Chordate and Vertebrate OriginsChordate and Vertebrate Origins

All are suspension feeding marine organismsAll are suspension feeding marine organisms

ProtochordatesProtochordatesPhylum Hemichordata: acorn worms (Phylum Hemichordata: acorn worms (BalanoglossusBalanoglossus))

• Subphylum Urochordata: Subphylum Urochordata: sea squirts (sea squirts (Ciona/MolgulaCiona/Molgula))

• Subphylum Vertebrata: Subphylum Vertebrata: vertebratesvertebrates

Larvae are planktonic, adults are usually benthicLarvae are planktonic, adults are usually benthic

Hemichordate larvae (tornaria) resemble Hemichordate larvae (tornaria) resemble echinoderm larvaechinoderm larva

• recently validated by molecular techniquesrecently validated by molecular techniques

Phylum Chordata:Phylum Chordata:

• cilia transports water, mucus collects food, cilia transports to GI tractcilia transports water, mucus collects food, cilia transports to GI tract

Common Protochordates CharacteristicsCommon Protochordates CharacteristicsPossess some or all chordate characteristicsPossess some or all chordate characteristics

• all chordate characteristics may not appear all chordate characteristics may not appear at onceat once

Subphylum UrochordataSubphylum Urochordata: Tunicates: Tunicates

3 groups: Ascidiaceans, Larvaceans, and Thaliaceans 3 groups: Ascidiaceans, Larvaceans, and Thaliaceans

Sea squirts, appendicularia, & salpsSea squirts, appendicularia, & salps

Planktonic larvae; sessile adultsPlanktonic larvae; sessile adultsSuspension feeder specialistsSuspension feeder specialists

Tunic: flexible outer layer, thick & can be brightly coloredTunic: flexible outer layer, thick & can be brightly colored

• approximately 2000 species; most are sea squirtsapproximately 2000 species; most are sea squirts

• wide distribution: fouling organisms to deep sea formswide distribution: fouling organisms to deep sea forms

• diagnostic for the groupdiagnostic for the group

Subphylum CephalochordataSubphylum Cephalochordata: : BranchiostomaBranchiostoma//AmphioxusAmphioxus (lancelets) (lancelets)Adults exhibit all chordate characteristicsAdults exhibit all chordate characteristics Cosmopolitan distribution in tropical and Cosmopolitan distribution in tropical and

warm temperate seas.warm temperate seas.

Cilia driven filter feeding apparatusCilia driven filter feeding apparatus1-way water flow thru pharyngeal slits1-way water flow thru pharyngeal slits

Oral hood w/buccal cirri encloses pharynxOral hood w/buccal cirri encloses pharynxFood & mucus:Hatschek’s & epibranchial Food & mucus:Hatschek’s & epibranchial groovegroove • wheel organwheel organ• thread to gutthread to gut

Water flows to atrium & out atrioporeWater flows to atrium & out atriopore

Start to see characters of vertebratesStart to see characters of vertebrates• chevron-shaped myomereschevron-shaped myomeres• precursors to vertebrate organsprecursors to vertebrate organs

-- endostyle: thyroid -- endostyle: thyroid -- midgut cecum: liver & pancreas-- midgut cecum: liver & pancreas

• cardiovascular: cardiovascular:

-- paired cardinal arteries; carotids-- paired cardinal arteries; carotids-- dorsal & ventral aorta-- dorsal & ventral aorta

-- overall circulation similar to -- overall circulation similar to vertebratesvertebrates

-- pharyngeal arches; aortic arches-- pharyngeal arches; aortic arches

BranchiostomaBranchiostoma//AmphioxusAmphioxusExcretory/Osmoregulatory System:Excretory/Osmoregulatory System:

-- single cell w/ pedicels surround-- single cell w/ pedicels surround the glomerulus vesselsthe glomerulus vessels

• parts: glomerulus, pedicel, solenocytes, parts: glomerulus, pedicel, solenocytes, nephridial tubule, atrioporenephridial tubule, atriopore

* bridge from glomerulus to nephridial tubule* bridge from glomerulus to nephridial tubule

• located in atria along pharyngeal slitslocated in atria along pharyngeal slits

Pedicels of the solenocytes differ from invertsPedicels of the solenocytes differ from inverts

