Giganotosaurus

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Not to be confused with Gigantosaurus.
Giganotosaurus
Temporal range: Late Cretaceous, 97 Ma
Giganothosaurus.jpg
Reconstructed skeleton, Natural History Museum, Helsinki
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Dinosauria
Order: Saurischia
Suborder: Theropoda
Family: Carcharodontosauridae
Subfamily: Carcharodontosaurinae
Tribe: Giganotosaurini
Genus: Giganotosaurus
Coria & Salgado, 1995
Species: † G. carolinii
Binomial name
Giganotosaurus carolinii
Coria & Salgado, 1995

Giganotosaurus (/ˌɡəˌntəˈsɔːrəs/ JY-gə--NOH-tə-SOR-əs[1] or GIG-ə-NOT-o-SAW-rus meaning "giant southern lizard"[2]) 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.

Giganotosaurus was one of the largest known terrestrial carnivores, but the exact size has been hard to determine, due to the incompleteness of its remains. Estimates for the most complete specimen range from a length of 12 and 13 m (39 and 43 ft), a skull length of 1.53 and 1.80 m (5.0 and 5.9 ft), and a weight of 6 and 13.8 tonnes (13,230 and 30,420 lb). Some researchers have found it to be bigger than Tyrannosaurus, which has historically been considered the largest theropod, while others have found them equal in size.

Giganotosaurus is a member of the family Carcharodontosauridae, together with other very large theropods such as Carcharodontosaurus and the closely related Mapusaurus.

Description

Size (orange) compared to that of other large theropods

Giganotosaurus is thought to have been one of the largest theropod dinosaurs, but the incompleteness of its remains have made it difficult to reliably estimate its size. It is therefore not possible to safely determine whether it was larger than for example Tyrannosaurus, which has historically been considered the largest theropod. Different size estimates have been reached by several researchers, based on various methods, and depending on how the missing parts of the skeleton have been reconstructed. The length of the holotype specimen has been estimated to have been between 12 and 13 m (39 and 43 ft) long, with a skull between 1.53 and 1.80 m (5.0 and 5.9 ft), a femur between 1.365 and 1.43 m (4.48 and 4.69 ft), and a weight between 6 and 13.8 tonnes (13,230 and 30,420 lb).[3][4][5][6] Fusion of sutures in the braincase indicates the holotype specimen was a mature individual.[3] A second specimen, consisting of a dentary bone from a supposedly larger individual, has been used to extrapolate a length of 13.2 m (43 ft), a skull 1.95 m (6.4 ft) long, and a weight of 8.2 tonnes (18,080 lb), but some writers have considered such sizes exaggerated.[5][7][8][9]

The neck of Giganotosaurus was strong, and the axis bone (the neck vertebra that articulates with the skull) was robust. The rear cervical (neck) vertebrae had short, flattened centra (the "bodies" of the vertebrae), with almost hemispherical articulations at the front, and pleurocoels (hollow depressions) divided by laminae (plates). The dorsal (back) vertebrae had high neural arches and deep pleurocoels. The caudal (tail) vertebrae had neural spines that where elongated from front to back and had robust centra. The transverse processes of the caudal vertebrae were long from front to back, and the chevrons on the front were blade-like. The pectoral girdle was proportionally shorter than that of Tyrannosaurus, with the ratio between the scapula (shoulder blade) and the femur (thigh bone) being less than 0.5. The blade of the scapula had parallel borders, a strong tubercle for insertion of the triceps muscle, and the lower end was thickened and projected forwards above the coracoid. The coracoid was small, hook-shaped, and had an open foramen.[4]

