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3/28/05
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• Phylum Chordata
Fig. 34.1
4 unifying anatomical features of chordates
• Notochord
• Dorsal hollow nerve cord
Fig. 34.2
•Pharyngeal slits
•Muscular, postanal tail.
• Several recent fossil finds in China provide
support for the second stage, from
cephalochordate to vertebrate.
– They appear to be “missing links” between groups.
– Features that appear in these
fossils include a more elaborate
brain, eyes, a cranium, and
hardened structures (“denticles”)
in the pharynx that may have
functioned somewhat like teeth.
– These fossils push the vertebrate
origins to Cambrian
explosion.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 34.5
Craniata - Vertebrata
*
*Hagfish has
cranium but no
vertebate column
Vertebrata characteristics:
•Vertebral column
•Neural crest
•Pronounced cephalization •Closed circulatory system
 Embryonic ectodermal plate
rolls together to form the
hollow neural tube.
 Neural crest forms near the
dorsal margins of the
closing neural tube  cells
migrate and form many
important structures - bones
and cartilages of the
cranium, neurons, ganglia,
melanocytes, and others
 Pronounced cephalization – anterior end of nerve
cord enlarged to form brain; concentration of sensory
and neural equipment in the head (cranium).
The cranium and vertebral column form the
vertebrate axial skeleton.
•Structural support for trunk, so large body size and fast
movements possible.
•Ribs of the axial skeleton - anchor muscles and protect
internal organs.
Most vertebrates also have an appendicular skeleton,
supporting 2 pairs of appendages (fins, legs, or arms).
 Vertebrate endoskeleton - made of bone and
cartilage; can grow continuously.
 Closed circulatory system - ventral, chambered
heart that pumps blood through arteries and
capillaries to provide nutrients and O2 to all body
tissues.
 Respiratory system – oxygenation of blood
through gills or lungs.
 Circulatory and respiratory adaptations support
aerobic respiration of mitochondria to produce ATP
for active life styles.
 Active lifestyle requires a large supply of organic
fuel - adaptations for feeding, digestion, and
nutrient absorption.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Agnathan
Vertebrate
diversity
Fig. 34.7
Overview of vertebrate diversity
Current vertebrate phylogeny based on anatomical,
molecular, and fossil evidence.
– Agnathan (lack hinged jaws) – primitive hagfishes and
lampreys.
– Gnathostomes (hinged jaws) - all other vertebrates.
Also have 2 sets of paired appendages:
- fins in fish; legs in tetrapods.
o Tetrapods:
 Non-amniotes – amphibians; eggs laid in water.
 Amniotes - mammals, reptiles, birds; shelled, waterretaining eggs for life cycles on land.
• Most modern mammals don’t lay eggs, but retain
many key features of the amniotic mode of
reproduction.
Jawless Vertebrates – Agnathan
1. Class Myxini: Hagfishes
2. Class Cephalaspidomorphi: Lampreys
3. Ostracoderm: Extinct jawless vertebrates that had
ossified teeth and body armor
ostracoderms
1. Class Myxini: Hagfishes - the most
primitive living “vertebrates”
• ~ 30 species, all marine
• scavengers, feeding on worms and sick or dead fish.
Fig. 34.8
Row of
Slime glands
slime
• Hagfish skeletal system
• Cartilaginous cranium, notochord.
• Lack vertebrae – “invertebrate” craniate.
• Hagfishes diverged from ancestors that produced
the vertebrate lineage ~ 530 mya, during early
Cambrian.
• Considered the most primitive living “vertebrate”.
• Salty as seawater.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
2. Class Cephalaspidomorphi: Lampreys provide
clues to the evolution of the vertebral column
• ~ 35 species, both marine and freshwater
• Sea lamprey – larva is free-living; adult is
ectoparasite.
Rows of thorny teeth, and rasping tongue, penetrates
skin of its fish prey, and ingests fish blood and
tissues.
Fig. 34.9
• Lamprey skeletal system
• Notochord persists as the main axial skeleton in
adult.
– Cartilaginous pipe surrounding the notochord.
– Pairs of cartilaginous projections extend dorsally,
partially enclosing the nerve cord - vestige of an
early stage vertebral column.
