Mahakala (from Sanskrit, named for Mahakala, one of eight protector deities (dharmapalas) in Tibetan Buddhism) is a genus of halszkaraptorine theropod dinosaurfrom the Campanian-age (about 80 million years ago) Upper Cretaceous Djadokhta Formation of Ömnögovi, Mongolia. It is based on a partial skeleton found in the Gobi Desert. Mahakala was a small dromaeosaurid (approximately 70 centimeters long (28 in)), and its skeleton shows features that are also found in early troodontids and avialans. Despite its late appearance, it is among the most basal dromaeosaurids. Its small size, and the small size of other basal deinonychosaurians, suggests that small size appeared before flight capability in birds.
Description
Mahakala is based on IGM 100/1033, a partial skeleton including skull bones, vertebrae, limb bones, and portions of the pelvis and shoulder girdle. Although this individual was small, comparable in size to Archaeopteryx, Caudipteryx, and Mei, it was close to adulthood. This genus can be distinguished from other paravians (dromaeosaurids, troodontids, and birds) by details of the ulna, thighbone, ilium, and tail vertebrae. Like Archaeopteryx and derived dromaeosaurids, but unlike basal troodontids and other dromaeosaurids, the middle (third) metatarsal was not compressed, suggesting that the uncompressed version was the basalversion. It had a typical dromaeosaurid form of the second toe, with an expanded claw.[1]
Classification
A phylogenetic analysis performed by Turner and colleagues, who described the specimen, found Mahakala to be the most basal known dromaeosaurid. Their results, along with the small size of other theropods found at the base of paravian lineages, suggest that small size was not an innovation of early birds, but a common trait of early paravians; small size would have preceded flight and would not have been a special avian autapomorphy as the result of a size squeeze. Like birds, troodontids and dromaeosaurids were not small throughout their evolutionary history, and showed size increases among several different lineages. Mahakala has a combination of characteristics found among basal troodontids and birds, but lacks some that are present in more derived dromaeosaurids.[1] A study in 2017 found that Mahakala was a member of the enigmatic, basal subfamily Halszkaraptorinae.[2]
The cladogram below is based on the phylogenetic analysis conducted in 2017 by Cau et al. using updated data from the Theropod Working Group in their description of Halszkaraptor.[2]
Paleoecology and paleobiology
The paleoenvironment of the Djadokhta Formation is interpreted as having a semiaridclimate, with sand dune and alluvial settings. The semiarid steppe landscape was drained by intermittent streams and was sometimes affected by dust and sandstorms, and moisture was seasonal. Animals present included terrestrial turtles and crocodilians, lizards, mammals, and a variety of dinosaurs; aquatic animals like fish were not present. The majority of the fauna was small to medium-sized.[3] Small coelurosaurians are the most diverse dinosaurs, including fellow dromaeosaurid Velociraptor, troodontids Byronosaurus and Saurornithoides, oviraptorids Citipati, Khaan, and Oviraptor, and alvarezsaurs Kol and Shuvuuia; other dinosaurs present included ceratopsiansProtoceratops and Udanoceratops, the hadrosaur Plesiohadros, and the ankylosauridPinacosaurus.[4] Like other dromaeosaurids, Mahakala would have been a small active predaceous carnivore.[5]
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