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Chordate and Vertebrate Origins. All are suspension feeding marine organisms Protochordates Phylum Hemichordata: acorn worms (Balanoglossus) Subphylum.

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Presentation on theme: "Chordate and Vertebrate Origins. All are suspension feeding marine organisms Protochordates Phylum Hemichordata: acorn worms (Balanoglossus) Subphylum."— Presentation transcript:

1 Chordate and Vertebrate Origins

2 All are suspension feeding marine organisms Protochordates Phylum Hemichordata: acorn worms (Balanoglossus) Subphylum Urochordata: Subphylum Urochordata: sea squirts (Ciona/Molgula) sea squirts (Ciona/Molgula) Subphylum Vertebrata: Subphylum Vertebrata: vertebrates vertebrates Larvae are planktonic, adults are usually benthic Hemichordate larvae (tornaria) resemble echinoderm larva recently validated by molecular techniques recently validated by molecular techniques Phylum Chordata: cilia transports water, mucus collects food, cilia transports to GI tract cilia transports water, mucus collects food, cilia transports to GI tract Common Protochordates Characteristics Possess some or all chordate characteristics all chordate characteristics may not appear all chordate characteristics may not appear at once at once

3 Subphylum Urochordata: Tunicates 3 groups: Ascidiaceans, Larvaceans, and Thaliaceans Sea squirts, appendicularia, & salps Planktonic larvae; sessile adults Suspension feeder specialists Tunic: flexible outer layer, thick & can be brightly colored approximately 2000 species; most are sea squirts approximately 2000 species; most are sea squirts wide distribution: fouling organisms to deep sea forms wide distribution: fouling organisms to deep sea forms diagnostic for the group diagnostic for the group

4 Subphylum Cephalochordata: Branchiostoma/Amphioxus (lancelets) Adults exhibit all chordate characteristics Cosmopolitan distribution in tropical and warm temperate seas. Cilia driven filter feeding apparatus 1-way water flow thru pharyngeal slits Oral hood w/buccal cirri encloses pharynx Food & mucus:Hatschek’s & epibranchial groove wheel organ wheel organ thread to gut thread to gut Water flows to atrium & out atriopore Start to see characters of vertebrates chevron-shaped myomeres chevron-shaped myomeres precursors to vertebrate organs precursors to vertebrate organs -- endostyle: thyroid -- midgut cecum: liver & pancreas cardiovascular: cardiovascular: -- paired cardinal arteries; carotids -- dorsal & ventral aorta -- overall circulation similar to vertebrates vertebrates -- pharyngeal arches; aortic arches

5 Branchiostoma/Amphioxus Excretory/Osmoregulatory System: -- single cell w/ pedicels surround the glomerulus vessels the glomerulus vessels parts: glomerulus, pedicel, solenocytes, parts: glomerulus, pedicel, solenocytes, nephridial tubule, atriopore nephridial tubule, atriopore * bridge from glomerulus to nephridial tubule * bridge from glomerulus to nephridial tubule located in atria along pharyngeal slits located in atria along pharyngeal slits Pedicels of the solenocytes differ from inverts similar to the processes of podocytes in similar to the processes of podocytes in vertebrate kidneys vertebrate kidneys

6 Phylogenetic History of Protochordates Living protochordates: 500 million years of evolution independent of vertebrates Actual chordate ancestors are extinct; minimal fossil record ancestral features can help determine phylogeny ancestral features can help determine phylogeny Early theories: Arthropods & Annelids to Chordates All 3: segmented, similar brain organization, similar All 3: segmented, similar brain organization, similar but inverted body plan but inverted body plan Weaknesses: annelid segmentation differs from myomere segmentation annelid segmentation differs from myomere segmentation mouth & anus positions differ; no evidence of migration mouth & anus positions differ; no evidence of migration during development during development overlooks protostome/deuterostome developmental patterns overlooks protostome/deuterostome developmental patterns

7 Phylogenetic History of Protochordates Cephalochordates from Echinoderms: W. Garstang Echinoderms are deuterostomes; a more likely chordate ancestor phylogenetic key are echinoderm larvae phylogenetic key are echinoderm larvae tornaria larvae share traits with echinoderm larvae tornaria larvae share traits with echinoderm larvae hemichordate larvae share traits with chordate larvae hemichordate larvae share traits with chordate larvae Chordate characteristics 1 st appeared in echinoderm larva Bilateral symm. 1-way gut Body elongation creates tail that could undulate; post anal tail formed According to Garstang “Chordates arose from larval echinoderms” causes CCB to elongate, move dorsal & fuse; forms a proto-dorsal nerve cord causes CCB to elongate, move dorsal & fuse; forms a proto-dorsal nerve cord causes CAB to elongate; forms a proto-endostyle causes CAB to elongate; forms a proto-endostyle Only needs pharyngeal slits & notochord to be considered a chordate Which selection pressures are involved?

