Giganotosaurus
Giganotosaurus is a theropod dinosaur genus discovered in Argentina in 1993, which lived 99.6-97.0 million years ago, during the early Cenomanian age of the Late Cretaceous period. Within Theropoda, Giganotosaurus belonged to the family Carcharodontosauridae. The meaning of the genus name Giganotosaurus is "giant southern lizard". It was one of the largest land predators, its size was comparable to the later Tyrannosaurus rex. Giganotosaurus is suspected to have been a terrifying warm-blooded, fast-moving carnivorous dinosaur, and probably it was the apex predator of its ecosystem.
Giganotosaurus carolinii. By Durbed - http://durbed.deviantart.com/art/Giganotosaurus-carolinii-290866454, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=37158111
Known occurrences of Giganotosaurus fossils.
Discovery
The first fossil remains of Giganotosaurus were discovered by Rubén D. Carolini near Villa El Chocón, in the Neuquén province of Patagonia, Argentina, in 1993. Then the specimen was excavated by the team of National University of Comahue. Though the partial skull was scattered in a relatively large area, and other parts of the skeleton were disarticulated, the skeleton of the found specimen was in nearly 70% complete, including the vertebral coloumn and bones of the legs and arms.
The finding was scientifically described by Coria and Salgado in 1995 and it became the holotype of a new genus and species, named Giganotosaurus carolinii[1]. The holotype specimen is now exhibited in the Ernesto Bachmann Paleontological Museum in Villa El Chocón, Argentina. Soon after its discovery, this dinosaur became to a kind of sensation, as it competed with Tyrannosaurus rex for the title of the largest land predator. Though they lived in different continents, Giganotosaurus and other newly discovered species helped a lot to understand the dinosaur ruled ecosystems of other continents than North America[2].
During the 90's, further material, a fragmentary dentary bone, was referred to Giganotosaurus, which was actually discovered in 1987, earlier, than the type specimen was found. Furthermore, large theropod trackways were also assigned to Giganotosaurus[3]. No complete skeleton of Giganotosaurus was found yet.
Mounted skeleton of Giganotosaurus carolinii. By Jonathan Chen - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=70480981
Classification and valid species
The sole species of the genus is Giganotosaurus carolinii, known from Patagonia, Argentina[1]. Giganotosaurus was originally placed closer to the clad Tetanure than to basal theropods[1]. However, subsequently a common family, called Carcharodontosauridae, was established for Giganotosaurus and some other species, Carcharodontosaurus and Arcocanthosaurus within the superfamily Allosauroidea, based to the following shared caharacteristics: lower jaw having a squared front, and a broad shelf formed by the lacrimal and postorbital bones over the orbit[4].
Later, in 2008, the tribe Giganotosaurini was formed for Giganotosaurus, Mapusaurus and Tyrannotitan within the subfamily Giganotosauridae of the family Charcarodontosauridae[5]. These species were probably the apex predators of the South American fauna. While carcharodontosaurids were also represented in the North American fauna by Acrocanthosaurus, the subfamily Carcharodontosaurinae seems to have been occured only in the southern continent of Gondwana, which was formed by the later South America and Africa[6].
Adaptation of Slate Weasel Size comparison of the two known specimens of Giganotosaurus. Silhouettes scaled and based on Franoys's Giganotosaurus skeletal. Human silhouette from File:Silhouette of man standing and facing forward. By KoprX - File:Giganotosaurus Scale.svg, CC0, https://commons.wikimedia.org/w/index.php?curid=79211379
Description
Giganotosaurus was a huge bipedal carnivor dinosaur with a robust and elongated head, strong hindlegs, small three-fingered arms and with a long tail. It was undoubtedly one of the largest land predators. Since it discovery an ongoing scientific debate exists about its exact size, which is hard to estimate due to the incompleteness of the fossil skeleton. However, the most important matter of this debate is that which dinosaur was the largest dryland carnivor ever from the following candidates: Tyrannosaurus , Giganotosaurus , Carcharodontosaurus, or Spinosaurus? Regardint the size of Giganotosaurus, different estimates are known based on various methods. The results of these largely depended on that how the missing elements of the fossil skeleton were reconstructed. The length of the holotype specimen has been estimated between 12-13 m, the length of its skull between 1.53-1.80 m, and its weight between 4.2-13.8 tons[1][7].
