Neuroanatomy of the spinosaurid Irritator challengeri (Dinosauria: Theropoda) indicates potential adaptations for piscivory

Abstract

Spinosauridae, a theropod group characterized by elongated snouts, conical teeth, enlarged forelimbs, and often elongated neural spines, show evidence for semiaquatic adaptations and piscivory. It is currently debated if these animals represent terrestrial carnivores with adaptations for a piscivorous diet, or if they largely lived and foraged in aquatic habitats. The holotype of Irritator challengeri, a nearly complete skull from the late Early Cretaceous Santana Formation of northeastern Brazil, includes one of the few preserved spinosaurid braincases and can provide insights into neuroanatomical structures that might be expected to reflect ecological affinities. We generated digital models of the neuroanatomical cavities within the braincase, using computer tomography (CT) data. The cranial endocast of Irritator is generally similar to that of other non-maniraptoriform theropods, with weakly developed distinctions of hindbrain and midbrain features, relatively pronounced cranial flexures and relatively long olfactory tracts. The endosseous labyrinth has a long anterior semicircular canal, a posteriorly inclined common crus and a very large floccular recess fills the area between the semicircular canals. These features indicate that Irritator had the ability for fast and well-controlled pitch-down head movements. The skull table and lateral semicircular canal plane are strongly angled to one another, suggesting a downward angling of approximately 45° of the snout, which reduces interference of the snout with the field of vision of Irritator. These neuroanatomical features are consistent with fast, downward snatching movements in the act of predation, such as are needed for piscivory. Spinosauridae is a large-bodied theropod group within Megalosauroidea known from the Cretaceous, although their phylogenetic relationships indicate that the clade must have originated in the Jurassic 1. Spinosaurids are characterized by a long and slender skull, conical teeth, strongly developed forelimbs with exceptionally large thumb claws and elongated neural spines 2-6. Due to superficial similarities in cranial form with piscivorous Crocodilia, such as the gharial, and the wealth of fossil fish within the assemblages they were found in, spinosaurids were repeatedly associated with a semiaquatic lifestyle and piscivory [e.g. 3,7-12 ]. Direct evidence for piscivory comes from acid-etched fish scales in the stomach contents of Baryonyx walkeri 3 , although the same individual also includes terrestrial dinosaur bones of a juvenile ornithopod. Predation on pterosaurs has also been shown for spinosaurids 13. Thus, direct evidence for spinosaurid diets indicates a mix, or opportunistic behaviour with a tendency towards relatively small prey items. Additional evidence to support semiaquatic adaptations beyond dietary preference in spinosaurids comes from: isotope signals acquired from tooth enamel of samples from different geographical contexts, which show that spinosaurids spent a significant amount of their lifetime in water 8,14,15...

Deep evolutionary diversification of semicircular canals in archosaurs

“…The small SCC sizes of these stem-archosaurs argues against the possibility that their avemetatarsalian-like SCC shapes evolved to optimize vestibular sensitivity. Based on these observations, we urge strongly against the practice of interpreting specific, detailed aspects of locomotion or foraging styles of extinct taxa from SCC geometry (as done by, e.g., Stocker et al, 41 Dudgeon et al, 50 and Schade et al 53 ).…”

Evaluating the ecology of Spinosaurus: shoreline generalist or aquatic pursuit specialist?

“…This therefore suggests why the data for orbit position for semiaquatic taxa lies away from both terrestrial and aquatic taxa (Figure 3). Posteriorly, but not dorsally, retracted nares would however allow an animal standing next to, or in, water to hold the snout below the surface and to strike at or forage for aquatic prey while still being able to breathe (Hone and Holtz, 2019;Schade et al, 2020) (Figure 7).…”

“…Based on the neuroanatomy of the skull, Schade et al (2020) posited that the large spinosaurine Irritator had a habitual head-down posture, and that the internal ear structure suggests an animal for which vertical head movement was important. This would support the wading model (indeed, it would be predicted as the standard wait-and-see posture while foraging), but would not fit with an aquatic pursuit model where a habitual head-down posture would be inappropriate for a swimming animal or one residing on the surface.…”

Histology of spinosaurid dinosaur teeth from the Albian-Cenomanian of Morocco: Implications for tooth replacement and ecology

“…Further, they assumed that integumentary mechanoreceptors were involved in prey detection in spinosaurs as they are in extant pike congers (Muraenesocidae; Vullo et al, 2016). A recent study that investigated the neuroanatomy of Irritator challengeri, more evidence for adaptations for piscivory was found (Schade et al, 2020). However, many of these features would generally be advantageous for a specialization on small, elusive prey that is being secured with the jaws before swallowing, as argued by Rauhut (2001a).…”