4-27-06Chapter 34Pages 671-700 Vertebrates
Overview: Half a Billion Years of Backbones By the end of the Cambrian period, some 540 million years ago an astonishing variety of animals inhabited Earth’s oceans One of these types of animals gave rise to vertebrates, a successful group that all of you are aware of.
Figure 34.1 The animals called vertebrates • Get their name from vertebrae, the series of bones that make up the backbone • There are approximately 52,000 species of vertebrates • Which include the largest organisms ever to live on the Earth
Vertebrates are a subphylum of the phylum Chordata Chordates have a notochord and a dorsal, hollow nerve cord Chordates are bilaterian animals that belong to the clade of animals known as Deuterostomia Two groups of invertebrate deuterostomes, the urochordates and cephalochordates are more closely related to vertebrates than to invertebrates Chordate Characteristics
Chordates Craniates Vertebrates Gnathostomes Osteichthyans Lobe-fins Tetrapods Amniotes Echinodermata(sister group to chordates) Chondrichthyes(sharks, rays, chimaeras) Cephalaspidomorphi(lampreys) Amphibia(frogs, salamanders) Cephalochordata(lancelets) Reptilia(turtles, snakes,crocodiles, birds) Actinopterygii(ray-finned fishes) Urochordata(tunicates) Actinistia(coelacanths) Dipnoi(lungfishes) Myxini(hagfishes) Mammalia(mammals) Milk Amniotic egg Legs Lobed fins Lungs or lung derivatives Jaws, mineralized skeleton Vertebral column Head Brain Notochord Ancestral deuterostome Figure 34.2 • A hypothetical phylogeny of chordates
Dorsal,hollownerve cord Brain Notochord Musclesegments Mouth Anus Pharyngealslits or clefts Muscular,post-anal tail Figure 34.3 Derived Characters of Chordates All chordates share a set of derived characters although some species possess some of these traits only during embryonic development
Notochord • The notochord • Is a longitudinal, flexible rod located between the digestive tube and the nerve cord • Provides skeletal support throughout most of the length of primitive chordates (lancelet and hagfish) • In most vertebrates, a more complex, jointed skeleton develops • And the adult retains only remnants of the embryonic notochord which are the fluid filled cartilaginous discs between the centra of the vertebrae
Dorsal, Hollow Nerve Cord • The nerve cord of a chordate embryo • Develops from a plate of ectoderm (neural plate) 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 by trapping food particles • Cartilagenous separations between gill slits (gill bars) • Are modified for gas exchange in aquatic vertebrates (gill bars between slits develop gills) • Develop into parts of the ear, head, and neck in terrestrial vertebrates
Muscular, Post-Anal Tail • Chordates have a tail extending posterior to 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 that swim (fishes, reptiles and some amphibians)
Invertebrate Chordates • Tunicates, subphylum Urochordata • Belong to the deepest-branching lineage of chordates • Adults are marine suspension feeders commonly called sea squirts, and lack most chordate characters. It is the larval form of this subphylum that resemble the lancelot which is the common example of an early chordate
Incurrentsiphonto mouth Excurrentsiphon Excurrent siphon Atrium Pharynxwith numerousslits Anus Intestine Tunic Esophagus Stomach (b) In the adult, prominent pharyngeal slits function in suspension feeding, but other chordate characters are not obvious. (a) An adult tunicate, or sea squirt, is a sessile animal (photo is approximately life-sized). Figure 34.4a, b Adult tunicate or sea squirt • A sessile tunicate draws in water through an incurrent siphon, filtering out food particles that are transported to the esophagus by cilia
Notochord Dorsal, hollownerve cord Tail Excurrent siphon Musclesegments Incurrentsiphon Intestine Stomach Atrium Pharynx with slits (c) A tunicate larva is a free-swimming butnonfeeding “tadpole” in which all fourchief characters of chordates are evident. • Tunicates most resemble chordates during their larval stage, but note they have incurrent and excurrent siphons like the adult Figure 34.4c
Tentacle 2 cm Mouth Pharyngeal slits Atrium Notochord Digestive tract Atriopore Dorsal, hollownerve cord Segmentalmuscles Anus Tail Figure 34.5 Lancelets • Lancelets, subphylum Cephalochordata • Are named for their bladelike shape
Lancelets are marine suspension feeders • That retain the characteristics of the chordate body plan as adults • They lack a brain and anterior sense organs
BF1 Otx Hox3 Nerve cord of lancelet embryo BF1 Hox3 Otx Brain of vertebrate embryo(shown straightened) Midbrain Forebrain Hindbrain Figure 34.6 Lancelet molecular evolution studies • Gene expression in lancelets holds clues to the evolution of the vertebrate form because they show same developmental hox genes as vertebrate fishes
Concept 34.2: Craniates are chordates that have a head • The evolution of a head with a skull, brain, eyes, and other sensory organs was important because it- Allowed for a completely new way of feeding for chordates: active predation
Neuraltube Neuralcrest Dorsal edgesof neural plate Ectoderm Ectoderm (a) The neural crest consists of bilateral bands of cells near the margins of the embryonic folds that form the neural tube. (b) Neural crest cells migrate todistant sites in the embryo. Migrating neuralcrest cells Notochord Figure 34.7a, b Derived Characters of Craniates • One feature unique to craniates • Is the neural crest, a collection of cells that appears near the dorsal margins of the closing neural tube in an embryo
The Origin of Craniates • Craniates evolved at least 530 million years ago • During the Cambrian explosion
(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 • The most primitive of the fossils • Are those of the 3 cm long Haikouella with a brain, eyes but not skull
5 mm (b) Haikouichthys.Haikouichthys had a skull and thus is considered a true craniate. Figure 34.8b • In other Cambrian rocks • Paleontologists have found fossils of even more advanced chordates, such as Haikouichthys with a brain,eyes and a skull (a true craniate).