• Both hagfishes and lampreys lack skeletonsupported jaws and paired appendages.
Comparison of craniate skeletal structure
Hagfish – skull of cartilage
bars (blue); brain
surrounded by fibrous
sheath (yellow). Notochord
underneath (green).
Lamprey - more elaborate
braincase; cartilaginous pipe
(early/vestige vertebral
column) partially surrounds
nerve cord. Has branchial
cartilage around gills
Gnathostomes - braincase
closed. Vertebral column.
Notochord vestigial.
Evolution of gnathostome skeletal structures
• Brain case in both agnathans and gnathostomes.
Lamprey has vertebrate-like structure.
• So cranium evolved first in the vertebrate lineage.
Followed by the vertebral column.
• Jaws, ossified skeleton, and paired appendages
evolved later.
• This sequence consistent with the early Cambrian
fossils.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Ostracoderms - extinct jawless
vertebrates
• Also called Agnathans because jawless. Lived ~ 450
to 375 mya.
• Bottom dwellers, mud suckers, filter feeders –
limited feeding modes because no hinged jaws.
• Some species had paired fins.
• Ossified teeth and body armor – from mineralization
(Ca phosphate secreted by special cells) of
connective tissues.
• So probably more advanced than the extant
agnathans.
Fishes and Amphibians
1.
Placoderms: - first jawed vertebrates.
2.
3.
Class Chondrichthyes: Sharks and rays - cartilaginous skeletons.
Class Osteichthyes: Bony fishes - ray-finned, lobe-finned, and lungfishes.
4. Tetrapods evolved from specialized fishes.
5.
Class Amphibia: Salamanders, frogs, and caecilians.
Rise of Gnathostomes
• During the late Silurian and early Devonian
periods (~440- 400 mya), gnathostomes largely
replaced the agnathans.
• Placoderms and Acanthodians –fishes with
hinged jaws.
• Besides jaws, fishes
have 2 pairs of fins
(extinct)
placoderm
• Jaws and paired fins were major evolutionary
breakthroughs.
– Jaws and teeth – firm grip on food.
– Range of food expands vs agnathan’s suspension
feeding.
– Paired fins, plus the tail, enable accurate swimming
maneuvers.
• With these adaptations, many fish species were
active predators  diversification of both
lifestyles and nutrient sources.
Evolution of Vertebrate Jaws
• Vertebrate jaws evolved by modification of the
skeletal rods that previously supported the
anterior pharyngeal slits (see lamprey).
– The remaining gill slits remained as the site of gas
exchange.
Fig. 34.10
Rises of the Fishes
• Devonian (360-400 mya) is the “age of fishes”.
• Placoderms and acanthodians radiated in both fresh
and salt water. Extinct ~360 mya
• Ancestors to the placoderms and acanthodians also
gave rise to sharks and bony fishes ~ 425-450
mya.
– Chondrichthyes - cartilaginous fishes.
– Osteichthyes - bony fishes.
– (To present day fishes)
Class Chondrichthyes
• ~ 750 extant species. Endoskeletons of cartilage, except teeth
(bony).
• Subclass Elasmobranchii – sharks, skates, rays (gill slits).
• Subclass Holocephalii – chimerae or ratfishes (gill cover over
4 gills)
• All have well-developed
jaws and paired fins.
• Chondrichthyes ancestors
have bony skeletons. So
cartilaginous skeleton of
extant chondrichthys is a
ratfish
derived characteristic, not
a primitive one.
• Shark Swimming
• Streamlined bodies and powerful axial muscles
allow swift swimming, but not fine maneuvers.
• Buoyancy:
– No swim bladder.Sinks!
– Some buoyancy provided
by low density oils in large
livers;
– Caudal and pectoral fins  lift.
Dorsal fins stablization.
Fig. 34.11a
• Shark feeding and digestion
• Most sharks are carnivores.
• The largest sharks and rays are suspension feeders
that consume plankton.
• The intestine of shark has a
spiral valve, a corkscrewshaped ridge that increases
surface area and prolongs
carnivorous
the passage of food along
the short digestive tract.
