Majungasaurus

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Majungasaurus
Temporal range:
Ma
↓
Mounted Skeleton, Stony Brook University
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Clade: †
Abelisauria
Family: †Abelisauridae
Subfamily: †Majungasaurinae
Genus: †Majungasaurus
Lavocat, 1955
Species:
†M. crenatissimus
Binomial name
†Majungasaurus crenatissimus
(Depéret, 1896) [originally Megalosaurus]
Synonyms
  • Megalosaurus crenatissimus
    Depéret, 1896
  • Majungatholus atopus
    Sues & Taquet, 1979

Majungasaurus (

junior synonym
of Majungasaurus.

Like other abelisaurids, Majungasaurus was a

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 (although it was thought to be a species of

cannibalism
.

Discovery and naming

Original material described in 1896

French

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.[1] This name is derived from the Latin word crenatus ("notched") and the suffix -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 genus Dryptosaurus, another poorly known taxon.[3]

MNHN
.MAJ 1), the right dentary of a subadult individual – Muséum national d'histoire naturelle, Paris

Numerous fragmentary remains from

MNHN.MAJ 1) 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 (meaning "lizard").[4] Hans-Dieter Sues and Philippe 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 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.[2] 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.[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]

MNHN
.MAJ 1)

Fieldwork in 1996 turned up a spectacularly complete theropod skull preserved in exquisite detail (

ICZN.[9][10]

Description

Majungasaurus compared in size to a human

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 relative Carnotaurus, possibly exceeding 8 m (26 ft) in length.[13]

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 60–70 centimeters (24–28 in) long. The tall

dentary of the lower jaw, more than in any other abelisaurid except Rugops.[14]

Life restoration

The

center of gravity over the hips. Although the cervical (neck) vertebrae had numerous cavities and excavations (pleurocoels) to reduce their weight, they were robust, with exaggerated muscle attachment sites and ribs that interlocked for strength. Ossified tendons 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.[15] The humerus (upper arm bone) 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.[16] 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.[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. The

calcaneum (ankle bones) were fused together, and the feet bore three functional digits, with a smaller first digit that did not contact the ground.[19]

Classification and systematics

Majungasaurus is classified as a member of the theropod

tyrannosaurids), and stocky hindlimb proportions, among other features.[21]

As with many dinosaur families, the

phylogeny.[22] The most recent analysis, using the most complete information, instead recovered Majungasaurus in a clade with Rajasaurus and Indosaurus from India, but excluding South American genera like Carnotaurus, Ilokelesia, Ekrixinatosaurus, Aucasaurus and Abelisaurus, as well as Rugops from mainland Africa. This leaves open the possibility of separate clades of abelisaurids in western and eastern Gondwana.[2]

Skeletal restoration showing known elements of two specimens

A cladogram by Tortosa et al. 2013 places Majungasaurus in a new subfamily, Majungasaurinae. A simplified version showing the taxa within the group is shown below.[23]

Majungasaurinae

Pourcieux abelisaurid

Arcovenator

Majungasaurus

Indosaurus

Rahiolisaurus

Rajasaurus

Paleobiology

Skull ornamentation

Skull cast of FMNH PR 2100

Majungasaurus is perhaps most distinctive for its skull ornamentation, including the swollen and fused nasals and the frontal horn. Other ceratosaurs, including Carnotaurus, Rajasaurus, and Ceratosaurus itself bore crests on the head. These structures are likely to have played a role in intraspecific competition, although their exact function within that context is unknown. The hollow cavity inside the frontal horn of Majungasaurus would have weakened the structure and probably precluded its use in direct physical combat, although the horn may have served a display purpose.[21] While there is variation in the ornamentation of Majungasaurus individuals, there is no evidence for sexual dimorphism.[13]

Feeding

Restoration of two Majungasaurus chasing a Rapetosaurus

Scientists have suggested that the unique skull shape of Majungasaurus 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.

canids, with the delivery of many bites weakening the prey animal.[25]

Abelisaurids, especially Majungasaurus, may instead have been adapted for a feeding strategy more similar to modern

felids
, 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 bone

fenestra (opening) on each side, as seen in other ceratosaurs, as well as synovial joints between certain bones that allowed a high degree of flexibility in the lower jaw, although not to the extent seen in snakes. 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 of Majungasaurus teeth prevented them from breaking off during a struggle. Finally, unlike the teeth of Allosaurus and most other theropods, which were curved on both the front and back, abelisaurids like Majungasaurus 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 of Majungasaurus 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]

