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4-27-06
Chapter 34
Pages 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.
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
Figure 34.1
Chordate Characteristics
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
Figure 34.2
Ancestral deuterostome
Brain
Notochord
Chordates
Craniates
Vertebrates
Gnathostomes
Osteichthyans
Lobe-fins
Tetrapods
Amniotes
Head
Vertebral column
Jaws, mineralized skeleton
Legs
Lobed fins
Lungs or lung derivatives
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
Milk
Amniotic egg
All chordates share a set of derived characters although some
species possess some of these traits only during embryonic
Derived Characters of Chordates
development
Dorsal,
hollow
nerve cord
Muscle
segments
Brain
Notochord
Mouth
Anus
Pharyngeal
slits or clefts
Muscular,
post-anal tail
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
Adult tunicate or sea squirt
– A sessile tunicate draws in water through an incurrent
siphon, filtering out food particles that are transported
Incurrent
to the esophagus by cilia
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.
• Tunicates most resemble chordates during their
larval stage, but note they have incurrent and
excurrent siphons like the adult
Notochord
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.
Lancelets
• Lancelets, subphylum Cephalochordata
– Are named for their bladelike shape
Tentacle
2 cm
Mouth
Pharyngeal slits
Atrium
Notochord
Digestive tract
Atriopore
Dorsal, hollow
nerve cord
Segmental
muscles
Anus
Tail
Figure 34.5
• Lancelets are marine suspension feeders
– That retain the characteristics of the chordate
body plan as adults
– They lack a brain and anterior sense organs
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
BF1
Otx
Hox3
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 evolution of a head with a skull, brain, eyes,
and other sensory organs was important because
itAllowed for a completely new way of feeding for
chordates: active predation
Derived Characters of Craniates
• One feature unique to craniates
– Is the neural crest, a collection of cells that appears
Neural
near the dorsal
marginsNeural
of
the
closing
neural tube in
Dorsal edges
tube
crest
an embryoof neural plate
Ectoderm
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.
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 with a brain, eyes but not skull
(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 more
advanced chordates, such as Haikouichthys with a
brain,eyes and a skull (a true craniate).
5 mm
(b) Haikouichthys. Haikouichthys had a
skull and thus is considered a true craniate.
Figure 34.8b
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
Hagfish lacks some derived characters
• 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 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.
• Lampreys are jawless vertebrates
– Inhabiting various marine and freshwater habitats
Figure 34.10
• 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.
Pteraspis
Pharyngolepis
Figure 34.12
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??
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.
Gill slits
Cranium
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 (were successful in their time but died
out)
(a) Coccosteus, a placoderm
Figure 34.14a
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?
• The largest and most diverse subclass of
Chondrichthyes
– Includes the sharks and rays
(a)
(b)
Blacktip reef shark (Carcharhinus melanopterus).
Fast swimmers with acute senses, sharks have
paired pectoral and pelvic fins.
Pelvic fins act as stabilizing planes
(c) 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
4.15a, 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.
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 LobeFins
• 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
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
Adipose fin
Dorsal fin
(characteristic of
trout)
Swim bladder
Nostril
Spinal cord
Brain
Cut edge of
operculum Gills
Heart
Figure 34.16
Gonad
Urinary
Anus bladder
Liver
Kidney
Intestine
Stomach Pelvic fin
Caudal
fin
Anal fin
Lateral
line
Ray-Finned Fishes
• Class Actinopterygii, the ray-finned fishes
– Includes nearly all the familiar aquatic osteichthyans.
