Endocranial anatomy of plesiosaurians (Reptilia, Plesiosauria) from the Late Cretaceous (Turonian) of Goulmima (southern Morocco)

Abstract

Despite recent advances in non-invasive imaging, Plesiosauria remains one of the least explored clade of reptiles with respect to palaeoneuroanatomy. Only partial endocasts, obtained from either latex casts or imprints left on the braincase, have been described so far. In this contribution, the digital endocasts of three plesiosaurian specimens were analysed: two referred to the Elasmosauridae Libonectes morgani and one undetermined Polycotylidae, all from the Late Cretaceous (Turonian) of Goulmima (Morocco). They were CT-scan generated in order to provide new anatomical information on the plesiosaurian endocast, endosseous labyrinth and cranial nerves. Results show that the three endocasts are very similar to each other. They appear anteroposteriorly elongated and horizontally oriented in lateral view, with long olfactory tracts, relatively small and incomplete olfactory bulbs, a reduced pineal organ, distinguishable optic lobes, and a possible large cerebellum constituting the main component in size of the endocast. The endocranial features reconstructed here are compared to those of other plesiosaurians, as well as other marine reptiles, notably to discuss their intraspecific and interspecific variability. This study provides pioneer data in order to estimate the impact of both phylogenetical and ecological constraints on the endocranial morphology of plesiosaurians and proposes a few preliminary paleobiological suggestions.

The cranial anatomy and relationships of Cardiocorax mukulu (Plesiosauria: Elasmosauridae) from Bentiaba, Angola

“…The horizontal semicircular canal is significantly atrophied relative to the anterior and posterior semicircular canals. This condition is similar to Libonectes morgani [ 73 , 87 ].…”

“…Lateral to the interpterygoid fenestra, the pterygoid is broad in being nearly the same width mediolaterally as the interpterygoid fenestra and forms a trough along the ventral surface (Figs 6 and 9E–9G ). A trough or excavation along the ventral surface is a common characteristic among elasmosaurids [ 10 , 13 , 41 , 71 , 73 , 74 ].…”

“…The endosseous labyrinth in MGUAN PA278 is compact, with a low aspect ratio (dorsoventral height:anteroposterior length) and is overall most similar to the endosseous labyrinth of Libonectes morgani and not Callawayasaurus colombiensis which exhibits a high aspect ratio and comparatively smaller ampullae [ 73 , 87 ]. The morphology of the endosseous labyrinth in MGUAN PA278 is distinct from that of Brancasaurus brancai Wegner, 1914 [ 84 , 88 ].…”

Braincase and endocranium of the Placodont Parahenodus atancensis de Miguel Chaves, Ortega & Pérez-García, 2018, a representative of the highly specialized clade Henodontidae

“…A weak development of this structure has also been observed in Nothosaurus marchicus (Voeten et al 2018). This contrasts with the identification of an unusually well-developed hypophysis in the placodont Placodus gigas (Neenan & Scheyer 2012), whose morphology could therefore be uncommon among Triassic sauropterygians, and also with the well-developed pituitary of such other Sauropterygia linages as the plesiosaurs (Otero et al 2018;Allemand et al 2019).…”

“…As in the case of Nothosaurus marchicus (Voeten et al 2018), Parahenodus atancensis lacks welldeveloped cerebral lobes, but unlike this small nothosaur, no optic lobes are distinguishable in the endocast of MUPA ATZ0104. The absence of well recognizable optic lobes in the midbrain of Parahenodus atancensis suggests a poorer vision than those of the nothosaurs and plesiosaurs, where these structures are well developed (Allemand et al 2019). No optic lobes have been identified in Placodus gigas so far.…”