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17.12.2015 15:34 - Encyclopedia Largest prehistoric animals Vol.1 Vertebrates part1 Mammals ch.10 Whales - King lizards whales and killer dolphins
Автор: valentint Категория: Забавление   
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Whales
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The evolution of whales has been a mystery. How did a large, big-brained mammal -- air-breathing, warm-blooded, giving birth to live young -- come to live entirely in water, when mammals evolved on land? The discovery of many fossils with transitional features documents the transformation of whales from land animals to ocean dwellers. Another indication of whales" evolutionary heritage can be seen in the way they move.
The basic theme of whale evolution is the development of large animals from much smaller ancestors--and nowhere is this more evident than in the case of multi-ton sperm and gray whales, whose ultimate forebears were small, dog-sized prehistoric mammals that prowled the riverbeds of central Asia 50 million years ago. Perhaps more intriguingly, whales are also a case study in the gradual evolution of mammals from fully terrestrial to fully marine lifestyles, with corresponding adaptations (elongated bodies, webbed feet, blowholes, etc.) at various key intervals along the way.
Until the turn of the 21st century, the ultimate origin of whales was shrouded in mystery, with scarce remains of early species. That all changed with the discovery of a huge trove of fossils in central Asia (specifically, the country of Pakistan), some of which are still being analyzed and described. These fossils, which date from only 15 to 20 million years after the demise of the dinosaurs 65 million years ago, prove that the ultimate ancestors of whales were closely related to artiodactyls, the even-toed, hooved mammals represented today by pigs and sheep.
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In most ways, Pakicetus (Greek for "Pakistan whale") was indistinguishable from other small mammals of the early Eocene epoch: about 50 pounds or so, with long, dog-like legs, a long tail, and a narrow snout. Crucially, though, the anatomy of this mammal"s inner ears closely matches that of modern whales, the main "diagnostic" feature that places Pakicetus at the root of whale evolution. One of Pakicetus" closest relatives was Indohyus ("Indian pig"), an ancient artiodactyl with some intriguing marine adaptations, such as a thick, hippopotamus-like hide.
Ambulocetus, aka the "walking whale," flourished a few million years after Pakicetus and already displayed some distinctly whale-like characteristics. Whereas Pakicetus led a mostly terrestrial lifestyle, occasionally dipping into lakes or rivers to find food, Ambulocetus possessed a long, slender, otter-like body, with webbed, padded feet and a narrow, crocodile-like snout. Ambulocetus was much bigger than Pakicetus--about 10 feet long and 500 pounds, much closer to a blue whale than a guppy--and probably spent a significant amount of time in the water.
Named after the region of Pakistan where its bones were discovered, Rodhocetus shows even more striking adaptations to an aquatic lifestyle. This prehistoric whale was genuinely amphibious, crawling up onto dry land only to forage for food and (possibly) give birth. In evolutionary terms, though, the most telling feature of Rodhocetus was the structure of its hip bones, which weren"t fused to its backbone and thus provided it greater flexibility when swimming.
The remains of Rodhocetus and its predecessors have been found mostly in central Asia, but the larger prehistoric whales of the late Eocene epoch (which were able to swim faster and farther) have been unearthed in more diverse locations. The deceptively named Protocetus (it wasn"t really the "first whale") had a long, seal-like body, powerful legs for propelling itself through the water, and nostrils that had already begun to migrate halfway up it forehead--a development foreshadowing the blowholes of modern whales.
Protocetus shared one important characteristic with two roughly contemporary prehistoric whales, Maiacetus, and Zygorhiza. The front limbs of Zygorhiza were hinged at the elbows, a strong clue that it crawled onto land to give birth, and a specimen of Maiacetus (meaning "good mother whale") has been found with a fossilized embryo inside, positioned in the birth canal for terrestrial delivery. Clearly, the prehistoric whales of the Eocene epoch had a lot in common with modern giant tortoises!
By about 35 million years ago, some prehistoric whales had attained gigantic sizes, bigger even than modern blue or sperm whales. The largest genus yet known is Basilosaurus, the bones of which (discovered in the mid-19th century) were once thought to belong to a dinosaur--hence its deceptive name, meaning "king lizard." Despite its 100-ton size, Basilosaurus possessed a relatively small brain and didn"t use echolocation when swimming. Even more important from an evolutionary perspective, Basilosaurus led a fully aquatic lifestyle, birthing as well as swimming and feeding in the ocean.
Contemporaries of Basilosaurus were much less fearsome, perhaps because there was only room for one giant mammalian predator in the undersea food chain. Dorudon was once thought to be a baby Basilosaurus; only later was it realized that this small whale (only about 16 feet long and half a ton) merited its own genus. And the much later Aetiocetus (which lived about 25 million years ago), though it weighed only a few tons, shows the first primitive adaptation to plankton feeding--small plates of baleen alongside its ordinary teeth.
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No discussion of prehistoric whales would be complete without a mention of a fairly new genus, the aptly named Leviathan, which was announced to the world in the summer of 2010. This 50-foot-long sperm whale weighed "only" about 25 tons, but it seems to have preyed on its fellow whales along with prehistoric fish and squids, and it may have been preyed on in turn by the largest prehistoric shark of all time, the Basilosaurus-sized Megalodon.
Baleen whales, which comprise the suborder Mysticeti, are all filter-feeders, sieving small organisms from water sucked in and expelled through a curtain of keratin bristles. The group contains a wide variety of species, including the right, humpback and blue whales.
Among biologists it is generally agreed that baleen whales evolved from toothed whales (the Odontoceti), although the estimation of when the two orders last shared a common ancestor varies.
A DNA analysis led by Michel Milinkovitch from Yale University, US, and published in the journal Nature in 1993 found that the last common ancestor may have lived as recently as 10 to 15 million years ago.
A more recent genetic study, published in the journal Royal Society Open Science in 2015, suggests the division occurred much earlier, at around 34 million years ago.
Now, research by palaeontologists Cheng-Hsiu Tsai and Ewan Fordyce from New Zealand’s University of Otago seems to support the older date.
In a paper the pair announce the discovery of an entirely new genus and species, which they call Toipahautea waitaki. Using a variety of dating methods, including the identification of fossilised microfauna found next to the whale, they place it at 27.5 million years ago – making it the earliest known baleen whale ever found.
The distinction, it should be said, is a comparatively narrow one. The area in which the fossil was found, known as the Kokoamu Greensand in the Waitaki region of New Zealand’s south island, is a bit of hotspot for ancient whales.
The area has also offered up Whakakai waipata, dated to 27.3 million years ago, and Horopeta umarere, a relative newcomer at just 25.2 million years old.
Tsai and Fordyce infer T. waitaki sported baleen, based on its jaw shape and evidence that its mouth was well supplied with arterial blood.
As an early – possibly the earliest – representative of the order, they suggest the species may have been a generalist and opportunist feeder – an intermediate form between fully carnivorous toothed whales and the full-time filter feeders that were its descendants.