Diverse life styles
(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
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)
Lobe-Fins
• The lobe-fins, class Sarcopterygii
– Have muscular pelvic and pectoral fins with rod
like bones in them
– Include coelacanths and lungfishes,
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 (not the lung
fish or coelacanth) evolved into the limbs and feet of
tetrapods
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
Bones
supporting
gills
Figure 34.19
Tetrapod
limb
skeleton
• Extraordinary fossil discoveries over the past
20 years
– Have allowed paleontologists to reconstruct the
origin of tetrapods
Millions of years ago
420 415 400 385 370 355 340 325 310 295 280 265
Silurian
Devonian
Carboniferous
Paleozoic
Permian
Ray-finned fishes
Coelacanths
Lungfishes
Acanthostega was
a transitional form
that showed both
fish and tetrapod
traits
Figure 34.20
Eusthenopteron
Panderichthys
Elginerpeton
Metaxygnathus
Acanthostega
lchthyostega
Hynerpeton
Greerpeton
Amphibians
Amniotes
To
present
Class Amphibia- 4,800 species
Frogs, toads and salamanders—moist skin
through which gas exchange occurs
• Order Urodela
– Includes salamanders, which have tails
(a)
Order Urodela. Urodeles
(b)
(salamanders) retain their tail as adults.
Figure 34.21a
Frogs and Toads
• Order Anura
– Lack tails
(b) Order Anura. Anurans, such as
this poison arrow frog, lack a tail as adults.
Figure 34.21b
Obscure Order (S. America)
• Order Apoda
–
Includes caecilians, which are legless and resemble worms
(c) Order Apoda. Apodans, or caecilians,
are legless, mainly burrowing amphibians.
Figure 34.21c
Reproduction in Amphibians
•
Amphibian means “two lives”
–
A reference to the metamorphosis of an aquatic larva into a terrestrial
adult
(b) The tadpole is
an aquatic
herbivore with
a fishlike tail and
internal gills.
(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
Figure 34.22a–c would desiccate in air.
(c) During metamorphosis, the
gills and tail are resorbed, and
walking legs develop.
• Concept 34.6: Amniotes are tetrapods that
have a terrestrially adapted egg
• Amniotes are a group of tetrapods
– Whose living members are the reptiles,
including birds, and the mammals
• A phylogeny of amniotes
note separation of reptiles
Most (turtles and crocodiles) fall out with dinosaurs and birds.
Snakes and lizards separate separate out with the tuatara)
Saurischians
Dinosaurs
Lepidosaurs
Archosaurs
Synapsids
Diapsids
Reptiles
Ancestral
amniote
Figure 34.23
Derived Characters of Amniotes
• Amniotes are named for the major derived character of the
clade, the amniotic egg
– Which contains specialized membranes that protect the embryo
from dessication
– Thus allows them to reproduce on land.
– This was a major evoutionary inovation that allowed for
colonization of the terrestrial environment
– Why haven’t amphibians evolved this sort of reproductive
strategy??? Don’t know. Maybe the evolutionary orgin of the
amniotic egg was and extremely rare event.
– Some amphibians have evolved other sorts of adaptation that
allow them to reproduce in the absence of water.
The Amnionic Egg
• The extraembryonic membranes
–
Have various functions
Allantois. The allantois is a disposal
sac for certain metabolic wastes produced by the embryo. The membrane
of the allantois also functions with
the chorion as a respiratory organ.
Amnion. The amnion protects
the embryo in a fluid-filled
cavity that cushions against
mechanical shock.
Extraembryonic membranes
Chorion. The chorion and the membrane of the
allantois exchange gases between the embryo
and the air. Oxygen and carbon dioxide diffuse
freely across the shell.
Yolk sac. The yolk sac contains the
yolk, a stockpile of nutrients. Blood
vessels in the yolk sac membrane transport
nutrients from the yolk into the embryo.
Other nutrients are stored in the albumen (“egg white”).
Embryo
Amniotic cavity
with amniotic fluid
Yolk (nutrients)
Albumen
Shell
Figure 34.24
• Amniotes also have other terrestrial
adaptations
– Such as relatively impermeable skin and the
ability to use the rib cage to ventilate the lungs
• Early amniotes appeared in the carboniferous
period and included large herbivores and
predators (reptiles).
Early Amniotes
• Early amniotes
– Appeared in the Carboniferous period
– Included large herbivores and predators