Fish Evolution and Characteristics

Angelica Peskov

Period 3

3-24-15

Fish evolution and characteristics

The first fish forms evolved about 550 million years ago. They came about in the Devonian period. These animals were extremely different from the fish we’re familiar with today. They belonged to the class Agnatha meaning “without a jaw.” There are species of Agnathans that belong in this group to this day. The word Agnathan combines all jawless fish still living and extinct.

Millions of years back, Agnathans very much like jawless fish today, they still had recognizable heads, 2 sets of eyes, and a notochord consistent throughout their lives. Between 245 and 570 million years Agnathans began developing unusual various bony plates aka armour plates to protect themselves. Class Conodonta of the phylum Ostercoderm are one of the first Agnathans to appear. These small fish were typically 7 cm to length; they had teeth, a tongue, and a caudal fin; however they lacked an external skeleton. Conodonts became extinct by 205 million years ago. Two other groups of jawless fish appeared shortly after, the Arandospida and the Astraspida. Both of these groups had large head shields with no fins except the caudal fin. They disappeared about 440 million years ago. Heterostraci from the class Pteraspidomorpha and phylum Ostracoderm followed. They had 2 good eyes, 2 good nostrils, a rostrum pointing over the mouth, and spines scattered over their body. These Agnathans were later overruled by the Cephalaspidomorpha, but both were extinct by 360 million years ago. Next the Anaspida (no scales) and Galeaspida were the first Agnathan fish to develop the lateral line on the side of their body rather than on the ventral side, then closely followed by the Osteostraci. This class of fish had a bony shield covering their head, and small plate-like scales. Anaspids were known to be active fish where Heterstraci and Galeaspida were known as slow moving, mostly at the bottom of bodies of water. They fed on rotting plants and ate materials from the mud. These Agnathans had lobe tailed fins which are closely related to those of modern fish. Around 425 million years ago the Condontia aka Gnathosomes began taking over and replacing Agnathans. These fish changed the evolution of fish overall. From gill arches they developed moveable jaws which allowed a much larger selection of food. At around the same time they developed paired fins which increased their mobility in the water. Next Placoderms meaning “plate skin” evolved 435 million years ago. They were the earliest of all jawed fish.  A swim bladder developed to help with buoyancy and they also had bony-like scales. Placoderms were known as bottom dwellers and grew between 15-40 inches in length. Long pectoral fins and weak jaws were common features. Shark and Ray-like fish were common Placoderms. They however became extinct by 360 million years ago. As the years progress to more modern time, the classes Chondrichthyes (shark), Osteichthyes (trout), and Cyclostomata (lamprey) evolved 20 million years ago. They still roam the earth’s waters today.

Today fish have fins that are used for balance and moving through the water. Most fish have 2 kinds of fins.  Paired fins on the ventral side of the fish, and single fins which can be found along the ventral and dorsal sides of the fish. The caudal fin aka the tail fin is mainly used to propel the fish forward in the water. The dorsal and anal fins aid in the fish’s balance that keep it from turning over. The paired fins help with steering and floating in the water.

 The external parts of the fish are covered with scales that overlap in a sequence type of way. These scales protect the fish from any injuries that can become infected. Some scales have sharp jagged edges, while others have smooth and rounded scales. Furthermore fish secrete a mucus substance over their scales to protect them from bacteria entering their body .This mucus covering reduces friction allowing the fish to move swiftly through the water.