Basilosaurus
was once recognized as one of the largest known extinctcetaceans at 20 metres (66 ft) in length.
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Basilosaurus ("king lizard") is a
genus of early whale that lived 40 to 34 million years ago in the late Eocene. The first fossil of B. cetoides was discovered in the United States and was initially believed to be some sort of reptile, hence the suffix -"saurus", but it was later found to be a marine mammal. Richard Owen wished to rename the creature Zeuglodon ("yoked tooth"), but, per taxonomic rules, the creature"s first name remained permanent. Fossils of B. isis have been found in Egypt and Jordan. B. drazindai was described based on a single vertebra found in Pakistan.
The species B. cetoides is the state fossil of Mississippi and Alabama in the United States.
Measuring 12–20 m (40–65 ft), Basilosaurus cetoides is the largest known ocean animal of the Late Eocene. B. isis is slightly smaller than B. cetoides.
The dental formula for Basilosaurus isis is 3.1.4.23.1.4.3. 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 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.
Fahlke et al. 2011 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 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. 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, 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. Kellogg 1936 estimated a total of 58 vertebrae, based on two partial and non-overlapping 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 7 cervical, 18 thoracic, 20 lumbar and sacral, and 25 caudal vertebrae. It can be assumed that the vertebral formula of B. cetoides is 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. 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 whales"s, but other reconstructions show a dorsal ridge.
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.
It is also believed that Basilosaurus relied on unusual modes of locomotion, relative to other cetaceans; similarly sized thoracic, lumbar, sacral and caudal vertebrae imply that it moved in an anguilliform (eel-like) 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 it is likely that they were 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. It is also believed that it was incapable of
terrestrial locomotion.
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 up skull bones of another species of prehistoric whale, named Dorudon. By the damage caused, researchers calculated that the aquatic predator could chomp down with a skull crushing force of more than 1600 kilograms.

Analyses of the stomach contents of B. cetoides has shown that this species fed exclusively on fishes and 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.
During the early 19th century, Basilosaurus cetoides fossils were so common (and sufficiently large) that they were regularly used as furniture in the American south. 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 hill side 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, 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 80–100 ft (24–30 m) long — and therefore suggested the name Basilosaurus, meaning “king lizard”.
Harlan brought his assembled specimens (including fragments of jaw and teeth, humerus, and fragments of rib) 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 erroneously with "herbivorous cetaceans" (all cetaceans are carnivorous) such as the dugong (a sirenian, not a cetacean).Consequently, Owen proposed renaming the find Zeuglodon cetoides (“whale-like yoke teeth” in reference to the double-rooted teeth) and Harlan agreed. Zeuglodon, now a junior synonym, is considered by many to be a more fitting name, but the first-published name always takes precedence.


Masracetus


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D
orudon atrox, Cynthiacetus und Masracetus markgrafi
Masracetus
(from Arabic Masr, "Egypt", and Greek ketos, "whale")is an extinct genus of  basilosauridancient whale known from the Late Eocene (Priabonian, 37.2 to 33.9million years ago) of Egypt.

