Modified skulls but conservative brains? The palaeoneurology and endocranial anatomy of baryonychine dinosaurs (Theropoda: Spinosauridae)

Abstract

The digital reconstruction of neurocranial endocasts has elucidated the gross brain structure and potential ecological attributes of many fossil taxa, including Irritator, a spinosaurine spinosaurid from the "mid" Cretaceous (Aptian) of Brazil. With unexceptional hearing capabilities, this taxon was inferred to integrate rapid and controlled pitch-down movements of the head that perhaps aided in the predation of small and agile prey such as fish. However, the neuroanatomy of baryonychine spinosaurids remains to be described, and potentially informs on the condition of early spinosaurids. Using micro-computed tomographic scanning (μCT), we reconstruct the braincase endocasts of Baryonyx walkeri and Ceratosuchops inferodios from the Wealden Supergroup (Lower Cretaceous) of England. We show that the gross endocranial morphology is similar to other non-maniraptoriform theropods, and corroborates previous observations of overall endocranial conservatism amongst more basal theropods. Several differences of unknown taxonomic utility are noted between the pair. Baryonychine neurosensory capabilities include low-frequency hearing and unexceptional olfaction, whilst the differing morphology of the floccular lobe tentatively suggests less developed gaze stabilisation mechanisms relative to spinosaurines. Given the morphological similarities observed with other basal tetanurans, baryonychines likely possessed comparable behavioural sophistication, suggesting that the transition from terrestrial hypercarnivorous ancestors to semi-aquatic "generalists" during the evolution of Spinosauridae did not require substantial modification of the brain and sensory systems.

Keywords: Baryonychinae; Spinosauridae; Theropoda; endocast; palaeoneurology; sensory anatomy.

FIGURE 1

Braincases of (a, c, e, f, h) Baryonyx walkeri (NHMUK PV R9951) and (b, d, g, i) Ceratosuchops inferodios (IWCMS 2014.95.1–3), in (a, b) right anterolateral and (c, d) right posterolateral, (e–g) right lateral and (h, i) posterior (right side) views, showing the major neurovascular features (and associated foramina) and braincase anatomy. Note that in Baryonyx, the separate cranial nerve trunks X—XI and XII open into a common fossa lateral to the occipital condyle, which is depicted here. Abbreviations: ce, cranial endocast; cul, cultriform process; ct, crista tuberalis; BO, basioccipital; BS, basisphenoid; car, cerebral internal carotid artery canal; F, frontal; fm, foramen magnum; fo, fenestra ovalis; lab, endosseous labyrinth; LS, laterosphenoid; ocv, orbitocerebral vein; oc, occipital condyle; OS, orbitosphenoid; OT, otoccipital; otc, olfactory tract; P, parietal; pit, pituitary; pmcv, posterior middle cerebral vein canal; PRO, prootic; II, optic nerve canal; IV, trochlear nerve canal; V, trigeminal nerve canal; V₁, ophthalmic nerve canal; VI, abducens nerve canal; VII, facial nerve canal; IX, glossopharyngeal nerve canal; X–XI, shared canal for the vagus and accessory nerves, and accompanying vessels; XII, hypoglossal nerve canal; ?, potential accessory hypoglossal nerve or venous canal. Scale bars: (a–d) 20 mm and (e–i) 50 mm.

FIGURE 2

Cranial endocast of Baryonyx walkeri (NHMUK PV R9951), reconstructed from CT scans, in (a) right lateral, (b) anterior, (c) ventral, (d) posterior and (e) dorsal views. Vascular structures and endosseous labyrinth also depicted. Abbreviations: c, cochlea; car, cerebral internal carotid artery canal; cer, cerebral hemisphere; de, dorsal expansion; fl, floccular lobe; lab, endosseous labyrinth; otc, olfactory tract; pit, pituitary; pmcv, posterior middle cerebral vein canal; sin, blind dural venous sinus of the hindbrain; V, trigeminal nerve canal; VI, abducens nerve canal; VII, facial nerve canal, VIII_co, cochlear ramus of the vestibulocochlear nerve; IX, glossopharyngeal nerve canal; X–XI, shared canal for the vagus, and accessory nerves, and accompanying vessels; XII, hypoglossal nerve canal. Asterisk (*) marks the disarticulated left otoccipital portion of the endocast. Scale bar: 50 mm.

FIGURE 3

Cranial endocast of Ceratosuchops inferodios (IWCMS 2014.95.1–3), reconstructed from CT scans, in (a) right lateral, (b) anterior, (c) ventral, (d) posterior and (e) dorsal views. Vascular structures and endosseous labyrinth also depicted. Abbreviations: c, cochlea; car, cerebral internal carotid artery canal; cer, cerebral hemisphere; de, dorsal expansion; fl, floccular lobe; lab, endosseous labyrinth; ocv, orbitocerebral vein; pit, pituitary; pmcv, posterior middle cerebral vein canal; sin, blind dural venous sinus of the hindbrain; II, optic nerve canal; IV, trochlear nerve canal; V, trigeminal nerve canal; V₁, ophthalmic nerve canal; VI, abducens nerve canal; VII, facial nerve canal, VIII_co, cochlear ramus of the vestibulocochlear nerve; IX, glossopharyngeal nerve canal; X–XI, shared canal for the vagus, and accessory nerves, and accompanying vessels; XII, hypoglossal nerve canal; ?, potential accessory hypoglossal nerve or venous canal. Scale bar: 50 mm.

FIGURE 4

The baryonychine rhombencephalon, in right lateral views. (a) Baryonyx walkeri, (b) Ceratosuchops inferodios. Abbreviations: c, cochlea; fl, floccular lobe; fo, fenestra ovalis; lab, endosseous labyrinth; pit, pituitary; sin, blind dural venous sinus of the hindbrain; V, trigeminal nerve canal; VI, abducens nerve canal; VII_hym, hyomandibular ramus of the facial nerve; VII_pal, palatine ramus of the facial nerve; VIII_vest, vestibular ramus of the vestibulocochlear nerve; IX, glossopharyngeal nerve canal; X–XI, shared canal for the vagus and accessory nerves, and accompanying vessels; XII, hypoglossal nerve canal; ?, potential accessory hypoglossal nerve or venous canal. Scale bar: 10 mm.

FIGURE 5

Baryonychine endosseous labyrinths. Baryonyx walkeri (a–d) and Ceratosuchops inferodios (e–h), in (a, e) lateral, (b, f) anterior, (c, g) posterior, (d, h) dorsal views. Note that the facial nerve (CN VII) trunk has been abbreviated in the Ceratosuchops model. Abbreviations: aam, anterior ampulla; asc, anterior semicircular canal; c, cochlea; cc, common crus; fo, fenestra ovalis; lam, lateral ampulla; lsc, lateral semicircular canal; pam, posterior ampula; psc, posterior semicircular canal; VII, facial nerve canal; VIII_co, cochlear ramus of the vestibulocochlear nerve; VIII_vest, vestibular ramus of the vestibulocochlear nerve. Scale bar 10 mm.