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20.12.2015 17:01 - Largest prehistoric animals Vol.1 Vertebrates part3 Dinosaurs ch.1 Theropods-Carcharodontosaurids-Giant sauropods hunters from South America
Автор: valentint Категория: Забавление   
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Последна промяна: 05.12.2016 23:09


South America-Giganotosaurinae

 The largest
Carcharodontosaurids were Giganotosaurus carolinii


Giganotosaurus (GIG-ə-NOT-o-SAW-rus meaning "giant southern lizard" is a genus of carcharodontosaurid dinosaurs that lived in what is now Argentina during the early Cenomanian age of the Late Cretaceous Period, approximately some 99.6 to 97 million years ago It included some of the largest known terrestrial carnivores, with known individuals equaling or slightly bigger than the size of the largest of the genera Tyrannosaurus and Carcharodontosaurus, but not as large as those of Spinosaurus.

The skeleton of the holotype specimen (MUCPv-Ch1) is about 70% complete and includes parts of the skull, a lower jaw, pelvis, hindlimbs and most of the backbone, missing only the premaxillae, jugals, quadratojugals, the back of the lower jaws and the forelimbs. A second specimen (MUCPv-95) has also been identified, found in 1988 by Jorge Calvo and consisting of a fragment of a lower jaw, said to be 8% larger than the corresponding part in the first specimen.

The skull of Giganotosaurus is large; that of the holotype was in 1995 estimated at 1.53 m (5.0 ft) in length. Even though the original authors briefly claimed the length to be up to 1.8 m (5.9 ft)—leading to an estimate of 1.95 m (6.4 ft) skull length for the referred specimen—this claim was not repeated by subsequent workers and one of the original authors was in 2002 co-writer of an article giving a holotype skull length of 1.6 m (5.2 ft).Some have claimed that even the original estimate was too long and believe the skull to be almost exactly comparable to the one of Tyrannosaurus in length.The skull is slender and elongated in build, with rugose areas on the edges of the snout top and above the eye. The supratemporal openings were overhung by the edges of the skull roof where the jaw muscles of each side directly attached instead of meeting each other at a midline skull crest. The back of the skull as preserved is strongly inclined forwards, bringing the jaw joints far behind the attachment point of the neck.The endocast of Giganotosaurus has a volume of 275 cc (16.8 cu in)and including the olfactory bulbs, it was 19% longer than that of the related theropod, Carcharodontosaurus saharicus.

The shoulder blade was very short and thick, with sudden kinks in its shaft. The ischium had a paddle-shaped end; the thigh bone, 1.43 m (4.7 ft) long in the holotype, had a head that was pointing relatively upwards.The mid dorsal vertebrae carried rather high spines. The total length of the holotype has been estimated between 12.2 and 12.5 m (40 and 41 ft),the largest specimen is 13.2 m (43 ft),and the weight between 6.5 and 13.8 tonnes (14,330 and 30,420 lb).

Titanosaur fossils belonging to Andesaurus and Limaysaurus have been recovered near the remains of Giganotosaurus, leading to speculation that these carnivores may have preyed on the giant herbivores. Fossils of the related carcharodontosaurid Mapusaurus grouped closely together may indicate pack hunting, a behavior that could possibly extend to Giganotosaurus itself.

Blanco and Mazzetta (2001) estimated that for Giganotosaurus a growing imbalance when increasing its velocity would pose an upper limit of 14 metres per second (50 km/h; 31 mph) to its running speed, after which minimal stability would have been lost.

In 2005 Franзois Therrien e.a. estimated that the bite force of Giganotosaurus was three times less than that of Tyrannosaurus and that the lower jaws were optimised for inflicting slicing wounds; the point of the mandibula was reinforced to this purpose with a "chin" and broadened to handle smaller prey.

Giganotosaurus, along with relatives like Tyrannotitan, Mapusaurus and Carcharodontosaurus, are members of the carnosaur family Carcharodontosauridae. Both Giganotosaurus and Mapusaurus have been placed in their own subfamily Giganotosaurinae by Coria and Currie in 2006 as more carcharodontosaurid dinosaurs are found and described, allowing interrelationships to be calculated.

The most complete find of Giganotosaurus was made by Rubйn Dario Carolini, an amateur fossil hunter who, on 25 July 1993, discovered a skeleton in deposits of Patagonia (southern Argentina) in what is now considered the Candeleros Formation.The discovery was scientifically reported in 1994.The initial description was published by Rodolfo Coria and Leonardo Salgado in the journal Nature in September 1995. The type species is Giganotosaurus carolinii. The generic name means "giant southern lizard", derived from the Ancient Greek. The specific name honours Carolini.

