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Silurian and Devonian Periods.docx

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Silurian and Devonian PeriodsSilurian PeriodFrom the end of Ordovician to the beginning of Devonian 443417 million years agoMajor extinction event Arthropods on landOxygen nears present day levelsGet the body forms we see todayDevonian PeriodFrom the end of Silurian to Carboniferous 417359 million years agoOxygen nears present day levelsLand vertebratesAppearance of amphibiansForests form The FishArmoured Fishes These fish were all armoured fish at this time no fish scales just heavy weighted boney fish and they swim by wagging their tails back and fourth so they squirm has a wide open mouth and it keeps it open to catch food in the water down the pharynges are openings and the water goes out through them and traps the food This becomes the gills and the first fish are jawlessJawless Fish Agnatha Primitive jawless vertebrates that include the lampreys hagfishes and many fossil fishlike forms Sticks itself to fish and sucks a hole through them to eat the blood and goop form the wound The beginning of parasitesChondrithyesa class of fish that includes those with a cartilaginous skeletonCartilaginous Fish A fish of a class distinguished by having a skeleton of cartilage rather than bone including sharks rays and chimera Bony Fish Their fins are help up by little rays of bones this is their support mechanism these have a few adaptions that improve from being a shark and they have paired fins that allowJaw The gnathostomes are the ones with jaws the whole genome of vertebrates double each of the upper and lower bony structures in vertebrates forming the framework of the mouth and containing the teeth it evolved so fishes didnt have to swim with their mouth open evolved from a pair of gills arches in the pharynx of a jawless ancestor one pair formed bones in the upper and lower jaws whereas another pair was transformed into the hypomandibular bones that brace the jaws against the cranium nerves and muscles of the ancestral suspensionfeeding pharynx control the movement and action of the jaws
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