| Majungasaurus | |
|---|---|
 | |
| Mounted skeleton,Stony Brook University | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Abelisauroidea |
| Family: | †Abelisauridae |
| Subfamily: | †Majungasaurinae |
| Genus: | †Majungasaurus Lavocat, 1955 |
| Species: | †M. crenatissimus |
| Binomial name | |
| †Majungasaurus crenatissimus (Depéret, 1896) [originallyMegalosaurus] | |
| Synonyms | |
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Majungasaurus (/məˌdʒʌŋɡəˈsɔːrəs/;lit. 'Mahajanga lizard') is agenus ofabelisauridtheropoddinosaur that lived inMadagascar from 70 to 66million years ago, at the end of theCretaceousPeriod, making it one of the last-known non-avian dinosaurs that went extinct during theCretaceous–Paleogene extinction event. The genus contains a singlespecies,Majungasaurus crenatissimus. This dinosaur is also calledMajungatholus, a name which is considered ajunior synonym ofMajungasaurus.
Like other abelisaurids,Majungasaurus was abipedalpredator 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 7 m (23 ft) long and weighing more than 1 t (1.1 short tons), it can be distinguished from other abelisaurids by its widerskull, 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 apachycephalosaur. It also had moreteeth 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 (although it was thought to be a species ofMegalosaurus) 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 theSouthern Hemisphere. It appears to be most closely related to abelisaurids from India rather than South America or continental Africa, a fact that has importantbiogeographical implications.Majungasaurus was theapex predator in itsecosystem, mainly preying onsauropods likeRapetosaurus, and is also one of the few dinosaurs for which there is direct evidence ofcannibalism.
FrenchpaleontologistCharles Depéret described the first theropod remains from northwesternMadagascar in 1896. These included two teeth, a claw, and some vertebrae discovered along theBetsiboka River by a French army officer and deposited in the collection of what is now theUniversité Claude Bernard Lyon 1. Depéret referred these fossils to the genusMegalosaurus, which at the time was awastebasket taxon containing any number of unrelated large theropods, as the new speciesM. crenatissimus.[1] This name is derived from theLatin wordcrenatus ("notched") and thesuffix-issimus ("most"), in reference to the numerous serrations on both front and rear edges of the teeth.[2] Depéret later reassigned the species to the North American genusDryptosaurus, another poorly known taxon.[3]
Numerous fragmentary remains fromMahajanga Province in northwestern Madagascar were recovered by French collectors over the next 100 years, many of which were deposited in theMuséum National d'Histoire Naturelle inParis.[2] In 1955,René Lavocat described a theropod dentary (MNHN.MAJ 1) with teeth from theMaevarano 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 bothMegalosaurus andDryptosaurus. Based on this dentary, Lavocat created the new genusMajungasaurus, using an older spelling of Mahajanga as well as theGreek wordσαυροςsauros (meaning "lizard").[4]Hans-Dieter Sues andPhilippe Taquet described a dome-shaped skull fragment (MNHN.MAJ 4) as a new genus of pachycephalosaur (Majungatholus atopus) in 1979. This was the first report of a pachycephalosaur in the Southern Hemisphere.[5]
In 1993, scientists from theState University of New York at Stony Brook and theUniversity of Antananarivo began the Mahajanga Basin Project, a series of expeditions to examine the fossils and geology of theLate Cretaceous sediments near the village of Berivotra, in Mahajanga Province.[2] Among these scientists was paleontologistDavid W. Krause of Stony Brook. The first expedition turned up hundreds of theropod teeth identical to those ofMajungasaurus, some of which were attached to an isolated premaxilla that was described in 1996.[6] 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.[2]
Fieldwork in 1996 turned up a spectacularly complete theropod skull preserved in exquisite detail (FMNH PR 2100). On top of this skull was a dome-shaped swelling nearly identical to the one described by Sues and Taquet asMajungatholus atopus.Majungatholus was redescribed as an abelisaurid rather than a pachycephalosaur in 1998. Although the nameMajungasaurus crenatissimus was older thanMajungatholus atopus, the authors judged the type dentary ofMajungasaurus too fragmentary to confidently assign to the same species as the skull.[7] 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 shedMajungasaurus teeth. Taken together, these remains represent nearly all the bones of the skeleton, although most of the forelimbs, most of thepelvis, and the tip of the tail are still unknown.[2] This fieldwork culminated in a 2007monograph consisting of sevenscientific papers on all aspects of the animal's biology, published in theSociety of Vertebrate Paleontology Memoirs. The papers are in English, although each has anabstract written inMalagasy.[8] In this volume, the dentary described by Lavocat was re-evaluated and determined to be diagnostic for this species. Therefore, the nameMajungatholus was replaced by the older nameMajungasaurus. 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.[2] The dentary was made theneotype specimen after a 2009 petition to theICZN.[9][10]
