Extinct fish {jawed fish}| (Gnasthostomes) lived in ocean and had jaws.
evolution
Gnasthostomes were cartilaginous-fish and bony-fish ancestors.
bone
Tissue calcification makes tissue firmer. Bone allows strong muscle attachments. Bone allows more shapes, because parts can be soft, medium, or hard. Retinoic acid became homeotic-gene regulator, allowing vertebrates to have head bone formation to create cranium to encase and protect brain and allow more muscle-attachment sites for head movement. Neural crest allows new skull bones, jaws, teeth, peripheral nerves, and dentine plates, under homeotic-gene control.
bone: jaw
Head bones evolved to make muscled and bony jaws, which opened larger and allowed grasping, for greater food intake. Bony jaws were possible because vertebrates had evolved heads separate from bodies and had evolved homeotic genes and gene regulators.
blood
By gene duplication, hemoglobin had four protein sequences.
senses
Jawed fish had eye muscles and eye lenses and so better vision. Vestibular system had three semicircular canals.
nervous system
Jawed fish had thalamuses. Cerebellum was larger. Early jawed fish evolved oligodendroglia, which make myelin, which allows faster saltatory conduction and requires less energy to restore ion balance.
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Date Modified: 2022.0225