3/28/05

1st after spring break

Fig. 34.1

• Phylum Chordata

• Notochord • Dorsal hollow nerve cord

4 unifying anatomical features of chordates

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

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

Vertebrata characteristics:•Neural crest•Pronounced cephalization

•Vertebral column•Closed circulatory system

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

Fig. 34.7

Vertebratediversity

Agnathan

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.

Overview of vertebrate diversity

o Tetrapods: Non-amniotes – amphibians; eggs laid in water. Amniotes - mammals, reptiles, birds; shelled, water-

retaining 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

• ~ 30 species, all marine• scavengers, feeding on worms and sick or dead fish.

1. Class Myxini: Hagfishes - the most primitive living “vertebrates”

Fig. 34.8

Row ofSlime 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

• ~ 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.

2. Class Cephalaspidomorphi: Lampreys provide clues to the evolution of the vertebral column

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 skeleton-supported jaws and paired appendages.

Hagfish – skull of cartilage bars (blue); brain surrounded by fibrous sheath (yellow). Notochord underneath (green).

Comparison of craniate skeletal structure

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

• 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.

Ostracoderms - extinct jawless vertebrates

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fishes and Amphibians

1. Placoderms: - first jawed vertebrates.

2. Class Chondrichthyes: Sharks and rays - cartilaginous skeletons.

3. Class Osteichthyes: Bony fishes - ray-finned, lobe-finned, and lungfishes.

4. Tetrapods evolved from specialized fishes.

5. Class Amphibia: Salamanders, frogs, and caecilians.

• 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)

Rise of Gnathostomes

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.

• 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.

Evolution of Vertebrate Jaws

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)

• ~ 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

derived characteristic, not

a primitive one.

Class Chondrichthyes

ratfish

• 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 corkscrew-shaped ridge that increases surface area and prolongs the passage of food along the short digestive tract.

carnivorous

Whale sharkFilter 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.

Horn shark

Oviparous - egg case

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

• 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

Osteichthyes – bony fishes

• 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?)

Shark scales

ctenoid

cycloid

Teleost scales

placoid

• Anatomical Differences between Sharks and Bony Fishes

Elasmobranchs - Gill slits Bony fishes - Gill cover (operculum)

Ratfish NZ elephant fish

Chimerids (Holocephalii) - Gill cover (almost sealed except for a single hole on each side

ctenoid

Even lobed caudal fin

Teleost

                                                               

SharkHeterocercal tail

Uneven lobed caudal fin

•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.

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)

Skeletal structure of lobe-fins

• 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.

Tetrapods evolved from specialized fishes that inhabited shallow water

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 34.15

• 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).

FISH - Lobe fin Eusthenopteron~ 300 mya

INTERMEDIATE hypothetical AMPHIBIAN- Erypos

primitive foot ~ 200 mya

Eusthenopteron

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 34.16

Acanthostega

• As the earliest terrestrial tetrapods, amphibians benefited from abundant food and relatively little competition.

• Carboniferous the age of amphibians

• ~ 4,800 species.

• Order Urodela (“tailed ones”) – salamanders, newts, axolot

• Order Anura (tail-less ones”) - frogs, toads Order Apoda (“legless ones”) - caecilians

Class Amphibia: Salamanders, frogs, and caecilians are the three extant

amphibian orders

• 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????