Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland.In contrast to excretion, the substance may have a certain function, rather than being a waste product.ecretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland.
The oldest shark fossils. The oldest fossilized evidence of prehistoric sharks comes from shark-like scales that date to 455 million years ago during the Ordovician Period, in Colorado. Some paleontologists do not agree that these scales are sufficiently shark-like to pronounce them sharks.
But there is no disagreement that scales found during the Silurian Period, aged 420 million years, are from sharks. Shark scales from this period have been found in Siberia and Mongolia. The oldest shark teeth are from the Devonian Period, about 400 million years old, found in Europe. Nothing beyond these scales or the teeth is known about these early sharks.