aCHAPTER 34 THE ORIGIN AND EVOLUTION OF VERTEBRATES HALF A BILLION YEARS OF BACKBONES Early in the Cambrian period some 530 million years ago an immense variety of invertebrate animals inhabited Earth s oceans Predators used sharp claws and mandibles to break apart prey and many had spikes or armor as well as modified mouthparts that enabled bearers to filter food Vertebrates were initially restricted to oceans but the evolution of limbs in one lineage allowed them to colonize land The vertebrates gave rise to 3 groups amphibians reptiles including birds and mammals 34 1 CHORDATES HAVE A NOTOCHORD AND A DORSAL HOLLOW NERVE CORD Vertebrates are members of the phylum Chordata the Chordates They are bilateralian and belong to the clade Deuterostomia DERIVED CHARCTERS OF CHORDATES All chordates share a set of derived characters though many species possess some of these traits only during embryonic The four key characteristics of chordates are notochord a dorsal hollow nerve cord pharyngeal slits or clefts and a muscular post anal tail Notochord Chordates are named due to a notochord being present in all chordate embryos as well as in some adult chordates The NOTOCHORD is a longitudinal flexible rod located between the digestive tube and the nerve cord that is composed of large fluid filled cells encased in fairly stiff fibrous tissue It provides skeletal support throughout most of the length of a chordate and provides a firm but flexible structure against which muscles can work for larvae and adults that retain it In most vertebrates a more complex jointed skeleton develops around the ancestral notochord and the adult retains only remnants of the embryonic notochord Dorsal Hollow Nerve Cord The nerve cord of a chordate embryo develops from a plate of ectoderm that rolls into a tube located dorsal to the notochord The resulting dorsal hollow nerve cord is unique to chordates The nerve cord of a chordate embryo develops into the central nervous system aka the brain and spinal cord Pharyngeal Slits or Clefts The digestive tube of chordates extents from the mouth to the anus The region just posterior to the mouth is the pharynx All chordate embryos have grooves called pharyngeal cleft that develop into slits that open into the pharynx PHARYNGEAL SLITS allow for water entering the mouth to exit the body without passing through the entire digestive tract In some invertebrate chordates these slits aid in filter feeding In vertebrates with the exception of tetrapods the slits have been modified for gas exchange In tetrapods the pharyngeal clefts do not develop into slits but instead develop into parts of the ear and other structures in the head and neck Muscular Post Anal Tail Have a tail that extends posterior to the anus although it is reduced in many species during embryonic development The chordate tail contains skeletal elements and muscles which help propel many aquatic species in the water LANCELETS Most basal group of living chordates are the lancelets Get their name form bladelike shape They are marine suspension feeders that use Cilia to draw seawater into the mouth TUNICATES Molecular studies indicate that the tunicates are more closely related to other chordates than they are to lancelets The chordate characteristics are most apparent during the larval stage The larva uses the notochord and muscles to swim in search for a suitable substrate Once it settles it undergoes a radical metamorphosis in which many of the chordate characters disappear tail and notochord are resorbed the nervous system degenerates and the remaining organs rotate 90 degrees The adults are suspension feeders THE LOSS OF CHORDATE CHARCTERS APPEARS TO HAVE OCCURRED AFTER THE TUNICATE LINEAGE BRANCHED OFF FROM OTHER CHORDATES TUNICATES HAVE 9 HOX GENES WHILE OTHER CHORDATES HAVE 13 HOX GENES EARLY CHORDATE EVOLUTION The ancestral chordate most likely looked something like a LANCELET Research about the lancelets revealed important information about the chordate brain Lancelts have a slightly swollen tip on the anterior end of the dorsal nerve cord However the same Hox genes that organize major regions of the forebrain midbrain and hindbrain are expressed in a corresponding pattern in this small cluster of cells in the lancelet s nerve cord Tunicates Ancestral chordates had genes associated with vertebrate organs such as the heart and thyroid gland These genes are found in tunicates and vertebrates but are absent from non chordate invertebrates In contrast tunicates lack many genes that in are associated with long range transmission of nerve impulses This suggests that such genes arose in early vertebrate and are unique to the vertebrate evolutionary lineage 34 2 VERTEBRATES ARE CHORDATES THAT HAVE A BACKBONE During the Cambrian period half a billion years ago a lineage of Chordates gave rise to vertebrates With a skeletal system and a more complex nervous system than that of their ancestors vertebrates become more efficient at 2 essential tasks capturing food and avoiding being eaten DERIVED CHARCTERS OF VERTEBRATES Possess 2 or more sets of Hox Genes o Lancelets and tunicates only have one Some genes produce transcription factors and signaling molecules are also duplicated in vertebrates Increased genetic complexity associated with innovations in the vertebrate nervous system and skeleton In majority of vertebrates the vertebrae enclose the spinal cord and have taken over the mechanical roles of notochord Faster swimming due to more efficient gas exchange system in gills HAGFISHES AND LAMPREYS Only lineages of living vertebrates that lack jaws Do not have a backbone Lampreys have a rudimentary backbone made out of cartilage while hagfishes lack one altogether However recent studies have shown that HAGFISHES HAVE RUDIMENTARY VERTEBRAE The Hagfishes and Lampreys are sister groups and form a clade of living jawless vertebrates called the CYCLOSTOMES Hagfishes Jawless vertebrates that have highly reduced vertebrate and a skull made out of cartilage Swim like snakes by suing their segmental muscles to exert force against their notochord which they retain in adulthood as a strong flexible rod of cartilage Secretes slime to protect it self from predators often resulting in suffocation Lampreys Most are parasitic Use rasping mouth and tongue to penetrate the skin of the fish and ingest the fish s blood and other tissues As larvae they live in freshwater streams as suspension feeders Move into larger bodies of water