Albia Dugger • Miami Dade College

Chapter 25Animal Evolution –

The ChordatesSections 1-5

25.1 Transitions Written in Stone

• Transitional fossils provide evidence of the evolution of birds from dinosaur ancestors• Archaeopteryx, an ancient winged dinosaur with feathers• Confuciusornis, a bird with claws on its wings• Sinosauropteryx, a dinosaur with feathers

• The structure, biochemistry, and genetic traits of living organisms also provide information about the evolution of modern animal groups

Archaeopteryx

Confuciusornis

Sinosauropteryx

25.2 Chordate Traits and Trends

• Chordates (phylum Chordata)• Most diverse lineage of deuterostomes• Some are invertebrates; most are vertebrates• Bilateral and coelomate• Cephalized and segmented• Complete digestive system• Closed circulatory system• Classified by embryonic characteristics

Chordate Characteristics

• Four characteristics of chordate embryos may not persist in adults• Notochord of stiff connective tissue that extends the

length of the body and supports it• Dorsal, hollow nerve cord parallels the notochord• Gill slits across the wall of the pharynx• Tail that extends beyond the anus

Invertebrate Chordates

• Lancelets (subphylum Cephalochordata) are the only group of chordates that retains all chordate characteristics as adults

• Tunicates (subphylum Urochordata) have typical chordate larvae, but adults retain only the pharynx with gill slits

Figure 25-2a p420

anus

tail extends past anus

eyespot

notochord

dorsal nerve cord

pharynx with gill slits

A Lancelet

ANIMATED FIGURE: Lancelet body plan

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Free-swimming tunicate larva

notochorddorsal nerve cord

pharynx with gill slits

postanal tail

Figure 25-3b1 p420

pharynx with gill slits

secreted“tunic”

waterflowsout

water flows in

Overview of Chordate Evolution

• Tunicates are the invertebrate lineage most closely related to the vertebrates

• Vertebrates are chordates with an internal skeleton (endoskeleton) of cartilage or bone

• Modern vertebrates (except lampreys) have jaws derived from gill-supporting structures

Overview of Chordate Evolution (cont.)

• Fins with bony supports evolved in a subgroup of jawed fishes

• Bony fins later evolve into limbs of the first four-legged walkers (tetrapods)

• The development of eggs allowed that enclosed embryos within waterproof membranes allowed amniotes to disperse widely on land

Evolutionary Tree for Chordates

Amniotes

ancestral chordate

Bony appendages

Amniote eggs

Swim bladder or lung(s)

Backbone

TunicatesReptiles

(with birds)Lancelets

Jaws

Four limbs

Cartilaginous fishes

Ray-finned fishes

Lobe-finned fishes Mammals

Jawlessfishes

ChordatesVertebratesTetrapods

Amphibians

ANIMATED FIGURE: Vertebrate evolution

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Take-Home Message: What traits define the major subgroups of chordates?

• All chordate embryos have a notochord, a dorsal tubular nerve cord, a pharynx with gill slits in its wall, and a tail that extends past the anus

• There are two groups of invertebrate chordates: lancelets and tunicates

• Most chordates also have a backbone and so are vertebrates; limbs evolved in one lineage that later colonized the land

• Amniotes, a tetrapod subgroup with specialized eggs, are the predominant vertebrates on land

25.3 Jawless Fishes

• Fishes are aquatic, nontetrapod vertebrates that typically have gills throughout their lifetime

• They are ectotherms, animals whose body temperature varies with that of their environment

• The first fishes were jawless; the skeleton consisted of cartilage, and the brain was enclosed in a cranium

Two Groups of Jawless Fishes

• Two groups of jawless fishes survived to the present: lampreys and hagfishes

• Both groups have a skeleton composed of cartilage, and lack the scales and paired fins typical of jawed fishes

• Their gill slits are uncovered and visible at the body surface

Lampreys

• Modern lamprey live their whole life in fresh water or live in the sea as larvae, then return to fresh water to breed

• Unlike most fish, lampreys undergo metamorphosis; their larvae resemble larval tunicates or adult lancelets

• Many adult lampreys are parasites that attach to other fish with an oral disk with toothlike structures made of keratin

Lamprey

Hagfishes

• Hagfishes are marine bottom-feeders with poor eyesight; they use sensory tentacles to locate worms and carcasses

• Their mouth has dental plates covered with sharp barbs of keratin

• The most recent genetic comparisons indicate that hagfishes and lampreys constitute a monophyletic group

Hagfish

Take-Home Message:What are jawless fishes?

