Chordates

body plan

• Fundamental similarities exist between amphioxus and all other chordates 

  • Sometimes modified or lost in adults 

• These features = chordate body plan 

• Features are family resemblance showing we all descended from a common ancestor species

5 key features

• Shared by all chordates at some point in their life 

• tells us all chordates are related

  • Notochord 

  • Dorsal, hollow nerve chord 

  • Overlapping muscle segments 

  • Pharyngeal slits 

  • Post-anal tail 

notochord

• Stiffened rod of tissue between nerve cord and gut 

• Supports back – gives muscles something to pull against during movement 

• Lost in some adult vertebrates, tunicates; present in all chordate embryos 

nerve chord

• Chordate nervous system has a hollow, dorsal nerve chord 

• Forms when ectoderm rolls up into a tube (neurulation) 

  • The space inside your spinal chord used to be your back 

• In vertebrates, nerve chord developed to form the spinal chord 

muscle bands

• Muscles form overlapping segments (myomeres) in Amphioxus 

• Retained in vertebrates 

  • Vertebrae and ribs correspond to muscle segments: one vertebrae and rib per segment 

  • Superficially similar to segments of worms and arthropods (convergent evolution) 

  • Primitive relatives of chordates, worms and arthropods lack segments 

  • Chordate bands are homologous (common evolutionary origin) 

  • Worm and insect segments are analogous and convergent (independent evolutionary origin) 

• Segments in humans 

  • Segmental muscles (serratus) in back correspond to segments of Amphioxus and fish, as do ribs and vertebrae 

  • Humans retain segmental structure of ancestral chordate 

pharyngeal slits

slits make an opening in throat to let water through

post-anal tail

• In most animals (worms, arthropods, echinoderms, hemichordates) anus lies at the end of the body 

• In chordates, the body projects past the anus, to make a tail to swim with 

• Retained in vertebrates 

• Even in humans and apes 

  • Coccyx (tailbone) 

  • Fused vertebrae at the base of the pelvis 

diversity deuterostomes

• Bilaterally symmetrical animals (bilateria) fit in 2 groups  

  • Protostomia (mouth first) 

    • Mollusca 

    • Antropoda 

    • Bachiopodia 

    • Tardigradia  

    • etc 

  • Deuterostomia (anus first) 

    • 3 phyla 

    • Echinodermata 

    • Chordata 

    • hemichordata 

• In both, embryo starts as hollow ball of cells (blastula), then one end is pushed in (gastrulation) 

• Deuterostomes include chordates, echinoderms and hemichordates 

hemichordates (acorn worms)

• Deuterostomes united by embryonic development 

• Help bridge gap between chordates and more primitve animals 

  • Have pharyngeal slits and a dorsal nerve tube like chordates 

  • Lack a tail, muscle bands and a notochord 

deuterostomes

3 major chordate groups

• Cephalochordata (lancelets) 

• Urchordata (tunicates) 

• Vertebrata (hagfish, lamprey, sharks, fish, tetrapod, humans) 

tunicates (sea squirts)

• Filter feeders 

  • Mouth sucks water through the pharynx (incurrent siphon), which is slotted due to pharyngeal slits 

  • Food is filtered and water is expelled through the excurrent siphon 

• Larvae have notochords, muscle bands and tails 

• Become sessile as they attach to rock and metamorphosise 

• Has thick, leathery skin, reinforced with cellulose 

• Salps 

  • Pelagic tunicates that do not become sessile 

  • Form gelatinous, transparent colonies that float through the sea 

  • Propel themselves with siphons 

• Larvaceans 

  • Free-swimming tunicates 

  • Thought to represent tunicates that lost metamorphosis  

lancelets

• Amphixious swims like fish 

• Filter feeders 

  • Bury tail in sand and stick snout out to feed 

did tunicates evolve from a lancet-like ancestor?

• We think of tunicates as primitive. 

• But ancestor of protostomes and deuterostomes- ancestral Bilateria- had head and tail end, could move 

• Immobile and headless isn’t primitive. It is advanced relative to ancestral forms. 

• Lacking head is a specialised condition, having head is ancestral condition 

• Ancestral condition can be more complex than descendant condition. 

• Humans evolved by losing many chordate features 

the fossil record

• Lack bones, but there are still a few fossils 

• Chordates appear in cambrian 

hagfish

• Blind, eel-like fish 

• Dorsal and ventral fin like advanced fish, but lack jaws and eyes 

• Cartilage skull (cranium) 

• No vertebrae 

  • Classified as craniates 

• Suggested to be highly modified, rather than primitive 

  • Losing vertebrae is useful for animals that burrow inside carcasses and tie themselves in knots 

  • Eyes are of little use for animals that burrow in the dark depths of the sea 

lampreys (petromyzontiformes)

• Have long, slender bodies with dorsal and ventral tail fins, cartilage skull (like hagfish) 

• More sophisticated than hagfish 

  • Have vertebrae 

  • Have eyes 

• Classified as vertebrates 

• Have sucker-like oral disk with spines to hook onto fish 

  • Once attached, use toothlike barbs on the tongue to scrape away skin and suck blood 

