Gastrulation and neurulation (frog)
Neurulation animation (human)
Phylum Chordata
Subphylum Urochordata (Tunicata)
Subphylum Cephalochordata
“Craniata” group
Subphylum Vertebrata
Superclass Agnatha
Superclass Gnathostomata
Class Placodermi
Class Chondrichthyes
Class Acanthodii
Class Osteichthyes
Class Amphibia
Class Reptilia
Class Mammalia
Class Aves
Urochordata
Urochordata
Sessile filter feeder – cilia move water and
food, filtering in pharyngeal ‘pouch’ (pharynx)
Water + Food
In
Gets
filtered
Water
Out
Urochordata
Endostyle – ciliated groove within pharynx
secretes mucous for food capture
metabolizes iodine - homologous to thyroid
Endostyle
Urochordata
Monoecious (hermaphroditic) - each
individual produces male and female
gametes.
Gametes
released
Tunic – polysacchrides w/tunicin
secreted by mantle
Urochordata
Adult lacks most chordate synapomorphies
Urochordata larvae
Eyespot (ocellus)
and
statocyst
Non-tunicate Urochordates

Phylum Chordata
Subphylum Urochordata (Tunicata)
Subphylum Cephalochordata
“Craniata” group
Subphylum Vertebrata
-Superclass Agnatha
Superclass Gnathostomata
Class Placodermi
Class Chondrichthyes
Class Acanthodii
Class Osteichthyes
Class Amphibia
Class Reptilia
Class Mammalia
Class Aves
Cephalochordata
Motile filterfeeders
 Ciliated wheel organ and pharynx
 Notochord – “hydroskeleton”
 Stiffness of notochord under neural
control
amphioxus
Notochord extends
into anterior end
Cephalochordata
Amphioxus

Tail musculature and associated nerves &
vessels are segmented.
myomeres
‘Metamerism”
Cephalochordata
Amphioxus

Circulatory system with dorsal and ventral
aorta.
Cephalochordata
Metapleural fold – stability for swimming
In text, Euchordates = Somitichordates
What were early chordates
like?
Pikaia gracilens - 530 million years ago
myomeres (muscle blocks)
skeletal notochord
cephalization

Cambrian explosion - ~550 million y.a.
Burgess shale

It’s a long way from amphioxus
Kiss off, you
dang
annelids!
Oh, a fish-like thing appeared among the annelids one day,
It hadn't any parapods or setae to display.
It hadn't any eyes or jaws or ventral nervous cord.
But it had a lot of gill slits and it had a notochord.
Chorus:
It's a long way from amphioxus, it's a long way to us.
It's a long way from amphioxus to the meanest human cuss.
It's good-bye to fins and gill slits, and welcome lungs and hair.
It's a long, long way from amphioxus, but we all came from there.
It wan't much to look at and it scarce knew how to swim.
And Nories was very sure it hadn't come from him.
The Molluscs wouldn't own it and the Arthropods got sore.
So the poor thing had to burrow in the sand along the shore.
"My notochord shall change into a chain of vertebrae,
And, as fins, my metapleural folds will agitate the sea."
"My tiny dorsal nervous cord shall be a mighty brain.
And the vertebrates shall dominate the animal domain."
Phylum Chordata
Subphylum Urochordata (Tunicata)
Subphylum Cephalochordata
“Craniata” group
Subphylum Vertebrata
-Superclass Agnatha
-Superclass Gnathostomata
Class Placodermi
Class Chondrichthyes
Class Acanthodii
Class Osteichthyes
Class Amphibia
Class Reptilia
Class Mammalia
Class Aves
Craniata
contains hagfish and all vertebrates
Chordates with skulls, neural crest
 cartilagenous, fibrous or bony
 encases brain & sense organs
Hagfish
Lamprey
Craniata
Sensory, digestive and respiratory anatomy
Neural crest cells
NC cells are found in all craniates and
give rise to a variety of structures
QuickTime™ and a Animation decompressor are needed to see this picture.
It’s nice to have a neural crest….
pigment cells
gill arches, jaw
ganglia in ANS
base of skull
induce skin ‘structures’
Craniata vs. ‘protochordates’

Selection for predatory characteristics
 active feeders
 muscular gut tube for filtering
Pikaia gracilens
Haikouella
Evolutionary scenarios
Craniates were originally linked
w/arthropods, annelids, mollusks

But essential differences in development
Chordate metamerism doesn’t involve
coelom as in annelids
Linking the subphyla
Garstang (p.44)
Euchordates (Somitochordates) evolved via
paedomorphosis (a type of heterochrony)
Paedomorphosis: Adult form of the descendant
species retains juvenile features of ancestral
species.
Adult salamander
w/gills
Garstang: Mutation caused development of
sexual maturity in a non-metamorphosing
lineage of Urochordata – better locomotion
 Early craniates – larger, more mobile than
cephalochordata. Driven by predation?
Conodonts - 540-230 m.y. ago
 after Pikaia, the next fossil Chordates
 microfossils of teeth, probably in pharynx
Cool thing about conodonts
A new hard substance appears:
mineralized tissue – calcium phosphate
(hydroxyapatite)