Trends in Chordate Evolution

Characteristic Features of the Phylum Chordata - Image

Single, hollow dorsal nerve cord

Flexible dorsal notochord

Pharyngeal slits or pouches

Muscular postanal tail

Evolutionary relationships among the chordates - Image

Non-Vertebrate Chordates  Fig. 25.38

Tunicates - Image

Lancelets - Image - Image   Fig 25.37

Subphylum Vertebrata- Phylogeny - Fig 26.8

Characteristics of the vertebrates

A vertebral column (Snake skeleton) replaces the notochord

Possess a distinct skull (cranium) enclosing the brain (cephalization)

Endoskeleton with axial and appendicular portions, which grows with organism

Hollow dorsal nerve cord (spinal cord) enclosed in a groove in vertebral column

Possess characteristic liver, kidneys, endocrine glands

Circulatory system with a heart and closed blood vessels

Endothermy versus ectothermy - Image

- Ectothermy = cold-blooded: Image

- Endothermy = warm-blooded

Trends in circulatory systems - Image

Trends in respiratory systems - gills to lungs

Characteristics of All Fishes Table 26.2

Gills - Fig 30.2

Single-loop blood circulation - 2-chambered heart

"Class" Agnatha: Jawless fishes: Hagfishes (Craniata) (Image) and Lampreys (Image)

Evolution of jaws - (Fig 26.10) occurred 410 million years ago.

Radiation of jawed fishes. - all ectothermic

Class Chondrichthyes: Sharks, skates and rays - Image Manta ray - the movie!

Class Osteichthyes: Bony fishes and Image and

Most fish are ray-finned fishes.

Lobe-finned fishes (coelocanth - Image)


A Devonian tetrapod fish - Image

Paired fins consist of fleshy, muscular lobe supported by bone core

The path to land.

Class Amphibia - Image - Image   Table 26.3

- Legs

- Lungs

- Partially divided heart - 3 chambers

- Ectothermic

- Water-bound reproduction - Image

Different views of the taxonomic classes of the amniotes - Image

Class Reptilia - Image   Table 26.4

- Amniotic egg - Fig 26.17

- Hatching reptile - Image

- Scales made of keratin

- Most with 3-chambered heart

- Ectothermic

- Septum totally divides ventricle in crocodiles and birds (and likely dinosaurs); makes 4 chambers

Class Aves - Birds - Image   Table 26.5

- Archaeopteryx - Image

- Feathers of keratin - Image

- Flight skeleton

- Lungs and air sacs near and in bones of back - Image

- Wall dividing ventricle is complete, two circulations do not mix

- Endothermic

- Amniotic egg with calcium carbonate for hardness

Class Mammalia - Mammals - Image   Table 26.6

- Hair

- Mammary glands

- Endothermic

- Four-chambered heart

- Keratin is structural material for claws, fingernails, hooves, horns, hair

- Differentiation of teeth (Fig. 26.23) and refinement of ear bones - Image

The Groups of Mammals

Monotremes: egg-laying mammals

- Includes duck-billed platypus and two species of spiny anteater - Image

- Lay shelled eggs

Marsupials: pouched-mammals (Image)

- Early embryo nourished by abundant yolk within shell-less egg and primitive placenta

- Born at early stage of development and most enter marsupium

- kangaroos, opossum, koalas, wallabies

- Most live in Australia where have evolved to fill niches filled by placentals in rest of world through convergent evolution. - Image

Eutherian (placental) mammals

- Produce true placenta that nourishes embryos Fig. 26.22

- Placenta (Image) is first organ to form during course of development

- Held in uterus of mother, contains abundant fetal and maternal blood vessels

- Fetus enclosed in amnion