Majungasaurus was a medium-sized theropod that typically reached 5.6–7 m (18–23 ft) in length and weighed 750–1,100 kg (1,650–2,430 lb).[2][11][12][13] Fragmentary remains of larger individuals indicate that some adults could have been similar in size to its relativeCarnotaurus, possibly exceeding 8 m (26 ft) in length.[13]
The skull ofMajungasaurus 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 inCarnotaurus. The skulls of large individuals measured 60–70 centimeters (24–28 in) long. The tallpremaxilla (frontmost upper jaw bone), which made the tip of the snout very blunt, was also typical of the family. However, the skull ofMajungasaurus was markedly wider than in other abelisaurids. All abelisaurids had a rough, sculptured texture on the outside faces of the skull bones, andMajungasaurus was no exception. This was carried to an extreme on thenasal bones ofMajungasaurus, 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 fusedfrontal bones on top of the skull as well. In life, these structures would have been covered with some sort ofintegument, possibly made ofkeratin.Computed tomography (CT scanning) of the skull shows that both the nasal structure and the frontal horn contained hollowsinus cavities, perhaps to reduce weight.[13] The teeth were typical of abelisaurids in having shortcrowns, althoughMajungasaurus bore seventeen teeth in both themaxilla of the upper jaw and thedentary of the lower jaw, more than in any other abelisaurid exceptRugops.[14]
Thepostcranial skeleton ofMajungasaurus closely resembles those ofCarnotaurus andAucasaurus, 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 thecenter of gravity over the hips. Although the cervical (neck)vertebrae had numerous cavities and excavations (pleurocoels) to reduce their weight, they were robust, with exaggeratedmuscle attachment sites andribs that interlocked for strength.Ossifiedtendons are attached to the cervical ribs, giving them a forked appearance, as seen inCarnotaurus. All of these features resulted in a very strong and muscularneck. Uniquely, the cervical ribs ofMajungasaurus had long depressions along the sides for weight reduction.[15] Thehumerus (upper arm bone) was short and curved, closely resembling those ofAucasaurus andCarnotaurus. 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.[16] The hand and finger bones ofMajungasaurus, 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.[17] 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.[18]
Like other abelisaurids, the hindlimbs were stocky and short compared to body length. Thetibia (lower leg bone) ofMajungasaurus was even stockier than that of its relativeCarnotaurus, with a prominent crest on the knee. Theastragalus andcalcaneum (ankle bones) were fused together, and the feet bore three functional digits, with a smaller first digit that did not contact the ground.[19]
Majungasaurus is classified as a member of the theropodclade Abelisauridae, which is considered afamily inLinnaean taxonomy. Along with the familyNoasauridae, abelisaurids are included in the superfamilyAbelisauroidea, which is in turn a subdivision of theinfraorderCeratosauria.[2][20] Abelisaurids are known for their tall skulls with blunt snouts, extensive sculpturing on the outer surfaces of the facial bones (convergent withcarcharodontosaurids), very reduced (atrophied) forelimbs (convergent withtyrannosaurids), and stocky hindlimb proportions, among other features.[21]
As with many dinosaur families, thesystematics (evolutionary relationships) within the family Abelisauridae are confused. Severalcladistic studies have indicated thatMajungasaurus shares a close relationship withCarnotaurus from South America,[20][21] while others were unable to firmly place it in thephylogeny.[22] The most recent analysis, using the most complete information, instead recoveredMajungasaurus in a clade withRajasaurus andIndosaurus from India, but excluding South American genera likeCarnotaurus,Ilokelesia,Ekrixinatosaurus,Aucasaurus andAbelisaurus, as well asRugops from mainland Africa. This leaves open the possibility of separate clades of abelisaurids in western and easternGondwana.[2]
A cladogram by Tortosaet al. 2013 placesMajungasaurus in a new subfamily,Majungasaurinae. A simplified version showing the taxa within the group is shown below.[23]
| Majungasaurinae |