Giganotosaurus was probably the apex predator in its ecosystem. It shared its environment with titanosaurian sauropod Andesaurus and the rebbachisaurid sauropods Limaysaurus and Nopcsaspondylus. Iguanodont and ornithischian remains have reportedly been found there too. Large abelisaurid theropod Ekrixinatosaurus also shared the environment, and was possibly a competitor at times. Smaller predators also inhabited the area. These included dromaeosaurid Buitreraptor, alvarezsaurid Alnashetri, and basal coelurosaurian Bicentenaria. The primitive snake Najash lived here as well, along with turtles, fish, frogs, and cladotherian mammals. Pterosaurs also lived in the area.



It was similar in size to its close relative Giganotosaurus, with the largest known individuals estimated as over 10.2 metres (33 ft) in length and weighing approximately 3 metric tons (3.3 short tons). The longest individual for which Coria and Currie (2006) provided an estimate is the animal to which femur MCF-PVPH-208.203 belonged; this individual is estimated as 10.2 metres (33 ft) long. Coria and Currie note the presence of isolated bones from at least one longer individual, but do not provide a figure, instead finding the larger bones comparable in size to those from a Giganotosaurus estimated at 12.2 metres (40 ft) in length. Maximum length is thus unknown but greater than 12.2 metres (40 ft). The weight estimate is from another femur (MCF-PVPH-208.234). It was furthermore concluded that the largest remain, a pubic shaft, was 110% the size of the giganotosaurus holotype.

Coria and Currie diagnosed Mapusaurus as follows: "Mapusaurus n. gen. is a carcharodontosaurid theropod whose skull differs from Giganotosaurus in having thick, rugose unfused nasals that are narrower anterior to the nasal/maxilla/lacrimal junction; larger extension of the antorbital fossa onto maxilla; smaller maxillary fenestra; wider bar (interfenestral strut) between antorbital and maxillary fenestrae; lower, flatter lacrimal horn; transversely wider prefrontal in relation to lacrimal width; ventrolaterally curving lateral margin of the palpebral; shallow interdental plates; higher position of Meckelian canal; more posteriorly sloping anteroventral margin of dentary. Mapusaurus roseae is unique in that the upper quadratojugal process of jugal splits into two prongs; small anterior mylohyoidforamen positioned above dentary contact with splenial; second and third metacarpals fused; humerus with broad distal end and little separation between condyles; the brevis fossa of the ilium extends deeply into excavation dorsal to ischial peduncle. It also differs from Giganotosaurus in having conical, slightly curving cervicalepipophyses that taper distally; axial posterior zygapohyses joined on midline; smaller and less elaborate prespinal lamina on midline of cervicals; remarkably sharp dorsal margin of cervical neural spines; tall, wider neural spines; curved ischiatic shaft; more slender fibula.

Mapusaurus was excavated between 1997 and 2001, by the Argentinian-Canadian Dinosaur Project, from an exposure of the Huincul Formation (Rio Limay Group, Cenomanian) at Canadon de Gato. It was described and named by paleontologistsRodolfo Coria and Phil Currie in 2006.

The name Mapusaurus is derived from the Mapuche word Mapu, meaning "of the Land" or "of the Earth" and the Greek sauros, meaning "lizard". The type species, Mapusaurus roseae, is named for both the rose-colored rocks, in which the fossils were found and for Rose Letwin, who sponsored the expeditions which recovered these fossils.

The designated holotype for the genus and type species, Mapusaurus roseae, is an isolated right nasal (MCF-PVPH-108.1, Museo Carmen Funes, Paleontologнa de Vertebrados, Plaza Huincul, Neuquйn). Twelve paratypes have been designated, based on additional isolated skeletal elements. Taken together, the many individual elements recovered from the Mapusaurus bone bed represent most of the skeleton.

The fossil remains of Mapusaurus were discovered in a bone bed containing at least seven individuals of various growth stages. Coria and Currie speculated that this may represent a long term, possibly coincidental accumulation of carcasses (some sort of predator trap) and may provide clues about Mapusaurus behavior.Other known theropod bone beds include the Allosaurus-dominated Cleveland Lloyd Dinosaur Quarry of Utah, an Albertosaurus bone bed from Alberta and a Daspletosaurus bone bed from Montana.

