Early fish ancestors obtained oxygen through their skin They used their gill slits for filter feeding a process of straining food particles out of the water But as vertebrates became larger they needed more oxygen The large surface area of gills enabled them to increase their oxygen uptake and gills became more important for respiration than for feeding Jawless fish originated far back in the Cambrian period more than 500 MYA and dominated the oceans for over 200 million years Only two types hagfish and lampreys remain today Other early fish included the now extinct placoderms or armored fish which had both jaws and protective armor Placoderms evolved during the Silurian period about 420 MYA but were extinct by about 360 MYA possibly because their heavy armor slowed them down A major advance in fish evolution was the rise of jawed fish with paired fins beginning about 425 MYA These pairs of pectoral and pelvic fins correspond to the forelimbs and hind limbs or arms and legs of tetrapods Paired fins improved fishes speed and mobility Jaws let them bite greatly increasing their food choices In addition developing jaws led to the separation of respiration and feeding Fish no longer had to suck in water to obtain food This enabled them to become true predators Spiny sharks were one important group of jawed fish They lacked heavy head armor and thus were much faster than the earlier placoderms They appeared around 450 MYA and became extinct about 275 MYA
The Arandaspis prehistoric fish lived about 500 MYA and was a precursor to true bony fish The two most important groups of modern fish evolved from different ancestors Both first appeared in the fossil record about 210 MYA Sharks and rays have skeletons made of cartilage rather than bone Ray finned fish have bony skeletons and other unique characteristics including fins composed of webs of skin supported by bony spines A separate group the lobe finned fish was most diverse between about 419 and 298 MYA during the Devonian and Carboniferous periods A few of these species still exist today Lobe fins are especially important because they were the ancestors of all the rest of today's vertebrates the tetrapods That is the common ancestor of amphibians reptiles birds and mammals was a lobe finned fish Of the six fish groups that have appeared and spread throughout the oceans in the more than 500 million years since Metaspriggina lived two groups placoderms and spiny
sharks are now extinct Another two jawless fish and lobe finned fish have declined to only a few families Today ray finned fish and sharks and rays dominate the oceans HOW EVOLUTION WORKS Evolution can most simply be summarized as descent with modification Over many generations the descendants of a common ancestor begin to diverge from the ancestor In the short term a few generations populations might show different color patterns or responses to environmental temperatures When populations are separated for many generations they eventually change so much that they form new species Changes in the genetic composition of populations and species over time result from natural selection a process first described by Charles Darwin in 1859. Natural selection requires two conditions genetic variation and selective pressure The differences within a population caused by individuals genetic variation or differences in their genes provides the raw material on which natural selection acts Selective pressure is outside pressure from other organisms or environmental forces that affects whether an individual can survive It can be positive or negative that is it can increase or decrease that organism's chances of survival The result is differential reproduction organisms with one form of a trait reproduce more successfully and
leave more offspring than those with other forms For example suppose a fish species shows variation in fin size and that the fish with larger fins are more agile More large finned fish avoid predators and reproduce They have more offspring than fish with smaller fins and their offspring inherit the large finned trait Over time the percentage of large finned fish in the THE IMPORTANCE OF JAWS population increases In natural selection the Fish that developed jaws could catch bigger more favorable trait eventually dominates faster prey than their ancestors because their mouths were larger and opened wider Evolution by natural selection has led The face was a key to vertebrate evolution to an astonishing diversity of fish species in A fossil from China of a fish that lived 419 million years ago has the same cheek the world's oceans brackish environments and jaw structure as today's vertebrates and fresh waters More than 30,000 species This fish Entelognathus primordialis had cheeks plus jaws with a mandible and maxilla representing more than 95 percent of all shaped much like these bones in humans It existing fish species are bony fish 4 There are was unlike other members of its family the another 1,259 species of cartilaginous fish placoderms Their simple cheeks and jaws had only a few large bones Entelognathus 123 species of jawless fish and only eight like today's bony fish had smaller bones with species of lobe finned fish 5 more complex arrangements
