Chordate and Vertebrate Origins Protochordates Phylum Hemichordata acorn

Chordate and Vertebrate Origins

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

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

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

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

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 Early theories: Arthropods & Annelids to Chordates • All 3: segmented, similar brain organization, similar but inverted body plan Weaknesses: • annelid segmentation differs from myomere segmentation • mouth & anus positions differ; no evidence of migration during development • overlooks protostome/deuterostome developmental patterns

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

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 • evolutionary solutions: notochord, segmented muscles, further elongation SA L 2 V L 3 SA V 2/3 24 cm 2 Same geometric principle applies for ciliated feeding • 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 • new phylogenetic direction How was metamorphosis into an adult echinoderm terminated? 48 cm 2

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

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

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 Ascidian larval morphology divergent • gut morphology is only analogous to Amphioxus • true gut does not develop in larvae Similarities of larval form can be explained by convergence alone • strong selection pressures in an aquatic environment

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

Chordate Clade Vertebrates arose within the deuterostome radiation • includes echinoderms & hemichordates Chordate evolved from a common echinoderm/chordate ancestor • 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