Paleo-neurology at the beginning of a new phase

Abstract

The material of paleoneurology has been assembled more or less by chance. Representing a random assortment of brains of various fossil vertebrates from different geological horizons, it was not a reliable basis for that special task of paleoneurology, the study of brain evolution through the ages. A new phase began for paleoneurology when the collaboration of American museums resulted in the preparation of endocranial casts from almost every stage of the well-established evolutionary history of the horse. Now it was possible to follow the evolution of a brain through a period of about 55 million years. In the gradual transformation of theHyracotherium brain into that ofEquus the main feature was found to have been greater expansion of cerebrum, neopallium and corresponding portion of the cerebellum than of the other brain portions. Comparison of the brains today characteristic of lower placental mammals with those today characteristic of the higher forms had suggested that this is the main trend in progressive brain evolution. However,Hyracotherium (and other early Tertiary representatives of the higher, specialized eutherian orders) had a brain which had not yet reached the evolutionary level of even the most primitive of Recent cutherian brains. The characteristics of theEquus brain were not present in the Equidae for the greater part of this family's history. In their gradual development, the equid brain passed through evolutionary stages which are not represented in the series of Recent mammal brains, until about 15 million years ago it reached the Recent level of ungulate brain evolution. The various results of this one investigation indicate that more studies of such phyletic brain histories will reveal further, still unknown data on brains and on evolution in general, facts of a kind which only a purposeful paleoneurology can furnish.