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25.1 Vertebrate Origins
KEY CONCEPT All vertebrates share common characteristics.
25.1 Vertebrate Origins
hollow nerve cord
tail
notochord
pharyngeal slits
The phylum Chordata contains all vertebrates and some invertebrates. • Chordates share four features at some stage of
development.– notochord– hollow nerve cord – pharyngeal slits– tail
25.1 Vertebrate Origins
hollow nerve cordtail
notochord
pharyngeal slits
• Most chordates lose some or all of these characteristics in adulthood.
Tunicates (Sea Squirts) only have these characteristics as larva. Once they settle into their sessile filter-feeding adults, none of these are visible.
25.1 Vertebrate Origins
All vertebrates share common features.
• An endoskeleton allows vertebrates to grow to large sizes.– internal– made of bone or cartilage
25.1 Vertebrate Origins
• An endoskeleton can be divided into four parts.
– braincase (cranium) braincase
vertebrae
bones
– gill arches (in fish and some amphibians)
– bones
– vertebrae
25.1 Vertebrate Origins
• There are seven classes of vertebrates.
– Agnatha are jawless fish, such as hagfish.– Cartilaginous fish (Chondrichthyes) include the sharks
and rays– Bony fish (Osteichthyes) obviously have bones.– Amphibians are characterized by the presence of four
limbs and water permeable skin.– Reptiles, birds, and mammals are characterized by the
presence of an amnion and water-tight skin.– Birds are characterized by the presence of feathers.– Mammals are characterized by the presence of hair.
25.1 Vertebrate Origins
Agnatha Chondrichthyes Osteichthyes Amphibia Aves Mammalia
JAWSJaws helped vertebrates to become successful predators.
VERTEBRAEVertebrates have a segmented backbone.
FOUR LIMBSFour limbs let animals move from the water to life on land.
FEATHERSFeathers insulate birds from the cold and allow for flight.
HAIRHair helps mammals to maintain constant body temperatures by roviding insulation from the cold.
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ibia
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What group seems to be missing from this slide?
25.1 Vertebrate Origins
Fossil evidence sheds light on the origins of vertebrates.
• Tunicates may be the closest relatives to vertebrates.• The first recognizable vertebrates were jawless fish.• Two groups of jawless fish still exist today.
– hagfish
– lampreys
25.1 Vertebrate Origins
KEY CONCEPT The dominant aquatic vertebrates are fish.
25.1 Vertebrate Origins
Fish are vertebrates with gills and paired fins. • Fish use specialized organs called gills to breathe
underwater.– sheets of thick, frilly tissue filled with capillaries – take in dissolved oxygen from water, release carbon
dioxide
waterflow
25.1 Vertebrate Origins
• Countercurrent flow is the opposite movement of water against the flow of blood in the fish’s gills.
25.1 Vertebrate Origins
• Fins are surfaces that project from a fish’s body.
dorsal fin
caudal fin
anal finpectoral fin pelvic fin
– keep fish stable– redirect water around fish as it swims– help fish maneuver in water
25.1 Vertebrate Origins
Jaws evolved from gill supports.
• Jaws developed from gill arches located around the pharynx.
cranium
mouth gill arches
cranium
mouth
cranium
mouth
• Jaws gave vertebrates a huge advantage as predators.
25.1 Vertebrate Origins
Only two groups of jawed fish still exist. • Cartilaginous fish such as sharks and rays (970 species).
• Cartilaginous fish have skeletons made of cartilage.• Bony fish make up the vast majority vertebrates at over
29,000 species!
25.1 Vertebrate Origins
• Cartilaginous fish include the Holocephali and Elasmobranchs.
– Holocephali include ratfish, a small group of deep-sea fish.– Elasmobranchs include sharks, rays, and skates.
