Chordates are
Bilaterian Deuterostomes
Amphioxus
Sea Squirt
5 Classes (before tetrapods)
1. Agnatha
Jawless fish;
hagfish and lampreys
2. Chrondrichthes
Sharks, skates and rays
3. Osteichthes
Bony fish
4. Actinopterygii
Ray- finned fish
5. Sarcopterygii
Lobe-finned fish
Lamprey
Hagfish
Chordates developed during
the Cambrian period,
around 560 Ma
Evolutionary issue
There 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 cord
2. Notochord
3. Pharyngeal slits
4. Post-anal tail
1.
2.
3.
4.
Egg becomes fertilized, then
divides repeatedly (cleavage)
Cells arranged into a round,
hollow ball (called a blastula)
with a fluid filled cavity (called
the coelom)
One side of the ball indents and
grows inward (gastrulation)
until an opening is formed (called
the blastopore)
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
Includes subphylums:
• Hemichordata
• Urochordata (tunicates)
• Cephalochordates
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)
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
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.
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
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
DNA confirms neither of these
diagrams are feasible
Why?
Larval Development basis
Pharyngeal slit development basis
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)
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)