Basilosaurus

Jurassic Park Builder Artwork

***Basilosaurus*** was a carnivorous, prehistoric cetacean that lived 40-35 million years ago in the Eocene epoch.

The head of *Basilosaurus* did not have room for a melon like modern day toothed whales, and the brain was smaller in comparison as well. It is believed that they therefore did not have the social capabilities of modern whales. The skull of *Basilosaurus* is asymmetrical like in modern toothed whales, and not, as previously assumed, symmetrical like in baleen whales and artiodactyls closely related to cetaceans. In modern toothed whale this asymmetry is associated with high-frequency sound production and echolocation, neither of which is thought to be present in *Basilosaurus*. This cranial torsion probably evolved in protocetids and basilosaurids together with directional underwater hearing and the sound receiving apparatus in the mandible (the auditory fat pad and the pan bone (thin portion of mandible). In the basilosaur skull, the inner and middle ear are enclosed by a dense tympanic bulla. The synapomorphic cetacean air sinus system is partially present in basilosaurids, including the pterygoid, peribullary, maxillary, and frontal sinuses. The periotic bone, which surrounds the inner ear, is partially isolated. The mandibular canal is large and laterally flanked by a thin bony wall, the pan bone or acoustic fenestra. The basilosaurid ear has a large external auditory meatus, strongly reduced in modern cetaceans, but, even-though this was probably functional, it can have been of little use under water. No complete *Basilosaurus* skeleton is known, but several attempts have been made to reconstruct the vertebral column from partial skeletons. More complete fossils uncovered in Egypt in the 1990s, allowed a more accurate estimation: the vertebral column of *B. isis* has been reconstructed from three overlapping skeletons to a total of 70 vertebrae with a vertebral formula interpreted as 7 cervical, 18 thoracic, 20 lumbar and sacral, and 25 caudal vertebrae.

*Basilosaurus* has an anguilliform (eel-like) body shape because of the elongation of the centra of the thoracic through anterior caudal vertebrae. In life, these vertebrae were filled with marrow, and, because of the enlarged size, this made them buoyant. From this it can be deduced that *Basilosaurus* swam predominantly in two dimensions at the sea surface, in contrast to the smaller *Dorudon* which was probably a diving, three dimensional swimmer. The skeletal anatomy of the tail suggests that a small fluke was probably present, which would have aided only vertical motion. Most reconstructions show a small, speculative dorsal fin similar to a rorqual whale’s, but other reconstructions show a dorsal ridge. A 16 m (52 feet) individual of B. isis had 35 cm (14 inches) long hindlimbs with fused tarsals and only three digits. The limited size of the limb and the absence of an articulation with the sacral vertebrae, makes a locomotory function unlikely.

## Description

Measuring 15–20 m (49–66 ft), *Basilosaurus* is one of the largest-known animals to exist between the K–Pg extinction event 66 million years ago (mya) and around 15 million years ago when modern cetaceans began to reach enormous sizes. *B. isis* is slightly smaller than *B. cetoides*, with *B. isis* being 15–18 m (49–59 ft) long and *B. cetoides* being 17–20 m (56–66 ft) long. *Basilosaurus* is distinguished from other basilosaurids genera by its larger body size and its more elongated posterior thoracic, lumbar, and anterior caudal vertebrae. *Basilosaurus* does not have the vertically oriented metapophyses seen in its closest relative the basilosaurid known as *Basiloterus*. *Basilosaurus* is considered to be the largest of archeocete whales.

### Cranium

The head of *Basilosaurus* did not have room for a melon like modern toothed whales, and the brain was smaller in comparison, as well. They are not believed to have had the social capabilities of modern whales.

A 2011 study concluded that the skull of *Basilosaurus* is asymmetrical like in modern toothed whales, and not, as previously assumed, symmetrical like in baleen whales and artiodactyls (which are closely related to cetaceans). In modern toothed whales, this asymmetry is associated with high-frequency sound production and echolocation, neither of which is thought to have been present in *Basilosaurus*. This probably evolved to detect sound underwater, with a fatty sound-receiving pad in the mandible.

In the skull, the inner and middle ear are enclosed by a dense tympanic bulla. The synapomorphic cetacean air sinus system is partially present in basilosaurids, including the pterygoid, peribullary, maxillary, and frontal sinuses. The periotic bone, which surrounds the inner ear, is partially isolated. The mandibular canal is large and laterally flanked by a thin bony wall, the pan bone or acoustic fenestra. These features enabled basilosaurs to hear directionally in water.

The ear of basilosaurids is more derived than those in earlier archaeocetes, such as remingtonocetids and protocetids, in the acoustic isolation provided by the air-filled sinuses inserted between the ear and the skull. The basilosaurid ear did, however, have a large external auditory meatus, strongly reduced in modern cetaceans, but, though this was probably functional, it can have been of little use under water.

