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16.01.2016 15:57 - Encyclopedia Largest prehistoric animals Vol.1 Vertebrates part9 Reptiles ch.4 Mosasaurs
Автор: valentint Категория: Забавление   
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Mosasaurus
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1- Halisaurus, 2 - Pannoniasaurus, 3 - Plioplatecarpus, 4 - Carinodens, 5 - Globidens, 6 - Platecarpus, 7 - Plesioplatecarpus, 8 - Plesiotylosaurus, 9 - Yaguarasaurus, 10 - Clidastes, 11 - Hainosaurus, 12 - Liodon, 13 - Prognathodon, 14 - Plotosaurus, 15 - Tylosaurus, 16 - Mosasaurus, 17 - Taniwhasaurus, 18 - Moanasaurus

Although they weren"t technically dinosaurs, the marine reptiles known as mosasaurs hold a unique place in paleontological history: it was the discovery of a specimen of Mosasaurus in 1764, in a Dutch quarry, that galvanized scientists into the realization that species could become extinct (and that the earth used to be populated by some very strange creatures well before Biblical times). Mosasaurus ("lizard from the Meuse River") was soon named by the renowned naturalist Georges Cuvier, and the general name "mosasaur" attached to other members of this ancient family.
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In evolutionary terms, mosasaurs were distinct from three other famous groups of marine reptiles, ichthyosaurs ("fish lizards"), long-necked plesiosaurs, and short-necked pliosaurs. These sleek, reptilian predators may have been responsible for the extinction of the ichthyosaurs by the end of the Cretaceous period (not necessarily by eating them, but by out-competing them for food), and their quick, agile, hydrodynamic builds gave plesiosaurs and pliosaurs a run for their money. Essentially, mosasaurs ruled the seas for about 20 million years, until the K/T Extinction expunged most giant reptiles (and all the marine varieties) from the face of the earth 65 million years ago.
While it would be tempting to speculate that mosasaurs evolved from ichthyosaurs and plesiosaurs, this doesn"t appear to be the case. The recent discovery of the small, amphibious Dallasaurus, which was capable of swimming as well as walking on land, hints that mosasaurs evolved from early Cretaceous reptiles very similar in appearance to modern monitor lizards (another transitional candidate is the European Aigialosaurus). Less certain is the proposed evolutionary relationship between ancient mosasaurs and modern snakes; the two reptile families share sleek body plans, scaly skin and the ability to open their mouths extra-wide, but the rest is a matter of debate.
In geological terms, one of the odd things about mosasaurs is that their fossils tend to turn up far inland, especially in the western United States and the interior of western Europe, along with other continents. In the case of the U.S., this is because, back in Cretaceous times, much of North America was covered by the "Great Interior Sea" (or the Sundance Sea, as it"s also called), a broad but shallow body of water that swamped large portions of modern-day Kansas, Nebraska, and Colorado. Kansas alone has yielded three major mosasaur genera, Tylosaurus, Platecarpus, and Clidastes.
As you might expect with such a long-lasting family of marine reptiles, not all mosasaurs were in the same weight class or pursued the same diet. The largest individuals of Mosasaurus attained lengths of 50 feet and weights of 15 or so tons, but other genera were considerably sleeker: Tylosaurus, for example, packed only about seven tons into its 35-foot length, and Platecarpus (judging by its fossil remains, the most common mosasaur of North America) was only about 14 feet long and a few hundred pounds. There are two main family groups of mosasaur that you must be familiar with. The first is the Mosasuroidea, which includes all mosasaurs, including the primitive ones like Dallasaurus and Komensaurus. The second group is the Mosasauridae, which includes all the advanced form mosasaurs that are defined by Mosasaurus, but excludes the primitive forms.