• similar to the processes of podocytes in similar to the processes of podocytes in vertebrate kidneysvertebrate kidneys

Phylogenetic History of ProtochordatesPhylogenetic History of Protochordates

Living protochordates: 500 million years of evolution independent of vertebratesLiving protochordates: 500 million years of evolution independent of vertebrates

Actual chordate ancestors are extinct; minimal fossil recordActual chordate ancestors are extinct; minimal fossil record• ancestral features can help determine phylogenyancestral features can help determine phylogeny

Early theories:Early theories:

Arthropods & Annelids to ChordatesArthropods & Annelids to Chordates

• All 3: segmented, similar brain organization, similarAll 3: segmented, similar brain organization, similar but inverted body planbut inverted body plan

Weaknesses:Weaknesses:

• annelid segmentation differs from myomere segmentationannelid segmentation differs from myomere segmentation

• mouth & anus positions differ; no evidence of migrationmouth & anus positions differ; no evidence of migration during developmentduring development• overlooks protostome/deuterostome developmental patternsoverlooks protostome/deuterostome developmental patterns

Phylogenetic History of ProtochordatesPhylogenetic History of ProtochordatesCephalochordates from Echinoderms: W. GarstangCephalochordates from Echinoderms: W. Garstang

Echinoderms are deuterostomes; a more likely chordate ancestorEchinoderms are deuterostomes; a more likely chordate ancestor

• phylogenetic key are echinoderm larvaephylogenetic key are echinoderm larvae

• tornaria larvae share traits with echinoderm larvaetornaria larvae share traits with echinoderm larvae

• hemichordate larvae share traits with chordate larvaehemichordate larvae share traits with chordate larvae

Chordate characteristics 1Chordate characteristics 1st st appeared in appeared in echinoderm larvaechinoderm larva

Bilateral symm.Bilateral symm.1-way gut1-way gut

Body elongation creates tail that could undulate; post anal tail formedBody elongation creates tail that could undulate; post anal tail formed

According to GarstangAccording to Garstang

““Chordates arose from larval echinoderms”Chordates arose from larval echinoderms”

• causes CCB to elongate, move dorsal & fuse; forms a proto-dorsal nerve cordcauses CCB to elongate, move dorsal & fuse; forms a proto-dorsal nerve cord• causes CAB to elongate; forms a proto-endostylecauses CAB to elongate; forms a proto-endostyle

Only needs pharyngeal slits & notochord to be considered a chordateOnly needs pharyngeal slits & notochord to be considered a chordate

Which selection pressures are involved?Which selection pressures are involved?

• evolutionary solutions: notochord, segmented muscles,evolutionary solutions: notochord, segmented muscles, further elongationfurther elongation

Phylogenetic History of ProtochordatesPhylogenetic History of Protochordates

Changes in body plan must have been advantageousChanges in body plan must have been advantageous

As size increases SA covered w/cilia cannot keep up As size increases SA covered w/cilia cannot keep up with increase in volume (Ch 4)with increase in volume (Ch 4)• individuals with alternate locomotion favoredindividuals with alternate locomotion favored

Same geometric principle applies for ciliated feedingSame geometric principle applies for ciliated feeding

24 cm24 cm22 48 cm48 cm22

SASA L L22

V V L L33

SA SA V V2/32/3

• endostyle & pharyngeal slits increase water flow; swimming increases flowendostyle & pharyngeal slits increase water flow; swimming increases flow

Garstang suggested adult form abandonedGarstang suggested adult form abandoned

If larvae > successful then adults then sexual maturity at larval stageIf larvae > successful then adults then sexual maturity at larval stage

• escapes an echinoderm adult lifeescapes an echinoderm adult life How was metamorphosis into How was metamorphosis into an adult echinoderm terminated?an adult echinoderm terminated?• new phylogenetic directionnew phylogenetic direction

Relative change in timing of developmental eventRelative change in timing of developmental event

-- paedomorphosis ≠ neotony-- paedomorphosis ≠ neotony

• change in ontogenetic onset, offset, or timing of a character appearance change in ontogenetic onset, offset, or timing of a character appearance • one process of evolutionary changeone process of evolutionary change

