Digital cranial endocast ofRiograndia guaibensis(Late Triassic, Brazil) sheds light on the evolution of the brain in non-mammalian cynodonts

Abstract

A digital cranial endocast of the specimen UFRGS-PV-596-T, Riograndia guaibensis, was obtained from µCT scan images. This is a small cynodont, closely related to mammaliaforms, from the Late Triassic of Brazil. Riograndia presents big olfactory bulb casts and the cerebral hemispheres region is relatively wider than in other nonmammaliaform cynodonts. Impressions of vessels were observed and a conspicuous mark on the dorsal surface was interpretated as the transverse sinus. The encephalization quotient calculated is greater than the range of the most nonmammaliaform cynodonts. The ratios between linear and area measurements of the dorsal surface suggests four evolutionary changes from a basal eucynodont morphology to mammaliaformes, evolving the increase of the relative size of the olfactory bulbs and the width of the cerebral hemispheres and cerebellum. The data supports the hypothesis of the neurological evolution of the mammlian lineage starting by a trend of the increase of the olfactory bulbs, which is associated to adaptations on the nasal cavity. This trend must be linked to the selective pressures for the small faunivorous, and probably nocturnal, animals, and represents an initial improvement of the sensory receptor system, leading to further development of the "superior" structures for sensorial processing and integration.

Virtual reconstruction of cranial endocasts of traversodontid cynodonts (Eucynodontia: Gomphodontia) from the upper Triassic of Southern Brazil

“…These characteristics are considered plesiomorphic in comparison to mammals (Kielan‐Jaworowska et al, ). In contrast, in Probainognathus , Riograndia , and Brasilodon the olfactory bulbs are more differentiated; the cerebral hemispheres have a clear division between them; and there is an increase in the olfactory bulbs and the cerebral hemispheres, mainly in Riograndia and Brasilodon (Rodrigues et al, , ). However, they still lack a cribriform plate, separating the endocast from the nasal cavity, and a full ossification of the anterior floor of the braincase (Quiroga, ; Rodrigues et al, , ).…”

“…In contrast, in Probainognathus , Riograndia , and Brasilodon the olfactory bulbs are more differentiated; the cerebral hemispheres have a clear division between them; and there is an increase in the olfactory bulbs and the cerebral hemispheres, mainly in Riograndia and Brasilodon (Rodrigues et al, , ). However, they still lack a cribriform plate, separating the endocast from the nasal cavity, and a full ossification of the anterior floor of the braincase (Quiroga, ; Rodrigues et al, , ). Usually, the midbrain is not visible on the dorsal surface of endocasts from cynodonts, probably because an anterior expansion of the cerebellum or a posterior expansion of the telencephalon overlaps this region, which also might be covered by the meninges and blood sinuses or by a combination of these structures (Edinger, ; Macrini, De Muizon, Cifelli, & Rowe, ; Rodrigues et al, , ).…”

“…However, they still lack a cribriform plate, separating the endocast from the nasal cavity, and a full ossification of the anterior floor of the braincase (Quiroga, ; Rodrigues et al, , ). Usually, the midbrain is not visible on the dorsal surface of endocasts from cynodonts, probably because an anterior expansion of the cerebellum or a posterior expansion of the telencephalon overlaps this region, which also might be covered by the meninges and blood sinuses or by a combination of these structures (Edinger, ; Macrini, De Muizon, Cifelli, & Rowe, ; Rodrigues et al, , ). A possible exception is Probainognathus , which would preserve a partial exposition of the midbrain because of the presence of the anterior colliculus (Quiroga, ).…”

“…In the last few years, several studies have been carried out, improving the knowledge of the endocranial morphology of several non‐mammaliaform therapsids through digital reconstruction of the endocast as follows: the biarmosuchian Lemurosaurus (Benoit, Fernandez, Manger, & Rubidge, ); the dinocephalian Moschops (Benoit, Manger, Norton, Fernandez, & Rubidge, ); the dicynodonts Pristerodon (Laaß, ), Diictodon (Laaß, Schillinger, & Kaestner, ), Kawingasaurus (Laaß & Kaestner, ), and Niassodon (Castanhinha et al, ); the gorgonopsian Cynariops (Bendel, Kammerer, Kardjilov, Fernandez, & Fröbisch, ); the early non‐mammaliaform cynodont Galesaurus (Pusch, Kammerer, & Fröbisch, ), and the prozostrodontid non‐mammaliaform cynodonts Riograndia (Rodrigues et al, ) and Brasilodon (Rodrigues, Ruf, & Schultz, ).…”

Endocranial morphology of the Brazilian Permian dicynodont Rastodon procurvidens (Therapsida: Anomodontia)

“…caused by an unossified zone of the braincase. This structure is commonly present and well developed in cynodonts (Watson, 1913;Olson, 1944;Quiroga, 1979;Macrini, 2006;Rodrigues et al 2018), being also reported in other nonmammalian synapsids (Castanhinha et al 2013;Laaß, 2015). It possibly housed the vermis (Kielan-Jaworowska et al 2004) or pineal gland (Kemp, 2009) as seen in mammals, or, in a more recent interpretation, it is seen as a terminal chamber for the superior sagittal and transverse sinuses (Macrini, 2006;.…”