Evidence for convergent evolution of a neocortex‐like structure in a late Permian therapsid

Abstract

The special sensory, motor, and cognitive capabilities of mammals mainly depend upon the neocortex, which is the six-layered cover of the mammalian forebrain. The origin of the neocortex is still controversial and the current view is that larger brains with neocortex first evolved in late Triassic Mammaliaformes. Here, we report the earliest evidence of a structure analogous to the mammalian neocortex in a forerunner of mammals, the fossorial anomodont Kawingasaurus fossilis from the late Permian of Tanzania. The endocranial cavity of Kawingasaurus is almost completely ossified, which allowed a less hypothetical virtual reconstruction of the brain endocast to be generated. A parietal foramen is absent. A small pit between the cerebral hemispheres is interpreted as a pineal body. The inflated cerebral hemispheres are demarcated from each other by a median sulcus and by a possible rhinal fissure from the rest of the endocast. The encephalization quotient estimated by using the method of Eisenberg is 0.52, which is 2-3 times larger than in other nonmammalian synapsids. Another remarkable feature are the extremely ramified infraorbital canals in the snout. The shape of the brain endocast, the extremely ramified maxillary canals as well as the small frontally placed eyes suggest that special sensory adaptations to the subterranean habitat such as a well developed sense of touch and binocular vision may have driven the parallel evolution of an equivalent of the mammalian neocortex and a mammal-like lemnothalamic visual system in Kawingasaurus. The gross anatomy of the brain endocast of Kawingasaurus supports the Outgroup Hypothesis, according to which the neocortex evolved from the dorsal pallium of an amphibian-like ancestor, which receives sensory projections from the lemnothalamic pathway. The enlarged brain as well as the absence of a parietal foramen may be an indication for a higher metabolic rate of Kawingasaurus compared to other nonmammalian synapsids.

Virtual endocasts of fossil Sciuroidea: brain size reduction in the evolution of fossoriality

“…The reason for this change remains unclear but may be related to an increase in body mass through time (Finarelli & Flynn ). In contrast, Laaß & Kaestner () found that a fossorial Permian therapsid had a higher EQ compared to other members of the group; however, the authors did not consider locomotor mode of the therapsids to which they made their comparisons, making it difficult to pinpoint fossoriality as the cause of their taxon's relatively large brain size.…”

What did the “Unossified zone” of the non‐mammalian therapsid braincase house?

“…1 and 2), which was according to Castanhinha et al (2013) a specimen of Diictodon feliceps. There is also evidence for the unossified zone in the anomodont Pristerodon mackayi (Laaß, 2015), but it seems to be absent in Niassodon mfumukasi (Castanhinha et al, 2013) and in K. fossilis (Laaß and Kaestner, 2017).…”

“…On the one hand, this is caused by the fact that therapsids did not possess a completely ossified endocranial cavity, whereas on the other hand it seems likely that most parts of the small, tubular brain were obviously not in close contact with the brain cavity (Hopson, 1979;Kemp, 2009). An exception might be the endocranial cavity of Kawingasaurus fossilis, which is almost completely ossified and due to the presence of impressions of soft-tissue structures in the braincase it seems reasonable that at least some parts of the enlarged brain were in close contact with the endocranial cavity (Laaß and Kaestner, 2017).…”

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

“…The endocranial morphology of R. procurvidens resembles the plesiomorphic pattern retained in most therapsids. Disparate endocast morphologies seem to be a consequence of cranial specializations and ecological variations: the inner ear morphology (plus stapes fitting) of Pristerodon suggests adaptations of the auditory apparatus and a higher sensitivity to airborne vibrations (Laaß, , ), whereas the brains of the cistecephalids Kawingasaurus and Cistecephalus are especially distinctive and reflect a fossorial lifestyle, with Kawingasaurus even presenting a convergent neocortex‐like structure, as suggested by Laaß & Kaestner ().…”

“…It is underdeveloped in R. procurvidens , being elliptically shaped and almost straight in the longitudinal axis. In this aspect, it resembles the horizontal canal of K. fossilis (Laaß, ; Laaß & Kaestner, ), differing from Niassodon mfumukasi (Castanhinha et al ) and P. mackayi (Laaß, ), which have a wider and more rounded horizontal semicircular canal (Fig. ).…”