Chordates

Chapter 23

I. Ancestry and Evolution

Anatomical, developmental, and molecular evidence indicate chordates arose about 570 mya from lineage related to echinoderms.

One lineage of fossil echinoderms has pharyngeal slits and other chordate characteristics

II. CharacteristicsA. Notochord

Rodlike, semirigid tissue enclosed in sheath extending length of body between gut and nervous system that stiffens body or acts as skeletal axis

Found at some embryonic stage (usually 1st part of endoskeleton to appear) of all chordates

Persists throughout life of protochordates and jawless fishes but in vertebrates, it becomes the invertebral discs between vertebrae

B. Dorsal Tubular Nerve Cord

Nerve cord is in dorsal position as opposed to ventral position in invertebrates

Anterior end enlarges to form brain

C. Pharyngeal Pouches and Slits

Slits form in pharynx in aquatic chordates which form filter feeding apparatus in protochordates

Fishes added a capillary network which eventually evolved into gills

In amniotes, grooves do not penetrate into pharynx

D. Post-Anal Tail

Post-anal tail along with muscles, provide motility for larvae

Tail increased in fishes but reduced or vestigial in later lineages

III. Subphylum UrochordataA. Diversity

3000 species found in all seas and all depths

Larvae bear the chordate characteristics but lose all but pharyngeal slits as adults, which are sessile 1. Ascidians--sea squirts

live on rocks, pilings in intertidal areas and are filter-feeders

2. Thalacians—salps live in open-ocean and filter-feed

3. Larvacea—resemble larval forms of tunicates and filter-feed

IV. Subphylum CephalochordataA. Characteristics

Lancelets (amphioxus) are slender, laterally flattened, translucent animals about 5-7 cm. Long

Live in sandy bottoms of coastal areas around the world

B. Form and Function

Filters water through pharyngeal slitsClosed circulatory system without heartNerve cord above notochordConsidered living descendant of ancestors that

produced cephalochordates and vertebrates

V. Subphylum VertebrataA. Characteristics

1. Endoskeleton Grows with individual, jointed

to allow scaffolding for muscles

Skull and rib cage enclose and protect organs

Tough integument also protect individual

Cartilage probably 1st endoskeleton material because it promotes fast growth; remains in sharks

Bone can store minerals and has added strength needed for terrestrial life

2. Pharynx & Efficient Respiration

As protovertebrates shifted from filter feeders to predators, pharynx modified into muscular feeding apparatus that could pump water

Circulation in internal gills improved with addition of capillary beds and aortic arches, which increased metabolic rate

3. Advanced Nervous System

Switch to predation created selective pressure for paired eyes with lenses and inverted retinas, pressure receptors, paired ears, electroreceptors, and chemical receptors

2 new vertebrate innovations in cells (extremely rare event in evolution) which resulted in cranium, cranial nerves, branchial skeleton, and aortic arches; also give rise to nose, eyes, ears, taste receptors, and lateral line mechanoreceptors

4. Paired Limbs Pectoral and pelvic

appendages originated as swimming stabilizers

Jointed limbs that developed are suited for life on land and permit fine movement

B. Evolutionary History1. Fossils

Only 1 invertebrate chordate fossil is knownIt has both a notochord, muscle bans, and is

similar to amphioxus

2. Garstang’s Hypothesis

Came up with the theory of paedomorphosis, the evolutionary retention of larval traits in an adult body

This occurs in some amphibians

Suggested that evolution occurs at some larval stages which is supported by embryological evidence

3. Amphioxus

Thought to be sister of vertebrates but more evidence indicates that it is a direct ancestor to earliest vertebrates

4. Lamprey Larvae Amnocoete (larval stage

of lampreys) resembles amphioxus

It has the most primitive set of organs seen in vertebrates:2-chambered heart, 3-part brain, median nostril, auditory vesicles, thyroid, pituitary gland, liver, gall bladder, and pancreatic tissue

5. Jawless Ostracoderms

Earliest articulated vertebral skeletons

Small,heavily armored, jawless fishlike animal

May have fed on invertebrates along ocean bottom