The sizes of the second specimen were extrapolated from its only known part, e.g. its incomplete dentary bone. Its length was estimated to have been 13.2 m, the skull length 1.95 m,and its weight for 8.2 tons[8]. Giganotosaurus had a low skul, with paralell upper and lower jaw edges. The surface of the nasal bone was wrinkled. Above the eyes there was a prominent, rugose, bony horn, that pointed upwards and a bit backwards. Giganotosaurus had a broad skull roof. unlike most theropod dinosaus, Giganotosaurus lacked a sagittal crest on the top of the skull. This dinosaur had a 19mm long and 64mm wide brain endocast with a volume of 275ml[9], which is bigger than that of Carcharodontosaurus, but much smaller tha the brain endocast of the Tyrannosaurus.
The teeth of Giganotosaurus were compressed sideways, sigmoid-shaped from frontal view, and serrated at its front and back edges[10]. Giganotosaurus had a quite strong neck, with a robust axis bone. Neural arches of dorsal vertebrae were high, and their pleurocoels were deep. Neural spines of caudal vertebrae were elongated from the front to backwards, and their centrae were robust[1].
Skull of Giganotosaurus carolinii. Source:https://www.flickr.com/photos/kabacchi/4674758502/in/photostream/. Licence: CC BY 2.0 https://creativecommons.org/licenses/by/2.0/
Paleobiology
Based on oxygen isotope patterns of the fossilized bones it is likely that Giganotosaurus was homeothermic, which is a type of warm-bloodedness with a metabolims taht is intermediate between that of mammals and reptiles, with a stable core body-temperature[11]. An other research suggests that Giganotosaurus could have the maximum running speed of 50 km/h, and at the same time, rejects the former hypothesys which stated that the risk of injuries involved in falling at top speed would have limited the velocity of these large land predators[12].
However, a more recent study from 2017 which was based on the biomechanical analysis of the running capabilities of Tyrannosaurus rex suggests that such large carnivore dinosaurs, like Tyrannosaurus , Giganotosaurus, Mapusaurus and Acrocanthosaurus probably were mechanically limited to walking gaits, and they were not able to pursuit their prey by running at high-speed[13]. Based on its cranial structure, in case of Giganotosaurus the jaw articulation was positioned backwards which resulted a faster jaw closure, in contrast with the Tyrannosaurus , as in case of the latter theropod the mass of the lower jaw muscles were increased, which resulted a more powerful bite[14]. Consequently, Giganotosaurus might have a weaker biting force than that of the Tyrannosaurus .
The discovery of Mapusaurus, a close relative of Giganotosaurus, suggested that these huge predators probably lived and hunted in packs, as remnants of several individuals of different growth stages were found in the same bonebed - the death of these dinosaurs probably was the result of a sudden catastrophic event[15]. Hunting in groups could have been a clear advantage in taking down gigantic sauropods, like Andesaurus, Limaysaurus, or Nopcsaspondylus, which were parts of the same ecosystem in which Giganotosaurus lived[16].
Mounted skeleton of Giganotosaurus carolinii in The Natural Science Museum at El Chocón, in the northwestern Argentine Patagonia (the Comahue region). By Simona.cerrato - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=20199607
Scientific references
[1] Coria, R.A.; Salgado, L. (1995): A new giant carnivorous dinosaur from the Cretaceous of Patagonia. Nature, 377 (6546): 224-226. doi: 10.1038/377224a0.
[2] Monastersky, R. (1995): New beast usurps T. rex as king carnivore. Science News, 148 (13): 199doi: 10.2307/3979427.
