The braincase ofMesosuchus browni(Reptilia, Archosauromorpha) with information on the inner ear and description of a pneumatic sinus

Abstract

Rhynchosauria is a group of archosauromorph reptiles abundant in terrestrial ecosystems of the Middle Triassic. Mesosuchus is one of the earliest and basalmost rhynchosaurs, playing an important role not only for the understanding of the evolution of the group as a whole, but also of archosauromorphs in general. The braincase of Mesosuchus has been previously described, albeit not in detail, and the middle and inner ears were missing. Here, we provide new information based on micro-computed tomography scanning of the best-preserved specimen of Mesosuchus, SAM-PK-6536. Contrary to what has been stated previously, the braincase of Mesosuchus is dorso-ventrally tall. The trigeminal foramen lies in a deep recess on the prootic whose flat ventral rim could indicate the articulation surface to the laterosphenoid, although no such element was found. The middle ear of Mesosuchus shows a small and deeply recessed fenestra ovalis, with the right stapes preserved in situ. It has a rather stout, imperforated and posteriorly directed shaft with a small footplate. These features suggest that the ear of Mesosuchus was well-suited for the detection of low-frequency sounds. The semicircular canals are slender and elongate and the floccular fossa is well-developed. This is indicative of a refined mechanism for gaze stabilization, which is usually related to non-sprawling postures. The most striking feature of the Mesosuchus braincase is, however, the presence of a pneumatic sinus in the basal tubera. The sinus is identified as originating from the pharyngotympanic system, implying ossified Eustachian tubes. Braincase pneumatization has not yet been a recognized feature of stem-archosaurs, but the potential presence of pneumatic foramina in an array of taxa, recognized here as such for the first time, suggests braincase sinuses could be present in many other archosauromorphs.

Cranial morphology of the tanystropheid Macrocnemus bassanii unveiled using synchrotron microtomography

“…The dorsal-most (sagittal) surface of the condyle forms the floor of the foramen magnum. No notochord pit is present on the condyle as in other early archosauromorphs including Prolacerta broomi, Mesosuchus browni and Euparkeria capensis 46,53,55 . The distance between the basal tubera of the basioccipital is roughly similar in width to the condyle.…”

“…The flange is relatively short and does not reach the dorsal margin of the main body of the quadrate as seen in Azendohsaurus madagaskarensis 34 . The orientation of the pterygoid flange is exclusively anterior; however, it is unclear whether this is due to compression of the element and it would have had a more antero-medial orientation in life as in other archosauromorphs 46 . A larger ellipsoidal flange protrudes antero-laterally; this is the tympanic flange.…”

“…Their contact through a horizontal ridge is more extensive than in Prolacerta broomi, in which this ridge is virtually absent 53 . More similar than Prolacerta broomi to M. bassanii is Mesosuchus browni as it displays a similar concave structure 46 .…”

The internal cranial anatomy of Champsosaurus lindoei and its functional implications

“…Champsosaurus lacked a tympanum, likely a retention of the basal amniote condition (Gardner et al, 2010). Sobral and Müller (2019) used osteological correlates established in Crocodyliformes (Montefeltro et al, 2016) to infer the presence of a tympanum in the rhynchosaur Mesosuchus, which lacks a distinct otic notch in the quadrate; however, they emphasized that this was done with caution due to the specialization of the crocodyliform outer ear (e.g., presence of an osseous meatal chamber and fleshy 'ear lid'), from basal archosauromorphs.…”

“…In some aquatic amniotes, thickening of ossifications in and around the middle ear better facilitates the transmission of sound vibrations to the inner ear (Mooney et al, 2012;Ladich and Winkler, 2017). Sobral and Müller (2019) inferred good auditory capabilities in the rhynchosaur Mesosuchus based partially on the location of the fenestra ovalis deep within the bone of the lateral wall of the otic capsule, which may have facilitated the transmission of sound vibrations. In Champsosaurus, the fenestra ovalis is not located deep within the bone of the otic capsule, and it is therefore unlikely that the bone of the otic capsule facilitated the detection of sound vibrations.…”