Whale shark
Filter feeder
• Shark Sensory System
• Acute senses
– Acute olfactory sense occurs in a pair of nostrils.
Huge olfactory lobes.
– Sharp B/W vision.
– Electric sensing of muscle movement of others.
– Lateral line system - a row of microscopic pressure
receptors along the midline of the body that can
detect low frequency vibrations.
– Whole body transmits sound to the hearing organs of
the inner ear.
• Shark Reproduction
• Internal fertilization.
– Males transfer sperm via claspers
on their pelvic fins to the reproductive
tract of the female.
– Oviparous - eggs in protective cases and laid
outside These hatch months later as juveniles.
– Ovoviviparous - fertilized eggs retained in oviduct.
Embryo completes development in the uterus,
nourished by the egg yolk.
– Viviparous - providing nutrients through a
placenta to the developing offspring.
Oviparous - egg case
Horn shark
Swell shark
Viviparous – life bearers
• Rays, skates – the other elasmobranchs;
different lifestyle.
– Flattened bottom dwellers; crush mollusks and
crustaceans in their jaws.
– Enlarged pectoral fins used like wings to propel
through water.
– Tail is whiplike and may bear
venomous barbs for
defense.
Fig. 34.11c
Osteichthyes – bony fishes
• Most numerous group of vertebrates, in number and
species (~30,000 species).
• Traditionally, a single class, Osteichthyes
• Now three extant classes recognized:
Ray-finned -- actinopterygii
Lobe-finned – sarcopterygii (text calls it actinista)
Lungfishes -- dipnoi
• Characteristics of bony fishes
• Ossified endoskeleton - hard matrix of calcium
phosphate.
• Skin often covered with thin, flattened bony scales.
• Possess lateral line system.
• Respiration by drawing water gills located in chambers
covered by a protective operculum.
(What fish is this?)
• Anatomical Differences between Sharks and Bony Fishes
Shark scales
Teleost scales
ctenoid
placoid
cycloid
Elasmobranchs - Gill slits
Bony fishes - Gill cover
(operculum)
Chimerids (Holocephalii) - Gill cover (almost sealed
except for a single hole on each side
Ratfish
NZ elephant fish
Shark
Heterocercal tail
Uneven lobed caudal fin
Teleost
Even lobed caudal fin
ctenoid
•Fundamental Physiological Difference Osmoregulation
• Elasmobranchs: evolved from marine origin
-- Blood and body fluids isosmotic with seawater.
-- Achieve isosmoticity by retaining urea
• Bony fishes: evolved from fresh water origin
-- Hypo-osmotic to seawater for marine fishes; need
to excrete salt (chloride cells in gills) and
conserve water.
-- Hyper-osmotic to freshwater for freshwater fishes;
need to conserve salt (Na pumps), and remove
excess water.
•Bony Fishes Reproduction
– Most species are oviparous - external fertilization.
Female sheds large numbers of small eggs and males
synchronously release clouds of sperm (milt).
– Also internal fertilization occurs in many fish groups;
some are even viviparous (live bearers).
Antarctic dragonfish Gymnodraco acuticeps
•Buoyancy and Swimming
• Most have an internal, air-filled swim bladder.
• Gas, primarily O2, actively secreted into swim bladder
to adjust volume and buoyancy so fish stays afloat at
different depths.
• Much better swimming maneuvers than sharks.
Swim bladder
The following slides (plus more) will be
covered on Monday April 7. So that we
can finish the Chapter, please review
these slides before class.
• Osteichthys (bony fish) classes:
• Class Actinopterygii - ray-finned fishes,
• Includes many of the common fishes we know bass, trout, perch, tuna, cods, puffer fish…….
• Fins are supported by long flexible ray.
• Class Actinistia - Lobe-finned fishes
• Muscular pectoral and pelvic fins supported by
extensions of the bony skeleton.
• Large, bottom dwellers that may have used their
paired, muscular fins to “walk” along the bottom.
• Mostly extinct at the end of Devonian.
• Coelocanth (Latimeria) discovered ~20 yrs ago –
living fossil.
Latimeria
• Class Dipnoi - 3 genera (live today in Oz, South American and
southern Africa)
– Inhabit stagnant ponds and swamps.