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The Mosasauridae can be further divided into what are currently six sub-families. First is the Mosasaurinae, which includes genera that are physically close to Mosasaurus, which means that regardless of differences in size, are noted for having proportionately robust builds meaning that members of the Mosasaurinae adapted to have an emphasis on strength and power. Members of the Mosasaurinae could be medium sized at around 6 to 7 meters long, to much larger at 12 to 15 meters long, with a few individuals exceeding even this.
Second is the Tylosaurinae, which includes the genera that are closer to Tylosaurus. So far tylosaurines and especially Tylosaurus itself make up in terms of body length some of the largest mosasaurs. However the tylosaurines are notably more gracile (lightly built) than the mosasaurines. This means that while tylosaurine mosasaurs were still powerful, there emphasis was more on speed and agility than raw strength. This means that a greater range of aquatic creatures was within their predatory scope, and indeed stomach contents of Tylosaurus do indicate that anything from fish to other marine reptiles to even sea birds were eaten by tylosaurine mosasaurs.
Third is the Plioplatecarpinae, and this includes genera such as Plioplatecarpus, Platecarpus and Angolasaurus amongst many others. These genera are notably smaller the previous two groups, with sizes commonly ranging between 3 and 6 meters long. Plioplatecarpines show some of the greatest adaptations for fast swimming, with some genera developing adaptations that were analogous to the earlier ichthyosaurs. It was the plioplatecarpine genus Platecarpus that was the first to reveal the presence of a fluked tail, something that now seems to have been a common feature for other mosasaurs. Plioplatecarpines also seem to have focused upon swimming forward using only their tails instead of their whole bodies, now an outdated idea.
Fourth, fifth and sixth are the Halisaurinae, Yaguarasaurinae and Tethysaurinae, and again these generally range from about 3 to 6 meters long. Like the Plioplatecarpine mosasaurs these seem to have been predators of fish and possibly smaller marine reptiles. A seventh group is called the Aigialosauridae after Aigialosaurus but this is a side group to the Mosasauridae which is a home for many of the primitive mosasaurs.
Why these variations? Reasoning by analogy with modern marine predators, like the Great White Shark, it"s likely that bigger mosasaur genera like Mosasaurus and Hainosaurus feasted on their fellow mosasaurs and marine reptiles, while smaller species like Clidastes made do with relatively harmless prehistoric fish. And to judge by the round, pebbly shapes of their teeth, it seems that other mosasaurs like Globidens and Prognathodon specialized in gobbling down shelled prey, ranging from small mollusks and ammonites to larger (and tougher) sea turtles.
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At the time they went extinct, mosasaurs were facing increased competition from prehistoric sharks, a good example being Cretoxyrhina (aka the "Ginsu Shark"). Not only were some of these sharks sleeker, faster and more vicious than the likes of Tylosaurus and Globidens, but they may have been smarter as well. The mass extinction of marine reptiles in the wake of the K/T Extinction allowed sharks, the new apex predators, to evolve to bigger and bigger sizes in the course of the Cenozoic Era, the culmination of this trend being the truly enormous (up to 50 feet long and 50 tons) Megalodon.



The largest known mosasaur is likely
Mosasaurus hoffmanni, estimated at 17.6 metres (58 ft) in length.
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Mosasaurus hoffmanni, one of the latest known mosasaurs, comes from the Upper Maastrichtian, Upper Cretaceous Chalk of The Netherlands. Although the first specimen was discovered over 200 years ago, it is here fully described for the first time to provide detailed insights into its anatomy, functional morphology and evolution. Many characters of the skull show that M. hoffmanni was among the most advanced mosasaurs. The skull is robustly constructed and is the least kinetic in the Mosasauridae and, with a tightly assembled palatal complex, provided greater cranial stability in this large-headed mosasaur. The cranial musculature is highly modified. The four-bar linkage system of lizards and early mosasaurs is non-functional in M. hoffmanni. The elements of the lower jaw are also more tightly united than in other mosasaurs. Tooth crowns are divided into several distinct, unique cutting surfaces or prisms. A functional analysis of the marginal teeth shows that they are particularly adapted to powerful bite forces although pterygoidal teeth are small and reduced in importance in ratchet feeding. Moderately large orbits and poorly developed olfactory organs suggest that Mosasaurus hoffmanni was a surface-swimming animal. A relatively lower level of binocular vision than in some other mosasaurs may indicate a somewhat uncomplicated habitat. Geological and palaeontological evidence indicates that M. hoffmanni lived in fairly deep nearshore waters of 40-50 m depth, with changing temperatures and rich vertebrate and invertebrate life. Several severely broken and healed mandibles suggest either a violent lifestyle in predation or in fighting.


