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

Abstract

The brain endocasts of the late Triassic (Carnian) traversodontids (Eucynodontia: Gomphodontia) Siriusgnathus niemeyerorum and Exaeretodon riograndensis from southern Brazil are described based on virtual models generated using computed tomography scan data. Their skull anatomy resembles that of other nonmammaliaform cynodonts, showing an endocranial cavity that is not fully ossified. A "V-shaped" orbitosphenoid, neither fully developed nor ossified is present in E. riograndensis. The nasal cavity is confluent with the encephalic cavity. Thus, the anterior limit of the olfactory bulbs is not definite. The brain endocast is elongated, being narrow anteriorly and wide posteriorly, with the maximum width at the parafloccular cast. The olfactory bulbs do not present a clear division between their counterparts, due to the absence of a longitudinal sulcus. A longitudinal sulcus in the forebrain delimiting the cerebral hemispheres, the pineal tube, and the parietal foramen are absent in both taxa. The large and well-developed unossified zone is partially separated from the remaining endocast by a notch formed by the supraoccipital. The encephalization quotients, as well as the endocranial volume/body mass relationships of S. niemeyerorum and E. riograndensis are within the range expected for nonmammaliaform Therapsida.

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

“…The resulting 3D-models were rendered using Design Spark Mechanical 2.0. As the ventral and anterior regions of the endocranial cavity of non-mammaliaform cynodonts are usually not ossified, both regions are delimited artificially (see Pavanatto et al 2019). The posteriormost region of the brain was not possible to be reconstructed confidently due to the fragmentation of the specimen.…”

“…With the advance of the CT-Scanning techniques, the endocranial study of therapsids has been significantly increased in the last years (e.g., Castanhinha et al 2013, Rodrigues et al 2014, Laaß 2015, Balanoff et al 2016, Benoit et al 2016, 2017a, b, Araújo et al 2017, Laaß & Kaestner 2017, Pavanatto et al 2019, Oliveira et al 2019, Hoffmann et al 2019. Now it is possible to reconstruct three-dimensional models of the endocranial cavities non-destructively, different from previous approaches (i.e., natural and artifi cial endocasts), with precise modeling of the impression of the soft tissues.…”

“…The morphology of the skull and post-cranial bones of R. guaibensis have been studied (Bonaparte et al 2001, Soares et al 2011, Guignard et al 2019, and recently the brain endocast morphology of one specimen (UFRGS-PV-596-T) was analyzed by Rodrigues et al (2019). As mentioned above, although a significant advance has been achieved in the last years, most of our knowledge of non-mammaliaform therapsids is based on the study of one specimen of each taxon, with some exceptions (see Pavanatto et al 2019 andHoffmann et al 2019 for a discussion on intraspecific variation). In most cases, this is a limitation of our science, and thus, the information on the intraspecific/ontogenetic variation is restricted until new specimens are discovered.…”

Novel Endocranial Data on the Early Therocephalian Lycosuchus vanderrieti Underpin High Character Variability in Early Theriodont Evolution

“…In the context of mammalian origins, these novel endocranial data have been instrumental in elucidating patterns of brain and ear evolution, facial innervation, and the evolution of endothermy in non-mammalian therapsids (e.g., Rodrigues et al, 2013Rodrigues et al, , 2014Crompton et al, 2015Crompton et al, , 2017Laaß, 2015aLaaß, ,b, 2016Benoit et al, 2016aBenoit et al, ,b, 2017aBenoit et al, ,b,c, 2018Benoit et al, , 2019Araújo et al, 2017Araújo et al, , 2018Bendel et al, 2018;Pusch et al, 2019). As the therapsid clade ancestral to (and including) modern mammals, Cynodontia has been the primary focus for CTassisted morphological studies in Synapsida (e.g., Rowe et al, 1993;Rodrigues et al, 2013Rodrigues et al, , 2014Crompton et al, 2015Crompton et al, , 2017Benoit et al, 2016aBenoit et al, , 2019Pavanatto et al, 2019;Pusch et al, 2019). By contrast, Therocephalia, generally considered the sister-group of Cynodontia (e.g., Hopson and Barghusen, 1986;Hopson, 1991;Huttenlocker, 2009;Huttenlocker and Sidor, 2016;Huttenlocker and Smith, 2017; but see Abdala, 2007;Botha et al, 2007), has received little study.…”

The nasal cavity of two traversodontid cynodonts (Eucynodontia, Gomphodontia) from the Upper Triassic of Brazil

“…The external osteology of E. riograndensis was studied by Abdala et al (2002), Oliveira et al (2007), and Liparini et al (2013), and that of S. niemeyerorum by Pavanatto et al (2018) and Miron et al (2020). Recently, the brain endocast morphology of both cynodonts was studied by Pavanatto et al (2019). In the present study, we aim to describe the morphology of the nasal cavities of E. riograndensis and S. niemeyerorum using computed tomography (CT) scanning and explore their paleobiological implications.…”