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Ancestral deuterostome Notochord Brain Head Vertebral column Jaws, mineralized skeleton Lobed fins Lungs or lung derivatives Legs Mammalia (mammals) Reptilia (turtles, snakes, crocodiles, birds) Amphibia (frogs, salamanders) Dipnoi (lungfishes) Actinistia (coelacanths) Actinopterygii (ray-finned fishes) Chondrichthyes (sharks, rays, chimaeras) Cephalaspidomorphi (lampreys) Myxini (hagfishes) Cephalochordata (lancelets) Urochordata (tunicates) Echinodermata (sister group to chordates) • A hypothetical phylogeny of chordates Chordates Craniates Vertebrates Gnathostomes Osteichthyans Lobe-fins Tetrapods Amniotes Milk Amniotic egg Derived Characters of Chordates • All chordates share a set of derived characters – Although some species possess some of these traits only during embryonic Brain Notochord Muscle development segments Dorsal, hollow nerve cord Mouth Anus Muscular, post-anal tail Figure 34.3 Pharyngeal slits or clefts • The notochordNotochord – Is a longitudinal, flexible rod located between the digestive tube and the nerve cord – Provides skeletal support throughout most of the length of a chordate • In most vertebrates, a more complex, jointed skeleton develops – 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 dorsal to the notochord – Develops into the central nervous system: the brain and the spinal cord Pharyngeal Slits or Clefts • In most chordates, grooves in the pharynx called pharyngeal clefts – Develop into slits that open to the outside of the body • These pharyngeal slits – Function as suspension-feeding structures in many invertebrate chordates – Are modified for gas exchange in aquatic vertebrates – Develop into parts of the ear, head, and neck in terrestrial vertebrates Muscular, Post-Anal Tail to • Chordates have a tail extending posterior the anus – Although in many species it is lost during embryonic development • The chordate tail contains skeletal elements and muscles – And it provides much of the propelling force in many aquatic species Tunicates • Tunicates, subphylum Urochordata – Belong to the deepest-branching lineage of chordates – Are marine suspension feeders commonly called sea squirts • Tunicates most resemble chordates during their larval stage Notochord – Which may be as brief as a few minutes Dorsal, hollow nerve cord Tail Excurrent siphon Incurrent siphon Muscle segments Intestine Stomach Atrium Pharynx with slits Figure 34.4c (c) A tunicate larva is a free-swimming but nonfeeding “tadpole” in which all four chief characters of chordates are evident. • As an adult – A tunicate draws in water through an incurrent siphon, filtering food particles Incurrent siphon to mouth Excurrent siphon Excurrent siphon Atrium Pharynx with numerous slits Tunic Anus Intestine Esophagus Stomach Figure 34.4a, b (a) An adult tunicate, or sea squirt, is a sessile animal (photo is approximately life-sized). (b) In the adult, prominent pharyngeal slits function in suspension feeding, but other chordate characters are not obvious. Lancelets • Lancelets, subphylum Cephalochordata – Are named forTentacle their bladelike shape Mouth Pharyngeal slits Atrium Notochord Digestive tract Atriopore Dorsal, hollow nerve cord Segmental muscles Anus Tail Figure 34.5 2 cm • Lancelets are marine suspension feeders – That retain the characteristics of the chordate body plan as adults Early Chordate Evolution • The current life history of tunicates – Probably does not reflect that of the ancestral chordate BF1 in lancelets • Gene expression Otx Hox3 – Holds clues to the evolution of the vertebrate form Nerve cord of lancelet embryo BF1 Hox3 Otx Brain of vertebrate embryo (shown straightened) Midbrain Figure 34.6 Forebrain Hindbrain • Concept 34.2: Craniates are chordates that have a head • The origin of a head – Opened up a completely new way of feeding for chordates: active predation • Craniates share some common characteristics – A skull, brain, eyes, and other sensory organs Derived Characters of Craniates • One feature unique to craniates – Is the neural crest, a collection of cells that Neural Neural Dorsal edges appears near the dorsal margins tube of the crest of neural plate Ectoderm closing neural tube in an embryo Ectoderm Notochord (a) The neural crest consists of bilateral bands of cells near the margins of the embryonic Figure 34.7a, b folds that form the neural tube. Migrating neural crest cells (b) Neural crest cells migrate to distant sites in the embryo. • Neural crest cells – Give rise to a variety of structures, including some of the bones and cartilage of the skull (c) The cells give rise to some of the anatomical structures unique to vertebrates, including some of the bones and cartilage of the skull. Figure 34.7c The Origin of Craniates • Craniates evolved at least 530 million years ago – During the Cambrian explosion • The most primitive of the fossils – Are those of the 3-cm-long Haikouella (a) Haikouella. Discovered in 1999 in southern China, Haikouella had eyes and a brain but lacked a skull, a derived trait of craniates. Figure 34.8a • In other Cambrian rocks – Paleontologists have found fossils of even 