Restoration

The ilium of the pelvis had a convex upper border, a low postacetabular blade (behind the acetabulum), and a narrow brevis-shelf (a projection where tail muscles attached). The pubic foot was pronounced and shorter at the front than behind. The ischium was straight and expanded hindwards, ending in a lobe. The femur was sigmoid-shaped, and had a very robust, upwards pointing head, with a deep sulcus (groove). The lesser trochanter of the femoral head was wing-like, and placed below the greater trochanter, which was short. The fourth trochanter was large and projected backwards. The tibia of the lower leg was expanded at the upper end, its articular facet (where it articulated with the femur) was wide, and its shaft was compressed from front to back.[4]

Skull

Partial holotype skull (front of left dentary is seen in the background), Ernesto Bachmann Museum

Though incompletely known, the skull of Giganotosaurus appears to have been proportionally low. The maxilla of the upper jaw had a 92 cm long tooth row, was deep from top to bottom, and its upper and lower edges were almost parallel. The maxilla had a pronounced process (projection) under the nostril, and a small, ellipse-shaped fenestra (opening), like in Allosaurus and Tyrannosaurus. The nasal bone was very rugose, and these rugosities continued backwards covering the entire upper surface of this bone. The lacrimal bone in front of the eye had a prominent, rugose crest (or horn) that pointed up at a backwards angle. The crest was ridge-like, and had deep grooves. The postorbital bone behind the eye had a down and backwards directed jugal process that projected into the orbit (eye opening), as seen in Tyrannosaurus, Abelisaurus, and Carnotaurus. The supraorbital bone above the eye that contacted between the lacrimal and postorbital bones was eave-like, and similar to that of Abelisaurus. The quadrate bone at the back of the skull was 44 cm long, and had two pneumatic foramina (openings) on the inner side.[4][10]

Partial teeth, EBM

The skull roof (formed by the frontal and parietal bones) was broad and formed a "shelf", which overhung the short supratemporal fenestrae at the top rear of the skull. The jaw articulated far behind the occipital condyle (where the neck is attached to the skull) compared to other theropods. The condyle was broad and low, and had pneumatic cavities. Giganotosaurus did not have a sagittal crest on the top of the skull, and the jaw muscles did not extend onto the skull roof, unlike in most other theropods (due to the shelf over the supratemporal fenestrae). These muscles would instead have been attached to the lower side surfaces of the shelf. The neck muscles that elevated the head would have attached to the prominent supraoccipital bones on the top of the skull, which functioned like the nuchal crest of tyrannosaurs.[3] A latex endocast of the brain cavity of Giganotosaurus showed that the brain was similar to that of Carcharodontosaurus, but larger. The endocast was 29 mm long, 64 mm wide, and had volume of 275 ml (9.7 imp fl oz).[11]

The dentary of the lower jaw expanded in height towards the front (by the mandibular symphysis), where it was also flattened, and it had a downwards projection at the tip (which has been referred to as a "chin"). The lower side of the dentary was concave, the outer side was convex in upper view, and a groove ran along it, which supported foramina that nourished the teeth. The inner side of the dentary had a row of interdental plates, where each tooth had a foramen. The Meckelian groove ran along the lower border. The curvature of the dentary shows that the mouth of Giganotosaurus would have been wide. Though not completely known, it is possible that each dentary had twelve alveoli (tooth sockets). Most of the alveoli were about 3.5 cm (1.3 in) long from front to back. The teeth of the dentary were of similar shape and size, except for the first one, which was smaller. The teeth were compressed sideways, were oval in cross-section, and had serrations at the front and back borders, which is typical of theropods.[5][12] One tooth had nine to twelve serrations per millimetre (0.039 in).[13] The side teeth of Giganotosaurus had curved ridges of enamel, and the largest teeth in the premaxilla had pronounced wrinkles (with their highest relief near the serrations).[14]

