Carcharodontosaurus

Skull of Carcharodontosaurus.

Carcharodontosaurus (meaning "shark-toothed reptile") is an extinct genus of carcharodontosaurid dinosaur that lived in North Africa around 100 to 93 million years ago, during the late Albian to early Cenomanian stages of the mid-Cretaceous Period. It was nearly as long as or even longer than Tyrannosaurus, growing to an estimated 11.1-13.5 meters (36-44 feet) and weighing up to 8 metric tons. The name Carcharodontosaurus means 'shark lizard', after the shark genus Carcharodon (from the Greek καρχαρο karcharo meaning 'jagged' and οδοντο odonto meaning 'teeth') and σαυρος sauros, meaning 'lizard'.

Description

Carcharodontosaurus includes some of the longest and heaviest known carnivorous dinosaurs, with various scientists proposing length estimates for the species C. saharicus ranging between 12 and 13 m (39 and 43 ft) and weight estimates between 6 and 15 metric tons.

Carcharodontosaurus were carnivores, with enormous jaws and long, serrated teeth up to eight inches long. Paleontologists once thought that Carcharodontosaurus had the longest skulls of any of the theropod dinosaurs. However, the premaxilla and quadrate bones were missing from the original African skull, which led to misinterpretation of its actual size by researchers. A more modest length of 1.6 meters (5.2 ft) has now been proposed for C. saharicus, and the skull of C. iguidensis is reported to have been about the same size.[5] Currently, the largest known theropod skull belongs to another huge carcharodontosaurid dinosaur, the closely related Giganotosaurus (with skull length estimates up to 1.95 m) (6.3 ft).

Brain and inner ear

In 2001, Hans C. E. Larsson published a description of the inner ear and endocranium of Carcharodontosaurus saharicus. Larsson observed that the C. saharicus braincase "completely encloses the endocranial region." This "high degree of ossification" made his analysis of its anatomy significantly easier to perform. The C. saharicus endocast is similar to that of a related dinosaur, Allosaurus fragilis. Larsson describes the olfactory bulbs and peduncles as lying "on approximately the same horizontal plane as the forebrain." The midbrain is angled downwards and towards the rear of the animal, while the hind brain is roughly parallel to the forebrain. The cephalic flexure, the bend between the fore- and midbrain, has an angle of 45 degrees. The pontine flexure, the bend between the mid- and hindbrain has an angle of about 40 degrees. Carcharodontosaurus had a large optic (II) nerve. The C. saharicus vena capitis dorsalis "drains the anterior neck muscles through a pair of long canals on the posterior surface of the endocast." This configuration is found in Allosaurus and Dromaeosaurus albertensis, although in C. saharicus and Troodon "the transverse sinus probably drained into a middle cerebral vein that exited the brain in the ridge present on the dorsal edge of the trigeminal foramen."

The three semicircular canals of the inner ear of Carcharodontosaurus saharicus, when viewed from the side, had a subtriangular outline.[This subtriangular inner ear configuration is present in Allosaurus, lizards, and turtles, but not in birds. The pointed apex "at the junction of the anterior and posterior semicircular canals" is caused by the near linearity of the canals and closely resembles the condition of modern crocodiles. The subtriangular configuration may be the basal condition of archosauromorphs. A recess which would have held the floccular lobe of the brain projects into the area surrounded by the semicircular canals. This condition is also present in other non-avian theropods, birds, and pterosaurs. The orientation of the lagena of C. saharicus resembles the condition in crocodilians and some birds. The extent of its perilymphatic duct resembled those of Varanus, crocodilians, and birds. The crista which would have supported the secondary tympanic membrane in C. saharicus was either absent, or not preserved. This contrasts with Troodon, whose crista were ossified at least in their dorsal and ventral regions and their remaining portions either cartilaginous or too delicate to be preserved. The metotic strut of C. saharicus is reduced and medial compared to the "laterally hypertrophied" condition of non-avian maniraptors like Dromaeosaurus and Troodon, as well as primitive birds like Archaeopteryx and Hesperornis.

Traditional comparisons of brain volume to body mass has estimated brain size as the volume of the endocast. However, the brain of Sphenodon fills only about half of its endocranial volume. Some paleontologists used the fifty percent estimate to ascertain the brain size of dinosaur endocasts. Other workers have observed that details on the endocranial surface indicates that some fossil reptiles had brains that occupied a much larger portion of the endocranium. Larsson notes that the transition from reptiles to birds prevents using a set ratio from being a valid approach to estimating the volume of the endocranium occupied by a dinosaur's brain.

Adding difficulty to examining the ratio of the brain volume of a dinosaur to its body mass is the wide range of estimates for live mass. Larsson observes that one study which estimated the live masses for many dinosaur genera typically had a fourfold range. Larsson laments that "[t]he broad ranges of body mass estimates, combined with the ambiguous ratio of endocranial volume occupied by the brain, present a high degree of uncertainty for [creating an] index of brain size." Consequently, he attempted to minimize errors in his study by making a different kind of comparison.

Noting that while it is difficult to estimate the absolute volume of the brain, the proportions of its various regions should be the same in the endocast as it was in the large brain Larsson's study compared the ratio of the cerebrum, which is highly demarcated, to the rest of the endocast's volume. However, if the thickness of the dura covering the various parts of the brain itself differed, then that could alter the relative proportions within the endocast. In Caiman the dura covering the medullary region seemed to increase proportionally in thickness compared to the dura covering the forebrain, although this might not impact the ratio between the regions. Nevertheless, Larsson reaffirmed the superiority of his technique to traditional comparisons of brain volume to estimated live body mass.

"As brain mass increases, cerebral mass increases with slight negative allometry in nonavian reptiles," Larsson concludes, larger nonavian reptile have proportionately smaller cerebra than smaller ones. Further, the opposite is true in birds, larger avian brains have cerebra which are slightly larger, proportionally speaking. Larsson found that both C. saharicus and Allosaurus lie within the 95% confidence limits of the nonavian reptile ratio, while Tyrannosaurus lies just outside it in the direction of a more avian proportion. The extinct crocodile Sebecus had a ratio similar to those of the non-coelurosaurian theropods studied. Since tyrannosaurs are coelurosaurs, this is evidence that the advent of the Coelurosauria marks the beginning of trend in theropod brain enlargement.

Gallery

Pathology

SGM-Din 1, a Carcharodontosaurus saharicus skull, has a circular puncture wound in the nasal and "an abnormal projection of bone on the antorbital rim".

External links

• Carcharodontosaurus at DinoData
• Student Identifies Enormous New Dinosaur December 7 2007 from the Science daily