Mounted skeletons of Majungasaurus and Rapetosaurus

Majungasaurus was the largest predator in its environment, while the only known large herbivores at the time were sauropods like Rapetosaurus. Scientists have suggested that Majungasaurus, and perhaps other abelisaurids, specialized on 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 of Majungasaurus, which were short and stocky, as opposed to the longer and more slender legs of most other theropods. While Majungasaurus 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 on Rapetosaurus bones confirm that it at least fed on these sauropods, whether or not it actually killed them.[27]

Cannibalism

Although sauropods may have been the prey of choice for Majungasaurus, discoveries published in 2007 detail finds in Madagascar that indicate the presence of other Majungasaurus in their diet. Numerous bones of Majungasaurus 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 in Majungasaurus jaws, are of the same size as Majungasaurus teeth, and contain smaller notches consistent with the serrations on those teeth. As Majungasaurus 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 the Triassic Coelophysis was a cannibal have been recently disproven, leaving Majungasaurus 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 if Majungasaurus actively hunted their own kind or only scavenged their carcasses.[27] However, some researchers have noted that modern Komodo monitors sometimes kill each other when competing for access to carcasses. The lizards will then proceed to cannibalize the remains of their rivals, which may suggest similar behavior in Majungasaurus and other theropods.[29]

Respiratory system

Comparison between the air sacs of Majungasaurus and a modern bird

Scientists have reconstructed the

lungs and air sacs. In birds, the neck vertebrae and ribs are hollowed out by the cervical air sac, the upper back vertebrae by the lung, and the lower back and sacral (hip) vertebrae by the abdominal air sac. Similar features in Majungasaurus 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 that oxygen-rich air inhaled from outside the body is never mixed with exhaled air laden with carbon dioxide. This method of respiration, while complicated, is highly efficient.[30]

The recognition of pneumatic foramina in Majungasaurus, besides providing an understanding of its respiratory biology, also has larger-scale implications for evolutionary biology. The split between the ceratosaur line, which led to Majungasaurus, and the tetanuran 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 dinosaurian origin of birds.[30]

Brain and inner ear structure

Head and neck of specimen FMNH PR 2836

flocculus, a region of the cerebellum that helps to coordinate movements of the eye with movements of the head. This suggests that Majungasaurus and other abelisaurids like Indosaurus, 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. The

lateral semicircular canal is usually parallel to the ground when the animal holds its head in an alert posture. When the skull of Majungasaurus 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 in Majungasaurus than in its more basal relative Ceratosaurus, indicating a greater sensitivity to side-to-side motions of the head.[13]

Pathology

A 2007 report described

phalanx (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 an

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 severe

physical trauma resulting in the loss of the tail tip, followed by osteomyelitis (infection) of the last remaining vertebrae. Alternatively, the infection may have come first and led to the end of the tail becoming necrotic 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 in Majungasaurus 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 injured Majungasaurus 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]

Ontogeny and growth

Restoration of an adult

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 of Majungasaurus (more specifically, the jugal, postorbital, and quadratojugal) 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'Emic et al indicates that it was among the slowest-growing theropods. Based on studies of the

lines of arrested growth in several bones, it was found that Majungasaurus 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]

Paleoecology

All Majungasaurus fossils have been found in the Mahajanga Province of Madagascar, most within 50 kilometers (31 mi) to the southeast of the provincial capital, Mahajanga (marked with a red dot on the map).

All specimens of Majungasaurus have been recovered from the Maevarano Formation in the

stage, 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 dinosaurs became extinct.[37]

Then as now, Madagascar was an island, having

marine environment.[37]

Besides Majungasaurus, fossil taxa recovered from the Maevarano include

dromaeosaurid Rahonavis,[41][42] the noasaurid Masiakasaurus[43] and two titanosaurian sauropods, including Rapetosaurus.[44] Majungasaurus was by far the largest carnivore and probably the dominant predator on land, although large crocodylomorphs like Mahajangasuchus and Trematochampsa might have competed with it closer to water.[37]

See also

References

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  4. ^ Lavocat, René (1955). "Sur une portion de mandibule de Théropode provenant du Crétacé supérieur de Madagascar" [On a portion of Theropod's mandible from the Upper Cretaceous of Madagascar]. Bulletin du Muséum National d'Histoire Naturelle (in French). 27: 256–259.
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External links

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