Chordate Phylogeny Chordates are Bilaterian Deuterostomes

Phylogeny (cont.)

5 Classes (before tetrapods)1. Agnatha

Jawless fish; hagfish and lampreys

2. Chrondrichthes Sharks, skates and rays3. Osteichthes Bony fish4. Actinopterygii Ray- finned fish5. Sarcopterygii Lobe-finned fish

Sea Squirt

Amphioxus

Lamprey

Hagfish

Chordate Origins

Chordates developed during the Cambrian period, around 560 Ma

Evolutionary issueThere is a poor fossil record for the origin of chordates, since they were most likely soft-bodied creatures

Primitive chordates (like the sea squirt) are studied closely, along with Hemichordates, to try and piece together their origins

Chordate characteristics:

1. Dorsal, hollow nerve cord2. Notochord

Chordate Basics

3. Pharyngeal slits4. Post-anal tail

Embryonic

Development

1. Egg becomes fertilized, then divides repeatedly (cleavage)

2. Cells arranged into a round, hollow ball (called a blastula) with a fluid filled cavity (called the coelom)

3. One side of the ball indents and grows inward (gastrulation) until an opening is formed (called the blastopore)

4. Indentation continues until it breaks through the wall on the other side, creating a second opening

The notochord forms out of the mesoderm • made up of a hard, fibrous material

The dorsal, hollow nerve cord is derived from the ectoderm (through ‘invagination’)• eventually forms the brain and spinal

cord

The Pharyngeal slits develop after the mouth and pharynx form, creating holes or slits into the pharynx• may become gills for respiration• used to filter food or eject water while

eating

The post-anal tail is created when the body elongates

Embryonic Dev. (cont.)

Considered to be “primitive” chordates (“proto” means first)

Larvae are generally planktonic, while adults may be sessile, benthic or they may burrow into the substrate

May be solitary or colonial, and feed by means of cilia and mucus

Male and female gonads are either found in a single individual (monoecious) or in separate individuals (dioecious)

Protochordata Includes subphylums:• Hemichordata• Urochordata

(tunicates)• Cephalochordates

Thought to be an intermediate group of species between the echinoderms and the chordates

• Acorn Worm• Pterobranch• Graptolites (extinct)

Has pharyngeal slits and a dorsal nerve cord (may not be hollow). However, they do not appear to have a notochord (merely a stomochord) and have no post-anal tail

Hemichordata

Acorn Worms(Enteropneusts - Hemichordata)

• # Species: 75-80• Evolved: 500-540 Ma• Size: can reach over 1 meter• Nutrition: Filter feeders or normal ingestion• Lifestyle: Planktonic larvae, burrowing adults• Support: Stomochord• Musculature: Circular or longitudinal orientation• Non-segmented muscles

• Reproduction: Dioecious or segmentation• Chordate Characteristics: Pharyngeal slits and dorsal nerve cord

Acorn Worms (cont.)(Enteropneusts - Hemichordata)

Pterobranch(Hemichordata)

• # Species: about 20• Evolved: 500-435 Ma• Size: can reach 1 mm• Nutrition: Filter feeders (ciliated tentacles)• Lifestyle: Planktonic larvae & sessile, colonial adults• Support: Stomochord and rigid, tube housing• Musculature: Circular or longitudinal orientation• Non-segmented muscles

• Reproduction: Dioecious or asexual• Chordate Characteristics: Pharyngeal slits and dorsal nerve cord

Graptolites EXTINCT(Graptolithina - Hemichordata )

•# Species: thought to be hundreds•Evolution: 490-545 Ma (extinct 298-354 Ma)•Lifestyle: Colonial and sessile or benthic

These were tiny, tubular creatures, which when fossilized may resemble a saw blade.

Subphylum

Urochordata

Over 20,000 species - thought to be the oldest living

chordates

Includes:•Tunicates (aka Sea Squirts; Ascidiacea)•Larvacea•Thaliacea

Tunicates (Sea Squirt)(Ascidiacea - Subphylum Urochordata)• # Species: about 2,000• Evolved: 540 Ma• Size: (small)• Nutrition: Filter feeders (complex straining apparatus); eat mostly plankton• Lifestyle: Planktonic larvae & sessile adults• Support: Notochord and tunic• Musculature: •Reproduction: hermaphroditic or asexual

Tunicates (cont.)(Ascidiacea - Subphylum Urochordata)

Larvacea and Thaliacea(Subphylum Urochordata)

• Larvacea look more like tunicates during adulthood• Thaliacea retain their tadpole-like body plan from larval state to their plankton-like adulthood (benthic)• Some are sexual and others are asexual, skipping the larval state altogether• May live solitary or colonial lives

Subphylum

Cephalochordata

About 25 species

This is the first group of chordates to retain all 4 traits into adulthood

Includes:•Lancelets (or Amphioxus)

Lancelets (Amphioxus)(Subphylum Cephalochordata)

• # Species: about 20• Evolved: 520 Ma• Size: up to 2 inches• Nutrition: Filter feeders - plankton and algae• Lifestyle: Burrowing adults• Support: Notochord (extends all the way to snout)

• Musculature: Segmented (myotomes)•Reproduction: Dioecious

Evolution of

Vertebrates Debate

DNA confirms neither of these diagrams are

feasible

Why? Larval Development basis Pharyngeal slit development basis

Class Craniata

The evolution of heads!

The evolution of the head (or skull) occurred around the same time that vertebrates evolved.

All species in the Subphylum Vertebrata convert their notochord into a backbone (except for the hagfish)

Subphylum Vertebrata

The notochord is now converted to vertebral plates.

With the inclusion of a head (or skull), species developed a brain, eyes and more complex nervous systems (the nerve cord becomes the spinal cord).

Includes:• Hagfish• Lampreys•Cartilaginous, lobe-finned and bony fish

Hagfish(Subphylum Vertebrata)

• # Species: about 72• Evolved: 330 Ma• Size: about 30 inches• Nutrition: Parasitic or scavengers; eat mainly worms or dead creatures• Lifestyle: Live in the muddy bottom of the ocean

There is no known larval state• Support: Notochord •Reproduction: Dioecious (has both male and female gonads, but not hermaphroditic)

Lampreys(Subphylum Vertebrata and Craniata)

• # Species: approx. 3• Evolved: approx. 330 Ma• Size: 5-40 inches• Nutrition: Filter feeders as larvae & parasitic adults (feeds on the blood of prey)• Lifestyle: Burrowing larvae & free swimming adults• Support: Cartilaginous skeleton•Reproduction: Dioecious (has both male and female gonads, but not hermaphroditic)

Questions?