The Spinosauridae is a specialized group of large tetanuran theropods known from the Berriasian to the Cenomanian of Africa, South America, Europe and Asia, characterised by a long, narrow skull, robust forelimbs with a hooked thumb claw, and tall neural spines forming a dorsal sail. The ecology of the group has been debated since the original discovery of Spinosaurus aegyptiacus in 1911. The recient description of a nearly complete and partially articulated tail of S. aegyptiacus reinforces the hypothesis that this giant theropod spent plenty of time underwater.
The holotype of Irritator challengeri (SMNS 58022; Staatliches Museum für Naturkunde Stuttgart, Stuttgart, Germany) from the Romualdo Member of the Santana Formation (Lower Cretaceous) in northeastern Brazil represents one of the few preserved spinosaurid braincases and can provide insights into neuroanatomical structures that might be expected to reflect the ecological affinities of the group.
The skull of Irritator is remarkably narrow, especially in the region of the elongated snout. The braincase is short anteroposteriorly but deep dorsoventrally, extending ventrally far below the occipital condyle. The cranial endocast of Irritator shows weakly demarcated brain regions, elongate olfactory tracts and pronounced cranial flexures that are consistent with the inferred phylogenetic position of spinosaurids as basal tetanurans. Irritator also exhibits enlarged floccular recesses, which is an unusual feature for basal tetanurans. The flocculus of the cerebellum plays a role in coordinate eye movements, and tends to be enlarged in taxa that rely on quick movements of the head and the body. Within non-avian theropod dinosaurs, large floccular recesses are common among coelurosaurs. Additionally, lateral semicircular canal orientation suggests a downward inclined snout posture, which enables unobstructed, stereoscopic forward vision, important for distance perception and thus precise snatching movements of the snout.
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