The Evolution of Fish

The Evolution of Fish — page 3 — notochord was an internal skeleton This unassuming little fish lived 518 million years ago MYA in the ocean waters covering what is now British Columbia Canada It is the oldest fish now known Metaspriggina or something similar was the ancestor of all of today's vertebrates not just fish but also tetrapods COMPREHENDING GEOLOGIC TIME A typical human family might produce four generations in a century But with a lifespan of a single century it can be difficult for a human to truly comprehend the stretch of time since fish first appeared on Earth Fossils indicate Metaspriggina appeared 518 MYA newspaper headlines often rounded this number to 500 MYA But the difference in the two dates is 18 million years not 18 years During that amount of time a human family would produce 720,000 additional generations More than 100 fossils of Metaspriggina were collected in 2014 from the Burgess Shale rock formation in British Columbia These represent a breakthrough in scientists understanding of fish origins The fossils show several traits found in humans vertebrate ancestors a notochord paired eyes and gillarches which are stiff structures that support the gills These fish were also involved in the transition from filter feeding to developing jaws for feeding The fossils show impressions of the fish's internal organs including the heart liver intestine and blood vessels Metaspriggina's notochord and dorsal

The Evolution of Fish — page 4 — nerve cord establish it as the earliest known member of the phylum Chordata called a chordate The dorsal nerve cord is the forerunner of the spinal cord It runs the length of the body along the top The first pair of Metaspriggina's gill arches is highly developed hinting at the later evolution of the jaw This fish lacks bony vertebrae the series of small bones making up the vertebral column or spine Thus it is not a vertebrate itself but it is definitely an ancestor to vertebrates According to paleontologist Simon Conway Morris of Cambridge University in many ways this is the most exciting find because it fills a really important gap in our knowledge of early chordate evolution 3 Morris and paleontologist Jean Bernard Caron of the Royal Ontario Museum collaborated on the 2014 study of this ancestral fish's fossils FISHY TERMINOLOGY The word fish is a common term not a scientific one Like worms fish refers to a variety of different animal types For example not all fish are bony vertebrates Jawless fish and sharks have skeletons made of cartilage not bone Scientists use the word fish to describe one or more members of a single fish species Twenty bluegills form a group of fish But scientists would describe the various species of minnows bluegills basses and catfish living in a pond as fishes

The Evolution of Fish — page 5 — Fossils allow paleontologists to learn about species of fish and other animals that Lived millions of years ago

The Evolution of Fish — page 6 — 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

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