Carcharodontosaurid dinosaur genus from the late Cretaceous period
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Giganotosaurus ( ) is a genus of large theropod dinosaur that lived in what is now Argentina, during the early Cenomanian age of the Late Cretaceous period, approximately 99.6 to 95 million years ago. The holotype specimen was discovered in the Candeleros Formation of Patagonia in 1993 and is almost 70% complete. The animal was named Giganotosaurus carolinii in 1995; the genus name translates to "giant southern lizard", and the specific name honors the discoverer, Ruben Carolini. A dentary bone, a tooth, and some tracks, discovered before the holotype, were later assigned to this animal. The genus attracted much interest and became part of a scientific debate about the maximum sizes of theropod dinosaurs.
"Giganotosaurus" was one of the largest known terrestrial carnivores, but the exact size has been hard to determine due to the incompleteness of the remains found so far. Estimates for the most complete specimen range from a length of , a skull in length, and a weight of . The dentary bone that belonged to a supposedly larger individual has been used to extrapolate a length of . Some researchers have found the animal to be larger than "Tyrannosaurus", which has historically been considered the largest theropod, while others have found them to be roughly equal in size and the largest size estimates for "Giganotosaurus" exaggerated. The skull was low, with rugose (rough and wrinkled) nasal bones and a ridge-like crest on the lacrimal bone in front of the eye. The front of the lower jaw was flattened and had a downward-projecting process (or "chin") at the tip. The teeth were compressed sideways and had serrations. The neck was strong and the pectoral girdle proportionally small.
Part of the family Carcharodontosauridae, "Giganotosaurus" is one of the most completely known members of the group, which includes other very large theropods, such as the closely related "Mapusaurus", "Tyrannotitan" and "Carcharodontosaurus". "Giganotosaurus" is thought to have been homeothermic (a type of "warm-bloodedness"), with a metabolism between that of a mammal and a reptile, which would have enabled fast growth. It would have been capable of closing its jaws quickly, capturing and bringing down prey by delivering powerful bites. The "chin" may have helped in resisting stress when a bite was delivered against prey. "Giganotosaurus" is thought to have been the apex predator of its ecosystem, and it may have fed on juvenile sauropod dinosaurs.
Discovery.
In 1993, the amateur Argentine fossil hunter Rubén Darío Carolini discovered the tibia (lower leg bone) 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 were sent to excavate the specimen after being notified of the find. The discovery was announced by the paleontologists Rodolfo Coria and Leonardo Salgado at a Society of Vertebrate Paleontology meeting in 1994, where science writer Don Lessem offered to fund the excavation, after having been impressed by a photo of the leg-bone. The partial skull was scattered over an area of about 10 m2 (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 was preliminarily described 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/" (meaning "giant"), "notos/" (meaning "austral/southern", in reference to its provenance) and "-sauros/-" (meaning "lizard"). The specific name honors Carolini, the discoverer. The holotype skeleton is now housed in the Ernesto Bachmann Paleontological Museum (where it is catalogued as specimen MUCPv-Ch1) in Villa El Chocón, which was inaugurated in 1995 at the request of Carolini. The specimen is the main exhibition at the museum, and is placed on the sandy floor of a room devoted to the animal, along with tools used by paleontologists during the excavation. A mounted reconstruction of the skeleton is exhibited in an adjacent room.
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 interest began with the discovery of one of the first known dinosaurs, "Megalosaurus", named in 1824 for its large size. More than half a century later in 1905, "Tyrannosaurus" was named, and it remained the largest known theropod dinosaur for 90 years, though other large theropods were also known. The discussion of which theropod was the largest was revived in the 1990s by new discoveries in Africa and South America. 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 , 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 to have been 12.5 m (41 ft) long, with a weight of about .
In 1996, the paleontologist Paul Sereno and colleagues described a new skull of the related genus "Carcharodontosaurus" from Morocco, a theropod described in 1927 but previously known only from fragmentary remains (much of its fossils were destroyed in World War II). They estimated the skull to have been long, 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. In an interview for a 1995 article entitled "new beast usurps "T. rex" as king carnivore", Sereno noted that these newly discovered theropods from South America and Africa competed with "Tyrannosaurus" as the largest predators, and would help in the understanding of Late Cretaceous dinosaur faunas, which had otherwise been very "North America-centric". In the same issue of the journal in which "Carcharodontosaurus" was described, the paleontologist 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 paleontologists 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.