Paleontologist Rodolfo Coria, of the Museo Carmen Funes, contrary to his published article, repeated in a press-conference earlier suggestions that this congregation of fossil bones may indicate that Mapusaurus hunted in groups and worked together to take down large prey, such as the immense sauropodArgentinosaurus. If so, this would be the first substantive evidence of gregarious behavior by large theropods other than Tyrannosaurus, although whether they might have hunted in organized packs (as wolves do) or simply attacked in a mob, is unknown. The authors interpreted the depositional environment of the Huincul Formation at the Canadon de Gato locality as a freshwater paleochannel deposit, "laid down by an ephemeral or seasonal stream in a region with arid or semi-arid climate". This bone bed is especially interesting, in light of the overall scarcity of fossilized bone within the Huincul Formation.

Cladistic analysis carried out by Coria and Currie definitively showed that Mapusaurus is nested within the cladeCarcharodontosauridae. The authors noted that the structure of the femur suggests a closer relationship with Giganotosaurus than either taxon shares with Carcharodontosaurus. They created a new monophyletic taxon based on this relationship, the subfamilyGiganotosaurinae, defined as all carcharodontosaurids closer to Giganotosaurus and Mapusaurus than to Carcharodontosaurus. They tentatively included the genus Tyrannotitan in this new subfamily, pending publication of more detailed descriptions of the known specimens of that form.

As previously mentioned, the Huincul Formation is thought to represent an arid environment with ephemeral or seasonal streams. The age of this formation is estimated at 97-94 mya. The dinosaur record is considered sparse here, despite the fact it has revealed some truly impressive animals. Mapusaurus shared its environment with sauropods Argentinosaurus (one of the largest sauropods, if not the largest), and Cathartesaura. Abelisauroid theropods Skorpiovenator and Ilokelesia also lived in the region.

With a name like Tyrannotitan you could be forgiven for thinking that this huge theropod was a tyrannosaurid, but in fact it was a carcharodontosaurid Giganotosaurus and Mapusaurus, both also from South America. Tyrannotitan seems to differ from other discovered carcharodontosaurids in that it has proportionately tiny forearms, something that is similar to the tyrannosaurids. Palaeontologists are confident that Tyrannotitan does not represent a tyrannosaurid because the geographical isolation of South America (it was not yet connected to North America and was long separated from Africa during this time) meant that the tyrannosaurids had no way of reaching the continent. The species name T. chubutensis is in reference to Chubut Province where Tyrannotitan’s fossils were found.
One key area of difference between Tyrannotitan and the other currently known carcharodontosaurids is the lack of pneumaticity in the sacral (hip) and caudal (tail) vertebrae. This essentially means that there are no air pockets that could have reduced weight resulting in a solid but heavier skeleton. Interestingly connected to this observation is the fact that a caudal vertebra of Tyrannotitan has a neural spine (the projection top of the vertebra) twice the height of the centrum, the main body of the vertebral bone that articulates with the other vertebra. The dorsal (back) and cervical (neck) vertebra do not display neural spines this high, and with this in mind the tail vertebrae may have had them to support a stronger muscle network that was required to carry the tail high and off the ground in the absence of weight saving features such as pneumaticity.
Tyrannotitan teeth do not seem to be as well developed as those of others of its group such as those seen in Carcharadontosaurus. Tyrannotitan teeth lack the clear curves that facilitate slicing however the general shape makes them part way similar to the teeth of allosaurids like Allosaurus itself. Given that the carcharodontosaurid theropods are thought to be descended from allosaurid theropods, the teeth of Tyrannotitan may represent a transitional form of the teeth from one changing into another.
The teeth of Tyrannotitan also have denitcles which themselves are dived into two by the presence of a groove. Denticles are essentially like teeth themselves and may have formed an additional cutting surface increasing the tooth’s ability to pierce flesh. These tooth denticles may represent an evolutionary experiment that did not carry forward into later descendants that had the flat curved teeth with serrated edges. As for feeding from a kill, Tyrannotitan probably would have been better at stripping the flesh from a carcass rather than crunching the bones.
Some researchers have asked the question could Tyrannotitan actually be the same as one of the other South American carcharodontosaurids. This question is based upon sexual dimorphism and on-going evolution of the group and comes about because the small number of specimens combined with their incomplete nature makes it difficult to be immediately certain. For now Tyrannotitan remains its own genus and will stay so unless fossils and further understanding of the group can prove otherwise.
Should it ever be found the same as another earlier discovered dinosaur then Tyrannotitan will become a synonym to that genus. Under international rules governing the naming of animals this means that the name Tyrannotitan could not be used for any other discoveries because it is already down in scientific literature as relating to this animal.



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