Phylum Chordata (49,400 species)

Ascidians: sessile filter feeders
bag like body covered by a tunic
Phylum Chordata (49,400 species)
(tunicin is a type of cellulose)
SubPhylum Vertebrata (48 K species)
SubPhylum Urochordata
SubPhylum Cephalochordata
•  Chordates are 1 of 4 deuterostome groups
•  1400 species of invertebrate chordates
(sea squirts)
(thyroid gland homologue
produces iodine rich mucus)
sessile filter feeders
bag-like body covered by a tunic
made of cellulose in some spp.
Water enters incurrent
siphon passes through
porous mucus and exits
into atrial cavity through
pharyngeal slits. Particles
trapped in mucus are
Megalodicopia hians (Megabyte sea squirt)
Many Ascidians are solitary, but
most are colonial (b) or
compound (c,d), the latter living
within a single tunic sheath an
often have a shared atrial
Tasmanian devil-squirt
catches prey using a “fly-trap” system
Deep sea tunicate with a pharynx that is lacking
ciliated stigmata
Chordate-like characters of Ascidians are
more prominent in the tadpole larva
Post anal
Nerve cord
Radical metamorphosis:
-- Involves loss of notochord, tail, dorsal
hollow nerve cord, swimming musculature
-- Rapid growth of region around buccal siphon
causes the body to
rotate bout 90 deg
Atrial siphon
--Atrium expands
--# gill slits increases
and intestine
Buccal siphon
Sub Phylum Urochordata
Class Larvacea: Small (5-6 mm) planktonic, tadpolelike; lives in midwater and secretes a gelatinous house.
One Hypothesis states that Larvaceans evolved from
ascidian tadpoles by a process known as Neoteny
but there is new evidence on this relationship (later)
SubPhylum Cephalochordata (lancelets; 2 dozen spp)
-- resemble urochordates in feeding biology
-- but have much in common with vertebrates:
distinct notochord that persists through life
dorsal hollow nerve chord, post anal tail….
Dorsal hollow nerve cord
Gill slit
Fig 24.9
Post anal tail
Reproductive organ
Transverse muscle
Gill slits
dorsal nerve cord
SubPhylum Cephalochordata
-- feeding biology resembles urochordates
-- but have much in common with vertebrates:
distinct notochord that persists through life
dorsal hollow nerve chord
Clearly not a Vertebrate, which have:
-- a nerve chord elaborated into a brain
-- a protective cranium
-- a notochord replaced by a vertebral column
But generally accepted as the closest living relative of vertebrates
and much studied as a model of vertebrate ancestors
Deuterostome Phylogeny
•  4 phyla, different from all others but also
considerably different from each other
Echinoderms, Hemichordates, Xenoturbella, Chordates
•  key characters are embryological, morphological, and more
recently genetic
•  A variety of scenarios; as early as 1890
• Key questions:
•  echinoderm-hemichordates-xenoturbellids
already discussed
•  chordate relationships (Urochordates?)??
•  ancestral chordate???
Summary : According to Garstang
how chordates and vertebrates evolved
How did vertebrate innovations evolve?
1. Chordates formed as a deuterostome larva in an
ascidian when its larva was modified to a tadpole-like
larva with a notocord, DNC, Endostyle (origin)
2.  Tadpole like larva becomes adult Larvacean-like
chordate through neoteny (a type of paedamorphosis)
3. Selection favors traits that continue to improve
locomotion and activity:
•  complex sensory organs, brain
•  complex musculature, respiration
•  addition of bone ....Vertebrate condition
(this also happenned in Urochordate line later)
Garstang speculated that at some point larvae developed
gonads and reproduced in the larval stage and a new group of
free swimming animals evolved, perhaps two separate times
Figure 23_05
Cladogram consistent
with Garstang’s view
•  Garstang
called this process neoteny (somatic development is slowed), a
type of paedomorphosis (adult assumes traits previously seen in juveniles)
in which juvenile or larval traits appear in the adult body
Selection for increased
swimming etc.
Neoteny (paedomorphosis)
Ascidacea (with tadpole
that has evolved chordate characters)
Dorsoventral Inversion in Adult:
Current Hypothesis that is contradictory to
Garstang’s hypotheses.
A dorsoventral inversion of body plan axis in an
adult worm like animal was a key event in the
evolution of deuterostomes.
Dpp (suppresses neural fate) and its homologue BMP-4 are expressed in a
pattern that supports inversion. SOG and its homologues (Chordin etc.) suppress
dpp action
Are Hemichordates like acorn worms representative of the common ancestor?
Cladogram Showing New “Molecular” View
Figure 23_05
Cladogram consistent
with Garstang’s view
Cladogram consistent
With Cameron’s view
Cameron, C.B., Garey, J.R. and Swalla, B. J.
(2000) Evolution of the chordate body
plan: New insights from phylogenetic
analyses of deuterostome phyla. Proc.
Natl. Acad. Sci.97: 4469-4474.
See also Fig. 22-10
Two major clades , Echinoderm-Hermichordates and Chordates
Common ancestors of chordate/ hemi-echinoderms had pharyngeal
slits, dorsal hollow nerve cord.
Larvacean like animal is ancestor of all chordates?
Key New Contributions of Molecular Studies
1. Echinoderm-Hemichordates are sister groups
therefore stem deuterostome:
• indirect development with auricularia
• adult with pharyngeal slits and dorsal nerve cord
2. Non-Larvacean Urochordates are united in a single
derived clade. Larvacean clade is the stem sister group.
Larvaceans have no larval stage. So larva played no
prominent role in evolution of chordate characters.
3. New phylogeny contradicts Garstang’s hypothesis in
several ways but is neutral with respect to D-V inversion