25.1 Vertebrate Origins
• All fish have a lateral line system.– sensory system– sensitive to small changes in water movement
lateral line
25.1 Vertebrate Origins
• Bony fish have skeletons made of bone.– operculum protects a bony fish’s gills– movements of operculum help bony fish move water
over gills
25.1 Vertebrate Origins
Ray-finned fish have a fan of bones in their fins.
• Ray-finned fish have fins supported by a fan-shaped array of bones.– embedded in a thin layer of skin and connective tissue– light, collapsible, and easy to move
25.1 Vertebrate Origins
• Ray-finned fish have a variety of body plans.– long torpedo-shaped bodies (barracuda)
25.1 Vertebrate Origins
• Ray-finned fish have a variety of body plans.– flattened bodies (plaice)
25.1 Vertebrate Origins
• Ray-finned fish have a variety of body plans.– elaborate camouflage (sea dragon)
25.1 Vertebrate Origins
• A swim bladder helps a fish float higher or lower in the water.
swim bladder
25.1 Vertebrate Origins
• Some ray-finned fish have both lungs and gills.– can breathe air and survive out of water for several
hours at a time– example: bichir found in West Africa
25.1 Vertebrate Origins
Lobe-finned fish have paired rounded fins supported by a single bone.
• Lobe-fins are paired pectoral and pelvic fins that are round in shape.– not as maneuverable as ray-fins– able to support weight
lobe fin
25.1 Vertebrate Origins
• Only seven species of lobe-finned fish exist today.
– coelacanths
– lungfish
25.1 Vertebrate Origins
KEY CONCEPT Amphibians evolved from lobe-finned fish.
25.1 Vertebrate Origins
Amphibians were the first animals with four limbs.
• Tetrapods are vertebrates that have four limbs.
The fossilized remains of Tiktaalik roseae indicate it was a transitional species between fish and tetrapods.
25.1 Vertebrate Origins
• A number of adaptations allow amphibians to live on land.– large shoulder
and hip bones– mobile, muscular
tongue– middle ear– breathe through
skin or with gills or lungs
25.1 Vertebrate Origins
Amphibians return to the water to reproduce.
• Amphibians use many strategies to keep their eggs wet.– lay eggs directly in water– lay eggs on moist ground– wrap eggs in leaves– brood eggs in pockets
on the female’s back
25.1 Vertebrate Origins
adult frog
young frog
tadpoles
fertilized eggs
• Tadpoles are aquatic larvae of frogs. • During metamorphosis, tadpoles develop into their adult
form.
• Not all amphibians undergo metamorphosis.
25.1 Vertebrate Origins
Modern amphibians can be divided into three groups.
• Salamanders have a long body, four walking limbs, and a tail.
• There are over 300 species of salamanders.
25.1 Vertebrate Origins
• Frogs are the largest amphibian group and include toads.• There are over 3000 species of frogs.• Glands in the skin of frogs and toads contain poisons that
help to protect them from predators.
25.1 Vertebrate Origins
• Caecilians are legless, burrowing, tropical amphibians.• There are 160 species of caecilians.
25.1 Vertebrate Origins
25.1 Vertebrate Origins
Amniotes can retain moisture.
• An amniote develops inside a thin, tough, membranous sac as an embryo or fetus.
Amnion Protectsand surroundsthe embryo
25.1 Vertebrate Origins
• Several characteristics help amniotes prevent water loss.– keratin forms a hydrophobic layer
– larger size of kidneys and intestines increases water absorption
25.1 Vertebrate Origins
Amniotes do not need to return to water to reproduce.
• The amniotic egg is an almost completely waterproof container.– prevents embryo from drying out as it develops– each egg represents a large investment of energy
25.1 Vertebrate Origins
• Not all amniotes lay eggs.– reptiles such as garter snakes retain their eggs– most mammals develop inside the mother’s reproductive
tract
25.1 Vertebrate Origins
• The placenta is a membranous organ that develops in female mammals during pregnancy.
placenta
umbilical cord
uterus
amniotic sac
– lines the uterine wall and partially envelops the fetus– carries nutrients from mother to embryo and removes
wastes