Cranial anatomy of the early cynodont Galesaurus planiceps and the origin of mammalian endocranial characters

Abstract

The cranial anatomy of the early non-mammalian cynodont Galesaurus planiceps from the South African Karoo Basin is redescribed on the basis of a computed tomographic reconstruction of the skull. Previously, little was known about internal skull morphology and the nervous and sensory system of this taxon. The endocranial anatomy of various cynodonts has been intensively studied in recent years to understand the origin of mammalian characters in the nasal capsule, brain and ear. However, these studies have focused on only a few taxa, the earliest of which is another Early Triassic cynodont, Thrinaxodon liorhinus. Galesaurus is phylogenetically stemward of Thrinaxodon and thus provides a useful test of whether the mammal-like features observed in Thrinaxodon were present even more basally in cynodont evolution. The cranial anatomy of G. planiceps is characterized by an intriguing mosaic of primitive and derived features within cynodonts. In contrast to the very similar internal nasal and braincase morphology of Galesaurus and Thrinaxodon, parts of the skull that seem to be fairly conservative in non-prozostrodont cynodonts, the morphology of the maxillary canal differs markedly between these taxa. Unusually, the maxillary canal of Galesaurus has relatively few ramifications, more similar to those of probainognathian cynodonts than that of Thrinaxodon. However, its caudal section is very short, a primitive feature shared with gorgonopsians and therocephalians. The otic labyrinth of Galesaurus is generally similar to that of Thrinaxodon, but differs in some notable features (e.g. proportional size of the anterior semicircular canal). An extremely large, protruding paraflocculus of the brain and a distinct medioventrally located notch on the anterior surface of the tabular, which forms the dorsal border of the large parafloccular lobe, are unique to Galesaurus among therapsids with reconstructed endocasts. These features may represent autapomorphies of Galesaurus, but additional sampling is needed at the base of Cynodontia to test this.

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

“…The endocranial morphology of R. procurvidens is similar to the pattern observed in most non-mammaliaform synapsids, resembling a 'reptilian' endocranium rather than that of mammals and mammal-like cynodonts. The resulting brain endocast is overall tubular and elongated, consistent with what is commonly seen in Permian therapsids (Olson, 1944;Kielan-Jaworowska et al 2004;Kemp, 2009;Rowe et al 2011;Castanhinha et al 2013;Laaß, 2015;Ara ujo et al 2017;Bendel et al 2018;Pusch et al 2019) (Fig. 7).…”

“…The midbrain is coronally oval‐shaped, and dorsally higher than the hindbrain, but laterally compressed. Posteriorly, the midbrain endocast bears a projection that fills the unossified zone (Kielan‐Jaworowska et al ; Macrini, ; Kemp, ; Castanhinha et al ; Laaß, ; Laaß et al ; Pusch et al ) between the parietals and supraoccipital (Fig. A).…”

“…Comparison of the endocast of Rastodon procurvidens and selected comparative therapsids. Endocasts redrawn based on: 1, Benoit et al (); 2, Pusch et al (); 3, Rodrigues et al (); 4, Kemp (); 5, Laaß (); 6, Laaß (); 7, Castanhinha et al (); 8, Lehman (). Phylogenetic affinities based on Kammerer ().…”

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

“…The elongated and tubular endocast becomes enlarged in the hindbrain, with poorly defined olfactory bulbs, undifferentiated cerebral hemispheres, and a cerebellum, resembling the condition described for other cynodonts such as Thrinaxodon and Galesaurus , the gomphodont Trirachodon , the traversodontids Massetognathus and E. argentinus , and the basal probainognathian cf. Probelesodon (Bonaparte, ; Hopson, ; Kielan‐Jaworowska et al, ; Pusch et al, ; Quiroga, , ). These characteristics are considered plesiomorphic in comparison to mammals (Kielan‐Jaworowska et al, ).…”

“…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, ).…”