Extinct animal: BasilosaurusModern animal: Killer WhaleFuture animal: Speckled Whale
The basilosaurus was the oldest known marine cetacean which lived 34 to 40 million years ago during the middle to late Eocene Epoch. Its fossilized remains were first discovered in 1840 in Louisiana, United States, and was mistaken for a reptile, hence the suffix in its name. It measured about 18 metres in length, and may have been the largest animal during that time.



Fig. 1: Location of cetacean fossils mapped on a reconstructed map of the Eocene epoch

During the middle Eocene epoch, the overall temperatures were much higher than today, where most of the time it would be warm and rainy. The global climate was much more even; the temperature difference from the equator to the pole was half of todays and deep ocean currents were remarkably warm. The lands were heavily forested, with temperate forests extending even into the Polar Regions. Continents during the middle Eocene epoch were much more connected than they were today. North America and Europe were connected through Greenland and perhaps North America and Asia were also connected through the Bering Strait. Antarctica and Australia, which were once connected, began to separate and created a deep water passage in between the two. This created the circum-Antarctic Current, which changed ocean circulation pattern and also affected the global heat transfer, leading to the cooling which occurred at the end of the Eocene.

During the late Eocene epoch, however, the mean annual temperature was lowered significantly due to the new ocean circulation, resulting in a larger variety of both temperature and seasons. The lower temperature led to the increase of body size in mammals, as this conserves heat more effectively. The lands, too, were affected, and shifted towards more of savannah-like vegetation, with the reduction of forests.

Basilosaurus were probably distributed throughout the tropical and subtropical seas in the world (fig. 1), around the Tethys Sea in particular where the waters were warm and full of life. They tended to remain in the warm shallow waters at the time, as there was a large variety of prey and more importantly, it was an ideal environment for their long, eel-like body shape.

The warm oceans were full of fish and sea life. Species of plankton, fish, crabs, snails, echinoderms, cephalopods, bivalves and crayfish thrived during this time. Larger marine animals, such as the first sharks (mako and giant-toothed white sharks) and sirenians, appeared. Researchers found fossilised stomach contents, which revealed that basilosaurus ate fish, sharks and mollusks. Teeth marks found on the skulls of some young Dorudon may suggest that they also fed on baby Dorudon, another cetacean. Being the largest animal during that time, the basilosaurus had no predators, similar to killer whales today. Nevertheless, animals such as sharks and crocodiles may be the basilosaurus’ competitors as they, too, hunt mostly on other fish.


Balisaurus have came to be after roughly 20 million years after the first cetacean, pakicetid, and within that amount of time, they have evolved to be well adapted to their environment, and posses qualities of both mammals and marine mammals (fig. 2).



Fig. 3: Drawing of the skeleton of the basilosaurus
Fig. 3: Drawing of the skeleton of the basilosaurus

The basilosaurus had strong teeth which helped it hunt. The sharp incisors at the front were used to seize and grab onto their prey, while the back molars were used to slice up their prey (unlike modern killer whales which swallow their prey in chunks) . One of its features which hint its land mammal origin was its set of primitive mammalian teeth, which consisted of incisors, canines, premolars and molars. This classification of teeth in their functional groups is called heterodonty.

Basilosaurus have incisors at the front, and molars at the back. Though the number of teeth compared to modern whales is much smaller, the largest difference is how modern whales have homodonty, the condition of having teeth all similar to one another.

How it helped the animal survive:
Without this set of strong teeth, the basilosaurus will have great difficulties in attacking and eating up larger prey, leading to its extinction or perhaps evolution.

Environmental pressure:
As basilosaurus were carnivorous and had such a large body, they required the strong teeth to prey on large, fast prey (such as sharks) which were escaping. If they had small, weak teeth will make it difficult to lock onto the prey and tear it apart.

Elongated Body Shape

Fig. 4: Basilosaurus were so long and elongated they were originally mistaken for a reptile
Fig. 4: Basilosaurus were so long and elongated they were originally mistaken for a reptile

One of the most prominent and curious structural adaptation of the basilosaurus is its excessively elongated, streamlined body shape , and were initially mistaken for being reptiles such as lizards or snakes (fig. 4). It measures about 18 metres in length, making it the largest animal living at that time. This adaptation makes it easier for basilosaurus to weave in and out of obstacles in the shallow warm waters.

How it helped the animal survive:
It is believed that its long, body shape was evolved to allow the basilosaurus to hunt with more flexibility in shallow warm waters where they generally lived in.

Environmental pressure:
Its extreme change in size may be due to the fact that they were once hunted by other marine animals, such as sharks. This adaptation had stopped it from being hunted down, it had the allowed the basilosaurus to hunt down the shark in return.

Inner Ears

Fig. 5: From left to right: a land mammal, a land cetacean, an early marine cetacean, a modern dolphin
Fig. 5: From left to right: a land mammal, a land cetacean, an early marine cetacean, a modern dolphin

The inner ear of humans consists of three canals which were filled with fluids. As the head tilts, the fluid in the canals moves accordingly. This is then registered by the nerve cells, which then send the signals back to the brain, and adjusts the balance. However, intensive acrobatic behaviour messes up these signals and causes dizziness.

