Ancestral deuterostome A hypothetical phylogeny of chordates Chordates Craniates Vertebrates Gnathostomes Osteichthyans Lobe-fins Tetrapods Amniotes Milk Amniotic egg Legs Lobed fins Lungs or lung derivatives Jaws, mineralized skeleton Vertebral column Head Brain Notochord Ancestral deuterostome Echinodermata (sister group to chordates) Urochordata (tunicates) Cephalochordata (lancelets) Myxini (hagfishes) Cephalaspidomorphi (lampreys) Chondrichthyes (sharks, rays, chimaeras) Actinopterygii (ray-finned fishes) Actinistia (coelacanths) Dipnoi (lungfishes) Amphibia (frogs, salamanders) Reptilia (turtles, snakes, crocodiles, birds) Mammalia (mammals) Figure 34.2

Derived Characters of Chordates All chordates share a set of derived characters Although some species possess some of these traits only during embryonic development Muscle segments Brain Mouth Anus Dorsal, hollow nerve cord Notochord Muscular, post-anal tail Pharyngeal slits or clefts Figure 34.3

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 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 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

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 Which may be as brief as a few minutes Pharynx with slits Notochord Tail Dorsal, hollow nerve cord Atrium Stomach Intestine Excurrent siphon Incurrent siphon Muscle segments (c) A tunicate larva is a free-swimming but nonfeeding “tadpole” in which all four chief characters of chordates are evident. Figure 34.4c

Pharynx with numerous slits As an adult A tunicate draws in water through an incurrent siphon, filtering food particles (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. Tunic Pharynx with numerous slits Atrium Excurrent siphon Incurrent siphon to mouth Stomach Esophagus Intestine Anus Excurrent siphon Figure 34.4a, b

Dorsal, hollow nerve cord Lancelets Lancelets, subphylum Cephalochordata Are named for their bladelike shape Tentacle Mouth Pharyngeal slits Atrium Digestive tract Atriopore Segmental muscles Anus Notochord Dorsal, hollow nerve cord Tail 2 cm Figure 34.5

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

Gene expression in lancelets Holds clues to the evolution of the vertebrate form BF1 Otx Hox3 Forebrain Midbrain Hindbrain Nerve cord of lancelet embryo Brain of vertebrate embryo (shown straightened) Figure 34.6

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 appears near the dorsal margins of the closing neural tube in an embryo Notochord (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 to distant sites in the embryo. Migrating neural crest cells Ectoderm Dorsal edges of neural plate Neural crest Neural tube Figure 34.7a, b

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 Figure 34.8a (a) Haikouella. Discovered in 1999 in southern China, Haikouella had eyes and a brain but lacked a skull, a derived trait of craniates.

In other Cambrian rocks Paleontologists have found fossils of even more advanced chordates, such as Haikouichthys Figure 34.8b (b) Haikouichthys. Haikouichthys had a skull and thus is considered a true craniate. 5 mm

Hagfishes The least derived craniate lineage that still survives Is class Myxini, the hagfishes Figure 34.9 Slime glands

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 Dental elements Figure 34.11

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 have jaws That evolved from skeletal supports of the pharyngeal slits Mouth Gill slits Cranium 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

(a) Coccosteus, a placoderm Fossil Gnathostomes The earliest gnathostomes in the fossil record Are an extinct lineage of armored vertebrates called placoderms Figure 34.14a (a) Coccosteus, a placoderm

(b) Climatius, an acanthodian Another group of jawed vertebrates called acanthodians Radiated during the Devonian period Were closely related to the ancestors of osteichthyans Figure 34.14b (b) Climatius, an acanthodian

Chondrichthyans (Sharks, Rays, and Their Relatives) Members of class Chondrichthyes 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 Figure 34.15a, b Pectoral fins Pelvic fins (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 their gaping mouth.

A second subclass Is composed of a few dozen species of ratfishes Figure 34.15c (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.

Most sharks Have a streamlined body and are swift swimmers Have acute senses

Ray-Finned Fishes and Lobe-Fins The vast majority of vertebrates Belong to a clade of gnathostomes called Osteichthyes

Nearly all living osteichthyans Aquatic 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 Located in chambers covered by a protective bony flap called the operculum Nostril Brain Spinal cord Swim bladder Dorsal fin Adipose fin (characteristic of trout) Caudal fin Cut edge of operculum Gills Heart Liver Kidney Stomach Intestine Gonad Anus Urinary bladder Lateral line Anal fin Pelvic fin Figure 34.16

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 (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

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