• Share more DNA with hagfish than sharks and bony fish 

  • Form a group to the exclusion of other fish = cyclostomes 

features of vertebrata

• cranium

• brain

• eyes

• nostrils

• tongue

• gills

• fin rays

• vertebrae

• teeth

cranium

brain is surrounded by a box of cartilage or bone

brain

• Enlarged anterior nerve clusters evolve independently in arthropods and mollusks but are absent in bilaterian 

• Useful for processing sensory information from eyes, nose, body (especially tracking prey) and coordinating a response 

eyes

• Grow from the brain 

• Useless for filter-feeding (food is too small to see) 

• Simple eyes are useful for detecting movement to avoid predation 

• Complex eyes are good for resolving images and tracking movement (being a predator) 

nostrils

nasal passages let water pass over olfactory tissues to detect chemicals

tongue

for pushing food down the throat

gills

• Slits on the side of head let water pass in through the mouth, out of the slits 

• Supported by cartilage arches (gill arches), each bearing a series of filaments (gills) 

• Blood circulates through gills, absorbing oxygen from seawater 

• Improved oxygenation of blood lets vertebrates become larger, faster and more active 

fin rays

• Cartilage or bone struts support fins 

• Permits larger, more elaborate fins; associated muscles let vertebrates control shape and move fin 

vertebrae

• Vertebrae form around the notochord 

• Notochord is kept in some, and lost in others 

• In bony fish, vertebrae ossify (first form as cartilage and then turn to bone) 

• Make a strong support for the body, allowing for more powerful swimming muscles 

teeth

• For tearing apart other animals 

• In jawed vertebrates = hardened with calcium phosphate 

• In lampreys and hagfish, made of keratin 

vertebrae evolution - ecological shift

• Filter feeding probably appeared in worm-like chordate ancestors (seen in some hemichordates) 

• Chordates evolve swimming- to escape, to move, exploit new habitats (e.g., shifting sand) where attached filter feeders can’t live 

• In vertebrates, adaptations for tracking prey (eyes, nose, brain), feeding (teeth, tongue), pursuing prey (gills, vertebrae, fin rays) reflect shift to predation 

• Primitive vertebrates aren’t very good predators: maybe why they have disgusting feeding habits like burrowing into carcasses, sucking blood 

jawed vertebrates (gnathostomes)

• Chondrichthyes (cartilagenous fish) 

  • Selachimorpha - Sharks (470 species) 

  • Batoidea – rays, skates and sawfish (560 species) 

  • Holocephali – chimaeras (50 species) 

• Osteichthyes (bony fish) 

  • Actinopterygii – ray-finned fish (30,000 species) 

  • Sarcopterygia – lobe-finned fish (30,000 species) 

features of gnathostomes

• have features not seen in cyclostomes

• pectoral and pelvic fins

• jaws

• mineralised teeth

• mineralised scales

pectoral and pelvic fins

• Effective for stabilising/steering 

• 3-axis control 

  • Pitch (up and down) 

  • Roll (around long axis) 

  • Yaw (left and right) 

• Create hydrodynamic lift to do banked turns 

• Improved manoeuvrability works for pursuing prey and to avoid becoming prey 

• Used for propulsion 

jaws

• Modified from first gill arch 

• In sharks, made of cartilage 

• In bony fish, cartilage (Meckel’s cartilage) forms a scaffold on which bones of the lower jaw grow 

• Unlike cyclostomes, which literally lick prey to death, jaws let you seize, cut, tear prey- and expand mouth to eat much larger prey items 

• adaptation for predation 

mineralised teeth

• In hagfish and lamprey, teeth (if they really are teeth) made of tough keratin 

• In sharks and bony fish, teeth hardened with mineralised tissues: mineralised dentine covered by a very hard enamel 

• Probably homologous with scales 

• They cut and pierce, even through armour like scales and bones, can crush armour shells 

• Combined with jaws, teeth make for deadly predators 

mineralised scales

• Bodies of gnathostomes typically covered with scales 

• May be covered with enamel (sharks, primitive bony fish) 

• May contain bone (many fish, stegosaur plates, etc.) 

• May be composed of tough keratin proteins (snakes and lizards, bird feathers… mammal fur?) 

mineralisation

• In cartilagenous fish (sharks and rays) scales and teeth are mineralised 

• In bony fish, the skeleton is also mineralised 

• In invertebrates, the mineral is calcium carbonate/limestone CaCO3 

• Vertebrate tissues are hardened with calcium phosphate/apatite CaPO4 

conodonts

• Conodonts are earliest example of mineralisation in vertebrates 

• Presence of mineralised toothplates led to proposal that mineralisation began with teeth, then extended to other parts of the body 

replacement

• Teeth are subject to heavy wear, especially when used on tougher objects 

• Vertebrates evolved the ability to shed worn-out teeth and replace them with newer, sharper and larger ones 

bone history

• Our ancestors evolved bony scales and armour plates to defend from predators 

• Later, adapt mineralised tissues for feeding, evolving jaws, then teeth 

• Finally, in bony fish, mineralisation of cranium, fin rays and vertebrae made possible a sturdier skeleton 

• Hard bone made precisely formed joints and hinges possible