ORIGIN OF CHORDATES
I. Phylum Chordata
A. Phylogeny (based primarily on embryonic criteria)
1. Coelomate (enterocoelous)
2. Deuterostome
3. Radial cleavage (generally)
B. Diagnostic characteristics: (present at some stage of life cycle)
1. Pharyngeal slits
a. Originally functioned for filter feeding.
b. Embryonic, may persist in some adult chordates.
2. Notochord: dorsal, hydrostatic organ - resists compression.
3. Dorsal, hollow nerve cord (dorsal to notochord).
4. Postanal tail - posterior elongation of body beyond anus.
C. Other characteristics (shared with some non-chordates)
1. Bilateral symmetry
2. Segmentation (metamerism)
II. Protochordates: possess some or all chordate characteristics; all marine
A. Subphylum Urochordata (tail+cord) = sea squirts, tunicates
1. Free-swimming larvae:
a. All chordate characteristics except possibly postanal tail.
b. Don't feed; metamorphosis into adult stage (usually sessile).
2. Adults:
a. No notochord or dorsal, hollow nerve cord.
b. Well-developed pharynx w/ slits.
3. Some urochordates retain larval characteristics as free-swimming adults.
B. Subphylum Cephalochordata (head+cord) = amphioxus (Branchiostoma)
1. Possess all chordate characteristics.
2. Some vertebrate features:
a. Hepatic cecum may be forerunner of liver and pancreas.
b. Similar circulatory pattern to vertebrates.
C. Phylum Hemichordata (half+chordate) = acorn worms
1. Similarities to chordates:
a. Pharynx w/ slits.
b. Embryonic invagination of nerve cord (usually solid, but may be tubular in some
species).
2. Similarities to echinoderms:
a. Larvae similar to tornaria larvae of echinoderms.
b. Deuterostomes - similar to echinoderms and chordates.
3. Similarities of larvae to echinoderms and adult chordates indicates possibility that hemichordates represent evolutionary route taken by early chordates. Exact ancestral affinities remain debatable.
III. Chordate Origins: must rely on similarities in anatomy and embryology among potentially ancestral invertebrates; actual ancestors are extinct.
A. Annelid/Arthropod Hypothesis
1. Evidence:
a. Segmentation.
b. Brain regionalization.
c. Similar (but inverted) body plans.
2. Problems:
a. Similarities due to analogy, not homology.
b. Exoskeletons, jointed appendages vs. endoskeletons, myotomes.
c. Solid nerve cords vs. hollow nerve cords.
d. Inversion of annelid/arthropod body plan would require migration of mouth and anus
ventrally or formation of new mouth and anus ventrally.
e. Different coelom formation.
f. Protostomes vs. deuterostomes.
B. Echinoderm Hypothesis (W. Garstang): chordates arose through larval echinoderms.
1. Evidence: (seen primarily in larval echinoderms)
a. Deuterostomes w/ similar cleavage and coelom formation.
b. Bilaterally symmetrical larvae similar to hemichordate larvae.
2. How to get from larval echinoderm to chordate?
a. Increase size of larvae: improve escape from predation & becoming established
on substrate once metamorphosis begins.
b. Surface ciliary propulsion would not keep up with increase in volume/mass, which would
favor development of alternate locomotor system: segmental musculature, elongated body,
stiffened support rod (notochord).
c. Increase in volume would also necessitate feeding change from ciliary to pharynx
w/slits.
d. Larval echinoderm --> chordate tadpole: paedomorphosis (attainment of sexual
maturity in the larva.
3. Problems:
a. Existing chordates should not be viewed literally as examples of ancestral forms.
b. Urochordate (ascidian) tadpoles were specialized for relatively short (safer)
planktonic life; doubtful that reverse selection would prolong this stage.
c. Structural problem: intestine of ascidian tadpole opens into atrium; not homologous to
intestine of other chordates such as amphioxus. Also, ascidian tail musculature is not
segmental. Ascidians seemed to be evolving away from acquisition of chordate features.
d. Similarity of echinoderm and protochordate larval types may be example of convergence
to a similar life style.
C. Dipleuruloid Hypothesis (Malcolm Jolly): embryonic similarities too close to be accidental, may result from retention of characteristics derived from ancestor common to both groups.
1. Dipleurula = hypothetical ancestor; small, ciliated bilateral, benthic or
pelagic.
2. Primitive features of dipleurula incorporated into larvae of echinoderms &
hemichordates.
3. Pharyngeal slits arose later in each group as aids in ciliary or filter feeding.
4. Notochord, tail, nerve tube arose to serve better locomotion; undulatory movement
accompanied by muscle segmentation.
5. Chordates evolved along two pathways:
a. Toward more specialized filter feeding - urochordates & cephalochordates.
b. Enhanced active predatory lifestyle - vertebrates.
D. Chordate to Echinoderm Hypothesis (W. Gutmann): see Box Essay 2.2, pp. 76-77.