Vertebrates: Animals with a head and vertebrae • A lineage of craniates evolved into vertebrates during the Cambrian along with the acraniate chordates
Most Vertebrates have the following derived characters • Vertebrae enclosing a spinal cord (notochord replaced and all that remains are the vertebral fluid filled cartilagenous discs) • An elaborate skull • Fin rays, in aquatic forms • Two pair appendages in terrestrial forms
Slime glands Figure 34.9 Hagfish lacks some derived characters • The least derived craniate lineage that still survives • Is class Myxini, the hagfishes
Hagfishes are jawless marine craniates • That have a cartilaginous skull and an axial rod of cartilage (the notochord) supporting the trunk and tail instead of vertebrae • That lack vertebrae • They have a sucker type mouth (no jaws), but teeth only on the sides of the mouth that they extend and retract and use to rasp holes in immobilized fish and dead organisms. • Slime glands—slime may prevent predators from eating them. • Bane of the halibut fishermen.
Lampreys (another jawless fish) • Lampreys, class Cephalaspidomorphi • Represent the oldest living lineage of vertebrates (25 species) • Have cartilaginous segments surrounding the notochord and arching partly over the nerve cord • Unlike hagfish have well developed eyes and fins. • Laval forms live in stream sediments and filter feed, but adults attach to fish, rasp a hole in their skin and suck blood and tissue. Attack lake trout in the Great Lakes.
Figure 34.10 • Lampreys are jawless vertebrates • Inhabiting various marine and freshwater habitats
Pteraspis Pharyngolepis Figure 34.12 • Armored, jawless vertebrates called ostracoderms present in fossil record but died out at the end of the Devonian (360 mya) • Had defensive plates of bone on their skin meaning mineralization occurred a long time ago. Successful even though no jaws.
Origins of Bone and Teeth • Mineralization in modern fishes • Appears to have originated with vertebrate mouth parts • The vertebrate endoskeleton • Became fully mineralized much later
Jawed Fishes • Concept 34.4: Gnathostomes are vertebrates that have jaws and far outnumber the jawless vertebrates. • The evolution of jaws must have given vertebrates a real advantage over jawless. • What are these advantages (active predation, modification of mouth for specialized feeding) • Today’s jawless fish represented by only a few passive suckers. • How did vertebrate jaws originate??
Gill slits Cranium Mouth Skeletal rods Figure 34.13 Derived Characters of Gnathostomes • Gnathostomes have jaws • That evolved from skeletal supports of the pharyngeal slits. One set became the upper/lower jaw and next set the supporting structures.
Other characters common to gnathostomes include • Enhanced sensory systems, including the lateral line system • An extensively mineralized endoskeleton • Paired appendages
(a) Coccosteus, a placoderm Figure 34.14a Fossil Gnathostomes • The earliest gnathostomes in the fossil record • Are an extinct lineage of armored vertebrates called placoderms (were successful in their time but died out)
Chondrichthyans (Sharks, Rays, and Their Relatives (750 species)) • Think “JAWS” =the great white shark, a voracious predator. Fear! • Members of class Chondrichthyes • Have a skeleton that is composed primarily of cartilage • The cartilaginous skeleton • Evolved secondarily from an ancestral mineralized skeleton, but they retain mineralized teeth, dermal “scales” and some on the surface of their vertebrae. Why did they loose their bony skeletons??? Bouyancy?
