1. Chordate and Vertebrate Origins

2. 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

3. 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

4. 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

5. 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

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
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

7. 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 1st 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?

8. Phylogenetic History of Protochordates
SA L2
V  L3
SA  V2/3
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
24 cm2
48 cm2
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
How was metamorphosis into
an adult echinoderm terminated?
new phylogenetic direction

9. 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

10. 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

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
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

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
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
Emphasizes trend toward predation
Instead of filter feeding
one group evolves as active predators

13. 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