Fossils allow paleontologists to learn about species of fish and other animals that Lived millions of years ago
THE RISE OF FISH The fish fossil record is sparse particularly for species without bones Their fossils like those of Metaspriggina consist of impressions left in mud or sand Over time the fish's organic matter was replaced by minerals from its surroundings Some impressions are well preserved with easily visible structures But even bony fossils are hard to come by Fish fossils date back as far as 518 million years Many are buried forever on the ocean bottom Accessible ones are on freshwater lake bottoms and in rock formations from which oceans have receded or which have been pushed up as mountains formed Fossilization is rare finding fossils is even rarer But paleontologists are persistent and continue to uncover new fossils PRIMITIVE VS. ADVANCED Cladograms are branching diagrams that show the evolutionary relationships among groups of organisms They show ancestors on the left and the various groups descending from those ancestors on the right Some people assume the leftmost groups of a cladogram are simple and primitive while the rightmost are specialized and advanced They see evolution as a steady march of improvement toward an advanced form This is not true First there are no scientific definitions for primitive or advanced These are not objective measurable terms Second every group on the tree regardless of location is specialized to fit its own environment Hagfish are on the far left of most fish phylogenies because they are oldest not because they are simple or primitive Nodes on a cladogram are points where two or more groups of organisms diverge from each other Nodes represent common ancestors of the organisms in the branching groups The groups connected by any node can be flipped from left to right and remain correct The tree only shows relationships It does not define one group as more primitive or advanced than others
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