History of discovery

Holotype skeleton with replica skull, arm, and feet, on a sandy floor at EBM

In 1993, the amateur fossil hunter Rubén D. Carolini discovered the tibia of a theropod dinosaur while driving a dune buggy in the badlands near Villa El Chocón, in the Neuquén province of Patagonia, Argentina. Specialists from the National University of Comahue where sent to escavate the specimen after being notified of the find.[15][16] The discovery was announced by the Argentinean paleontologists Rodolfo Coria and Leonardo Salgado at a Society of Vertebrate Paleontology meeting in 1994, where American science writer Don Lessem offered to fund the excavation, after having been impressed by a photo of the leg-bone.[15][17] The partial skull was scattered over an area of about 10 square meters (110 sq ft), and the postcranial skeleton was disarticulated. The specimen preserved almost 70% of the skeleton, and included most of the vertebral column, the pectoral and pelvic girdles, the femora, and the left tibia and fibula. In 1995, this specimen (MUCPv-Ch1) was preliminarily described in Nature by Coria and Salgado, who made it the holotype of the new genus and species Giganotosaurus carolinii (parts of the skeleton were still encased in plaster at this time). The generic name is derived from the Ancient Greek words gigas/γίγας (meaning "giant"), notos/νότος (meaning "austral/southern", in reference to its provenance) and -sauros/-σαύρος (meaning "lizard"). The specific name honours Carolini, the discoverer.[3][4][18] The holotype skeleton is now housed in the Ernesto Bachmann Museum in Villa El Chocón, which was inaugurated in 1995 on the request of Carolini. The specimen is the main exhibition at the museum, and is placed on the sandy floor of a room fully devoted to the animal, along with elements used by palaeontologists during the excavation. A mounted reconstruction of the skeleton is exhibited in an adjacent room.[16]

Reconstructed skeleton, EBM

One of the features of theropod dinosaurs that has attracted most scientific interest is the fact that the group includes the largest terrestrial predators of the Mesozoic Era. This already began with the discovery of one of the first known dinosaurs, Megalosaurus, named for its large size in 1824. More than half a century later in 1905, Tyrannosaurus was named, and remained the largest known theropod dinosaur for 90 years, though other large theropods were also known. The discussion of what was the largest theropod was revived in the 1990s by new discoveries in Africa and South America.[4] In their original description, Coria and Salgado considered Giganotosaurus at least the largest theropod dinosaur from the southern hemisphere, and perhaps the largest in the world. They conceded that comparison with Tyrannosaurus was difficult due to the disarticulated state of the cranial bones of Giganotosaurus, but noted that at 1.43 m (4.7 ft), the femur of Giganotosaurus was 5 cm (2 in) longer than that of "Sue", the largest known Tyrannosaurus specimen, and that the bones of Giganotosaurus appeared to be more robust, indicating a heavier animal. They estimated the skull to have been about 1.53 m (5 ft) long, and the whole animal as 12.5 m (41 ft) long, with a weight of about 6 to 8 tonnes.[4]

In 1996, Paul Sereno and colleagues described a newly discovered skull of Carcharodontosaurus from Morocco, a theropod described in 1927 but previously only known from fragmentary remains (the original fossils were destroyed in WW 2). They estimated the skull to have been 160 cm (5.2 ft), similar to Giganotosaurus, but perhaps exceeding that of the Tyrannosaurus "Sue", with a 1.53 m (5 ft) long skull. They also pointed out that carcharodontosaurs appear to have had the proportionally largest skulls, but that Tyrannosaurus appears to have had longer hind limbs.[19] In a 1995 interview for a Science News article entitled "New Beast Usurps T. Rex as King Carnivore", Sereno noted hat these newly discovered theropods from South America and Africa competed with Tyrannosaurus as the largest predators, and would help in understanding of Late Cretaceous dinosaur faunas, which had otherwise been very "North America-centric".[20] In the same issue of Science as Carcharodontosaurus was described in, the Canadian palaeontologist Philip J. Currie cautioned that it was yet to be determined which of the two animals were larger, and that the size of an animal is less interesting to palaeontologists than for example adaptations, relationships, and distribution. He also found it remarkable that the two animals were found within a year of each other, and were closely related, in spite of being found on different continents.[21]

