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Chordate/Vertebrate Diversity COMPLETE SUMMARY
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Notes covering the chordate and vertebrate diversity module of the BIO1004S course.
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Module 4 — Chordate DiversityCHAPTER 34 — Vertebrates
COCNEPT 34.1 CHORDATES
• Vertebrates —> derive name from vertebrae (series of bones that make up the backbone/
vertebral column)
• Much less diverse than insects…
• Great disparity!! —> enormous variation in characteristics (eg. Body mass)
• Vertebrates = member of chordata
phylum
• Chordates =
• Bilaterian (bilaterally symmetrical)
• Belong to clade of deuterostomina
• Deuterostomes = second mouth
• After fertilisation, cell division
begins
• At 32C stage, embryo = hollow
ball of cells (gastrulation)
• Opening called blastopore
forms + extends throughout
development opening on the
other end to form the gut
• In protostomes - rst opening =
mouth
• In deuterostomes - rst
opening = anus
DERIVED CHARACTERISTICS OF CHORDATES —> all possess these structural trademarks at
some point during development
• Notochord
• Longitudinal, exible rod located between the digestive tube and the nerve cord
• Composed of large, uid- lled cells encased in sti brous tissue (made of collagen +
cartilage)
• Provides skeletal support throughout majority of chordate’s length + allows muscles to work
against it
• Fulcrum for locomotion
• In most vertebrates —> more complex joined skeleton develops around the ancestral
notochord —> adult only retains remnants of embryonic notochord (nucleus pulposus of
intervertebral disks)
• Persists in shes and amphibians
• Replaced by vertebral column in reptiles, birds, mammals
• In air (less dense than water), need a stronger more supportive structure
• In humans = reduced + forms part of gelatinous disks between vertebrae
• Dorsal hollow nerve cord
• Develops from a plate of ectoderm that rolls into a tube dorsal to the notochord
• Unique to chordates
• Develops into a central nervous system (brain + spinal cord)
• Unique origin = neurulation (process leading to formation of nerve chord)
• Notochord send biochemical signals upwards causing a thickening of ectoderm unto
neural plate —> invaginated and rolls up to form a hollow tube
• Then leads to cephalisation
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, • Pharyngeal slits/clefts
• Digestive tube of chordates extends from mouth to anus
• Pharynx = posterior to mouthing chordate embryos —> series of arches separated by
grooves from along outer surface of pharynx = pharyngeal clefts
• Can develop into slits that open into pharynx
• Slits allow water entering mouth to exit body w/out passing through digestive tract
• In invertebrate chordates —> Function as suspension feeding devices (>50 in non
craniate chordates)
• Vertebrate chordates (without limbs) —> arches + slits modi ed into gills for gas
exchange
• Tetrapods —> pharyngeal arches develop into parts of ear, sinuses, tonsils + some
glands
• Post anal tail
• Greatly reduced in many species
• Non chordates have a digestive tract that extends almost full body length
• Chordate tail contains skeletal elements + muscles (propulsion in water)
• Balance, support + signalling
• Embryonic in humans + other great apes
• Muscle segments
• Work against notochord
• Muscle blocks with nerves (somites/myomeres) in cephalochordates + craniates
• V-shaped + simple in lancelets (early form)
• W shaped in vertebrate sh
• NOT homologous to invertebrate segmentation
• Did not develop from same germ layers or in same sequence of developmental events
(Non craniate chordates) = PROTOCHORDATES = Subphylum Urochordata (tunicates) +
Subhylum Cephalochordates
• SUBPHYLUM CEPHALOCHORDATA -
lancelets
• Notochord extends into head
• Tail with myomeres w/ segmented
nerves
• No larval stage —> free swimming
• Pharyngeal bars lack gills ( lter
feeders)
• Blade-like shape
• Burrowers —> trap particulate
matter in mucus nets secreted
across pharyngeal slits to lter feed
• Respire through external body
surface
• Closed circulatory system
• Have all chordate characteristics throughout lifetime
• Light receptive pigments (no eyes)
• Move via contraction of muscles against notochord (muscles arranged like chevrons <<<
and ex notochord to produce side to side undulations)
• Serial arrangement of muscles is evidence of segmentation
• Muscle segments develop from blocks of mesoderm —> somites (each side of the
notochord in all chordate embryos)
• External fertilisation
• Simple brain (swollen tip of dorsal nerve chord) but same genes as craniates in brain
development (HOX GENES!!)
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, • SUBPHYLUM UROCHORDATA (tunicates eg. Ascidians)
• Tunciates = Urochordates
• More closely related to chordates than lancelets
• Ascidians, sales, larvaceans
• Most resemble chordates during larval stage
• Larvae use tail muscles + notochord to swim in order to nd suitable substrate (guided by
light-gravity sensitive cells)
• Attaches to substrate with sucker on the anterior
• Once settle on substrate = retrograde metamorphosis (many of its chordate
characteristics disappear) (advanced to simple body plan)
• Tail + notochord resorbed
• Nervous system degenerates
• Remaining organs undergo torsion by 90°
• As adult —> only remaining chordate characteristic = pharyngeal slits
• Draws water in via incurrent siphon, passes through pharyngeal slits to chamber
(atrium) and exists via excurrent siphon
• Food particles ltered by a mucus net and transported to oesophagus via cilia
• Anus empties into excurrent siphon
• Some can expel water via excurrent siphon when attacked (sea quirts)
• RESPIRATION + FEEDING VIA PHARYNX
• Loss of chordate characteristics in adult occurs after the tunicate lineage branched o from
other chordates
• Tunicates = 9 hox genes
• Other chordates = 13 hox genes
• Loss of four hox genes = chordate body plan of tunicate larva is built using a di erent set
of genetic controls than other chordates
• Reproduction —>
• Generally hermaphroditic
• External or internal fertilisation
CONCEPT 34.2
• SUBPHYLUM CRANIATA (main characteristic = cranium/brain case)
• Great disparity
• Diverse morphologies
• Colonised nearly all habitats
• Ecological impacts (eg. Elephants pushing over trees) + important to humans
DERIVED TRAITS OF CRANIATES
• Head with cranium, brain and sophisticated sensory organs
• Eyes —> evolved early in craniate evolution
• Inner/middle ears (hearing, balance + vestibular senses)
• Noses —> smell
• Allows for completely new way of feeding —> active predation
• Neural crest cells —> unique tissue type (42 di erent structures)
• Teeth, bones, skull bones/cartilages, jaws
• Rods supporting gill bars, pharyngeal muscles, ventilation of lungs, gill pouches —> improved
respiration and water ow (allows for more active lifestyle)
ORIGINS OF CRANIATES
• Yannanozoans
• All chordate characteristics and some derived craniate features
• Large brain
• Well de ned eyes
• Muscular pharynx + gill bars
• Derived from neural crest cells??
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