Majungasaurus
Majungasaurus is a genus of abelisaurid theropod dinosaur that lived in Madagascar from 70 to 66 million years ago, at the end of the Cretaceous Period, making it one of the last-known non-avian dinosaurs that went extinct during the Cretaceous–Paleogene extinction event. The genus contains a single species, Majungasaurus crenatissimus. This dinosaur is also called Majungatholus, a name which is considered a junior synonym of Majungasaurus.
Like other abelisaurids, Majungasaurus was a bipedal predator with a short snout. Although the forelimbs are not completely known, they were very short, while the hind limbs were longer and very stocky. Measuring around long and weighing more than, it can be distinguished from other abelisaurids by its wider skull, the very rough texture and thickened bone on the top of its snout, and the single rounded horn on the roof of its skull, which was originally mistaken for the dome of a pachycephalosaur. It also had more teeth in both upper and lower jaws than most abelisaurids.
The genus is one of the first abelisaurs to be discovered, first being found in 1896 and being named in 1955. Known from several well-preserved skulls and abundant skeletal material, Majungasaurus has recently become one of the best-studied theropod dinosaurs from the Southern Hemisphere. It appears to be most closely related to abelisaurids from India rather than South America or continental Africa, a fact that has important biogeographical implications. Majungasaurus was the apex predator in its ecosystem, mainly preying on sauropods like Rapetosaurus, and is also one of the few dinosaurs for which there is direct evidence of cannibalism.
Discovery and naming
French paleontologist Charles Depéret described the first theropod remains from northwestern Madagascar in 1896. These included two teeth, a claw, and some vertebrae discovered along the Betsiboka River by a French army officer and deposited in the collection of what is now the Université Claude Bernard Lyon 1. Depéret referred these fossils to the genus Megalosaurus, which at the time was a wastebasket taxon containing any number of unrelated large theropods, as the new species M. crenatissimus. This name is derived from the Latin word crenatus and the suffix -issimus, in reference to the numerous serrations on both front and rear edges of the teeth. Depéret later reassigned the species to the North American genus Dryptosaurus, another poorly known taxon.File:Majungasaurus.jpg|thumb|Neotype specimen of M. crenatissimus, the right dentary of a subadult individual in the Gallery of Paleontology and Comparative Anatomy – Muséum national d'histoire naturelle, Paris
Numerous fragmentary remains from Mahajanga Province in northwestern Madagascar were recovered by French collectors over the next 100 years, many of which were deposited in the Muséum National d'Histoire Naturelle in Paris. In 1955, René Lavocat described a theropod dentary with teeth from the Maevarano Formation in the same region where the original material was found. The teeth matched those first described by Depéret, but the strongly curved jaw bone was very different from both Megalosaurus and Dryptosaurus. Based on this dentary, Lavocat created the new genus Majungasaurus, using an older spelling of Mahajanga as well as the Greek word σαυρος sauros. Hans-Dieter Sues and Philippe Taquet described a dome-shaped skull fragment as a new genus of pachycephalosaur in 1979. This was the first report of a pachycephalosaur in the Southern Hemisphere.
In 1993, scientists from the State University of New York at Stony Brook and the University of Antananarivo began the Mahajanga Basin Project, a series of expeditions to examine the fossils and geology of the Late Cretaceous sediments near the village of Berivotra, in Mahajanga Province. Among these scientists was paleontologist David W. Krause of Stony Brook. The first expedition turned up hundreds of theropod teeth identical to those of Majungasaurus, some of which were attached to an isolated premaxilla that was described in 1996. The following seven expeditions would turn up tens of thousands of fossils, many of which belonged to species new to science. The Mahajanga Basin Project claims credit for quintupling the known diversity of fossil taxa in the region.