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Majungasaurus is perhaps most distinctive for its skull ornamentation, including the swollen and fused nasals and the frontal horn. Other ceratosaurs, includingCarnotaurus,Rajasaurus, andCeratosaurus itself bore crests on the head. These structures are likely to have played a role inintraspecific competition, although their exact function within that context is unknown. The hollow cavity inside the frontal horn ofMajungasaurus would have weakened the structure and probably precluded its use in direct physical combat, although the horn may have served adisplay purpose.[21] While there is variation in the ornamentation ofMajungasaurus individuals, there is no evidence forsexual dimorphism.[13]
Scientists have suggested that the unique skull shape ofMajungasaurus and other abelisaurids indicate different predatory habits than other theropods. Whereas most theropods were characterized by long, low skulls of narrow width, abelisaurid skulls were taller and wider, and often shorter in length as well.[13] The narrow skulls of other theropods were well equipped to withstand the vertical stress of a powerful bite but not as good at withstandingtorsion (twisting).[24] In comparison to modernmammalian predators, most theropods may have used a strategy similar in some ways to that of long- and narrow-snoutedcanids, with the delivery of many bites weakening the prey animal.[25]
Abelisaurids, especiallyMajungasaurus, may instead have been adapted for a feeding strategy more similar to modernfelids, with short and broad snouts that bite once and hold on until the prey is subdued.Majungasaurus had an even broader snout than other abelisaurids, and other aspects of its anatomy may also support the bite-and-hold hypothesis. The neck was strengthened with robust vertebrae, interlocking ribs, and ossified tendons, as well as reinforced muscle attachment sites on the vertebrae and the back of the skull. These muscles would have been able to hold the head steady despite the struggles of its prey.
Abelisaurid skulls were also strengthened in many areas by bonemineralized out of theskin, creating the characteristic rough texture of the bones. This is particularly true ofMajungasaurus, where the nasal bones were fused and thickened for strength. On the other hand, the lower jaw ofMajungasaurus sported a largefenestra (opening) on each side, as seen in other ceratosaurs, as well assynovial joints between certain bones that allowed a high degree of flexibility in the lower jaw, although not to the extent seen insnakes. This may have been an adaptation to prevent the fracture of the lower jaw when holding onto a struggling prey animal. The front teeth of the upper jaw were more robust than the rest to provide an anchor point for the bite, while the low crown height ofMajungasaurus teeth prevented them from breaking off during a struggle. Finally, unlike the teeth ofAllosaurus and most other theropods, which were curved on both the front and back, abelisaurids likeMajungasaurus had teeth curved on the front edge but straighter on the back (cutting) edge. This structure may have served to prevent slicing and instead holding the teeth in place when biting.[13] Examination of the teeth ofMajungasaurus indicates that the theropod replaced its teeth anywhere from 2 to 13 times faster than other theropods, replacing the entire set within a span of two months. Gnawing on bone may have been a significant reason for such rapid tooth replacement.[26]
Majungasaurus was the largest predator in its environment, while the only known large herbivores at the time were sauropods likeRapetosaurus. Scientists have suggested thatMajungasaurus, and perhaps other abelisaurids, specialized in hunting sauropods. Adaptations to strengthen the head and neck for a bite-and-hold type of attack might have been very useful against sauropods, which would have been tremendously powerful animals. This hypothesis may also be supported by the hindlegs ofMajungasaurus, which were short and stocky, as opposed to the longer and more slender legs of most other theropods. WhileMajungasaurus would not have moved as fast as other similar-sized theropods, it would have had no trouble keeping up with slow-moving sauropods. The robust hindlimb bones suggest very powerful legs, and their shorter length would have lowered the animal's center of gravity. Thus,Majungasaurus may have sacrificed speed for power.[13]Majungasaurus tooth marks onRapetosaurus bones confirm that it at least fed on these sauropods, whether or not it actually killed them.[27]
Although sauropods may have been the prey of choice forMajungasaurus, discoveries published in 2007 detail finds in Madagascar that indicate the presence of otherMajungasaurus in their diet. Numerous bones ofMajungasaurus have been discovered bearing tooth marks identical to those found on sauropod bones from the same localities. These marks have the same spacing as teeth inMajungasaurus jaws, are of the same size asMajungasaurus teeth, and contain smaller notches consistent with the serrations on those teeth. AsMajungasaurus is the only large theropod known from the area, the simplest explanation is that it was feeding on other members of its own species.[27] Suggestions that theTriassicCoelophysis was a cannibal have been recently disproven, leavingMajungasaurus as the only non-avian theropod with confirmed cannibalistic tendencies,[28] although there is some evidence that cannibalism may have occurred in other species as well.[29]