Reconstructed skull in Japan, based on large size estimates

In a 1997 Science News interview, Coria estimated Giganotosaurus to have been 13.7 (45 ft) to 14.3 (47 ft) m long and weighing 8 to 10 tonnes based on new material, larger than Carcharodontosaurus. Sereno countered that it would be difficult to determine a size range for a species based on few, incomplete specimens, and both palaeontologists agreed that other aspects of these dinosaurs were more important than settling the "size contest".[22] In 1998, Jorge O. Calvo and Coria referred a partial left dentary containing some teeth (MUCPv-95) to Giganotosaurus. It had been collected by Calvo near Los Candeleros in 1988 (found in 1987), who briefly described it in 1989, while noting it may have belonged to a new theropod taxon. Calvo and Coria found the dentary to be identical to that of the G. carolinii holotype, though 8% larger at 62 cm (24 in). Though the rear part of it is incomplete, they proposed that the skull of the holotype specimen would have been 180 cm (5.9 ft) long, and estimated the skull of the larger specimen to have been 195 cm (6.4 ft) long, the longest skull of any theropod. In 1999, Calvo also referred an incomplete tooth (MUCPv-52) to Giganotosaurus; this specimen was discovered near lake Ezquiel Ramos Mexia in 1987 by A. Delgado, and is therefore the first known fossil of the genus.[5][13][23]

Restoration with size comparison

In 2001, the physician-scientist Frank Seebacher proposed a new polynomial method of calculating body-mass estimates for dinosaurs (using body-length, depth, and width), and found Giganotosaurus to have weighed 6.6 tonnes (based on the original 12.5 m length estimate).[24] In their 2002 description of the braincase of Giganotosaurus, Coria and Currie gave an estimate of 1.60 m for the skull of the holotype.[3] In 2004, Gerardo V. Mazzetta and colleagues pointed out that though the femur of the Giganotosaurus holotype was larger than that of "Sue", the tibia was 8 cm shorter at 1.12 m. They found the holotype specimen to have been equal to Tyrannosaurus in size at 8 tonnes (marginally smaller than "Sue"), but that the larger dentary might have represented an animal of 10 tonnes, if geometrically similar to the holotype specimen. By using multivariate regression equations, these authors also suggested an alternative weight of 6.5 tonnes for the holotype and 8.2 tonnes for the larger specimen, and that the latter was therefore the largest known terrestrial carnivore.[25] In 2005, Christiano Dal Sasso and colleagues described new skull material (a snout) of Spinosaurus (the original fossils of which were also destroyed during WW2), and concluded this dinosaur would have been 16-18 m long and weighed 7-9 t, exceeding the maximum size of all other theropods.[26] In 2006, Coria and Currie described Mapusaurus, a large theropod closely related to Giganotosaurus and of approximately the same size.[10]

Reconstructed skeleton, Australian Museum, Sydney

In a 2007, François Therrien and Donald M. Henderson found that Giganotosaurus and Carcharodontosaurus would both have approached 13 m in length and 13.8 tonnes in weight (surpassing Tyrannosaurus), and estimated the skull of the Giganotosaurus holotype to have been 1.56 m. They cautioned that these measurements depended on whether the incomplete skulls of these animals had been reconstructed correctly, and that more complete specimens were needed for more accurate estimates. They also found that Dal Sasso and colleagues had reconstructed Spinosaurus as too large, and instead estimated it to have been 14.3 m long and weighed 20.9 t, and possibly as low as 12.6 m in length and 12 tonnes in weight. They concluded that these dinosaurs had reached the upper biomechanical size limit attainable by a strictly bipedal animal.[6] In 2012, Matthew T. Carrano and colleagues noted that though Giganotosaurus had received much attention due to its enormous size, and in spite of the holotype being relatively complete, it had not yet been described in detail, apart from the braincase. They pointed out that many contacts between skull bones were not preserved, which lead to the total length of the skull being ambiguous. They instead found that the skulls of Giganotosaurus and Carcharodontosaurus were exactly the same size as that of Tyrannosaurus. They also measured the femur of the Giganotosaurus holotype to be 1.36,5 cm long, in contrast to the original measurement, and proposed that the body mass would have been smaller overall.[9]

