Summary: A new study reveals an early relative of mammals possessed an expanded brain with a neocortex like structure. The researchers reported the neocortex like structure developed in a late Permian therapsid 25 million years prior to the emergence of the first mammals.
According to a recent study an early relative of mammals already possessed an extraordinarily expanded brain with a neocortex-like structure. This has been discovered by Michael Laaß from the Institute of General Zoology at the University of Duisburg-Essen (UDE).
Today, mammals possess large and efficient brains. But, what was the bauplan of the brain of their far relatives, the therapsids? When and why evolved the neocortex?
For his doctoral thesis the palaeontologist Michael Laaß invesitgated a ca. 255 million years old fossil skull of the therapsid Kawingasaurus fossilis in collaboration with Dr. Anders Kaestner from the Paul Scherrer Institute in Switzerland by means of neutron tomography and reconstructed the internal cranial anatomy in 3D.
The results were amazing: The relative brain volume of Kawingasaurus was about two or three-times larger than in other non-mammalian therapsids. Laaß: “Interestingly, Kawingasaurus already possessed a large forebrain with two distinct cerebral hemispheres”. Obviously, a neocortex-like structure at the forebrain similar to the mammalian neocortex was present in this animal.
Why is this brain structure evolved in Kawingasaurus? “Kawingasaurus was a burrower and special sensory adaptations were crucial for life under ground”, explained the UDE scientist. For example, this therapsid possessed frontally placed eyes, which were probably useful for binocular vision in dimlight environments as it is known from modern cats and owls. Furthermore, extremely ramified trigeminal nerve endings penetrated the snout, which might be an indication for a well developed sense of touch. The inner ear vestibules were also very large, which suggests that Kawingasaurus was well adapted to detect seismic vibrations from the ground.
Laaß: “These special sensory adaptaions also required a more efficient neural processing of the brain than in other therapsids.” It seems reasonable that these special adaptations of the sense organs and the brain to underground life triggered the expansion of the brain. Interestingly, a similar scenario for the origin of the neocortex has been also proposed for early mammals. Consequently, the recent study at the UDE supports this hypothesis.
Moreover, the new discovery also shows that a neocortex-like structure already developed in the therapsid Kawingasaurus about 25 million years earlier before the emergence of the first mammals. However, Kawingasaurus was not a direct ancestor of mammals. Consequently, neocortex-like structures evolved several times independently in pre-mammalian and mammalian evolution.
Image Source: NeuroscienceNews.com image is credited to Michael Laaß / Verlag Wiley-VCH.
Original Research: Abstract for “Evidence for convergent evolution of a neocortex-like structure in a late Permian therapsid” by Michael Laaß, and Anders Kaestner in Journal of Morphology. Published online June 16 2017 doi:10.1002/jmor.20712
Evidence for convergent evolution of a neocortex-like structure in a late Permian therapsid
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.
“Evidence for convergent evolution of a neocortex-like structure in a late Permian therapsid” by Michael Laaß, and Anders Kaestner in Journal of Morphology. Published online June 16 2017 doi:10.1002/jmor.20712