On the brain of a primitive bird from the upper Cretaceous of European Russia

Abstract

Cerebavis cenomanica gen. et sp. nov. from the Middle Cenomanian of the Volgograd Region (Russia) is described based on a brain mold. The brain of Cerebavis is characterized by a mosaic combination of primitive and advanced features. The brain weight is estimated as approximately 1 g. The cerebrum is relatively very large, but lacks sulci. The brain mold has long olfactory lobes with large olfactory bulbs, a well-developed epiphysis, and a parietal organ. The auditory tubercles on the dorsal surface of the midbrain are well developed. The optical lobes are located under the auditory lobes, caudoventral to the cerebral hemispheres. The cerebellum is not preserved, but its imprints just behind the midbrain suggest that it was probably relatively small and extended dorsoventrally. The brain of Cerebavis is similar in some features to that of Archaeopteryx, but is substantially more advanced and more specialized. Cerebavis is similar to living ornithurine birds in the large cerebral hemispheres, but differs in the absence of a well-developed neostriatum, the presence of excessively developed olfactory lobes, and in the pattern of the midbrain. Thus, senses of smell, eyesight, and hearing were well developed in Cerebavis. It could have been equally active in the afternoon and at night. The unique brain design demonstrated by Cerebavis has not been repeated in subsequent evolution. It provides evidence for a wide diversity of feathered creatures in the past. Cerebavis probably belongs to the Enantiornithes.