In 2013, the American palaeontologist and palaeoartist Scott Hartman published a Graphic Double Integration mass estimate (based on drawn skeletal reconstructions), wherein he found Tyrannosaurus ("Sue") to have been larger than Giganotosaurus overall. He estimated the Giganotosaurus holotype to have weighed 6.8 tonnes, and the larger specimen 8.2 tonnes. Tyrannosaurus was estimated to have weighed 8.4 tonnes, and Hartman noted that it had a wider torso, though the two seemed similar in side view. He also pointed out that the Giganotosaurus dentary that was supposedly 8% larger than that of the holotype specimen would rather have been 6.5% larger, or could simply have belonged to a similarly sized animal with a more robust dentary. He conceded that with only one good Giganotosaurus specimen known, it is possible that larger individuals will be found, as it took most of a century to find "Sue" after Tyrannosaurus was discovered.[8] In 2014, Nizar Ibrahim and colleagues estimated the length of Spinosaurus to have been 15 m, by extrapolating from a new specimen scaled up to match the snout described by Dal Sasso and colleagues.[27]

Classification

Coria and Salgado originally found Giganotosaurus to group closer with the theropod clade tetanurae than to more basal (or "primitive") theropods such as ceratosaurs, due to shared features (synapomorphies) in the legs, skull, and pelvis. Other features showed that it was outside the more derived (or "advanced") clade coelurosauria.[4] In 1996, Sereno and colleagues found Giganotosaurus, Carcharodontosaurus, and Acrocanthosaurus to be closely related within the superfamily Allosauroidea, and grouped them in the family Carcharodontosauridae (named in 1931). Features shared between these genera include a the lacrimal and postorbital bones forming a broad "shelf" over the orbit, and the squared front end of the lower jaw.[19] After analysing the braincases of Giganotosaurus in 2002, Coria and Currie found Acrocanthosaurus to be only distantly related.[3]

As more carcharodontosaurids were discovered, their interrelationships became clearer. In 2006, Coria and Currie united Giganotosaurus and Mapusaurus in the carcharodontosaurid subfamily Giganotosaurinae, based on shared features of the femur such as a weak fourth trocanther and a shallow, broad groove on the lower end.[10] In 2008, Sereno and Stephen R. Brusatte united Giganotosaurus, Mapusaurus, and Tyrannotitan in the tribe Giganotosaurini.[28]

The following cladogram after Apesteguía et al., 2016, shows the placement of Giganotosaurus within Carcharodontosauridae.[29]



Allosaurus


Carcharodontosauria

Neovenatoridae


Carcharodontosauridae

Concavenator





Acrocanthosaurus



Eocarcharia





Shaochilong


Carcharodontosaurinae

Carcharodontosaurus saharicus



Carcharodontosaurus iguidensis


Giganotosaurini

Tyrannotitan




Mapusaurus



Giganotosaurus










Coria and Salgado suggested that the convergent evolution of gigantism in theropods in could have been linked to common conditions in their environments or ecosystems.[4] Sereno and colleagues found that the presence of charcarodontosaurids in Africa (Carcharodontosaurus) North America (Acrocanthosaurus), and South America (Giganotosaurus), showed the group had a trans-continental distribution by the Early Cretaceous period. Dispersal routes between the northern and southern continents appear to have been severed by ocean barriers in the Late Cretaceous, which lead to more distinct, provincial faunas, by preventing exchange.[19] Previously, it was thought that the Cretaceous world was biogeographically separated, with the northern continents being dominated by tyrannosaurids, South America by abelisaurids, and Africa by charcarodontosaurids.[21]

