Scimitar-crested Spinosaurus species from the Sahara caps stepwise spinosaurid radiation.

We describe a close relative of Spinosaurus aegyptiacus, the sail-backed, fish-eating giant from nearshore deposits of northern Africa. Spinosaurus mirabilis sp. nov., discovered in the central Sahara alongside long-necked dinosaurs in a riparian habitat, is distinguished by a scimitar-shaped bony crest projecting far above its skull roof. We discern three discrete phases in spinosaurid evolution. During the first phase with roots in the Jurassic, an elongate fish-snaring skull emerged that soon was modified along divergent paths. During a second Early Cretaceous phase, spinosaurids became the dominant predators in circum-Tethyan habitats. The final phase began just before the Late Cretaceous during the opening of the Atlantic Ocean, when spinosaurines attained maximum body size as shallow water ambush specialists limited geographically to northern Africa and South America. Editor's summary: Recent descriptions of and debates about the massive, fish-eating dinosaur Spinosaurus have brought this striking predator to the forefront of the dinosaur pantheon. Its huge size and distinctive morphology have stimulated much debate about the degree to which it lived an aquatic lifestyle. Sereno et al. describe a crested fossil Spinosaurus found in northern Africa as a new species. The researchers argue that this group of dinosaurs underwent three phases of evolution with increasing aquatic adaptations and existence in habitats around the Tethys Sea. —Sacha Vignieri INTRODUCTION: The fossils of Spinosaurus aegyptiacus, a giant sail-backed, fish-eating theropod dinosaur from northern Africa, have inspired competing lifestyle interpretations, either as a semiaquatic ambush predator stalking shorelines and shallows or a fully aquatic predator in pursuit of prey underwater. Its bones and teeth have been found only in coastal deposits near marine margins, a locale potentially consistent with either lifestyle interpretation. RATIONALE: In the central Sahara, a new fossiliferous area (Jenguebi) was discovered in beds equivalent in age [Farak Formation; Cenomanian ~95 million years ago (Mya)] to those yielding fossil remains of S. aegyptiacus. We describe from this area a new species, Spinosaurus mirabilis sp. nov., which is very similar to S. aegyptiacus in skeletal form but with a much taller, scimitar-shaped cranial crest. Two new sauropods were found in close association with the new spinosaurid buried in fluvial sediments indicative of an inland riparian habitat. RESULTS: S. mirabilis sp. nov. is distinguished by the low profile of its snout, a hypertrophied nasal-prefrontal crest, greater spacing of posterior maxillary teeth, and other features. Its features highlight the extraordinary specializations of both species of the genus Spinosaurus, including interdigitating upper and lower teeth. Principal component analysis of body proportions places spinosaurids between semiaquatic waders (e.g., herons) and aquatic divers (e.g., darters) distant from all other predatory dinosaurs. A time-calibrated phylogenetic analysis resolves three evolutionary phases: an initial Jurassic radiation when their distinctive elongate fish-snaring skull evolved and split into two distinctive designs, baryonychine and spinosaurine; an Early Cretaceous circum-Tethyan diversification when both reigned as dominant predators; and a final early Late Cretaceous phase when spinosaurines attained maximum body size as shallow water ambush specialists limited geographically to northern Africa and South America. CONCLUSION: The discovery of the tall-crested S. mirabilis sp. nov. in a riparian setting within an inland basin supports a lifestyle interpretation of a wading, shoreline predator with visual display an important aspect of its biology. At the end of the Cenomanian about 95 million years ago, an abrupt eustatic rise in sea level and the attendant climate change brought the spinosaurid radiation to an end. Sheathed bony head crests in extinct and living dinosaurs.: S. mirabilis sp. nov., evolved the tallest head crest of any theropod dinosaur, drawing attention to the midline ornamentation that characterizes the cranium and axial skeleton of all spinosaurids. In life, the crest would have been extended to some degree by a keratinous sheath, as in the living helmeted guinea fowl (Numida meleagris). Visual signaling, as is the case in guinea fowl and other crested avians, was likely the function of spinosaurid cranial crests and trunk and tail sails. Scale bar, 20 cm for S. mirabilis and 3 cm for N. meleagris. FLESH RENDERING AND LAYOUT BY DANI NAVARRO; ADULT SKULL CAST BY THE FOSSIL LAB; HELMETED GUINEA FOWL IMAGES BY TODD GREENE [ABSTRACT FROM AUTHOR]