• A hypothetical phylogeny of chordatesChordates

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

Ech

ino

de

rmat

a(s

iste

r g

rou

p to

cho

rda

tes)

Uro

ch

ord

ata

(tu

nica

tes)

Ce

ph

alo

ch

ord

ata

(lan

cele

ts)

Myx

ini

(hag

fish

es)

Ce

ph

ala

spid

om

orp

hi

(lam

pre

ys)

Ch

on

dri

ch

thy

es(s

hark

s, r

ays

, ch

ima

era

s)

Ac

tin

op

tery

gii

(ray

-fin

ned

fish

es)

Ac

tin

isti

a(c

oela

can

ths)

Dip

no

i(l

ungf

ishe

s)

Am

ph

ibia

(fro

gs,

sala

man

der

s)

Re

pti

lia

(tu

rtle

s, s

nak

es,

croc

odi

les,

bird

s)

Mam

mal

ia(m

amm

als)

Derived Characters of Chordates

• All chordates share a set of derived characters– Although some species possess some of

these traits only during embryonic developmentMuscle

segments

Brain

Mouth

Anus

Dorsal,hollow

nerve cord

Notochord

Muscular,post-anal tail

Pharyngealslits 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

Figure 34.4c

• Tunicates most resemble chordates during their larval stage– Which may be as brief as a few minutes

Pharynx with slits

Notochord

Tail

Dorsal, hollownerve cord

AtriumStomach

Intestine

Excurrent siphon

Incurrentsiphon

Musclesegments

(c) A tunicate larva is a free-swimming butnonfeeding “tadpole” in which all fourchief characters of chordates are evident.

• 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

Pharynxwith

numerousslits

Atrium

Excurrentsiphon

Incurrentsiphonto mouth

StomachEsophagus

IntestineAnus

Excurrent siphon

Figure 34.4a, b

Lancelets

• Lancelets, subphylum Cephalochordata– Are named for their bladelike shapeTentacle

Mouth

Pharyngeal slits

Atrium

Digestive tract

Atriopore

Segmentalmuscles

Anus

Notochord

Dorsal, hollownerve 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

BF1Otx

Otx Hox3

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 todistant sites in the embryo.

Migrating neuralcrest cells

Ectoderm

Ectoderm

Dorsal edgesof neural plate

Neuralcrest

Neuraltube

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 structuresunique 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 viewof head

DentalelementsFigure 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 slitsMouth

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

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

• 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 thatcrush 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 greaterthan 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– 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 operculumNostril Brain

Spinal cordSwim bladder

Dorsal fin Adipose fin(characteristic oftrout)

Caudal fin

Cut edge of operculum Gills

HeartLiver

KidneyStomach

Intestine

GonadAnus

Urinary bladder

Lateral line

Anal fin

Pelvic finFigure 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 habitatFigure 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