a hypothetical phylogeny of chordates chordates craniates vertebrates gnathostomes osteichthyans...
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• 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