Masracetus was briefly described in an addendum by Gingerich 2007 and is known from just an assemblage of vertebrae and a poorly reconstructed skull from 1908. The lumbar vertebrae are large but relatively short (anteroposteriorly) compared to those of other archaeocetes; the diameter is almost the same as for Basilosaurus isis but the length is less than half of the latter. Masracetus is larger than Cynthiacetus.
Cynthiacetus is an extinct genus of  basilosauridearly whale that lived during the Upper Eocene (Bartonian-Priabonian, 40.4 to 33.9 million years ago.)Specimens has been found in the south-eastern United States, Egypt and Peru.
The skull of Cynthiacetus was similar in size and morphology to that of Basilosaurus, but Cynthiacetus lacked the elongated vertebrae of Basilosaurus. Uhen 2005 erected the genus to avoid the nomen dubium Pontogeneus.


Basiloterus (other king)image
Recently discovered an ancient whale from near Kiev in Ukraine.Basilotritus uheni was quite large animals. Previously estimated its size by scientists at 7-10 meters. Based on the structure of the spine, this whale swam well, wavy bending body from side to side.It is worth noting that similar shape elongated vertebrae are characteristic of modern whales, such as sperm whales, killer whales and baleen whales.Since Basilotritus remnants were found in relatively deep sediments, paleontologists have suggested that at least part of the time, these whales spend at sea. A large body of evidence is entirely aquatic life, and well-developed vertebral spines indicate muscle. In all likelihood, Basilotritus uheni was a good swimmer. It is unlikely that its dimensions and proportions, he could develop a high speed, but he took his swim range, being fully adapted to migrate over long distances.
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The description of the whale genus Basiloterus was only based upon the description of two lumber vertebrae. These vertebrae have been described as being close to the form of the vertebrae of Basilosaurus, though not as long. This may indicate that although similar in form, Basiloterus was quite a bit smaller than the huge Basilosaurus. Whale fossils from the Barton Beds Formation of England have also been identified as possibly belonging to the Basiloterus genus.
The genus name Basilosaurinae means ‘other king’, a reference to the relative genus Basilosaurus which means ‘king lizard’ (information for why a whale was called lizard is on the Basilosaurus page). The type species name is in honour of Dr S. Taseer Hussain.


Macrodelphinus
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Macrodelphinus kelloggi was an orca-sized relative of the cetacean Eurhinodelphis. It lived off the coast of what is now California during the early Miocene. Because of its size, and inch-long teeth, it is believed to have been an apex predator. Macrodelphinus is known from a fragmentary skull from southern California.
Roughly the size of a modern orca, about 7m long (23′), Macrodelphinus was probably an apex predator. Known from California, 23 million years ago, it was a member of a an odd group of toothed whales called the Eurhinodelphinidae – cetaceans with elongated swordfish-like upper jaws. Their exact evolutionary relationships are unclear, although they might be related to the beaked whales.
The long snout may have been used in a similar manner to the swordfish it resembles, slashing to injure and stun its prey.


Saurocetes gigas

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Saurocetes was a prehistoric river dolphin from the Miocene. Its fossil remains have been found in Argentina.
It is notorious for its large size, for it could measure over 5 meters long, being the size of an average great white shark much bigger than any river dolphin today.



Macrokentriodon

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Macrokentriodon was a kentriodontid (cetaceans related to dolphins) which could grow up to be as large as a modern day killer whale- being one of the largest and most powerful predators of its time (not counting Megalodon, that is).
In life, it would have looked a lot like a gigantic, very robust bottlenose dolphin.


Squalodon (Shark tooth).

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Squalodon is the type genus of the Squalodontidae, a group of prehistoric whales that in evolutionary terms of are intermediary between the older Archaeoceti whales like Basilosaurus and Zygorhiza, and the later whales of the Odontoceti which includes modern toothed cetaceans like the killer whale (Orcinus orca). Exactly how Squalodon and the other relatives of the genus were related to modern cetaceans is still uncertain however due to many differing opinions.
Squalodon is represented by numerous species, though there is sometimes question over which ones are valid because often Squalodon remains are only of teeth and jaw segments. Squalodon would have been predators of other marine organisms including fish and possibly other marine mammals. They also show an early development towards echolocation, but it is still unknown if they had the ability to echolocate prey themselves, or if that was a later development of Odontoceti whales. The broad geographic and temporal distribution of the genus however suggest that Squalodon were very successful.
Although predators, themselves, Squalodon may not have been the top predators of the ocean during their time. Prehistoric sharks were growing to very large sizes during the time of Squalodon, and include such examples as C. angustidens, C. chubutensis to the fearsome and massive C. megalodon. The disappearance of Squalodon after the early Miocene also corresponds to a development of even more advanced predatory whales such as Brygmophyseter and Livyaten.

















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Автор: valentint
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