CVA ch.2: origin of chordates

chordate characteristics (synapomorphies)

1. notochord (mesoderm)

2. pharyngeal slits (endoderm)

3. endostyle/thyroid gland (endoderm)

4. dorsal hollow nerve cord (ectoderm)

5. post anal tail and myomeres (mesoderm)

1. notochord (chordate synapomorphy)

• mesoderm

• provides scaffolding, above gastrointestinal tract

• allows animal to flex it body left and right

• all triploblastic animals have this in some point in their life history

• in humans, it’s a gel-filled pad remnant in vertebral disk (nucleus pulposus)

  • mainly replaced by the vertebral column

• hydrostatic organ w/ elastic properties

• 

2. pharyngeal slits (chordate synapomorphy)

• endoderm

• slits in throat region that used to help us feed (suspension feeding), had mucus and cilia

  • helped early chordates filter feed 

• first formed as pharyngeal pouches along throat area

  • closed up more to form slits

• in more derived vertebrate groups, goes on to form other things 

3. endostyle/thyroid gland (chordate synapomorphy)

• endoderm

• grove that forms in bottom of the mouth/upper area of throat region depending on the animal

• lined with cilia/mucus

• young lamprey have endostyle, adult ones have thyroid

• in our tracheal region, butterfly shaped soft tissue that secretes lots of hormones

4. dorsal hollow nerve chord

• ectoderm

• dorsal relative to GI tract (on top of it)

• nerve cord is hollow, a lot quicker at propagating action potentials

5. post anal tail + myomeres (chordate synapomorphy)

• posterior elongation of the body extending beyond the anus 

• myotomes= cuts of muscle, delineate dif sections of muscle

• myomeres= actual muscle bt each of the segments

• myomeres/myotomes derived from mesoderm

chordates

• “string + gut”?

• 541 mya

• 5 main characteristics 

• fluid filled internal body cavity called coelom

• 3 types:

  • urochordates (tail string gut)- tuncates/”sea squirts”

    • 30-35 sp,

  • cephalochordates (head string gut)- amphoxi/lancets

    • 2,000 sp

  • vertebrates (“verte”= articulations?)- fishes/amphibians/reptiles/mammals

    • 66,000 sp

• urochordates + cephalochordates are technically invertebrates

ambulacraria

• deuterostome→ _______ → echinodermata + hemichordata

• sister to chordata

• sea stars + hemichordates

protochordates

• “first chordates”

• paraphyletic group- not looking at a mrca and all of its descendants, just some of them 

  • leaves out vertebra and echinodermata 

• consists of:

  • hemichordata: pterobranchia + enteropneusta

  • chordata (only 2/3 lineages, actual chordates): cephalochordata + urochordata 

ceplalochordata

• chordata→ ________ + urochordata + vertebra

• pikia, mid-cambrian

• had v tiny head

• extant lancelets (Branchiostoma Amphioxus lanceolatus) have all 5 chordate traits + bigger head “protochordate”

urochordata 

• chordata→ cephalochordata + ________ + vertebra

• “tail backstring”

• 5 synapomorphies, during some portion of life history

• marine suspension feeders

• tunicates (tunic), ascidiacea (sea squirts)

  • larvae: ascidian tadpole (all synapomorphies simultaneously)

  • short-lived (hrs)

• “protochordate”

• sister group to vertebrates

hemichordata 

• ambulacraria → _______ → pterobranchia + enteropneusta

• “marine worms”

• sister to echinodermata

• pharyngeal slits

• postanal appendage in some, but not for movement

• tornaria larvae (resembles auricularia larva of echinoderms), planktonic

• proboscis, collar, trunk

  

enteropneusta 

• ambulacraria → hemichordata → pterobranchia + _________

• “acorn worms”

