Temporal range: Late Cretaceous, 67 Ma
|Skull diagram showing known parts in brown|
Wilson et al., 2003
Wilson et al., 2003
Rajasaurus is a genus of carnivorous abelisaurid theropod dinosaur from the Late Cretaceous of India, containing one species: Rajasaurus narmadensis. The bones were excavated from the Lameta Formation in the Gujarat state of Western India, probably inhabiting what is now the Narmada River Valley. It was formerly described by paleontologist Jeffrey A. Wilson in 2003 based on a partial skeleton comprising post-cranial remains beyond the skull–a first for Indian theropods. It was a part of the subfamily Majungasaurinae, and, likewise, it bore a resemblance to the Madagascan Majungasaurus. The dinosaur likely measured 6.6 metres (22 ft), and had a single horn on the forehead. India at this time was an island, due to the break-up of the supercontinent Gondwana. The Lameta Formation has yielded several other dinosaur species, including abelisaurids and titanosaurian sauropods, similar to other Gondwanan landmasses.
Discovery and naming
The remains of Rajasaurus were first identified in 1981 by geologists working for the Geological Survey of India (GSI), G. N. Dwivedi and Dhananjay Mahendrakumar Mohabey, after being given limestone structures–later recognized as dinosaur eggs–by workers of the ACC Cement Quarry in the town of Rahioli in the Gujarat state of Western India. The geologists also found a fossil-rich limestone bed–the Lameta Formation–to which GSI geologist Suresh Srivastava was assigned to excavate on two separate trips from 1982–1983 and 1983–1984. In 2001, teams from the American Institute of Indian Studies and the National Geographic Society, with the support of the Punjab University, joined the study in order to reconstruct the excavated remains. Fragments of Rajasaurus were also found near Jabalpur in Madhya Pradesh in the northern part of the Lameta Formation. Rajasaurus was then formerly described in 2003 by geologist Jeffrey A. Wilson and paleontologist Paul Sereno. The generic name Rajasaurus derives from the Sanskrit raja, meaning prince or princely, and Ancient Greek saurus, meaning lizard; and its specific name narmadensis refers to the Narmada River in central India where it was discovered. Only one specimen exists, though the dubious genus Lametasaurus described in 1923, based on fragmentary bone and armor scute remains, may actually represent a stout Rajasaurus individual.
The discovery of Rajasaurus could lead to additional information on the evolutionary relationships of abelisaurids, since previously described specimens from India were mainly isolated bones. At a press conference held in 2003 on the discovery of Rajasaurus, Sereno stated:
The discovery, which will be put for examination before global experts, was important since it would help in adding to the current knowledge of dinosaur belonging to the family of Abelisaur predators and adding a new angle to dinosaurs in the Indian subcontinent.
Rajasaurus was an abelisaurid theropod of the Late Cretaceous Indian subcontinent, which, at the time, had separated from Madagascar during the break-up of Gondwana; likewise, the dinosaur was endemic to the island and probably developed unique attributes. The volcanic rocks in the Lameta Formation were radiometrically dated to around 65.5 mya, placing Rajasaurus in the Maastrichtian age of the latest Cretaceous. Rajasaurus closely resembled the Madagascan abelisaurid Majungasaurus, with a 20 million year separation.
The holotype specimen of Rajasaurus comprises a partial skeleton, GSI Type No. 2114111-33, which includes the braincase, spine, hip bones, parts of the hind legs, and tail. It is the first Indian theropod to have preserved post-cranial remains. In 2010, palaeontologist Gregory S. Paul estimated its body length at 11 metres (36 ft) and weight at 4 metric tons (4.4 short tons). In 2016, its length was estimated to be 6.6 metres (22 ft) in a comprehensive analysis of abelisaur size.