As cetaceans, which were mammals whose inner ears are quite similar in over size to humans, require the ability to perform extreme movements, the canals of their inner ear become so small that the fluid held inside barely moves (fig. 5). This allows the basilosaurus to twist and turn at high speeds without getting disorientated.

How it helped the animal survive:
After adapting to living in the sea, basilosaurus have shrunken their inner ears, or sometimes called the organ of balance, which enabled them to hunt down prey while being highly aerobic and agile swimmers without becoming dizzy.

Environmental pressure:
After starting to live in the water for prolonged amounts of time, ancestors of cetaceans have evolved to shrink the size of its inner ear canals, which contributes greatly to its speed and agility underwater that helped it escape from previous predators.

Reduced Hind Legs

Fig. 6: Skeleton of the basilosaurus with magnification on legs
Fig. 6: Skeleton of the basilosaurus with magnification on legs

The hind legs of the basilosaurus had evolved into small and useless hind legs (fig. 6) which, although they remain visible as small flaps near the end, cannot be used to walk or to aid swimming. However, there had been numerous suggestions that these small hind “legs” were used to aid copulation by locking their long, narrow bodies.

How it helped the animal survive:
The reduction of hind legs made the basilosaurus a faster swimmer, as it became more streamlined while it relied more on the movement of its body to swim rather than its limbs. One theory of how these small hind legs were used was that they may have been used as sexual claspers, where they can hold onto each other when mating.

Environmental pressure:
The reduction of hind legs was most likely due to the overall adaptation of the basilosaurus living in the water, where the hind legs were not needed and instead, prevented the streamline of the body. As a result, while the tail grew longer for propelling and swimming, the hind legs shrunk into small, useless flaps.

Raised Nostrils

Fig. 7: A crocodile hides while breathing through its raised nostrils

As basilosaurus were once land animals which breathed through the oral and naval cavities, basilosaurus have began to evolve so that their nostrils are higher up along their head (fig. 2). This makes it easier for basilosaurus to breathe, as they wouldn't have to lift their entire head out of the water, similar to the way a crocodile's nostrils and eyes are raised out of the water (fig. 7).
How it helped the animal survive:
The nasal cavities allow the basilosaurus to survive by allowing it to breathe conveniently. Without them, they will die from suffocation as they had evolved so that they are unable to breathe through their mouth.

Environmental pressure:
As the land mammals which basilosaurus had originally evolved from spent more time in the water, the nasal opening began to shift upwards (though it has not reached the point of modern cetaceans such as blue whales, killer whales and dolphins which have a blowhole on the top) which allows it to inhale, and then submerge with more convenience than if it were at the tip of the snout.

Possible Selection Pressure

The extravagantly elongation of the basilosaurus’ body aided their ability to hunt in shallow environments, but it may also be due to this characteristic that the basilosaurus became extinct at around the end of the Eocene epoch. During that time, Earth began to cool down, and oceans became dramatically colder. Antarctica split from South America, and had formed its own permanent ice cap, sapping away the water which once flowed down the oceans. This caused a dramatic drop in global sea levels, where shallow coastal waters, the hunting grounds of the basilosaurus, disappeared, forcing them to hunting in deeper waters. However, due to its long, eel-like body, it struggles to control itself in the stronger currents of the oceans, and lacks the power to dive, thus became extinct due to its inability to survive in the change in environmental conditions.

On the other hand, dorudon, another type of cetacean which lived at the same time, survived. Though they had a smaller body shape that spans 5 metres long, their shorter structure and muscular tail allowed them to survive in the oceans even after the cooling down of Earth. Furthermore, they developed the behaviour of living in groups, which protected them from predators such as the megalodon, an ancestor of the shark. Another way they escaped from their predators is thanks to their mammalian ancestors. As a mammal, dorudons are warm blooded, which is why they are able to live even in the colder regions of the earth. However, megalodons, which are cold blooded, cannot live in such cold waters without their body functions shutting down.


Images Sources

Fig. 1: Location of cetacean fossils mapped on a reconstructed map of the Eocene epoch. Source: http://people.trentu.ca/sarahdungan/Trent_marine_mammals/Lectures_files/whale%20evolution%20overview.pdf
Fig. 2: Labelled diagram of a reconstructed image of a basilosaurus. Source:
Fig. 3: Drawing of the skeleton of the basilosaurus. Source:
Fig. 4: Basilosaurus were so long and elongated they were originally mistaken for a reptile. Source: http://i291.photobucket.com/albums/ll314/mjsjobs/whale-evolution/JPG_LENS_we_basilosaurus-woodcut-2.jpg
Fig. 5: From left to right: a land mammal, a land cetacean, an early marine cetacean, a modern dolphin. Source: http://news.sciencemag.org/sciencenow/assets/2002/05/10/200251011.jpg
Fig. 6: Skeleton of the basilosaurus with magnification on legs. Source: http://palaeo.gly.bris.ac.uk/palaeofiles/whales/pictures/basilosaurus.gif
Fig. 7: A crocodile hides while breathing through its raised nostrils. Source: http://www.abc.net.au/reslib/200812/r326188_1461991.jpg

By Joyce Ng 10A