### Hind legs

A 16 m (52 ft) individual of *B. isis* had 35 cm (14 in) long hindlimbs with fused tarsals and only three digits. The limited size of the limb and the absence of an articulation with the sacral vertebrae, makes a locomotory function unlikely. Analysis has shown that the reduced limbs could rapidly adduct between only two positions.

### Spine and movement

A complete *Basilosaurus* skeleton was found in 2015, and several attempts have been made to reconstruct the vertebral column from partial skeletons. Kellogg 1936 estimated a total of 58 vertebrae, based on two partial and nonoverlapping skeletons of *B. cetoides* from Alabama. More complete fossils uncovered in Egypt in the 1990s allowed a more accurate estimation: the vertebral column of *B. isis* has been reconstructed from three overlapping skeletons to a total of 70 vertebrae with a vertebral formula interpreted as seven cervical, 18 thoracic, 20 lumbar and sacral, and 25 caudal vertebrae. The vertebral formula of *B. cetoides* can be assumed to be the same.

*Basilosaurus* has an anguilliform (eel-like) body shape because of the elongation of the centra of the thoracic through anterior caudal vertebrae. In life, these vertebrae were filled with marrow, and because of the enlarged size, this made them buoyant. *Basilosaurus* probably swam predominantly in two dimensions at the sea surface, in contrast to the smaller *Dorudon*, which was likely a diving, three-dimensional swimmer. The skeletal anatomy of the tail suggests that a small fluke was probably present, which would have aided only vertical motion.

Similarly sized thoracic, lumbar, sacral, and caudal vertebrae imply that it moved in an anguilliform fashion, but predominantly in the vertical plane. Paleontologist Philip D. Gingerich theorized that *Basilosaurus* may also have moved in a very odd, horizontal anguilliform fashion to some degree, something completely unknown in modern cetaceans. The vertebrae appear to have been hollow, and likely also fluid-filled. This would imply that *Basilosaurus* typically functioned in only two dimensions at the ocean surface, compared with the three-dimensional habits of most other cetaceans. Judging from the relatively weak axial musculature and the thick bones in the limbs, *Basilosaurus* is not believed to have been capable of sustained swimming or deep diving, or terrestrial locomotion. Basilosaurus did still have an elbow joint in its flipper like a seal.

## Paleobiology

### Feeding

The cheek teeth of *Basilosaurus* retain a complex morphology and functional occlusion. Heavy wear on the teeth reveals that food was first chewed then swallowed. Scientists were able to estimate the bite force of *Basilosaurus* by analyzing the scarred skull bones of another species of prehistoric whale, *Dorudon*, and concluded they could bite with a force of 3,600 pounds per square inch (25 MPa).

Analyses of the stomach contents of *B. cetoides* has shown that this species fed exclusively on fish and large sharks, while bite marks on the skulls of juvenile *Dorudon* have been matched with the dentition of *B. isis*, suggesting a dietary difference between the two species, similar to that found in different populations of modern killer whales. It was probably an active predator rather than a scavenger. The discovery of juvenile *Dorudon* at Wadi Al Hitan bearing distinctive bite marks on their skulls indicates that *B. isis* would have aimed for the skulls of its victims to kill its prey, and then subsequently torn its meals apart, based on the disarticulated remains of the *Dorudon* skeletons. The finding further cements theories that *B. isis* was an apex predator that may have hunted newborn and juvenile *Dorudon* at Wadi Al Hitan when mothers of the latter came to give birth. The stomach contents of an elderly male *B. isis* not only includes Dorudon but the fish *Pycnodus mokattamensis*.

### Paleoecology

Basilosaurus would have been the top predator of its environment. *Basilosaurus* lived in the warm tropical environment of the Eocene in areas abundant with sea grasses, such as *Thalassodendron*, *Thalassia* (also known as turtle grass) and *Halodule*. *Basilosaurus* would have coexisted with the dolphin-like *Dorudon*, the whales *Cynthiacetus* and *Basiloterus*, the primitive sirenian *Protosiren*, the early elephant *Moeritherium*, the sea turtle *Puppigerus* and many sharks, such as *Galeocerdo alabamensis*, *Physogaleus*, *Otodus*, *Squatina prima*, *Striatolamia*, *Carcharocles sokolovi* and *Isurus praecursor*.

## Extinction

Basilosaurus fossil record seems to end at about 35–33.9 mya. Basilosaurus extinction coincides with the Eocene–Oligocene extinction event which happened 33.9 mya, which also resulted in the extinction of almost all other archaeocetes. The event has been attributed to volcanic activity, meteor impacts or a sudden change in climate (such as the environment getting cooler), the latter of which might have caused changes in the ocean by disrupting oceanic circulation. *Basilosaurus* went extinct leaving no relatives. Despite this new currents and deep ocean upwelling favored the diversification of modern cetaceans such as early toothed and baleen whales.

## Appearance in other media

### *Jurassic Park*

* *Basilosaurus* appeared in the game Jurassic Park Builder.