For almost a century, the knowledge of the South American non-avian theropods was limited to the fragmentary remains of the ceratosaur Genyodectes; just until the 1980s new discoveries made in the Argentinean Patagonia begun to reveal a more diverse fauna of theropods, specially from the Cretaceous, like the abelisauroids Noasaurus, Abelisaurus and Carnotaurus; this allow formulate the idea of the different faunas of dinosaurs in the Southern and Northern Hemispheres, reflecting the division between the ancient landmasses of Laurasia and Gondwana (the "pan-Gondwanan hypothesis" mentioned initially by José Bonaparte). According with this idea, abelisaurs were the main predators in the South, being equivalents of the tyrannosaurids from the North. The discovery of Giganotosaurus lead to the first record of a member of the more advanced group Tetanurae from the Cretaceous of South America, and the idea of the dominance and diversification of the abelisaurs in South America occurs until the end of the Cretaceous, just after the extinction of the giant primitive tetanurans like Giganotosaurus.[30]

Palaeobiology

Casts of Giganotosaurus and the contemporary sauropod Limaysaurus, Hungarian Natural History Museum

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.

In 1999, Reese E. Barrick and William J. Showers found that the bones of Giganotosaurus and Tyrannosaurus had very similar oxygen isotope patterns, with similar heat distribution in the body. These thermoregulatory patterns indicate that these dinosaurs had a metabolism intermediate between that of mammals and reptiles, and were therefore homeothermic (with a stabke core body-temperature, a type of "warm-bloodedness"). The metabolism of an 8 tonne Giganotosaurus would be comparable to that of a 1,000 kilograms (0.98 long tons; 1.1 short tons) mammalian carnivore, and would have supported rapid growth.[31]

Restoration of a walking individual

In 2001, R. Ernesto Blanco and Mazzetta evaluated the cursorial (running) capability of Giganotosaurus, and found it to be rather limited, due to the risk of injuries involved in such a large animal falling while on a run. They found that imbalance would grow by increasing velocity, and by calculating the time it would take for a leg to gain balance after the retraction of the opposite leg, they found the upper limit of the running speed to be 14 metres per second (50 km/h; 31 mph), a speed where there would be no danger of falling. They also found comparison between the running capability of Giganotosaurus and birds like the ostrich based on the strength of their leg-bones to be of limited value, since theropods had heavy tails to counterbalance their weight, unlike birds.[32]

In 2002, Coria and Currie found that various features of the rear part of the skull (such as the frontwards slope of the occiput and low and wide occipital condyle) indicate that Giganotosaurus would have had a good capability of moving the skull sideways in relation to the front neck vertebrae. These features may also have been related to the increased mass and length of the jaw muscles; the jaw articulation of Giganotosaurus and other charcarodontosaurids was moved hindwards to increase the length of the jaw musculature, enabling faster closure of the jaws, whereas tyrannosaurs increased the mass of the lower jaw musculature, to increase the power of their bite.[3]

Restoration of Giganotosaurus feeding on the sauropod Andesaurus

In 2005 Therrien and colleagues estimated the bite force of theropods and found that Giganotosaurus and some other taxa had adaptations for capturing and bringing down prey by delivering powerful bites, whereas tyrannosaurs had adaptations for resisting tortsional stress and crushing bones. The bite force of Giganotosaurus was weaker than that of Tyrannosaurus, and the force decreased hindwards along the tooth row. The lower jaws were adapted for slicing bites, and it probably captured and manipulated prey with the front part of the jaws. These authors suggested that Giganotosaurus and other allosaurs may have been generalised predators that fed on a wide spectrum of prey smaller than themselves, such as juvenile sauropods. The ventral process (or "chin") of the lower jaw may have been an adaptation for resisting tensile stress when the powerful bite was delivered with the front of the jaws against the prey.[33]

Palaeoecology

Giganotosaurus was found in deposits in what is now considered the Candeleros Formation, previously assigned to the Río Limay Formation. It 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.[34]

References

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