File:P1050192 - Majungatholus.JPG|thumb|Type specimen of Majungatholus atopus: a Majungasaurus frontal horn misidentified as a pachycephalosaur dome in the Gallery of Paleontology and Comparative Anatomy
Fieldwork in 1996 turned up a spectacularly complete theropod skull preserved in exquisite detail. On top of this skull was a dome-shaped swelling nearly identical to the one described by Sues and Taquet as Majungatholus atopus. Majungatholus was redescribed as an abelisaurid rather than a pachycephalosaur in 1998. Although the name Majungasaurus crenatissimus was older than Majungatholus atopus, the authors judged the type dentary of Majungasaurus too fragmentary to confidently assign to the same species as the skull. Further fieldwork over the next decade turned up a series of less complete skulls, as well as dozens of partial skeletons of individuals ranging from juveniles to adults. Project members also collected hundreds of isolated bones and thousands of shed Majungasaurus teeth. Taken together, these remains represent nearly all the bones of the skeleton, although most of the forelimbs, most of the pelvis, and the tip of the tail are still unknown. This fieldwork culminated in a 2007 monograph consisting of seven scientific papers on all aspects of the animal's biology, published in the Society of Vertebrate Paleontology Memoirs. The papers are in English, although each has an abstract written in Malagasy. In this volume, the dentary described by Lavocat was re-evaluated and determined to be diagnostic for this species. Therefore, the name Majungatholus was replaced by the older name Majungasaurus. Although the monograph is comprehensive, the editors noted that it describes only material recovered from 1993 through 2001. A significant quantity of specimens, some very complete, were excavated in 2003 and 2005 and await preparation and description in future publications. The dentary was made the neotype specimen after a 2009 petition to the ICZN.
Description
Majungasaurus was a medium-sized theropod that typically reached in length and weighed. Fragmentary remains of larger individuals indicate that some adults could have been similar in size to its relative Carnotaurus, possibly exceeding in length.The skull of Majungasaurus is exceptionally well known compared to most theropods and generally similar to that of other abelisaurids. Like other abelisaurid skulls, its length was proportionally short for its height, although not as short as in Carnotaurus. The skulls of large individuals measured long. The tall premaxilla, which made the tip of the snout very blunt, was also typical of the family. However, the skull of Majungasaurus was markedly wider than in other abelisaurids. All abelisaurids had a rough, sculptured texture on the outside faces of the skull bones, and Majungasaurus was no exception. This was carried to an extreme on the nasal bones of Majungasaurus, which were extremely thick and fused together, with a low central ridge running along the half of the bone closest to the nostrils. A distinctive dome-like horn protruded from the fused frontal bones on top of the skull as well. In life, these structures would have been covered with some sort of integument, possibly made of keratin. Computed tomography of the skull shows that both the nasal structure and the frontal horn contained hollow sinus cavities, perhaps to reduce weight. The teeth were typical of abelisaurids in having short crowns, although Majungasaurus bore seventeen teeth in both the maxilla of the upper jaw and the dentary of the lower jaw, more than in any other abelisaurid except Rugops.
The postcranial skeleton of Majungasaurus closely resembles those of Carnotaurus and Aucasaurus, the only other abelisaurid genera for which complete skeletal material is known. Majungasaurus was bipedal, with a long tail to balance out the head and torso, putting the center of gravity over the hips. Although the cervical vertebrae had numerous cavities and excavations to reduce their weight, they were robust, with exaggerated muscle attachment sites and ribs that interlocked for strength. Ossified tendons are attached to the cervical ribs, giving them a forked appearance, as seen in Carnotaurus. All of these features resulted in a very strong and muscular neck. Uniquely, the cervical ribs of Majungasaurus had long depressions along the sides for weight reduction. The humerus was short and curved, closely resembling those of Aucasaurus and Carnotaurus. Also, like related dinosaurs, Majungasaurus had very short forelimbs with four extremely reduced digits, first reported with only two very short external fingers and no claws. The hand and finger bones of Majungasaurus, like other majungasaurines, lacked the characteristic pits and grooves where claws and tendons would normally attach, and its finger bones were fused together, indicating that the hand was immobile. In 2012, a better specimen was described, showing that the lower arm was robust, though short, and that the hand contained four metatarsals and four, probably inflexible and very reduced, fingers, possibly with no claws. The minimum phalanx formula was 1-2-1-1.
Like other abelisaurids, the hindlimbs were stocky and short compared to body length. The tibia of Majungasaurus was even stockier than that of its relative Carnotaurus, with a prominent crest on the knee. The astragalus and calcaneum were fused together, and the feet bore three functional digits, with a smaller first digit that did not contact the ground.