Evolution of Skeletal System. Disclosure 19
The evolution of the nervous system, as treated in the previous webblog, is parallel to the apparition and evolution of the skeletal system. The utility of the ganglion pairs of the brainstem related with the sight, smell and ear is linked with the apparition of the cranial rigid support with holes from where the nervous system receives outer information and the appearance of the backbone to support the extremities.
The marine fossils of Pikaia gracilens (570 million years), Haikouichthys ercaicunensis (530 million years) and Myllokunmingia fengjiaoa (525 million years); such as marine fish fossils Pteraspidomorphi (443 million years) are the most ancient ancestors of the current vertebrates and therefore the origin of the current bones system.
Pikaia gracilens was classified initially as a polichaete Word because its regular segmentation of the body, but in 1979 was included in the chordate group, becoming an most ancient ancestry of the current vertebrates, because of its primitive backbone with an inside spinal medulla.
Haikouichthys ercaicunensis had differenced head and trunk. The head had between six and nine regular segments while the stem was composed by segments in "V" shape.
Myllokunmingia fengjiao had cranial segments differenced from the 25 segments in "V" shape in the stem. All these segments were gristly without evidences of bio mineralization in the elements of the skeleton.
Between those marine fossils previous and the apparition of the fish fossils without mandibles (pteraspidomorphs) run about 90 million years, and with them the truth first bone shield. Although we are talking about extinguished species, we can consider as their live represent the agnat fishes with interesting characteristics as the presence of the notocorda in larval and adult ages and the presence of a pineal eye sensitive to the light (counterpart to the pituitary or second ganglion pair of the current brainstem)
Finally, the current skeleton system is formed by few differenced cells, so primitive, called osteoblast with ovoid shape and with plenty of branched extensions that are to connect with each other.
This type of cells and this skeleton system have vital importance for the later development of the metabolism of calcium and the development of nervous system and locomotor system.