• Jawless fishes are gilled, aquatic vertebrates with a cartilage skeleton; they do not have jaws or scales

• Lampreys and hagfishes have hard mouthparts made of keratin

• Lampreys undergo metamorphosis and some parasitize other fish as adults

• Hagfishes are marine scavengers

25.4 Evolution of Jawed Fishes

• Jaws evolved from gill arches, skeletal elements that support a fish’s gills

• Jawed fishes typically have a body covered with scales and two pairs of fins: pectoral fins and pelvic fins

• Armored placoderms were the most numerous vertebrates in the seas during the Devonian period – the “Age of Fishes”

• Another group of early jawed fish lineages is the acanthodians (spiny fins)

Evolution of Jaws

jaw

location of spiracle (modified gill slit)

jaw supportjaw, derived from support structure

supporting structure for gill slitsgill slits

Stepped Art

ANIMATED FIGURE: Evolution of jaws

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Dunkleosteus, a Placoderm

Placoderm with Paired Fins

pectoral finspelvic fins

Take-Home Message: What traits characterized jawed fishes?

• Jaws evolved during the Silurian period by the modification of the first pair of gill arches in a jawless ancestor

• Jawed fishes were the first vertebrates with paired fins

• Placoderms were an early group of jawed fishes that had bony plates on their head and neck; some grew to great size

• Acanthodian lineages were smaller and lacked bony armor

25.5 Modern Jawed Fishes

• Cartilaginous fishes (Chondrichthyes) have a cartilage skeleton, gill slits, and teeth that shed• Sharks and rays

• Bony fishes (Osteichthyes) have a bony skeleton, gill covers, and a swim bladder• Ray-finned fishes, lungfishes, coelacanth

Relationships among Jawed Vertebrates

lobe-finned fishes“bony fishes”

tetrapods

placoderms (extinct)

cartilaginous fishes

acanthodians (extinct)

ray-finned fishes

Cartilaginous Fishes: Predatory Shark

Cartilaginous Fishes:Plankton-Feeding Shark

Cartilaginous Fishes: Manta Ray

Bony Fishes

• In bony fishes, bone replaces cartilage in the skeleton and gill slits are hidden beneath a gill cover

• Most bony fishes have a swim bladder that allows it to adjust its buoyancy

• Modern bony fishes include two lineages: ray-finned fishes and fleshy-finned fishes

Ray-Finned Fishes

• Ray-finned fishes have thin, membranous fins with flexible fin supports derived from skin

• Sturgeons are members of one ancient ray-finned lineage; gars are members of another early ray-finned lineage

• Most ray-finned fishes (including salmon, perch, and sardines) belong to the most recently evolved lineage, the teleosts – some have a highly modified body plan

Ray-Finned Fishes: Perch

heart

kidney

anus

swim bladder

ovary

intestine gills

brain

stomach liver

nerve cord

Ray-Finned Fishes: Gar

Highly Modified Ray-Finned Fishes

ANIMATED FIGURE: Bony fish body plan

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Lobe-Finned Fishes

• Lobe-finned fishes, the fish most closely related to tetrapods, have fleshy fins supported by bones

• There are two lineages, the marine coelacanths and the freshwater lungfishes

• Lungfishes have both gills and air sacs, modified outpouchings of the gut wall that function in respiration

Coelacanth

Lungfish

Take-Home Message: What are the characteristics of jawed fishes?

• Jawed fishes are cartilaginous fishes and bony fishes. Both groups typically have scales

• The ray-finned lineage of bony fishes is the most diverse group of vertebrates

• Lobe-finned fishes are the fish closest to the tetrapods