Aquatic adaptation in the skull of carnivorous dinosaurs (Theropoda: Spinosauridae) and the evolution of aquatic habits in spinosaurids

Abstract

The Spinosauridae is a specialized group of theropod dinosaurs characterised by a long, narrow skull, robust forelimbs with a hooked thumb claw, and tall neural spines forming a dorsal sail. The ecology of these unusual dinosaurs has been debated since the original discovery of Spinosaurus aegyptiacus in 1912. Morphological similarities to crocodilians, including tooth shape and an elongated rostrum, indicate a piscivorous diet, and in the giant Spinosaurus, a long body and short limbs suggest semi-aquatic habits. However, the hypothesized aquatic habits of Spinosaurus have been called into question, and the distribution of aquatic habits within Spinosauridae remain unclear. Here, new spinosaurid specimens from the Kem Kem beds of Morocco reveal aquatic adaptations in the cranium. Elevated orbits and bending of the frontals placed the eyes atop the skull, as in semiaquatic animals such as crocodiles and hippos. Two morphologies are present, a smaller morph characterized by narrow, triangular frontals, and a larger morph characterized by broad, subrectangular frontals overlapping the prefrontals. The two morphs suggest two distinct spinosaurine taxa, and are tentatively referred to the spinosaurines Spinosaurus cf. aegyptiacus and Sigilmassasaurus brevicollis, respectively. Semiaquatic habits were widespread within the Spinosaurinae and at least two distinct aquatic spinosaurines inhabited the Cenomanian of North Africa, challenging previous assumptions that non-avian dinosaurs were solely terrestrial. The appearance of giant semiaquatic dinosaurs may have followed the disappearance of giant pholidosaurid crocodylomorphs, suggesting that the extinction of large crocodylomorphs was associated with the rise of dinosaurs as apex predators in the freshwater ecosystem in North Africa.