8 evolutionary solutions: notochord, segmented muscles, evolutionary solutions: notochord, segmented muscles, further elongation further elongation Phylogenetic History of Protochordates Changes in body plan must have been advantageous As size increases SA covered w/cilia cannot keep up with increase in volume (Ch 4) individuals with alternate locomotion favored individuals with alternate locomotion favored Same geometric principle applies for ciliated feeding 24 cm 2 48 cm 2 SA  L 2 V  L 3 SA  V 2/3 endostyle & pharyngeal slits increase water flow; swimming increases flow endostyle & pharyngeal slits increase water flow; swimming increases flow Garstang suggested adult form abandoned If larvae > successful then adults then sexual maturity at larval stage escapes an echinoderm adult life escapes an echinoderm adult life How was metamorphosis into an adult echinoderm terminated? new phylogenetic direction new phylogenetic direction

9 Relative change in timing of developmental event -- 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 change one process of evolutionary change Paedomorphosis (“child form”): juvenile or embryonic features are present in adults (axlotls:gill retention in adults) Heterochrony 1. progenesis: body growth ends earlier; sexual maturity achieved earlier than normal sexual maturity achieved earlier than normal 3. postdisplacement: features appear late; features maintain juvenile characteristics features maintain juvenile characteristics 3 Mechanisms:

10 Peramorphosis (“beyond form”): appearance of ancestral features in adults; exaggerated features or characters acceleration: rate of growth increases acceleration: rate of growth increases predisplacement: onset of growth is early; predisplacement: onset of growth is early; characters appear earlier than normal characters appear earlier than normalHeterochrony 3 Mechanisms best example is Irish best example is Irish elk elk example of example of hypermorphosis hypermorphosis

11 Phylogenetic History of Protochordates Garstang viewed protochordate evolution as a series of paedomorphic steps However! Urochordate larvae (ascidian) have decreased larval time Garstang’s theory requires a reversal of time spent as larvae -- unlikely Garstang’s theory requires a reversal of time spent as larvae -- unlikely Ascidian larval morphology divergent gut morphology is only analogous to Amphioxus gut morphology is only analogous to Amphioxus true gut does not develop in larvae true gut does not develop in larvae Similarities of larval form can be explained by convergence alone strong selection pressures in an strong selection pressures in an aquatic environment aquatic environment

12 Phylogenetic History of Protochordates Malcolm Jollie’s Dipleuruloid Theory “Similarities of echinoderm & chordate deuterostome development too strong to ignore” proposes a theoretical ancestor: Dipleurula proposes a theoretical ancestor: Dipleurula Dipleurula: small, bilateral, & ciliated larval characters found in both echinoderm and hemichordate larvae larval characters found in both echinoderm and hemichordate larvae Pharyngeal slits arose among Hemichordates assists ciliary and mucus feeding system assists ciliary and mucus feeding system Notochord, tail, nerve cord, & myomeres develop to serve adult forms Chordates Split: one group secondarily moves back to one group secondarily moves back to filter feeding niche filter feeding niche -- cephalochordates & urochordates Emphasizes trend toward predation Instead of filter feeding one group evolves as active predators one group evolves as active predators

13 Chordate Clade Vertebrates arose within the deuterostome radiation includes echinoderms & hemichordates includes echinoderms & hemichordates Chordate evolved from a common echinoderm/chordate ancestor chordates did not evolve from echinoderms (sensu Garstang) chordates did not evolve from echinoderms (sensu Garstang) Chordate body plan established early in time, among invertebrates Basic chordate plan includes: pharyngeal slits, notochord, dorsal hollow nerve cord, and a post anal tail. Locomotion relied upon a notochord and serially segmented musculature


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