• some thought to have a dorsal hollow nerve cord

• proboscis, collar, trunk 

• pharyngeal slit

• no notochord or thyroid/endostyle

• close relatives of echinodermata

• not colonial

pterobranchia 

• ambulacraria → hemichordata → _________ + enteropneusta

• evolved from acorn worms→ no, just say sister taxa or shared mrca

• colonial (zooid)= start off as trunk but multiple animals sprout from it 

• proboscis, collar, trunk

  

echinodermata 

• ambulacraria → _________

• pentaradial, symmetrical body plan (sea stars)

• endoskeleton of calcium carbonate, ossicles

• water-vascular system, tube feet

• in some fossils, pharyngeal slits present 

synapomorphies

• shared derived characteristics 

parsimony 

• things usually connected/behave in the simplest/most economical way

• occam’s razor: the simplest explanation has the least number of steps

(fill in the blanks)

major metazoan groups (add?)

• parazoa

• eumetazoa

  • radiata 

  • bilateria 

    • protostomia

    • deuterostomia

species diversity

• arthropods (crustaceans/insects) most diverse (millions of species)

• nematoda (roundworms) second most diverse (1/2 million species)

• chordata (vertebrates + others) (~50,000 species)

bilateria

• eumetazoa → _________

• left + right side

• front + back

• top + bottom

• triploblastic= 3 germ layers

• ______ → deuterostomia= anus forms first from blastopore

• ______ → protostomia= mouth forms first from blastopore

bilateria development (protostomes + deuterostomes)

• protostomes

  • blastopore forms mouth

  • spiral cleavage

  • schizocoelic coelom (cell clusters)

  • ectodermal skeleton

• deuterostomes

  • blastopore forms anus

  • radial cleavage 

  • enterocoelic coelom (outpouching)

  • mesodermal skeleton

triploblastic

• 3 germ layers:

  • endoderm= inside, digestive tracts (liver + pancreas), all eumetazoa have this

  • mesoderm= middle, organs, blood, muscle, bones, heart

  • ectoderm= outside, nervous system (eyes + skin), all eumetazoa have this

• bilateria are this

• more complex, mesoderm gives rise to specific character traits 

radiata

• eumetazoa → _______ → cnidaria= jellyfish/hydra (nida=stinging)

• diploblastic= 2 germ layers

protostomes (ADDD)

• eumetazoa → bilateria → ______

• “first mouth”

  • mouth arises from blastopore

• molluscs, annelids, arthropods,

• solid nerve cord

deuterostomes  (ADDDDD)

• eumetazoa → bilateria → ______

• hollow nerve cord 

• “mouth second” → anus develops from blastopore as first opening, mouth forms later

• ambulacraria (echinoderms + hemichordates) + chordates (cephalochordates, urochordates + vertebrates)

chordate origins (2 debated):

1. annelid or arthropod origin

2. echinoderm origin

1. annelid or arthropod origin (chordate origin debate)

• arguments:

  • segmented body

  • brain regionalization: forebrain, hindbrain

  • nerve cord and gut present

• counter arguments:

  • nerve/gut flipped

  • nerve cord of chordates is hollow, in annelids/arthropods its solid

    • also develops a dif way in each

  • protostome vs deuterostome development 

  • many of the supposed linking similarities between chordates and annelids or arthropods result from homoplasy rather than homology

2. echinoderm origin (chordate origin debate) (idk)

• W. Garstang’s arguments

  • deuterostomes

    • share embryonic similarities of cleavage + mesodermal/coelomic formation

  • auricularia hypothesis (appears similar to tornaria larvae?)

    • bilateral symmetry (left + right side)- echinoderm larvae + chordates

    • body elongation grew out of circumoral band (dorsal hollow nerve cord)

• counter argument

  • larvae→ chordate plan?

• W.Garstang’s argument

  • paedomorphosis- larval characteristics observed in adult form 

another phylogenetic tree to review

evolution of feeding image

another version of the phylogenetic tree

(note that chordates are deuterostomes along with hemichordates and echinoderms,but the protostomes are a separate lineage)