Chapter 34 The Origin and Evolution of Vertebrates 1 Chapter 34 The Origin and Evolution of Vertebrates Introduction Half a billion years of Backbones Vertebrates derive their name from vertebrae the series of bones that make up the vertebral column or backbone The evolution of limbs in one lineage of vertebrates set the stage for these vertebrates to colonize land There they diversified into amphibians reptiles including birds and mammals Concept 34 1 Chordates have a notochord and a dorsal hollow nerve cord Vertebrates are members of the phylum Chordata the chordates Chordates are bilaterian bilaterally symmetrical animals and within Bilateria they belong to the clade of animals known at Deuterostomia The cephalochordats and urochordates are two groups of invertebrate Deuterostomes that are more closely related to vertebrates than to other invertebrates Derived Characters of Chordates All chordates share a set of derived characters though many possess some of these traits only during embryonic development Notochord Chordates are named for a skeletal structure the notochord present in all chordate embryos as well as in some adult chordates In most vertebrates a more complex jointed skeleton develops around the ancestral notochord and the adult retains only remnants of the embryonic notochord The notochord is a longitudinal flexible rod located between the digestive tube and the nerve cord It is composed of large fluid filed cells encased in fairly stiff fibrous tissue In humans the notochord is reduced and forms part of the gelatinous disks sandwiched between the vertebrate Dorsal Hollow Nerve Cord Pharyngeal Slits or Clefts The nerve cord of a chordate embryo develops from a plate of ectoderm that rolls into a tube located dorsal to the notochord The nerve cord of a chordate embryo develops into the central nervous system the brain and spinal cord The digestive tube of chordates extends from the mouth to the anus The region posterior to the mouth is the pharynx In all chordate embryos a series of pouches separated by grooves forms along the sides of the pharynx pharyngeal clefts then develop into slits that open to the outside of the body Pharyngeal slits allow water entering the mouth to exit the body without passing through the entire digestive tract In vertebrates these slits and the structures that support them have been modified for gas exchange and are known as gill slits In tetrapods the pharyngeal clefts do not develop into slits Instead they develop parts of the ear and other structures in the head and neck Muscular Post Anal Tail Chordates have a tail that extends posterior to the anus although in many species it is greatly reduced during embryonic development The chordate tail contains skeletal elements and muscles and it helps propel many aquatic species in the water The most basal earliest diverging group of living chordates are animals called lancelets cephalochordata which get their name from their bladelike shape As larvae the lancelets develop a notochord a dorsal hollow nerve cord numerous pharyngeal slits and a post anal tail Following metamorphosis an adult lancelet swims down to the seafloor and wriggles backward into the sand Cilia draw seawater into the lancelet s mouth The pharynx and pharyngeal slits play a minor role in gas exchange which occurs mainly across the external body surface The muscle segments develop from blocks of mesoderm called somites which are found along each side of the notochord in all chordate embryos Recent molecular studies suggest that the tunicates urorchordata are more closely related to other chordates than are lancelets Once a tunicate has settled on a substrate it undergoes a radical metamorphosis in which many of its chordate characters disappear Its tail and notochord are resorbed its nervous system degenerates and its remaining organs rotate 90 Tunicates draw in water through an incurrent siphon the water then passes through the pharyngeal slits into a chamber called the atrium and exits through an excurrent siphon Some tunicate species shoot a jet of water through their excurrent siphon when attacked earning the name Sea squirts The loss of chordate characters in the adult stage of tunicates appears to have occurred after the tunicate lineage branched off from other chordates Lancelets Tunicates Chapter 34 The Origin and Evolution of Vertebrates 2 Early Chordate Evolution Ancestral chordate may have looked something like a lancelet that is it had an anterior end with a mouth a notochord a dorsal hollow nerved cord pharyngeal slits and a post anal tail Rather than a full fledged brain lancelets have only a slightly swollen tip on the anterior end of their dorsal nerve cord Tunicates lack many genes that in vertebrates are associated with the long range transmission of nerve impulses This suggests that such genes arose in an early vertebrate and are unique to the vertebrate evolutionary lineage Concept 34 1 Summary Chordates have a notochord and a dorsal hollow nerve cord Chordates notochord dorsal hollow nerve cord pharyngeal slits post anal tail Clade Cephalochordata Lancelets Clade Urochordata Tunicates Concept 34 2 Craniates are chordates that have a head Chordates with a head are known as craniates Description Basal chordates marine suspension feeders that exhibit four key derived characters of chordates Description Marine suspension feeders larvae display the derived traits of chordates The origin of a head consisting of a brain at the anterior end of the dorsal nerve cord eyes and other sensory organs and a skull enabled chordates to coordinate more complex movement and feeding behaviors Derived Characters of Craniates One feature unique to craniates is the neural crest a collection of cells that appears near the dorsal margin of the closing neural tube in an embryo Neural crest cells disperse throughout the body where they give rise to a variety of structures including teeth some of the bones and cartilage of the skull the inner layer of skin dermis of the facial region several types of neurons and the sensory capsules in which eyes and other sense organs develop In aquatic craniates the pharyngeal clefts evolved into gill slits that are associated with muscles and nerves that allow water to be pumped through the slits Craniates have a higher metabolic rate and a much more extensive muscular system Muscles lining their digestive tract aid digestion by moving food through the tract Craniates also have a heart