CHAPTER ONE From Fish to Tetrapods Metaspriggina walcotti was an unimpressive looking fish except for its large bulging eyes It had no obvious fins and looked more wormlike than fishlike It measured only 2.4 inches 6 cm long so its life in the ancient oceans was hazardous 1 It shared the ocean with gigantic predators such as Anomalocaris a 6 foot 1.8 m insect like creature with sharp teeth The Burgess Shale in British Columbia contains the best record of animal fossils from the Cambrian period 2 But Metaspriggina was a fast swimmer and its seven pairs of gills provided plenty of oxygen to keep its energy high It also boasted an exciting new evolutionary development It had a notochorda flexible rod extending the length of its body that provided support and a place for its strong bands of muscle to attach That is the
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
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
