Fish were amongst the first known chordates about 500 million years ago. Therefore, they have a very elaborate and complicated evolutionary history. The first type of fish to appear during the Ordovician era were called Ostracoderms. They had their head and flanks covered with a bony armor while they had a cartilaginous back. The shield was used to keep off predators. The extinction of the Ostracoderms during the Devonian period saw the emergence of Placoderms. These types of fish were also armored, but their functional jaws distinguished them from their predecessors. They also have gills derived from their jaws that took over the specialized role of gaseous exchange. Evolution of fish occurred very fast during the Devonian era. The process was necessitated by the high competition for food, dynamic climatic and geological conditions.
2. What major phylogenetic group of fish does the shark belong to? When did this group appear in evolutionary history?
Sharks belong to the Elasmobranchii phylogenetic group of fish. It is a s...
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