpart
Basilosaurus might have the same life cycle as its modern day counterpart | Dominik Hammelsbruch via Wikipedia CC BY-SA 4.0

But it was probably similar to that of their modern relatives. 

Evolution and History

Basilosaurus represents a fairly low branch in the evolutionary tree of whales. 

Roughly 10 million years before the genus evolved, their ancestors were still walking on land in their terrestrial homes. 

A possible terrestrial ancestor of the Basilosaurus is the Ambulocetus.‭ 

This is a prehistoric cetacean that lived both on land and in the water during the Early Eocene. 

Fully marine species, like the Pakicetus, evolved during the Middle Eocene and gave birth to Late Eocene groups like the Basilosaurus. 

One piece of evidence for the relative primitiveness of the Basilosaurus is the presence of their unusually long front flippers, which still had a rudimentary elbow. 

Skeleton of a Basilosaurus showcasing its short limbs
Skeleton of a Basilosaurus showcasing its short limbs | Tim via Wikipedia CC BY-SA 2.0

Most of the modern whale species that evolved after the Basilosaurus did not have this feature (except the pinnipeds). 

Basilosaurus did not have the telescoping skull seen in present-day whales. 

Instead, they had an elongated jaw similar to that of prehistoric crocodilians. 

Their jaws were also quite powerful, with a well-developed dentition different from that of their ancestors. 

The large jaws of the Basilosaurus seem like they were adapted to a diet that included large fish and smaller whales.

Basilosaurus's large jaw clearly indicated that it fed on large aquatic life forms
Basilosaurus’s large jaw clearly indicated that it fed on large aquatic life forms | Ghedoghedo et al. via Wikipedia CC BY-SA 3.0

Basilosaurus was a specialized marine mammal with no direct living ancestor. 

It went extinct along with other prehistoric whale species during the Eocene–Oligocene extinction event, which occurred roughly 34 million years ago. 

Interactions With Other Species

As an apex predator, Basilosaurus was most likely big enough to take on any other animal in the Late Eocene Tethys Sea. 

The pelagic environment of this warm tropical sea had an abundance of seagrasses which would have attracted numerous marine animals like sea turtles, fish, and various prehistoric mammals. 

Prehistoric sharks like the Galeocerdo alabamensis, Otodus, Squatina prima, and Isurus praecursor were particularly numerous in the Tethys Sea and may have formed the bulk of this giant whale’s diet. 

Other prehistoric whales, such as the Dorudon, lived alongside the Basilosaurus

However, most of these whales were smaller in size. 

Basilosaurus isis (top) and Dorudon atrox (bottom) skeletons compared
Basilosaurus isis (top) and Dorudon atrox (bottom) skeletons compared | Manja Voss et al. via Wikipedia CC BY-SA 4.0

Although it was a predator itself, the difference in size meant the Basilosaurus likely preyed on both adults and juvenile Dorudon

Scientists have found fossils of Dorudon with distinctive bite marks on their skulls, which suggests that they were prey for the Basilosaurus

The finding confirms that the Basilosaurus was an apex predator in its ecosystem, capable of killing other competitors. 

Despite this, it was probably not entirely invincible.

Marine mammals were particularly vulnerable during childbirth, and juveniles probably fell prey to other giant predators as well. 

Cultural Significance

Basilosaurus was officially named and described during the 19th century, which means it was one of the first prehistoric whales known to science. 

Interestingly, fossils of this ancient whale had been around even before it was officially identified. 

Skeleton of B. isis at Wadi El Hitan
Skeleton of B. isis at Wadi El Hitan | Mohammed ali Moussa via Wikipedia CC BY-SA 3.0

Several decades before it was officially recognized as a fossil, residents of the southeastern United States used petrified materials that were later identified as prehistoric whale bones as andirons for fireplaces and foundation posts for houses.

When Basilosaurus was eventually identified in the 19th century, it suffered a major identity crisis. 

The earliest description of this whale identified it as a reptile because of its anatomical similarities to prehistoric marine reptiles like the mosasaurs. 

Although this was later found to be inaccurate, it was already too late to rename the king lizard, and an attempt to reintroduce a new name did not work. 

Basilosaurus is well known in the world of paleontology. 

The fact that it lived relatively early during the evolution of marine mammals makes it quite an interesting animal to study. 

It represents a pivotal evolutionary stage in the transition from terrestrial ancestors to fully marine aquatic mammals. 

Basilosaurus is also well-known to the general public, especially in parts of the United States. 

Fossils of this whale were used as part of a popular hoax in 1845 by Dr. Albert Koch. 

Albert Koch's "Hydrarchos" fossil skeleton from 1845, two basilosaurus or Pontogeneus skeletons tied together
Albert Koch’s “Hydrarchos” fossil skeleton from 1845, two basilosaurus or Pontogeneus skeletons tied together | Image via Wikipedia Public Domain

Koch made a composite from numerous Basilosaurus bones and remains from other animals. 

He went on a touring display with this bone which he presented to the general public as a sea serpent. 

Today, Basilosaurus is recognized as the official state fossil of Mississippi and Alabama due to its abundance in both locations. 

Basilosaurus has been featured in the BBC documentary “Walking with Beasts and Sea Monsters.”

It was also referenced in the popular Moby Dick novel written by Herman Melville. 


Basilosaurus is a prehistoric whale genus that lived during the Late Eocene Epoch. 

Although closely related to modern whales, Basilosaurus looked superficially similar to prehistoric crocodiles, with a narrow skull and powerful dentition efficient for tearing prey apart. 

This led to confusion about the whale’s identity during the early years of its discovery. 

Given its size and formidable appearance, Basilosaurus was undoubtedly an apex predator in its ecosystem.

It lived in the prehistoric Tethys Sea, a warm tropical environment during the Late Eocene. 

Basilosaurus preyed on different types of fish, sharks, and other marine mammals. 

Studying the Basilosaurus and its unique anatomical features has helped scientists better understand the evolution of modern whales and the relationship between the different members of this group. 

As one of the most well-known marine creatures, this prehistoric whale continues to fascinate paleontologists and the general public.


Was Basilosaurus bigger than a blue whale?

No. the Basilosaurus wasn’t bigger than the blue whale. 

Although it was the biggest whale from the Eocene Epoch, it wasn’t as big as many of the other whale species that evolved after it. 

With a maximum length of about 29.9 meters and a weight of up to 199 tonnes, the blue whale is the largest animal to have ever existed. 

How did Basilosaurus go extinct?

Climate change is the main factor responsible for the Basilosaurus’ extinction. 

An abrupt cooling of the earth’s climate caused a rapid change in oceanic circulation at the end of the Eocene Epoch. 

This led to the disappearance of the Basilosaurus and other ancient whale species. 

Did Basilosaurus have any natural predators?

As a large and formidable predator itself, Basilosaurus likely had few natural predators in its ecosystem. 

However, young or weak individuals might have been susceptible to predation by larger marine predators of the time, such as the Dorudon.


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