[3] Calvo, J. O. (1999): Dinosaurs and other vertebrates of the Lake Ezequiel Ramos Mexía area, Neuquén-Patagonia, Argentina. National Science Museum Monographs, 15: 13-45.
[4] Sereno, P. C.; Dutheil, D. B.; Iarochene, M.; Larsson, H. C. E.; Lyon, G. H.; Magwene, P. M.; Sidor, C. A.; Varricchio, D. J.; Wilson, J. A. (1996): Predatory Dinosaurs from the Sahara and Late Cretaceous Faunal Differentiation. Science, 272 (5264): 986-991. doi: 10.1126/science.272.5264.986.
[5] Brusatte, S. L.; Sereno, P. C. (2008): Phylogeny of Allosauroidea (Dinosauria: Theropoda): Comparative analysis and resolution. Journal of Systematic Palaeontology, 6 (2): 155-182. doi: 10.1017/S1477201907002404.
[6] Novas, F. E.; Agnolín, F. L.; Ezcurra, M. D.; Porfiri, J.; Canale, J. I. (2013): Evolution of the carnivorous dinosaurs during the Cretaceous: The evidence from Patagonia. Cretaceous Research, 45: 174-215. doi: 10.1016/j.cretres.2013.04.001.
[7] Therrien, F.; Henderson, D.M. (2007): My theropod is bigger than yours...or not: estimating body size from skull length in theropods. Journal of Vertebrate Paleontology, 27 (1): 108-115. doi: 10.1671/0272-4634(2007)27[108:MTIBTY]2.0.CO;2.
[8] Calvo, J. O.; Coria, R. A. (1998): New specimen of Giganotosaurus carolinii (Coria & Salgado, 1995), supports it as the largest theropod ever found. Gaia, 15: 117-122.
[9] Paulina Carabajal, A.; Canale, J. I. (2010): Cranial endocast of the carcharodontosaurid theropod Giganotosaurus carolinii Coria & Salgado, 1995. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen. 258 (2): 249-256. doi: 10.1127/0077-7749/2010/0104.
[10] Canale, J. I.; Novas, F. E.; Pol, D. (2014): Osteology and phylogenetic relationships of Tyrannotitan chubutensis Novas, de Valais, Vickers-Rich and Rich, 2005 (Theropoda: Carcharodontosauridae) from the Lower Cretaceous of Patagonia, Argentina. Historical Biology, 27 (1): 1-32. doi: 10.1080/08912963.2013.861830.
[11] Barrick, R.E.; Showers, W.J. (1999): Thermophysiology and biology of Giganotosaurus: Comparison with Tyrannosaurus. Palaeontologia Electronica, 2 (2).
[12] Blanco, R. Ernesto; Mazzetta, Gerardo V. (2001): A new approach to evaluate the cursorial ability of the giant theropod Giganotosaurus carolinii. Acta Palaeontologica Polonica, 46 (2): 193-202.
[13] Sellers, W. I.; Pond, S. B.; Brassey, C. A.; Manning, P. L.; Bates, K. T. (2017): Investigating the running abilities of Tyrannosaurus rex using stress-constrained multibody dynamic analysis. PeerJ. 5: e3420. doi: 10.7717/peerj.3420.
[14] Coria, R. A.; Currie, P. J. (2002): The braincase of Giganotosaurus carolinii (Dinosauria: Theropoda) from the Upper Cretaceous of Argentina. Journal of Vertebrate Paleontology, 22 (4): 802-811. doi: 10.1671/0272-4634(2002)022[0802:TBOGCD]2.0.CO;2.
[15] Coria, R.A.; Currie, P.J. (2006): A new carcharodontosaurid (Dinosauria, Theropoda) from the Upper Cretaceous of Argentina. Geodiversitas, 28 (1): 71-118.
[16] Leanza, H. A; Apesteguía, S.; Novas, F. E; de la Fuente, M. S (2004): Cretaceous terrestrial beds from the Neuquén Basin (Argentina) and their tetrapod assemblages. Cretaceous Research, 25 (1): 61-87. doi: 10.1016/j.cretres.2003.10.005.