On the braincase, only the left sides of the parietal and frontal bones were preserved, though the opposite is true for the horn. The braincase was thick, with the frontal bone achieving a maximum thickness of 4 centimetres (1.6 in) above the eye socket. A trough-like groove is present where the frontal meets the nasal bone, with the walls decreasing towards the center of the groove, serving to support the horn on the nasal bone. This low horn on its forehead was primarily made of nasal bone than frontal bone, unlike the horn on Majungasaurus. The rims of the temporal fossae, depressions on either side of the top of the skull, formed a low sagittal crest along the middle of the top of the skull. The front rims of the fossae were unusually steep. Abelisaurids, typically, had elongated fenestrae (holes in the skull) below the quadrate bone near the bottom of the skull, but Rajasaurus had elongated supratemporal fenestrae near the top of the skull. The right side of the skull, near the midline, preserves a path for the olfaction track necessary for smelling, which would have had below it the orbitosphenoid bones.
Only one neck vertebra was preserved, and it is proportionally shorter than other carnosaurians. Several partial thoracic vertebrae were found, but only one–most likely the fourth thoracic vertebra–is well preserved; the two ends of the vertebra (articular processes) are concave, and more deeper than broad, in contrast to the neck vertebra which is the opposite. The bottom side of six sacral vertebrae were preserved; they are elongated and grow thinner towards the articular processes, and at least five sacral vertebrae have no arching. Partial tail vertebrae were found, and they are also concave at the articular processes.
Wilson, in 2003, assigned Rajasaurus to the subfamily Carnotaurinae, being more closely related to abelisaurids like Majungasaurus and Carnotaurus than to African abelisaurids, on the basis of several similarities such as the presence of a sagittal crest, neck vertebrae with two air spaces, the configuration of the nasal bones, a fleshy growth ("excrescence") on the frontal bone, and a thick skull roof. In 2014, the subfamily Majungasaurinae was erected to include the newly discovered European Arcovenator, Majungasaurus, Indosaurus, Rahiolisaurus, and Rajasaurus, despite large geographical barriers between Europe, India, and Madagascar. It has been suggested that a migration of abelisaurids took place in the Late Cretaceous between Africa, Europe, Madagascar, and India; it is possible that migration occurred between India and Africa given their close proximity, and the volcanic Dras-Kohistan island arc may have allowed island hopping and an indirect path to Asia, though these are still questionable explanations.
Rajasaurus is distinguished from other genera by its single forehead horn (though Majungasaurus also only has one), the elongated supratemporal fenestrae (holes in the upper rear of the skull), and the ilia bones on the hip which feature a ridge separating the brevis shelf from the hip joint.
India, during the Late Cretaceous, had separated from Madagascar and South America during the break-up of Gondwana, and Rajasaurus lived on an isolated island, likely causing endemism and unique characteristics not seen in other abelisaurids. The leg bones of Majungasaurus were comparatively short to other similarly-sized theropods, suggesting the dinosaur was comparatively slower–this same condition was seen in Rajasaurus.
Rajasaurus has been found in the Lameta Formation, a rock unit representing an arid or semi-arid landscape with a river flowing through it–probably providing shrub cover near the water–which formed between episodes of volcanism in the Deccan Traps. The formation is known for being a sauropod nesting site, yielding several dinosaur eggs, and sauropod herds likely chose sandy soil for nesting. Rajasaurus likely inhabited what is now the Narmanda River Valley.
Several dinosaurs have been described from this formation, such as the noasaurid Laevisuchus; abelisaurids Indosaurus, Indosuchus, Lametasaurus, and Rahiolisaurus; and the titanosaurian sauropods Jainosaurus, Titanosaurus, and Isisaurus. The diversity of abelisauroid and titanosaurian dinosaurs in Cretaceous India indicates they shared close affinities to the dinosaur life of the other Gondwanan continents, which had similar inhabitants.
Sauropod coprolite remains indicate they lived in a forested landscape, consuming plants such as Podocarpus, Araucaria, Cheirolepidiaceae conifers; cycads; palm trees; early grass; and Caryophyllaceae, Sapindaceae, and Acanthaceae flowering plants. The similarity between European and Indian sauropod egg taxa suggest an inter-continental migration of animals between India, Europe, and South America during the Cretaceous, despite water barriers.
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