The Origin and Early Evolution of the Chordata

Deuterostome Synapomorphies

• Enterocoely
• Blastopore forms anus
• Tripartite coelom

Deuterostome Relationships

Echinoderm origins

• Three assumptions
• 1) Motile, bilateral ancestor with tripartite coelom
• 2) Radial symmetry, skeleton are adaptations to sessile lifestyle
• 3) Water-vascular system arose as a feeding device

Clues from Embryology

• Coelom tripartite
• Protocoel (Axocoel)
• Mesocoel (Hydrocoel)
• Metacoel (Somatocoel)

Clues from Embryology

• Left side -> oral surface
• Left metacoel becomes coelom around mouth
• Left protocoelom/ mesocoelom form ring canal, stone canal, radial canals, tube feet
• Right side ->aboral surface
• Right metacoel forms coelom on "top"
• Right protocoel, mesocoel reduced

A scenario of echinoderm origins

• Ancestor became attached
• Developed radial symmetry, skeleton
• Rotation so that left side became "upper" side
• Left "lophophore" -> ring canal, tube feet; right "lophophore" lost
• Note: Both the lophophore and water vascular system are derived from the mesocoel
• In Eleutherozoa, switch to oral surface down, use of tube feet for locomotion

Deuterostome Relationships

Phylum Hemichordata

• Two Groups
• Pterobranchia
• Enteropneusta
• Monophyly disputed, but …
• Share tripartite plan
• Proboscis, collar, trunk

Pterobranchia

• About 25 species
• Small (about 1cm)
• Colonial
• Zooids may be connected
• Secrete protective tube
• U-shaped gut

Pterobranchia

• Ciliated tentacles very lophophore-like
• Mesocoel
• Ciliary suspension feeding
• Generate current with cilia
• Trap food in mucus
• Move to mouth with cilia
• Gill slit

Enteropneusta

• Acorn worms
• About 75 species
• Up to 2 m
• Direct deposit feeding
• Suspension feeding
• Food trapped in mucus
• Moved to mouth by cilia

Enteropneusta

• Rows of gill slits
• Exit for feeding current
• Not used for respiration
• Dorsal hollow nerve cord
• Probably homologous to that of chordates

Hemichordates - review

• Echinoderm and/or lophophorate like features
• Mesocoel extensions for feeding
  • Like lophophore of lophoporates
  • Like tube feet of echinoderms
  • Mesocoel extensions may be primitive for deuterostomes
• Larvae very similar to larvae of Echinoderms

Hemichordates - review

• Echinoderm and/or lophophorate like features
• Mesocoel extensions for feeding
• Larvae very similar to larvae of Echinoderms
• Chordate like features
• Gill slits
• Dorsal, hollow nerve cord
• Chordate like features appear to be synapomorphies

Phylum Chordata

• Three major groups

Phylum Chordata

• Clearly monophyletic
• Primitive similarities
• Gill slits
• Dorsal, hollow nerve cord
• Synapomorphies
• Notochord
• Post anal tail
• Endostyle

Subphylum Urochordata

• Adults sessile
• Suspension feeding
• Ciliary current
• Particles trapped in mucus secreted by endostyle
• Collected dorsally

Subphylum Urochordata

• Adults sessile
• Suspension feeding
• Ciliary current
• Particles trapped in mucus secreted by endostyle
• Collected dorsally

Subphylum Urochordata

• Adults sessile
• Suspension feeding
• Ciliary current
• Particles trapped in mucus secreted by endostyle
• Collected dorsally
• Don’t seem much like Chordates, but ...

Subphylum Urochordata

• Free-swimming "tadpole" larvae
• Exhibits the basic chordate characteristics
• Notochord
• Post anal tail
• Endostyle
• Gill slits
• Dorsal hollow nerve cord

From Free-swimming to sessile
Urochordate Metamorphosis

• Lose or greatly reduce
• Notochord
• Tail
• Nervous system

From sessile to free swimming adults
Paedomorphosis

• Retain ancestral free-swimming "juvenile" form as adults of descendants

Subphylum Cephalochordata

• Sister group of Craniates
• Excellent insight into craniate origins!
• Has the classic chordate characters, plus …
• Can be active, even as adult
• Myomeres

Subphylum Cephalochordata

• Cephalochordate-like fossils in Cambrian
• Pikaia
• Amphioxus/Pikaia are our baselines for understanding the origins of craniates

Cephalochordates - Locomotion

• Notochord
• Provide axial support
• Body bends rather than shortens when muscles contract

Cephalochordates - Locomotion

• Notochord
• Myomeres
• Segmentally arranged muscle blocks
• Shared with Craniates!

Cephalochordates - Feeding

• Many gill slits
• Suspension feeding
• Ciliary current
• Particles trapped in mucus secreted by endostyle
• Collected dorsally
• Similar to Urochordates

Cephalochordates
Respiration/Circulation

• No specialized respiratory organs
• Small, so use body surface

Cephalochordates
Respiration/Circulation

• Closed circulation
• No heart, just contractile vessels
• Pattern similar to "fishes"
• Ventral aorta -> Gill bars -> Dorsal aortae -> Body -> …
• No blood pigments

Cephalochordates
Sensory/Nervous systems

• Sensory systems fairly rudimentary
• No eyes, ears, etc.
• Dorsal, hollow nerve cord, but …No brain

Chordate evolution

• Phase 1: Origin of ...
• Gill slits, Dorsal nerve cord
• Base of Hemichordates/chordates
• Phase 2: Origin of …
• Notochord
• Endostyle
• Post-anal tail, swimming phase
• Base of Chordates
• Phase 3: Origin of …
• Myomeres/segmentation
• Distinctly "Craniate-looking" circulation
• Base of Cephalochordates and Craniates

Early Chordate Evolution

The origin of Craniates

• Transition from "Protochordates" to craniates marked by a dramatic change in …
• Activity level
• Sensory systems
• Locomotor efficiency
• Feeding strategies
• Size

The origin of Craniates - Two key features

• 1) Neural crest cells

The origin of Craniates - Two key features

• 1) Neural crest cells

The origin of Craniates - Two key features

• 1) Neural crest cells
• Gill arch supports
  • More efficient respiration
  • Eventually jaws
• Some bone, eventually
  • More effective protection
  • More effective locomotion
  • Parts of Teeth
• Myelinating cells, many neurons
  • More effective movement
  • More effective digestion

The origin of Craniates - Two key features

• 1) Neural crest cells
• 2) Placodes
• Contribute to much of the sensory system, including
• Ear, lateral line
  • Balance, hearing, pressure sense
• Olfactory
  • Sense of smell
• Optic
  • Lens of eye

The origin of Craniates - Other changes

• Respiratory structures
• Respiratory pigments
• Heart
• Muscular gut
• Cartilage and eventually bone for protection and support

Chordate evolution - Summary

Chordate evolution - Summary

Midterm 2 Information

• Exam mean = 67
• Exam Standard Deviation = 14
• High Score = 93
• Key will be posted Tuesday
• Regrade requests due at end of lecture on Thursday, December 2