488-444 million years ago
THE NERVOUS SYSTEM
The early gnathostomes evolved a number of modifications in their nervous systems. These included a larger cerebellum, a more complex midbrain, additional nuclei in the diencephalon, an increase in the areas of the cerebrum dedicated to functions other than olfaction, a lens and iris in the eye, a saccule and utricle in the ear, and an organization of the gray matter of the spinal cord.
The cerebellum increased greatly in size and was composed of an unpaired corpus and 2 lateral auricles. The corpus cerebellum contained a pars anterior and pars posterior. The neurons of the cerebellar cortex formed molecular, Purkinje, and granular layers (Ariens, p. 720). The cerebellum increased its connections to the body through dorsal and ventral spinocerebellar tracts, although they were largely uncrossed (Ariens, p. 722).
In the early gnathostomes, the midbrain underwent a number of changes. The tectum was better developed, the basal region of the midbrain increased in size, cerebellar tracts to diencephalon passed through the midbrain, connections of the vestibulocochlear nerve to the hypothalamus traveled through the midbrain, the midbrain was able to affect body position through connections with cerebellum and medulla, the ependymal part of the midbrain was lost (Ariens, p. 1192-3), and the formation of cuneiform, intercollicular, red nuclei, a substantia nigra, and a locus coeruleus formed in the midbrain (Butler, 1996, p. 207-17)
In early gnathostomes, the cerebrum underwent a variety of changes. There was greater development in the non-olfactory regions of the cerebrum (Romer p. 588) and olfactory fibers no longer projected to all pallial areas (Butler, 1994b). Other changes included the increase in interconnections between regions of telencephalon, the formation of a tractus olfacto-habenularis anterior and posterior, and increased connections between the dorsal thalamus and striatum (Ariens, p.1266). The area periventricularis ventrolateralis and the nucleus superficialis basalis in primitive gnathostomes may be equivalent to the dorsal and ventral striatal regions in mammals (Butler, 1996, p. 272). Gnathostomes formed medial and lateral septal nuclei (Butler, 1996, p. 440), the dorsal and medial pallium received visual input (Butler, 1996, p. 370), the striatum and pallidum formed distinct regions of basal ganglia (Medina, 1995), pyramidal cells were produced (in hippocampus at first) (Hassler, p. 117), and the bed nucleus of anterior hippocampal commissure developed (Hassler, p. 121).
The eyes of gnathostomes were much more developed than those of primitive
vertebrates. Gnathostome modifications included smooth muscle in the iris,
an attached lens (Romer, p. 508-9), and primitive eyelids (Romer, p. 516).
Gnathostomes developed a saccule and utricle in the inner ear (Romer,