It is unknown ifMajungasaurus actively hunted their own kind or onlyscavenged their carcasses.[27] However, some researchers have noted that modernKomodo monitors sometimes kill each other when competing for access to carcasses. Thelizards will then proceed to cannibalize the remains of their rivals, which may suggest similar behavior inMajungasaurus and other theropods.[29]
Scientists have reconstructed therespiratory system ofMajungasaurus based on a superbly preserved series of vertebrae (UA 8678) recovered from the Maevarano Formation. Most of these vertebrae and some of the ribs contained cavities (pneumaticforamina) that may have resulted from the infiltration ofavian-stylelungs and air sacs. In birds, the neck vertebrae and ribs are hollowed out by the cervical air sac, the upper back vertebrae by thelung, and the lower back andsacral (hip) vertebrae by the abdominal air sac. Similar features inMajungasaurus vertebrae imply the presence of these air sacs. These air sacs may have allowed for a basic form of avian-style 'flow-through ventilation,' where air flow through the lungs is one-way, so thatoxygen-rich air inhaled from outside the body is never mixed with exhaled air laden withcarbon dioxide. This method of respiration, while complicated, is highly efficient.[30]
The recognition of pneumatic foramina inMajungasaurus, besides providing an understanding of its respiratory biology, also has larger-scale implications for evolutionary biology. The split between the ceratosaur line, which led toMajungasaurus, and thetetanuran line, to which birds belong, occurred very early in the history of theropods. The avian respiratory system, present in both lines, must therefore have evolved before the split, and well before the evolution of birds themselves. This provides further evidence of the dinosaurianorigin of birds.[30]
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Computed tomography, also known as CT scanning, of a completeMajungasaurus skull (FMNH PR 2100) allowed a rough reconstruction of itsbrain andinner ear structure. Overall, the brain was very small relative to body size, but otherwise similar to many other non-coelurosaurian theropods, with a very conservative form closer to moderncrocodilians than to birds. One difference betweenMajungasaurus and other theropods was its smallerflocculus, a region of thecerebellum that helps to coordinate movements of theeye with movements of the head. This suggests thatMajungasaurus and other abelisaurids likeIndosaurus, which also had a small flocculus, did not rely on quick head movements to sight and capture prey.[13]
Inferences about behavior can also be drawn from examination of the inner ear. Thesemicircular canals within the inner ear aid in balance, and thelateral semicircular canal is usually parallel to the ground when the animal holds its head in an alert posture. When the skull ofMajungasaurus is rotated so that its lateral canal is parallel to the ground, the entire skull is nearly horizontal. This contrasts with many other theropods, where the head was more strongly downturned when in the alert position. The lateral canal is also significantly longer inMajungasaurus than in its morebasal relativeCeratosaurus, indicating a greater sensitivity to side-to-side motions of the head.[13]
A 2007 report describedpathologies in the bones ofMajungasaurus. Scientists examined the remains of at least 21 individuals and discovered four with noticeable pathologies.[31] While pathology had been studied in largetetanuran theropods likeallosaurids andtyrannosaurids,[32] this was the first time an abelisauroid had been examined in this manner. No wounds were found on any skull elements, in contrast to tyrannosaurids where sometimes gruesome facial bites were common. One of the specimens was aphalanx (toe bone) of the foot, which had apparently been broken and subsequently healed.[31]
Most of the pathologies occurred on the vertebrae. For example, a dorsal (back) vertebra from a juvenile animal showed anexostosis (bony growth) on its underside. The growth probably resulted from the conversion ofcartilage or aligament to bone during development, but the cause of the ossification was not determined.Hypervitaminosis A andbone spurs were ruled out, and anosteoma (benign bone tumor) was deemed unlikely. Another specimen, a small caudal (tail) vertebra, was also found to have an abnormal growth, this time on the top of itsneural spine, which projects upwards from the vertebrae, allowing muscle attachment.[31] Similar growths from the neural spine have been found inspecimens ofAllosaurus[33] andMasiakasaurus, probably resulting from the ossification of a ligament running either between the neural spines (interspinal ligament) or along their tops (supraspinal ligament).[31]