5 mm more advanced chordates, such as Haikouichthys (b) Haikouichthys. Haikouichthys had a skull and thus is considered a true craniate. Figure 34.8b Hagfishes • The least derived craniate lineage that still survives Slime glands – Is class Myxini, the hagfishes Figure 34.9 • Hagfishes are jawless marine craniates – That have a cartilaginous skull and axial rod of cartilage derived from the notochord – That lack vertebrae • Concept 34.3: Vertebrates are craniates that have a backbone • During the Cambrian period – A lineage of craniates evolved into vertebrates Derived Characters of Vertebrates • Vertebrates have – Vertebrae enclosing a spinal cord – An elaborate skull – Fin rays, in aquatic forms Lampreys • Lampreys, class Cephalaspidomorphi – Represent the oldest living lineage of vertebrates – Have cartilaginous segments surrounding the notochord and arching partly over the nerve cord • Lampreys are jawless vertebrates – Inhabiting various marine and freshwater habitats Figure 34.10 Fossils of Early Vertebrates • Conodonts were the first vertebrates – With mineralized skeletal elements in their mouth and pharynx Dorsal view of head Figure 34.11 Dental elements • Armored, jawless vertebrates called ostracoderms – Had defensive plates of bone on their skin Pteraspis Pharyngolepis Figure 34.12 Origins of Bone and Teeth • Mineralization – Appears to have originated with vertebrate mouthparts • The vertebrate endoskeleton – Became fully mineralized much later • Concept 34.4: Gnathostomes are vertebrates that have jaws • Today, jawless vertebrates – Are far outnumbered by those with jaws Derived Characters of Gnathostomes • Gnathostomes haveGill jaws slits Cranium – That evolved from skeletal supports of the pharyngeal slits Mouth Skeletal rods Figure 34.13 • Other characters common to gnathostomes include – Enhanced sensory systems, including the lateral line system – An extensively mineralized endoskeleton – Paired appendages Fossil Gnathostomes • The earliest gnathostomes in the fossil record – Are an extinct lineage of armored vertebrates called placoderms (a) Coccosteus, a placoderm Figure 34.14a • Another group of jawed vertebrates called acanthodians – Radiated during the Devonian period – Were closely related to the ancestors of osteichthyans (b) Climatius, an acanthodian Figure 34.14b • Chondrichthyans (Sharks, Rays, Membersand of class Chondrichthyes Their Relatives) – Have a skeleton that is composed primarily of cartilage • The cartilaginous skeleton – Evolved secondarily from an ancestral mineralized skeleton • The largest and most diverse subclass of Chondrichthyes – Includes the sharks and rays (a) Blacktip reef shark (Carcharhinus melanopterus). Fast swimmers with acute senses, sharks have paired pectoral and pelvic fins. (b) Southern stingray (Dasyatis americana). Most rays are flattened bottom-dwellers that crush molluscs and crustaceans for food. Some rays cruise in open water and scoop food into Figure 34.15a, b their gaping mouth. Pectoral fins Pelvic fins • A second subclass – Is composed of a few dozen species of ratfishes (c) Spotted ratfish (Hydrolagus colliei). Ratfishes, or chimaeras, typically live at depths greater than 80 m and feed on shrimps, molluscs, and sea urchins. Some species have a poisonous spine at the front of their dorsal fin. Figure 34.15c • Most sharks – Have a streamlined body and are swift swimmers – Have acute senses Ray-Finned Fishes and LobeFins • The vast majority of vertebrates – Belong to a clade of gnathostomes called Osteichthyes • Nearly all living osteichthyans – Have a bony endoskeleton • Aquatic osteichthyans – Are the vertebrates we informally call fishes – Control their buoyancy with an air sac known as a swim bladder • Fishes breathe by drawing water over four or five pairs of gills Caudal – Located in chambers covered by a protective fin Swim bladder Spinal cord bony flap called the operculum Brain Dorsal fin Adipose fin (characteristic of trout) Nostril Cut edge of operculum Gills Heart Figure 34.16 Gonad Urinary Anus bladder Liver Kidney Intestine Stomach Pelvic fin Anal fin Lateral line Ray-Finned Fishes • Class Actinopterygii, the ray-finned fishes – Includes nearly all the familiar aquatic osteichthyans (a) Yellowfin tuna (Thunnus albacares), a fast-swimming, schooling fish that is an important commercial fish worldwide (b) Clownfish (Amphiprion ocellaris), a mutualistic symbiont of sea anemones Figure 34.17a–d (c) Sea horse (Hippocampus ramulosus), unusual in the animal kingdom in that the male carries the young during their embryonic development (d) Fine-spotted moray eel (Gymnothorax dovii), a predator that ambushes prey from crevices in its coral reef habitat • The fins, supported mainly by long, flexible rays – Are modified for maneuvering, defense, and other functions Lobe-Fins • The lobe-fins, class Sarcopterygii – Have muscular and pectoral fins – Include coelacanths, lungfishes, and tetrapods Figure 34.18 • Concept 34.5: Tetrapods are gnathostomes that have limbs and feet • One of the most significant events in vertebrate history – Was when the fins of some lobe-fins evolved into the limbs and feet of tetrapods