In a 1997 interview, Coria estimated "Giganotosaurus" to have been 13.7 (45 ft) to 14.3 (47 ft) m long and weighing 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 paleontologists agreed that other aspects of these dinosaurs were more important than settling the "size contest". In 1998, the paleontologist Jorge O. Calvo and Coria assigned a partial left dentary bone (part of the lower jaw) containing some teeth (MUCPv-95) to "Giganotosaurus". It had been collected by Calvo near Los Candeleros in 1988 (found in 1987), who described it briefly 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 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 long, and estimated the skull of the larger specimen to have been long, the longest skull of any theropod.
In 1999, Calvo referred an incomplete tooth, (MUCPv-52), to "Giganotosaurus"; this specimen was discovered near Lake Ezequiel Ramos Mexia in 1987 by A. Delgado, and is therefore the first known fossil of the genus. Calvo further suggested that some theropod trackways and isolated tracks (which he made the basis of the ichnotaxon "Abelichnus astigarrae" in 1991) belonged to "Giganotosaurus", based on their large size. The largest tracks are long with a pace of , and the smallest is long with a pace of . The tracks are tridactyl (three-toed) and have large and coarse digits, with prominent claw impressions. Impressions of the digits occupy most of the track-length, and one track has a thin heel. Though the tracks were found in a higher stratigraphic level than the main fossils of "Giganotosaurus", they were from the same strata as the single tooth and some sauropod dinosaurs that are also known from the same strata as "Giganotosaurus".
Continued size estimations.
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 (based on the original length estimate). In their 2002 description of the braincase of "Giganotosaurus", Coria and Currie gave a length estimate of for the holotype skull, and calculated a weight of by extrapolating from the circumference of the femur-shaft. This resulted in an encephalization quotient (a measure of relative brain size) of 1.9. In 2004, the paleontologist Gerardo V. Mazzetta and colleagues pointed out that though the femur of the "Giganotosaurus" holotype was larger than that of "Sue", the tibia was shorter at . They found the holotype specimen to have been equal to "Tyrannosaurus" in size at (marginally smaller than "Sue"), but that the larger dentary might have represented an animal of , if geometrically similar to the holotype specimen. By using multivariate regression equations, these authors also suggested an alternative weight of for the holotype and for the larger specimen, and that the latter was therefore the largest known terrestrial carnivore.
In 2005, the paleontologist Cristiano Dal Sasso and colleagues described new skull material (a snout) of "Spinosaurus" (the original fossils of which were also destroyed during World War II), and concluded this dinosaur would have been long with a weight , exceeding the maximum size of all other theropods. In 2006, Coria and Currie described the large theropod "Mapusaurus" from Patagonia; it was closely related to "Giganotosaurus" and of approximately the same size. In 2007, the paleontologists François Therrien and Donald M. Henderson found that "Giganotosaurus" would have approached in length and in weight, while "Carcharodontosaurus" would have approached in length and in weight (surpassing "Tyrannosaurus"), and estimated the "Giganotosaurus" holotype skull to have been long. 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' reconstruction of "Spinosaurus" was too large, and instead estimated it to have been long, weighing , and possibly as low as in length and in weight. They concluded that these dinosaurs had reached the upper biomechanical size limit attainable by a strictly bipedal animal. In 2010, the paleontologist Gregory S. Paul suggested that the skulls of carcharodontosaurs had been reconstructed as too long in general.
In 2012, the paleontologist 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 found instead 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 long, in contrast to the original measurement, and proposed that the body mass would have been smaller overall. In 2013, the paleontologist Scott Hartman published a Graphic Double Integration mass estimate (based on drawn skeletal reconstructions) on his blog, wherein he found "Tyrannosaurus" ("Sue") to have been larger than "Giganotosaurus" overall. He estimated the "Giganotosaurus" holotype to have weighed , and the larger specimen . "Tyrannosaurus" was estimated to have weighed , and Hartman noted that it had a wider torso, though the two seemed similar in side view. He also pointed out that the "Giganotosaurus"