Masiakasaurus lived around 70 million years ago, along with animals such as Majungasaurus, Rapetosaurus, and Rahonavis. Masiakasaurus was a member of the group Noasauridae, small predatory ceratosaurs found primarily in South America.
Masiakasaurus were small-bodied ceratosaurs that grew up to around 1.8 to 2 metres (5.9 to 6.6 ft) in length. The most distinctive characteristic of Masiakasaurus is the forward-projecting, or procumbent, front teeth. The teeth are heterodont, meaning that they have different shapes along the jaw. The first four dentary teeth of the lower jaw project forward, with the first tooth angled only 10° from horizontal. These teeth are long and spoon-shaped with hooked edges. They have carinae, or sharp edges, that are weakly serrated. Serrations are more evident in the posterior teeth, which are also recurved and laterally compressed. The margin of the dentary curves downward so that the tooth sockets of the front teeth are directed forward. The front teeth of the upper jaw are also procumbent, and the margin of the premaxilla curves slightly upward to direct them outward.
Unlike the skulls of abelisaurids, which are very deep, the skull of Masiakasaurus is long and low. The lacrimal and postorbital bones around the eye are textured with bumpy projections. Not including the highly modified jaws and teeth, the skull of Masiakasaurus possesses many general ceratosaurian characteristics. Overall, its morphology is intermediate between abelisaurids and more basal ceratosaurs.
The neck is relatively narrow in comparison to abelisaurids and bear stout neck ribs. While many theropods have s-shaped necks, the ribs would make the neck rather stiff in Masiakasaurus, and the back of the neck is positioned almost horizontally, giving it only a slighter curve. Like those of other abelisauroids, the vertebrae are heavily pneumaticized, or hollowed, and have relatively short neural spines. Pneumaticity is limited to the neck and foremost back vertebrae, however. Pneumatic cavities are also present in the braincase.
The scapulocoracoid, a bone that makes up the pectoral girdle, is large and broad. While abelisaurids have arms that are extremely reduced in size, Masiakasaurus and other noasaurids had longer forelimbs. The humerus is slender and the bones of the hand are relatively short. The related genus Noasaurus has a raptorial ungual which was originally interpreted as a sickle-like claw on the foot, but has more recently been considered a claw of the hand. The penultimate phalanx, a finger bone that immediately precedes the raptorial ungual in Noasaurus, is also known in Masiakasaurus and has a similar appearance. The enlarged ungual, however, is unknown in Masiakasaurus. The hand bore three large fingers with stout - but not particularly sharp - claws, and a highly reduced fourth finger.
Remains of Masiakasaurus have been found in the Late Cretaceous Maevarano Formation in northwestern Madagascar and were first described in the journal Nature in 2001. Fragmentary bones comprising around 40% of the skeleton were collected near the village of Berivotra. Several parts of the skull, including the distinctive teeth, were found. The humerus, pubis, hind limbs, and several vertebrae were also collected.
In 2011, additional specimens of Masiakasaurus were described. The braincase, premaxilla, facial bones, ribcage, portions of the hands and pectoral girdle, and much of the cervical and dorsal vertebral column were described for the first time. The discovery of this new material clarified many aspects of noasaurid anatomy and made the genus among the best known dinosaurs. The new finds did however not allow for a detailed study of its evolutionary relationships among ceratosaurs. With the new material, around 65% of the skeleton is currently known.