A close contender in size is
Hainosaurus bernardi, estimated at 15 metres (49 ft) in length.
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Hainosaurus is an extinct genus of marine lizard belonging to the mosasaur family. It is one of the largest mosasaurs, though its size has been revised more than once. At first it was estimated to be 17 metres (56 ft), and the largest mosasaurid. During the 1990s, its size was revised to 15 metres (49 ft) long;more recently, Johan Lindgren estimated that it reached lengths of up to 12.2 metres (40 ft) It was one of the top marine predators of the Late Cretaceous. Like other giant mosasaurs, this giant predator preyed on turtles, plesiosaurs, pterosaurs, cephalopods, sharks, fish, and smaller mosasaurs.
Hainosaurus is a member of the subfamily Tylosaurinae, and it is related to the wholly North American Tylosaurus. However, it has more vertebrae from the neck to the part of the tail with chevrons (53) than Tylosaurus (35).Both genera are large marine superpredators. Hainosaurus" tail has less chevron-bearing vertebrae, making it shorter than that of Tylosaurus. The only known and type species of Hainosaurus is H. bernardi, named after the Belgian Lйopold Bernard, owner of the phosphate chalk exploitation where the fossil was unearthed.


Another giant mosasaur is
Tylosaurus, estimated at 10–14 metres (33–46 ft) in length.
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Tylosaurus (Greek "protuberance, knob" + Greek "lizard") was a mosasaur, a large, predatory marine lizard closely related to modern monitor lizards and to snakes.
Along with plesiosaurs, sharks, fish, and other genera of mosasaurs, it was a dominant predator of the Western Interior Seaway during the Late Cretaceous. Tylosaurus proriger was among the largest of all the mosasaurs (along with Hainosaurus and Mosasaurus hoffmannii), reaching maximum lengths of 15 meters or (50 ft).A distinguishing characteristic of Tylosaurus is its elongated, cylindrical premaxilla (snout) from which it takes its name and which may have been used to ram and stun prey and also in intraspecific combat.
Early restorations showing Tylosaurus with a dorsal crest were based on misidentified tracheal cartilage, but when the error was discovered, depicting mosasaurs with such crests was already a trend.
Stomach contents associated with specimens of Tylosaurus proriger indicate that this ferocious mosasaur had a varied diet, including fish, sharks, smaller mosasaurs, plesiosaurs, and flightless diving birds such as Hesperornis. In some paleoenvironments, Tylosaurus seems to have preferred shallow, nearshore waters (as with the Eutaw Formation and Mooreville Chalk Formation of Alabama), while favoring deeper water farther out from shore in other environments (as with the Niobrara Chalk of the western U.S.).