The most serious pathology discovered was in a series of five large tail vertebrae. The first two vertebrae showed only minor abnormalities with the exception of a large groove that extended along the left side of both bones. However, the next three vertebrae were completely fused together at many different points, forming a solid bony mass. There is no sign of any other vertebrae after the fifth in the series, indicating that the tail ended there prematurely. From the size of the last vertebrae, scientists judged that about ten vertebrae were lost. One explanation for this pathology is severephysical trauma resulting in the loss of the tail tip, followed byosteomyelitis (infection) of the last remaining vertebrae. Alternatively, the infection may have come first and led to the end of the tail becomingnecrotic and falling off. This is the first example of tail truncation known in a non-avian theropod dinosaur.[31]
The small number of specimens preserved with pathologies inMajungasaurus suggest that the multitude of injuries that occurred were obtained over the course of the lives of the individuals studied. Furthermore, the small number of injuredMajungasaurus specimens observed amongst those recovered indicates most well preserved individuals generally lack observable pathologies, while a few select individuals were shown to have possessed multiple pathologies, a general pattern also noted in other large, nonavian theropods. Such patterns may be the result of a snowball effect where one injury or an infection increased the likelihood of additional maladies and injuries due to functional impairment or compromised immune systems in individuals once an initial injury had occurred.[34]
Majungasaurus, being known from many well-preserved specimens of different ages, is well studied in regards to its growth and development. Throughout ontogeny, the skull ofMajungasaurus (more specifically, thejugal,postorbital andquadratojugal) seems to have become taller and more robust; additionally, the skull bones became more fused and the eye sockets became proportionally smaller. This indicates a shift in dietary preferences between juveniles and adults.[35]
Research by Michael D'Emicet al indicates that it was among the slowest-growing theropods. Based on studies of thelines of arrested growth in several bones, it was found thatMajungasaurus took twenty years to reach maturity, which may have been a result of the harsh environment in which it lived. However, other abelisaurids have also been found to have comparably slow growth rates.[36]
Subadults ofMajungasaurus crenatissimus were probably eaten by adults of the snakeMadtsoia madagascariensis, though such large prey would have been injurious for the snake.[37]
All specimens ofMajungasaurus have been recovered from the Maevarano Formation in theMahajanga Province in northwestern Madagascar. Most of these, including all of the most complete material, came from the Anembalemba Member, althoughMajungasaurus teeth have also been found in the underlying Masorobe Member and the overlying Miadana Member. While these sediments have not been datedradiometrically, evidence frombiostratigraphy andpaleomagnetism suggest that they were deposited during theMaastrichtianstage, which lasted from 70 to 66 Ma (million years ago).Majungasaurus teeth are found up until the very end of the Maastrichtian, when all non-avian dinosaursbecame extinct.[38]
Then as now, Madagascar was an island, havingseparated from theIndian subcontinent less than 20 million years earlier. It was drifting northwards but still 10 to 15degrees more southerly inlatitude than it is today. The prevailingclimate of the time was semi-arid, with pronouncedseasonality in temperature and rainfall.Majungasaurus inhabited a coastalfloodplain cut by many sandyriverchannels.[38] Strong geological evidence suggests the occurrence of periodicdebris flows through these channels at the beginning of the wet season, burying the carcasses of organisms killed during the preceding dry season and providing for their exceptional preservation as fossils.[39] Sea levels in the area were rising throughout the Maastrichtian, and would continue to do so into thePaleocene Epoch, soMajungasaurus may have roamed coastal environments liketidal flats as well. The neighboringBerivotra Formation represents the contemporaneousmarine environment.[38]
BesidesMajungasaurus, fossil taxa recovered from the Maevarano includefish,frogs, lizards, snakes,[38] seven distinct species ofcrocodylomorphs,[40]five or six species of mammals,[40]Vorona[41] and several other birds,[38] the possibly flighteddromaeosauridRahonavis,[42][43] the noasauridMasiakasaurus[44] and twotitanosaurian sauropods, includingRapetosaurus.[45]Majungasaurus was by far the largest carnivore and probably the dominant predator on land, although large crocodylomorphs likeMahajangasuchus andTrematochampsa might have competed with it closer to water.[38]
Media related toMajungasaurus at Wikimedia Commons
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