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CARBONIFEROUS PERIOD

359-299 million years ago

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amphibian eating fish

Early amphibians evolved a number of novel characteristics of the nervous system which are shared among modern tetrapods. These include a larger brain with additional nuclei, a better olfactory system with a separate vomeronasal system, hair cells to perceive sound, and new types of receptors for general senses.

Ancestral amphibians evolved reticular regions in the midbrain shared with amniotes and the optic tectum expanded to cover the tori semicirculares (Ariens, p. 1196). Afferent fibers lead to the optic tectum, even in blind amphibians (Ariens, p. 1196). In the diencephalon, the dorsal thalamus increased in size, increased connections were made between the dorsal thalamus and the telencephalon, olfactory pathways connected the diencephalon to the telencephalon, such as the stria terminalis (Ariens, p. 1197) the nucleus ventrolateralis was divided into a dorsal and ventral regions (Butler, 1996, p. 262), and a lateral hypothalamic nucleus evolved (Butler, 1996, p. 336).
Olfaction was improved with the development of an accessory olfactory bulb, a vomeronasal organ and vomeronasal nerves (Ariens, p. 1293-1300). Ancestral amphibians increased the non-olfactory regions of the cerebrum as well (Romer p. 588) with the additions of an amygdaloid complex, caudate nucleus, nucleus accumbens, connections of stria medullaris conserved in higher vertebrates, an amygdalo-habenularis tract, a median forebrain bundle, a fornix, the bed nucleus of the anterior commissure, the striohypothalamic tract, and the primordium pallii dorsalis (Ariens, p. 1298-1311).
Amphibians developed a single deep cerebellar nucleus (Butler, 1996, p. 194), a nucleus accumbens, an entopeduncular nucleus (Butler, 1996, p. 272), and the posterior ramus of cranial nerve VIII carried information from the cochlea (Ariens).Amphibians developed cervical and lumbar enlargements of the spinal cord (Ariens) and increased the volume of the dorsal column (Ariens, p. 188).

Amphibians increased the ability of their skin to perceive sensations with the evolution of Pacinian corpuscles (Ariens, p.186) and the first proprioceptors (Romer, p. 498). Amphibians incorporated hair cells in the perception of sound (Romer, p. 532) and an auditory ossicle involved in hearing (the stapes) (Romer, p.532). The papilla basilaris may represent a primitive homolog of the organ of Corti in amniotes (Kardong, p. 684). Vision was enhanced with the evolution of moveable eyelids, lacrimal glands, and lacrimal ducts.


Ancestral amphibians evolved parathyroid glands (Weichert, 1970, p. 246-7).