The Talkeetna Mountains Hadrosaur was a hadrosaurid of indeterminate classification whose carcass appeared to have been deposited in a bathyal or outer shelf environment that later became Alaska"s Matanuska Formation.Every element of its skeleton not found in a concretion bore many closely spaced ovualar conical depressions ranging in diameter from 2.12 to 5.81 mm and 1.64 to 3.62 deep.These depressions are probably bite marks.The depressions are not symmetrical enough for gastropod drill marks and are not shaped like sponge borings.None of the preserved fish fossils of the Matanuska Formation fit the size or geometry of the borings.The size and spacing and shape by contrast, however, closely resembles the teeth of Tylosaurus proriger.The apparent tooth marks are unlikely to have occurred before the carcass was washed out to sea since that would have punctured the body, preventing the buildup of bloating gases that allowed the carcass to drift out to sea in the first place.The distribution of bite marks corresponds inversely to the presence of flesh in the animal.For instance, lower limb bones sustained the most damage because there was the least amount of flesh shielding the bones at those locations.The concretions formed as the flesh chemically reacted to the seafloor on the largest parts of the animal where the scavenging mosasaur would be unable to fully wrap its jaws around the carcass.Bones pulled free from the carcass were buried in the mud and preserved in mudstone.
Like many other mosasaurs, the early history of this taxon is complex and involves the infamous rivalry between two early American paleontologists, Edward Drinker Cope and Othniel Charles Marsh. Originally, the name "Macrosaurus" proriger was proposed by Cope for a fragmentary skull and thirteen vertebrae collected from near Monument Rocks in western Kansas in 1868. It was placed in the collections of the Harvard Museum of Comparative Zoology. Only a year later, Cope redescribed the same material in greater detail and referred it, instead, to the English mosasaur taxon Liodon. Then, in 1872, Marsh named a more complete specimen as a new genus, Rhinosaurus ("nose lizard"), but soon discovered that this name had already been used for a different animal. Cope suggested that Rhinosaurus be replaced by yet another new name, Rhamposaurus which also proved to be preoccupied. Marsh finally erected Tylosaurus later in 1872, to include the original Harvard material as well as additional, more complete specimens which had also been collected from Kansas.A giant specimen of T. proriger, recovered in 1911 by C. D. Bunker near Wallace, Kansas is one of the largest skeleton of Tylosaurus ever found. It is currently on display at the University of Kansas Museum of Natural history.
In 1918, Charles H. Sternberg found a Tylosaurus, with the remains of a plesiosaur in its stomach. The specimen is currently mounted in the United States National Museum (Smithsonian) and the plesiosaur remains are stored in the collections. Although these important specimens were briefly reported by C. H. Sternberg (1922), the information was lost to science until 2001. This specimen was rediscovered and described by Everhart (2004a). It is the basis for the story line in the 2007 National Geographic IMAX movie Sea Monsters, and a book by the same name (Everhart, 2007).
A photograph of a Tylosaurus skull was taken by George F. Sternberg about 1926 after he collected and prepared the specimen. It was discovered in the Smoky Hill Chalk of Logan County, Kansas. Sternberg offered the specimen to the Smithsonian and included a photograph in his letter to Charles Gilmore. Copies of the original photos are in the archives of the Sternberg Museum of Natural History (FHSM). The specimen is FHSM VP-3, the exhibit specimen in the same museum.
A 34 ft. long Tylosaurus found in Kansas by Alan Komrosky in 2009 is now on display at the Museum of World Treasures in Wichita, Kansas.
Though many species of Tylosaurus have been named over the years, only a few are now recognized by scientists as taxonomically valid. They are as follows: Tylosaurus proriger (Cope, 1869), from the Santonian and lower to middle Campanian of North America (Kansas, Alabama, Nebraska, etc.); Tylosaurus nepaeolicus (Cope, 1874 ), from the Santonian of North America (Kansas); Tylosaurus kansasensis Everhart, 2005, from the late Coniacian of Kansas.


Plotosaurus
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Plotosaurus ("swimmer lizard") is an extinct genus of mosasaur from the Upper Cretaceous (Maastrichtian) of Fresno County, California. Originally named Kolposaurus (meaning "bay lizard") by Berkeley paleontologist Charles Lewis Camp in 1942, it was changed to Plotosaurus in 1951 when Camp discovered the name had already been assigned to a type of nothosaur.
