Chordate Origins

• Closest relatives - Echinodermata and Hemichordata (acorn worms, pterbranchs)
• Key traits:
• Notochord 
  • Longitudinal flexible chord between nerve chord and digestive tube
  • Large-fluid filled cells
  • Maintain hydrostatic presssure, provide support and act as structure for muscles to act against
  • Replaced by vetebral column in higher vertebrates
• Dorsal nerve chord (spinal column)
  • Develops by invagination of dorsal ectoderm
  • Hollow nerve chord develops into CNS
• Pharyngeal slits (throat slits)
  • Allow water to exit body and nutrients remain - filter feeding
  • Slits -> gills -> jaws -> ear bones
• Muscular post-anal tail
  • No digestive tract in tail
  • Propelling force -> concentration of sensory organs (cephalisation)
• Segmentation
  • Only key trait if evolved independently

Sea squirts, tunicates (2000 species)

• Either this or cephalochordata most pirimitive chordates
• Sessile filter feeders
• Adults - pharyngeal slits
• No CNS 
• Larva have all other traits - disappear during metamorphosis when settles
• One group free living and retains traits

 Lancelets (amphioxus)

• Plankton feeders
• Burrow in sand
• Anterior tip of nerve chord swollen - brain beginnings

• Pharyngeal slits = gill slits (no longer filter feeding)
• Neural crest cells migrate to head = cephalisation

EXTINCT - Haikouella (oldest crainiate)

• Eyes and brain
• No skull
• More fossils needed to see if brain or skull comes first

Hagfish (30-60 species)

• Pirimative craniates
• Cartillage skull
• No jaws
• No vertebrae
• Notochord
• Scavangers or parasites
• Produces slime as defence

• More extensive skull - still cartilage
• Vertebrae in addition to/replacing notochord
• Fin rays stiffen fins

Lampreys (35 species)

• Primitive vertebrata
• Parasites
• Cartilage skeleton
• Cartilagionous pipe around notochord - beginnings of vertebral column

• Abundant tooth-like fossils
• Late Cambrian to late Triassic
• Complex dental system - first instance of mineralisation

• Anterior gill slits = jaws
• Increasing mineralisation
• Two sets pair fins

Extinct Placoderms

• <23m long
• Big bony heads
• First instance of internal fertilisation

Extinct Acanthodians

• Smaller
• Less armoured with bony fin spines

• Mostly cartilage skeleton (suggests anscestors had bone but evolved back)
• Marine, some freshwater

Extinct Helicprion shark/ratfish

• Weird spiral mouthparts like teeth conveyorbelt

• Fully mineralised endoskeleton
• Varying lifestyles
• No notochord - vertebrate instead
• Skull with lots of bones
• Swim bladder - same organ as lung

• Lobe-finned fish
• Rod-shaped bones surrounded by muscles in pectoral/pelvic fins
• Yes, we're fish

• Thought extinct
• 2 living species near islands

• Gills
• Swim bladder can act as a lung
• Survive drought in mud cocoon

• Limbs/feet
• Gil slit supports - ear bones
• First appeared in Devonian period

Extinct Tiktaalik 

• Canadian fossil 
• Croocodile fish mutant
• Intermediate both in time and physiology

Extinct Gogonausus

• ~380mya 

• Breathing (no gills)
• Drying out
• Egg fertilisation
• Embryo survival 
• Body support

• Adults live on land and breathe with lungs/skin
• Larvae live in water and breath with gills/skin
• Metamorphosis

So magical

• Lungs
• Scales to protect against desication
• Membranous embryoes
• No obligatory water stage
• Early carboniferous ~345mya
• Extraembryonic membranes surround embryo water
• Egg shell protection
• Internal fertilisation

• Extinct parareptiles - herbivore
• Extinct pterosaurs - flight
• Extinct plesiosaurs - swim
• Extinct ichtyosaurs
• Dinosauria - dinos, birds
• Lepidosauria - snakes, lizards
• Crocodilia - crocodiles
• Testudines - turtles

• Produce milk from modified sweat glands
• Endothermic - high metabolism
• Hair
• Mostly viviparous - live young
• 34 bones in skull - 2 main ones cranium and lower jaw (simplification)
• Tiny ear bones

Extinct Synapsidia:

• No hair
• Oviparous - lays eggs
• Dominant land tetrapods during Permian
• Few survived and evolved

Monotremata - platapus and echidana

• Hair 
• Produce milk from pores everywhere
• Oviparous
• Australia and New Guinea

Marsupalia - opposum, wombat, kangoroo

• Viviparous - underdeveloped young - move to pouch for second pregnancy
• Australia, South/North America

Eutheria - cats, humans, dolphins

• Placental mammals - interface between embryo and mother, feeds embryo
• Viviparous
• Embryo develops in uterus
• Woldwide because of endothermy and efficient circulatory system - environ independence
• Lrrge brain size - behavioural adaptation
• Teeth differentiation - evolutionary adaptation