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The Devonian was the age of fishes. Sharks, skates and rays evolved from jawless fish about 390 mya. Although their ancestors had bone, the shark group at some stage lost the ability to make bone, and their skeletons are made of cartilage. Cartilage is what we call “gristle” and in humans it makes the ears, nose and much of the skeletons of babies.
Sharks have no swim bladder and must swim perpetually to keep from sinking to the bottom.
Image of shark courtesy of Suneko
These are the animals we normally think of as . They evolved about 390 mya. Unlike the , their fins are adapted for swimming, not crawling.
Image of stout beardfish, Polymixia nobilis from Oceanic Ichthyology, Brown Goode and Bean, 1896
Lobe finned fish evolved from about 390 mya. They lived in fresh waters subject to seasonal droughts. Their swim bladder evolved into a sac able to breathe air, similar to the lungfish of today. They had fleshy bases to their fins which were strong and flexible, letting them leave drying pools to find those ponds with water. They evolved into the . Most are now extinct, except for coelacanths and the lungfishes.
Image of Coelacanth in public domain
Lobe fins were the ancestors of the and all higher types of , including man.
Contrast this with the other main group of fish, the .
At the end of the Devonian period one of the major mass extinction events occurred. Nobody knows why. Many families of living thing disappeared probably in events spread over millions of years.
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