Plotosaurs possessed several adaptations to marine life not seen in other mosasaurs. Compared with their relatives, they had narrower flippers, large tail fins and a streamlined fusiform body shape.These features probably enabled them to be faster swimmers than most other mosasaurs. They also had relatively large eyes for keen eyesight, and impressions found with their fossils suggest that they had scaly skin.
Based on cladistic analysis, plotosaurs are considered to be the most derived branch of mosasaur evolution.
The type species, P. bennisoni, was named for Allan Bennison, a fossil hunter who discovered the first remains in 1937. It was around 9 metres (30 ft) long, and was the first known mosasaur from California (a year previously, Bennison had also discovered the state"s first dinosaur, a species of Saurolophus that is now known as Augustynolophus).
A second species, P. tuckeri, was also found in 1937 by Frank Paiva and Professor William M. Tucker. Although, not quite as advanced in aquatic adaptations as P. bennisoni it was about 40% larger, reaching lengths of around 13 metres (43 ft).However, a recent analysis by Lindgren, Caldwell and Jagt (2008) considers P. tuckeri to be a junior synonym.

Prognathodon
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Prognathodon was a late mosasaur that showed a trend towards a different kind of predation that saw it living like the much earlier basal placodont reptiles of the Triassic such as Placodus. This means that Prognathodon specialised in eating tough shelled prey items like shellfish, ammonites and turtles. The diet of Prognathodon was for a long time just speculation based upon the teeth and jaw construction, but two discoveries in Canada in the early years of the twenty-first century not only revealed the full body shape of Prognathodon but the diet as well. One specimen revealed the presence of turtle and ammonite fossils located where its stomach would have been. Interestingly it also had a one-hundred and sixty centimetre long fish in its gut, suggesting that while Prognathodon was a specialised predator, it was also opportunistic in its feeding.
Prognathodon had a robust and heavy jaw that would have been capable of withstanding a high bite force supplied by powerful jaw muscles. However it’s the teeth that should receive special note as not only are they strong and well adapted for crushing, they have serrations which can be seen under much more detailed inspection. This makes the teeth specialised for a dual purpose, destroying the protective shells of prey while shearing the flesh within. Another specialisation is the presence of bony rings around the eye sockets. This is seen as a deep water adaptation for the eyes to better withstand the higher water pressure of deep water, something which may have often been necessary when diving for ammonites.
Why Prognathodon shifted towards this kind of diet when mosasaurs are generally perceived to be apex predators of other reptiles and fish remains uncertain. It could have been that competition for the ecological niche of apex predator was so fierce that the only way Prognathodon could evolve and survive was by adapting to a different food source, removing the need for competition with other predators. It could also be that numbers of large prey animals that mosasaurs are traditionally associated with began to fall, necessitating a need to switch to a different diet. It could have course been to simply exploit an abundant food supply. What is certain is that Prognathodon was not the only mosasaur to adjust to this diet with another named Globidens also having particularly large and rounded crushing teeth in its mouth.
For a time a particularly large specimen of Prognathodon was split off from P. curri into its own genus named Oronosaurus, and a consequence of this resulted in the upper size estimate being reduced to ten metres long. However further study of the material has revealed that the original interpretation was the correct one. Today Oronosaurus is now regarded as a synonym to Prognathodon. The largest species of Prognathodon at the time of writing is P. saturator which has an estimated body length of thirteen meters and seventy centimetres.
2013 saw the description of a Prognathodon specimen first discovered in 2008 which led to the announcement that like with the genus Platecarpus, Prognathodon too had a bi lobed tail. This basically means that the tail of Prognathodon did not just curve downwards but had and additional growth upwards, something quite more advanced than more primitive formed mosasaurs. Although not as well developed as the bi lobed tails of fish and advanced ichthyosaurs, this tail would have still provided extra push against the water meaning more efficient and faster swimming. This probably allowed Prognathodon to dive deeper faster so that it could hunt for prey longer before returning to the surface for air.








 






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Автор: valentint
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