The history of the bird brain is not as well-known as that of mammals, and the present diversity in brains in birds does not appear to be as dramatic. However, the brain of Archaeopteryx was bird-like primarily because it filled the cranial cavity and was larger than that in comparable reptiles. There was no Wulst—that is, the dorsal enlargement of the forebrain in living birds that functions equivalently to primary visual cortex in mammals. The next significant evidence in the history of birds is from an Eocene whimbril-like bird, Numenius gypsorum, in which the brain is somewhat smaller than in comparable living birds of its body size but, most dramatically, its forebrain is clearly much smaller so that its optic lobes are more fully exposed than in living birds. Endocasts of later birds are indistinguishable from those of their living relatives.
In their early history, the mammals were small, probably nocturnal insectivorous creatures. Their adaptation for life in nocturnal niches could have been the major selection pressure to explain the "advance" to a mammalian grade of brain morphology and encephalization. The characteristic morphological feature of the brain of living mammals is the presence of neocortex, the six-layered neuronally rich outer covering of the forebrain. Its presence can often be established on an endocast because a major fissure, the rhinal fissure, is its ventral boundary.
The earliest mammalian brain is known from an Upper Jurassic endocast of Triconodon mordax and is about 150 million years old. The lateral surface of this endocast is not preserved well enough to indicate whether or not there was a rhinal fissure; thus, positive evidence regarding the presence of neocortex is not available. In encephalization, however, its brain was comparable to that of small-brained living species such as opossums and hedgehogs, in which neocortex is present. It is therefore likely that neocortex appeared at least 150 Ma. The best assumption from available information is that neocortex is, in fact, part of the suite of traits that characterized the mammals from the beginning of their evolution, at least 50 million years earlier.
Mammals in which the endocasts are sufficiently complete to show a rhinal fissure, if present, are about 75 Ma. They are from a unique assemblage of Late Cretaceous mammals from the Gobi desert, which includes early placentals. The most common mammals of the time, the multituberculates, were unrelated to any living species. Superficially, multituberculates probably looked like living rodents or insectivores. Their life span as an order was about 120 million years, between about 150 and 30 Ma, a very long span for a mammalian group. The specimen in which the endo-cast is best known, Chulsanbaatar, weighed no more than about 15 g, a small mammal even for the Mesozoic and smaller than most living species of mice. There is a suggestion of a rhinal fissure in its endocast, although there is disagreement about where it is located. Whether or not one can see a rhinal fissure, from its grade of encephalization it is very likely that neocortex was present in its brain.
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