Blacktip reef shark (Carcharhinus melanopterus).Fast swimmers with acute senses, sharks have paired pectoral and pelvic fins. • Pelvic fins act as stabilizing planes Pelvic fins Pectoral fins (c) Southern stingray (Dasyatis americana).Most rays are flattened bottom-dwellers thatcrush molluscs and crustaceans for food. Some rays cruise in open water and scoop food into their gaping mouth. Figure 34.15a, b • The largest and most diverse subclass of Chondrichthyes • Includes the sharks and rays
(c) Spotted ratfish(Hydrolagus colliei). Ratfishes, or chimaeras, typically live at depths greaterthan 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 • A second subclass • Is composed of a few dozen species of ratfishes
Most sharks • Have a streamlined body and are swift swimmers. Carnivores (great white eats marine mammals (sea lions, elephant seals and fur seals) and occasionally humans. Broken teeth are replaced from a new row that grow behind the front row. • Have acute senses (taste or smell and electrical field detection). • No swim bladder, but do have fatty livers, but still dense enough so they sink. Thus must keep swimming to maintain their level in the water column. • Fins only for steering no maneuvering like in fish. • Reproduction: Internal fertilization, oviparous (hatch outside), ovoviviparous (hatch inside) and viviparous feed from a placenta attached to their yolk –also eat their siblings • Spiral valve, rectal gland and cloaca.
Ray-Finned Fishes and Lobe-Fins • The vast majority of vertebrates (25,000 species) • Belong to a clade of gnathostomes called Osteichthyes
Nearly all living osteichthyans • Arose in fresh water and migrated to the sea. Evidence is that their blood has 1/3rd the salt that is found in seawater. • Have a bony endoskeleton • Control their buoyancy with an air sac known as a swim bladder • Reproduction-most eggs fertilized externally, few internal and live bearers
Adipose fin(characteristic oftrout) Dorsal fin Caudal fin Swim bladder Spinal cord Brain Nostril Cut edge of operculum Anal fin Gills Anus Gonad Liver Heart Lateral line Stomach Urinary bladder Kidney Intestine Pelvic fin Figure 34.16 Modern bony fishes • Fishes breathe by drawing water over four or five pairs of gills • Located in chambers covered by a protective bony flap called the operculum
(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 (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 Figure 34.17a–d Ray-Finned Fishes • Class Actinopterygii, the ray-finned fishes • Includes nearly all the familiar aquatic osteichthyans. Diverse life styles
Fish fins mainly for swimming • The fins, supported mainly by long, flexible rays • Are modified for maneuvering, defense, and other functions • Some modified enough to walk on mud flats (mud skipper)
Figure 34.18 Lobe-Fins • The lobe-fins, class Sarcopterygii • Have muscular pelvic and pectoral fins with rod like bones in them • Include coelacanths and lungfishes,
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 (not the lung fish or coelacanth) evolved into the limbs and feet of tetrapods
Bonessupportinggills Tetrapodlimbskeleton Figure 34.19 The Origin of Tetrapods • In one lineage of lobe-fins • The fins became progressively more limb-like while the rest of the body retained adaptations for aquatic life
Millions of years ago 420 370 340 310 295 280 265 415 400 385 355 325 Silurian Devonian Carboniferous Permian To present Paleozoic Ray-finned fishes Coelacanths Lungfishes Eusthenopteron Panderichthys Elginerpeton Metaxygnathus Acanthostega lchthyostega Hynerpeton Greerpeton Amphibians Amniotes Figure 34.20 • Extraordinary fossil discoveries over the past 20 years • Have allowed paleontologists to reconstruct the origin of tetrapods Acanthostega was a transitional form that showed both fish and tetrapod traits
Order Urodela. Urodeles • (salamanders) retain their tail as adults. Figure 34.21a Class Amphibia- 4,800speciesFrogs, toads and salamanders—moist skinthrough which gas exchange occurs • Order Urodela • Includes salamanders, which have tails
(b) Order Anura. Anurans, such as this poison arrow frog, lack a tail as adults. Figure 34.21b Frogs and Toads • Order Anura • Lack tails
(c) Order Apoda. Apodans, or caecilians, are legless, mainly burrowing amphibians. Figure 34.21c Obscure Order (S. America) • Order Apoda • Includes caecilians, which are legless and resemble worms
(b) The tadpole is an aquatic herbivore witha fishlike tail and internal gills. (c) During metamorphosis, the gills and tail are resorbed, andwalking legs develop. (a) The male grasps the female, stimulating her to release eggs. The eggs are laid and fertilized in water. They have a jelly coat but lack a shell and would desiccate in air. Figure 34.22a–c Reproduction in Amphibians • Amphibian means “two lives” • A reference to the metamorphosis of an aquatic larva into a terrestrial adult