Basilosaurus


Basilosaurus is a genus of large, predatory, prehistoric archaeocete whale from the late Eocene, approximately 41.3 to 33.9 million years ago. First described in 1834, it was the first archaeocete and prehistoric whale known to science. The first fossils were discovered along the Gulf Coast of the United States, along with a few fossils in the eastern U.S., attributed to the type species B. cetoides. They were originally thought to be of a giant reptile, hence the suffix "-saurus", Ancient Greek for "lizard". The animal was later found to be an early marine mammal, which prompted attempts at renaming the creature, which failed as zoological nomenclature dictates using the original name given. Fossils were later found of the second species, B. isis, in 1904 in North Africa, more specifically the countries of Egypt, Jordan, Tunisia and Morocco. Basilosaurus has a possible record in New Zealand and Seymour Island in Antarctica.
Basilosaurus is considered to have been common in the Tethys sea.
It was one of the largest, if not the largest, animals of the Paleogene. It was the top predator of its environment, preying on sharks, large fish and other marine mammals, namely the dolphin-like Dorudon, which seems to have been their predominant food source.
Basilosaurus was at one point a wastebasket taxon, before the genus slowly started getting reevaluated, with many species of different Eocene cetacean being assigned to the genus in the past, however they are invalid or have been reclassified under a new or different genus, leaving only 2 confirmed species.
Basilosaurus may have been one of the first fully aquatic cetaceans. Basilosaurus, unlike modern cetaceans, had various types of teeth–such as canines and molars–in its mouth, and it probably was able to chew its food in contrast to modern cetaceans which swallow their food whole.

Description

Measuring, Basilosaurus is one of the largest-known animals to exist from K–Pg extinction event 66 million years ago to 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 long and B. cetoides being 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 dental formula for B. isis is. The upper and lower molars and second to fourth premolars are double-rooted and high-crowned.
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. 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 limbs

A individual of B. isis had hind limbs with fused tarsals and only three digits. The limited size of the limb and the absence of an articulation with the sacral vertebrae make a locomotory function unlikely. Analysis has shown that the reduced limbs could rapidly adduct between only two positions. Possible uses for the structure have been given, such as clasper-like body functions. These limbs would have been used to guide the animals' long bodies during mating.

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. 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 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.
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 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, 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.

Classification

Below is the phylogenetic analysis on the placement of Basilosaurus. Two subfamilies exist in Basilosauridae: Basilosaurinae which includes Basilosaurus, and Dorudontinae. These groups have been declared invalid in the past. Dorudon remains were once thought to represent juvenile Basilosaurus.

Taxonomic history

Etymology

The two species of Basilosaurus are B. cetoides, whose remains were discovered in the United States, and B. isis, which was discovered in Egypt. B. cetoides is the type species for the genus. The Holotype Harlan found, was found in Ouachita County, Louisiana. Vertebrae were sent to the American Philosophical Society by a Judge Bry of Arkansas and Judge John Creagh of Clarke County, Alabama. Both fossils ended up in the hands of the anatomist Richard Harlan, who requested more examples from Creagh. The first bones were unearthed when rain caused a hillside full of sea shells to slide. The bones were lying in a curved line "measuring upwards of four hundred feet in length, with intervals which were vacant." Many of these bones were used as andirons and destroyed; Bry saved the bones he could find, but was convinced more bones were still to be found on the location. Bry speculated that the bones must have belonged to a "sea monster" and supplied "a piece having the appearance of a tooth" to help determine which kind.
Harlan identified the tooth as a wedge-shaped shell and instead focused on "a vertebra of enormous dimensions" which he assumed belonged to the order "Enalio-Sauri of Conybeare", "found only in the sub-cretaceous series." He noted that some parts of the vertebra were similar to those of Plesiosaurus and skull was similar to Mosasaurus, but that they were completely different in proportions. Comparing his vertebra to those of large dinosaurs such as Megalosaurus and Iguanodon, Harlan concluded that his specimen was considerably larger—he estimated the animal to have been no less than long—and therefore suggested the name Basilosaurus, meaning "king lizard".
Harlan brought his assembled specimens to the UK where he presented them to anatomist Richard Owen. Owen concluded that the molar teeth were two-rooted, a dental morphology unknown in fishes and reptiles, and more complex and varied than in any known reptile, and therefore that the specimen must be a mammal. Owen correctly associated the teeth with cetaceans, but he thought it was an herbivorous animal, similar to sirenians. Consequently, Owen proposed renaming the find Zeuglodon cetoides and Harlan agreed.

Wadi El Hitan

Wadi El Hitan, Arabic وادي الحيتان, "Valley of the Whales", is an Egyptian sandstone formation where many early-whale skeletons were discovered. German botanist Georg August Schweinfurth discovered the first archaeocete whale in Egypt in 1879. He visited the Qasr el Sagha Formation in 1884 and 1886 and missed the now famous Wadi El Hitan by a few kilometers. German paleontologist Wilhelm Barnim Dames described the material, including the type specimen of Z. osiris, a well-preserved dentary.
Hugh Beadnell, head of the Geological Survey of Egypt 1896–1906, named and described Zeuglodon isis in based on a partial mandible and several vertebrae from Wadi El Hitan in Egypt. described a skull and some vertebrae of a smaller archaeocete and named it Prozeuglodon atrox, now known today as Dorudon atrox. discovered deciduous teeth in this skull and it was then believed to be a juvenile zeuglodon isis for decades before more complete fossils of mature Dorudon were discovered.
In the 1980s, Elwyn L. Simons and Philip D. Gingerich started to excavate at Qasr el-Sagha and Wadi El Hitan with the hope of finding material that could match archaeocete fossils from Pakistan. Since then, over 500 archaeocete skeletons have been found at these two locations, of which most are B. isis or D. atrox, several of the latter carrying bite marks assumed to be from the former. A 1990 paper described additional fossils including foot bones and speculated that the reduced hind limbs were used as copulatory guides. One thing that was noted, was that whale fossils were so common, that when a mason company looked at their newest table counter, they realized that they had created a cross section of a 40 million year old Basilosaurid fossil. This find was another thing that caught the eye of Gingerich.
In 2016, a complete skeleton, the first-ever find for Basilosaurus, was uncovered in Wadi El Hitan, preserved with the remains of its prey, including a Dorudon and several species of fish. The whale's skeleton also shows signs of scavenging or predation by large sharks such as the otodontid Carcharocles sokolovi.

Wastebasket taxa

Many dubious species have been assigned to Basilosaurus in the past which have since been invalidated or were too incomplete to determine anything.

''Nomina dubia''

A nomen dubium is a scientific name that is of unknown or doubtful application. There are a few documented cases of this being applied to Basilosaurus in the past.
The species B. cetoides is the state fossil of Alabama and Mississippi. Basilosaurus is featured in the BBC's Walking with series in Walking with Beasts and Sea Monsters. During the early 19th century, B. cetoides fossils were so common that they were regularly used as furniture in the American South.

Cited literature

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