Synchrotron microtomography of a Nothosaurus marchicus skull informs on nothosaurian physiology and neurosensory adaptations in early Sauropterygia

Abstract

Nothosaurs form a subclade of the secondarily marine Sauropterygia that was well represented in late Early to early Late Triassic marine ecosystems. Here we present and discuss the internal skull anatomy of the small piscivorous nothosaur Nothosaurus marchicus from coastal to shallow marine Lower Muschelkalk deposits (Anisian) of Winterswijk, The Netherlands, which represents the oldest sauropterygian endocast visualized to date. The cranial endocast is only partially encapsulated by ossified braincase elements. Cranial flattening and lateral constriction by hypertrophied temporal musculature grant the brain a straight, tubular geometry that lacks particularly well-developed cerebral lobes but does potentially involve distinguishable optic lobes, suggesting vision may have represented an important sense during life. Despite large orbit size, the circuitous muscular pathway linking the basisphenoidal and orbital regions indicates poor oculomotor performance. This suggests a rather fixed ocular orientation, although eye placement and neck manoeuvrability could have enabled binocular if not stereoscopic vision. The proportionally large dorsal projection of the braincase endocast towards the well-developed pineal foramen advocates substantial dependence on the corresponding pineal system in vivo. Structures corroborating keen olfactory or acoustic senses were not identified. The likely atrophied vomeronasal organ argues against the presence of a forked tongue in Nothosaurus, and the relative positioning of external and internal nares contrasts respiratory configurations proposed for pistosauroid sauropterygians. The antorbital domain furthermore accommodates a putative rostral sensory plexus and pronounced lateral nasal glands that were likely exapted as salt glands. Previously proposed nothosaurian ‘foramina eustachii’ arose from architectural constraints on braincase development rather than representing functional foramina. Several modifications to brain shape and accessory organs were achieved through heterochronic development of the cranium, particularly the braincase. In summary, the cranium of Nothosaurus marchicus reflects important physiological and neurosensory adaptations that enabled the group’s explosive invasion of shallow marine habitats in the late Early Triassic.

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

“…Plesiosaurian olfactory tracts, elongated anteroposteriorly and not separated along their length (Fig. 3), are similar to those reported in mosasauroids and nothosaurians (Camp, 1942;Voeten et al, 2018) but differ from the short olfactory tracts observed in turtles (Paulina-Carabajal et al, 2013) and the more or less separated olfactory tracts found in snakes (Allemand et al, 2017c), thalattosuchians (e.g., Pierce et al, 2017) and ichthyosaurs (e.g., Marek et al, 2015). Plesiosaurian endocasts are nearly linear (Fig.…”

“…Plesiosaurian endocasts are nearly linear (Fig. 3), both the anterior and posterior parts being at the same dorsoventral level, similarly as reported in snakes (Allemand et al, 2017c), thalattosuchians (e.g., Pierce et al, 2017), nothosaurians (Voeten et al, 2018) and turtles (Paulina-Carabajal et al, 2013), a condition differing from that of Placodus gigas (Neenan and Scheyer, 2012), in which the structure appears anteroposteriorly inclined and sigmoidal. The horizontal projection of the plesiosaurian pituitary differs from the tilted one observed in snakes (Allemand et al, 2017c) and thalattosuchians (e.g., Pierce et al, 2017).…”

“…A pineal complex has been reported for various clades of extinct marine reptiles such as mosasauroids (Camp, 1942;Allemand et al, unpub. data 2017b), ichthyosaurians (Marek et al, 2015) and Triassic sauropterygians (Neenan and Scheyer, 2012;Voeten et al, 2018). On the contrary, although a pineal complex occurs in extant turtles and snakes (Wyneken, 2003(Wyneken, , 2007, the structure is covered by the dorsal longitudinal vein (Aurboonyawat et al, 2008) and not visible from the endocast (Paulina-Carabajal et al, 2013;Allemand et al, 2017c).…”

“…Although recent advances in non-invasive imaging have increased the number of taxa for which the endocranial morphology is accessible (e.g., Paulina-Carabajal et al, 2013;Cuthbertson et al, 2015;Kishida et al, 2015;Marek et al, 2015;Voeten et al, 2018), only the plesiosaurian endosseous labyrinth has been so far analysed (e.g., Neenan et al, 2017), and the current understanding of their neuroanatomy remains poor. Partial endocasts are known for Brancasaurus brancai (Edinger, 1928see Hopson, 1979, Libonectes morgani (Carpenter, 1997) and Terminonatator ponteixensis (Sato, 2003).…”

Triassic (Anisian and Rhaetian) palaeomagnetic poles from the Germanic Basin (Winterswijk, the Netherlands)

“…This well-dated section, palynologically constrained at the Anisian (247.2-242 Ma;Herngreen et al 2005), has a rich fossil fauna, which has been subject to numerous palaeontological studies. These recovered ichnofossils and tetrapod footprints (Oosterink 1976;Demathieu and Oosterink 1988;Diedrich 2000;Schulp et al 2017), reptiles (nothosaurs and placodonts) (Albat 1999;Bickelmann and Sander 2008;Klein 2009Klein , 2012Klein and Albers 2009;Albers 2011;Klein and Scheyer 2013;Klein and Sichelschmidt 2014;Sander et al 2014;Klein et al 2015Klein et al , 2016Voeten et al 2015Voeten et al , 2018Maxwell et al 2016;During et al 2017;Klein and Griebeler 2018;Heijne et al 2019), but also lobsters (Klompmaker and Fraaije 2011), jellyfish (Klompmaker and Fraaije 2011), and insects (van Eldijk et al 2017). The Muschelkalk Formation contains widespread secondary mineralizations in cracks and pores, containing like pyrite and celestine (Kloprogge et al 2001).…”