A digital reconstruction of the nasal cavity of Riograndia guaibensis (Eucynodontia, Ictidosauria) and nasal cavity evolution in nonmammaliaform cynodonts

Abstract

With the aid of modern technologies such as micro-CT scanning and 3D modelreconstruction, the internal anatomy of fossilized remains can be easily and non-invasivelystudied. Here we describe the internal anatomical structures and associated bones ofthe nasal cavity of Riograndia guaibensis, a non-mammaliaform cynodont for the LateTriassic of Brazil. The specimen (UFRG-PV-596-T) is represented by an almost completeskull, articulated right and isolated left hemimandibles that were micro-CT scanned atthe University of Bristol in 2016. The 3D reconstruction of the nasal cavity highlights theimpression of ridges and canals on the internal surface of the bones that constitute the nasalpassage. The presence, dimensions and morphology of these ridges have implications forthe possible respiratory patterns, metabolic rate and ecology of this taxon. It is generallyaccepted that the acquisition of ossifed maxilloturbinals is related to the appearance ofendothermy in mammaliaforms. However, the presence of cartilaginous maxilloturbinals inRiograndia, as indicated by the preserved bony ridges, is very probable. The comparativestudy of these characteristics in various taxa before and after the appearance of mammalshelps to shed light on this much debated moment and is crucial in mammalian evolution.