Phylum Chordata

Group Protochordata (Acraniata)

Subphylum Urochordata

Subphylum Cephalochordata

Group Craniata

Subphylum Vertebrata

Superclass Agnatha

Class Myxini

Class Cephalaspidomorphi

Superclass Gnathostomata

Four Distinct Characteristics of Chordates:

1. Notochord

2. Dorsal Hollow Nerve Cord

Inverts -

Chordates -

3. Pharyngeal Pouches and Gill Slits

Animals that use gills for respiration - two pockets break through forming slits

In others - may not break through.

Tetrapods - pouches give rise to eustachian tubes, middle ear cavity, tonsils, and parathyroid glands.

4. Postanal tail

Humans - coccyx

Vertebrate origins

Two living protochordate groups:

Subphylum Urochordata (Tunicata)

Larval form has all four chordate characteristics

Adult does not; notochord and tail disappear and DHNC reduced to single ganglion

Subphylum Cephalochordata

Lancelets

Amphioxus -------->> Branchiostoma

Has all four chordate characteristics

Nerve chord expands at the anterior end to form a simple brain.

Other features that suggest vertebrate plan:

1. liver (hepatic) cecum - food particles are phagocytized and digested.

2. resemblance to a vertebrate pancreas

3. segmented trunk muscles

4. basic circulatory plan of advanced chordates.

Subphylum Vertebrata (Craniata)

Evolution of Vertebrates: specialized adapations

1. Endoskeleton

Living vs the dead exoskeleton of Arthropods

Grows with body

Scaffolding for muscle attachment

Skull and rib cage protects viscera

Cartilage -------------->> Bone

2. Pharynx and efficient respiration

Gill slits - suspension feeding using cilia = filter feeders

Larger food items needed for a predatory lifestyle -- muscular pharnyx

Capillary beds in gill pouches - increased circulation of gases needed for increased metabolic rate

3. Advanced Nervous System

Increased sensory and motor controls

Paired eyes paired ears electroreceptors chemical receptors

4. Paired limbs

Most vertebrates have pectoral and pelvic appendages - fins or jointed legs

Fish used fins to help maintain stability in water

Tetrapods used legs for locomotion on land

Primitive vertebrate ancestral stock???

Ostracoderms = earliest vertebrate fossils

Pikaia - Burgess Shale of British Columbia

notochord segmented muscles = myotomes

resembles amphioxus

Tunicates?

Garstang's hypothesis - paedomorphosis

Cephalochordate - amphioxus?

No longer considered direct ancestor but a sister group of vertebrates

No brain or other sensory structures associated with vertebrates

No pharyngeal musculature to pump water - use cilia

Ammocoete Larva of Lamprey (Subphylum Vertebrata, Superclass Agnatha)

Characteristics lacking in amphioxus but homologous to those of vertebrates:

1. muscles for feeding instead of cilia

2. two chambered heart

3. three part brain (fore, mid and hind brain)

4. special sense organs

5. thyroid gland

6. pituitary gland

7. true liver replacing hepatic cecum of amphioxus, gall bladder and pancreatic tissue

Earliest Vertebrates - Superclass Agnatha - Jawless Vertebrates

Ostracoderms = several groups of heavily armored extinct jawless fishes

Lacked paired fins

Armor of bony plates (endoskeleton)

Muscular pump to pull water through pharynx

Earliest Jawed Vertebrates (Superclass Gnathostomata) Extinct

Jaws - from modification of first two gill arches

Placoderms among first jawed vertebrates. Heavily armored like ostracoderms but had jaws.