An additional brain endocast of the ictidosaur Riograndia guaibensis (Eucynodontia: Probainognathia): intraspecific variation of endocranial traits

Abstract

Recently, the morphology and encephalization of the brain endocast of the Triassic non-mammaliaform probainognathian cynodont Riograndia guaibensis were studied. Here, we analyzed the brain endocast of an additional specimen of this species.The new endocast shows well-defi ned olfactory bulbs and a median sulcus dividing the hemispheres, traits that were not clearly observed in the fi rst studied specimen.Encephalization quotients were also calculated, revealing similar values to other nonmammaliaform cynodonts and lower than those of the fi rst analyzed specimen. The analyzed cranium is slightly larger than the fi rst studied one and may represent an advanced ontogenetic stage. Hence, these differences may be related to the intraspecifi c variation of this cynodont or alternatively, to the preservation of each specimen.

A new specimen provides insights into the anatomy of Irajatherium hernandezi, a poorly known probainognathian cynodont from the Late Triassic of southern Brazil

“…This crest is reflected as a longitudinal groove on the dorsal surface of the cranial endocast, dividing the cerebral hemispheres (Figure 4i). This pattern is similar to that of other probainognathians (Kerber et al, 2021; Quiroga, 1979, 1980, 1984; Rodrigues et al, 2014).…”

Cranial anatomy of Nythosaurus larvatus Owen, 1876, an Early Triassic cynodont preserving a natural endocast

“…While the dorsal surface of the forebrain cast suggests a close contact with the missing skull roof in lateral aspect based on the concave impression of the latter anterior and posterior to the cast of the pineal body, the exact ventral limit of the forebrain region remains obscure due to the absence of any delimiting bony structure between the frontals and pterygoids (Figs. 2, 3), as is frequently the case in nonmammalian therapsids (e.g., Kielan-Jaworowska et al, 2004; Rowe et al, 2011; Rodrigues et al, 2014; Laaß, 2015b; Araújo et al, 2017; Crompton et al, 2017b; Bendel et al, 2018; Rodrigues et al, 2018; Pavanatto et al, 2019; Pusch et al, 2019; Huttenlocker & Sidor, 2020; Kerber et al, 2021). The maximum width of the forebrain cast at the level of the orbital vacuity appears relatively well defined in dorsal view in the actual specimen (Fig.…”

“…1,2,3,8A,C,D). The presence of two distinct cerebral hemispheres is a derived condition in cynodonts, described only in advanced probainognathians such as Probainognathus (Quiroga, 1980a(Quiroga, , 1980bKielan-Jaworowska et al, 2004), Brasilitherium (Rodrigues et al, 2014), and Riograndia (Rodrigues et al, 2018;Kerber et al, 2021), which are also lacking the cast of the pineal body on the dorsal surface of the forebrain based on the absence of a parietal foramen encircling it. The loss of the latter is usually considered a synapomorphy for all Probainognathia (e.g.…”

Endocranial anatomy of the early prozostrodonts (Eucynodontia: Probainognathia) and the neurosensory evolution in mammal forerunners

“…The interbulbar sulcus is more commonly observed in cranial endocasts of probainognathians than in cynognathians (Pavanatto et al, 2019). In general, the olfactory bulbs of CAPPA/UFSM 0210 are morphologically similar to Probainognathus jenseni (Figure 6), Therioherpeton cargnini, Pseudotherium argentinus, and Riograndia guaibensis (Kerber et al, 2021;Quiroga, 1980aQuiroga, , 1984Wallace, 2018), but distinct from B. quadrangularis (Rodrigues et al, 2014). The latter has a larger volume, resulting in much more marked and welldefined and oval-shaped bulbs.…”

“…Early studies, especially based on natural cranial endocasts, often excluded the volume of the olfactory bulbs and tracts to calculate the Encephalization Quotients (EQ) (see Jerison, 1973;Quiroga, 1980aQuiroga, , 1984. On the other hand, some studies, such as Rowe et al (2011), Rodrigues et al (2014Rodrigues et al ( , 2019, Wallace (2018), andKerber et al (2021), which were exclusively based on virtual models, used the volume of the entire endocast. By contrast, Pavanatto et al (2019) and Hoffmann et al (2019) presented the values with and without the olfactory bulbs.…”