Paedomorphosis (“child form”)Paedomorphosis (“child form”): juvenile or: juvenile orembryonic features are present in adultsembryonic features are present in adults(axlotls:gill retention in adults)(axlotls:gill retention in adults)

HeterochronyHeterochrony

1. progenesis: body growth ends earlier; 1. progenesis: body growth ends earlier; sexual maturity achieved earlier than normalsexual maturity achieved earlier than normal

3. postdisplacement: features appear late;3. postdisplacement: features appear late; features maintain juvenile characteristicsfeatures maintain juvenile characteristics

3 Mechanisms3 Mechanisms::

Peramorphosis (“beyond form”): appearancePeramorphosis (“beyond form”): appearanceof ancestral features in adults; exaggerated of ancestral features in adults; exaggerated features or charactersfeatures or characters

• acceleration: rate of growth increasesacceleration: rate of growth increases

• predisplacement: onset of growth is early; predisplacement: onset of growth is early; characters appear earlier than normalcharacters appear earlier than normal

HeterochronyHeterochrony

3 Mechanisms3 Mechanisms

• best example is Irishbest example is Irish elkelk• example of example of hypermorphosishypermorphosis

Phylogenetic History of ProtochordatesPhylogenetic History of Protochordates

Garstang viewed protochordate evolution as a series of paedomorphic stepsGarstang viewed protochordate evolution as a series of paedomorphic steps

However!However!

Urochordate larvae (ascidian) have decreased larval timeUrochordate larvae (ascidian) have decreased larval time• Garstang’s theory requires a reversal of time spent as larvae -- unlikelyGarstang’s theory requires a reversal of time spent as larvae -- unlikely

Ascidian larval morphology divergentAscidian larval morphology divergent• gut morphology is only analogous to gut morphology is only analogous to AmphioxusAmphioxus• true gut does not develop in larvaetrue gut does not develop in larvae

Similarities of larval form can beSimilarities of larval form can beexplained by convergence aloneexplained by convergence alone• strong selection pressures in anstrong selection pressures in an aquatic environmentaquatic environment

Phylogenetic History of ProtochordatesPhylogenetic History of ProtochordatesMalcolm Jollie’sMalcolm Jollie’s Dipleuruloid Dipleuruloid Theory Theory

““Similarities of echinoderm & chordate deuterostome development too strong to ignore”Similarities of echinoderm & chordate deuterostome development too strong to ignore”

• proposes a theoretical ancestor: proposes a theoretical ancestor: DipleurulaDipleurulaDipleurulaDipleurula: small, bilateral, & ciliated: small, bilateral, & ciliated

• larval characters found in both echinoderm and hemichordate larvaelarval characters found in both echinoderm and hemichordate larvae

Pharyngeal slits arose among HemichordatesPharyngeal slits arose among Hemichordates• assists ciliary and mucus feeding systemassists ciliary and mucus feeding system

Notochord, tail, nerve cord, & myomeres Notochord, tail, nerve cord, & myomeres develop to serve adult formsdevelop to serve adult forms

Chordates Split: Chordates Split: • one group secondarily moves back to one group secondarily moves back to filter feeding nichefilter feeding niche

-- cephalochordates & urochordates-- cephalochordates & urochordatesEmphasizes trend toward predationEmphasizes trend toward predationInstead of filter feedingInstead of filter feeding• one group evolves as active predatorsone group evolves as active predators

Chordate CladeChordate CladeVertebrates arose within the deuterostome radiationVertebrates arose within the deuterostome radiation• includes echinoderms & hemichordatesincludes echinoderms & hemichordates

Chordate evolved from a common echinoderm/chordate ancestorChordate evolved from a common echinoderm/chordate ancestor• chordates did not evolve from echinoderms (chordates did not evolve from echinoderms (sensusensu Garstang) Garstang)

Chordate body plan established early in time, among invertebratesChordate body plan established early in time, among invertebrates

Basic chordate plan includes:Basic chordate plan includes:pharyngeal slits, notochord,pharyngeal slits, notochord,dorsal hollow nerve cord, and adorsal hollow nerve cord, and apost anal tail.post anal tail.

Locomotion relied upon a notochordLocomotion relied upon a notochordand serially segmented musculatureand serially segmented musculature