– Gulp air into lungs connected to the pharynx of the digestive tract.
– Also have gills - main organs for
gas exchange in Australian lungfishes.
Some obligate air breathers
– Estivate in mud burrow during
dry season.
• The ancestor of amphibians and all other tetrapods was probably a
lungfish from the Devonian, when they were dominant predators.
Skeletal structure of lobe-fins
The Sarcopterygians (Actinistia) are the
so-called lobe-finned fishes. Do not be
confused, just because they aren't called
ray-finned fishes does not mean that
their fins do not have rays, because they
certainly do. The difference between the
two fins is that the lobe-fin has the bones
and the muscles that operate the fin
mostly on the outside of the body (think
about how our arm muscles work)
Tetrapods evolved from specialized
fishes that inhabited shallow water
• Amphibians were the first tetrapods to spend a
substantial portion of their time of land.
• The earliest vertebrate tetrapods are fish-like,
with sturdy, skeleton-supported legs instead of
paired fins, and which lived in shallow aquatic
habitats.
Fig. 34.15
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
•
•
•
•
•
•
• At the Water’s Edge………
During Devonian, plant flourished.
Plants at the edges of ponds and swamps created
new living conditions and food for fishes living
near the water’s edge.
Lobe-fin and lungfish like fishes evolved and lived
in these shallow habitats.
Lungs evolved to gulp in air.
Leg- like appendages evolved - better than fins for
paddling and crawling through the dense
vegetation in shallow water.
The fossil record chronicles the transition to land
over a 50-million-year period from 400 to 350
million years ago (mostly Greenland mountains).
Eusthenopteron
FISH - Lobe fin
Eusthenopteron
~ 300 mya
INTERMEDIATE
hypothetical
AMPHIBIAN- Erypos
primitive foot ~ 200 mya
Acanthostega
Fig. 34.16
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• As the earliest terrestrial tetrapods, amphibians
benefited from abundant food and relatively
little competition.
• Carboniferous  the age of amphibians
Class Amphibia: Salamanders, frogs,
and caecilians are the three extant
amphibian orders
• ~ 4,800 species.
• Order Urodela (“tailed ones”) – salamanders,
newts, axolot
• Order Anura (tail-less ones”) - frogs, toads
Order Apoda (“legless ones”) - caecilians
• Urodeles - some of the 500 species are entirely
aquatic, but others live on land as adults or
throughout life, but must come to water to lay
eggs.
Fig. 34.17a
• Anurans - ~ 4,200 species of anurans. More
specialized than urodeles for moving on land.
• Among adaptations that reduce predation color camouflages, distasteful, and/or
poisonous mucus from skin glands.
– Many poisonous species
are also bright warning
colors
Fig. 34.17b
• Apodans – caecilians, ~ 150 species; legless
and nearly blind.
– The reduction of legs evolved secondarily from a
legged ancestor.
• Superficially resembling earthworms,
• most species burrow in moist forest
soil in the tropics
• Amphibian means “two lives,” metamorphosis of frogs from an aquatic
tadpole, to the terrestrial adult.
– Tadpoles aquatic herbivores with gills, a lateral line
system, and swim by undulating its tail.
– During metamorphosis:
tadpole develops legs,
lateral line disappears,
tail resorbed and
gills replaced by lungs.
Fig. 34.18
• Amphibian Respiration
• Have lungs, but also rely on cutaneous
respiration. When resting cutaneous
sufficient.
• Most amphibians live in damp habitats. Gas
exchange through moist skin.
• Terrestrial toads – some have waxed skin to
conserve water and depend more on lungs.
• Amphibian Reproduction
• Non-amniotes - eggs lack a shell and dehydrate
quickly in dry air.
– Mostly external fertilization; eggs shed in ponds or
swamps, or other moist environments.
– Vast numbers of eggs in temporary pools where
mortality is high.
– Some species – parental care.
• Males or females house eggs on their back, in the mouth, or
even in the stomach.
• A few ovoviviparous or viviparous.
• Why didn’t they evolve an amniotic egg ( a water proof
egg) ?????? Evolutionary barrier????