What is a Chordate?
Shared Derived Characters:
•Paired pharyngeal gill slits
•Dorsal tubular nerve cord
Post-anal muscular tail
•Subpharyngeal gland that binds iodine (endostyle or thyroid)
Pharyngula Stage in Vertebrate Development
• pharyngula stage shows the basic Chordate characteristics present in all
Subphylum Tunicata/Urochordata tunicates
Subphylum Cephalochordata amphioxus
Subphylum Vertebrata animals with backbones
•Outgroups/sister groups are the most distant relatives to the other
•Sympleisiomorphy — shared primitive trait such as the vertebrae
(backbone) of tetrapods. This represented by a node on the
Cladogram. Every species on the right of the branch shares the
•Synapomorphy - a shared derived trait found among two or more
taxa and their most recent common ancestor, whose own ancestor in
turn does not possess the trait. i.e. shared traits that have originated
with the last common ancestor
Cladograms are hypotheses which use synapomorphies to establish
relationships among clades.
•Clades are referred to as monophyletic groups They contain a
common ancestor and all the descendants e.g. mammals are
monophyletic — contains all living and extinct mammals plus their
•Parisomy- the least number of charges is most desirable when
forming a Cladogram. •Polyphyletic - unrelated organisms descended from more than one
ancestor. e.g. artificial group of birds and bats (both have wings) in
which common ancestor of the two groups not included
•Paraphyletic — some descendant groups are not included (e.g. birds
and mammals not included in Reptilia)
•Evidence for phylogeny (evolutionary relationships) comes from
morphology, fossil evidence, molecular biology (e.g. RNA, DNA,
proteins) and development (embryology and patterns in gene
regulation “evo-devo”= evolutionary developmental biology).
Hemichordates (e.g. acorn worms and pterobranchs); 80
•Pharyngeal gill slits
•Deuterostomes, but not chordates
•Filter feeders that burrow. Fill collar celum to anchor and use circular
muscles of proboscis to move forward and tighten, pulling forward.
•Proboscis is ciliated to make water and food particles enter mouth
before proceeding to the pharynx where it will be covered in mucus
before moving to the gut.
•Lacks post-anal tail.
•They have stomochord ("mouth cord"), composed partially of chitin.
Extant Non-Vertebrate Chordates
The Protochordates (“Lower Chordates”)
marine filter feeders — use cilia to create a current to bring in food
particles which are then trapped on mucus strands produced by an
Tunicates/Urochordates — 2000 species
•Ascidians- sea squirts (1900 sp.)
o Tunic: Tunicin and rare vanadium metal
o Pharynx takes up most of body
Incurrent siphon and excurrent siphon for filter feeding
o Gill slits are spirals to increase surface area.
o Has chordate characters
o Dispersal phase in which larvae move to a new area o Adhesive cilia help them plant themselves to ground for
• Ascidian Diversity
o Solitary or Colonial (with a stalked configuration)
o Carnivorous Ascidians with an incurrent siphon that acts
as a trap.
o Invasive Tunicates- lack of predators and warming has
allowed numbers to explode. These tunicates attach to things
and go from port to port quickly, entering new niches. They
have become a problem for shellfish fishermen by out
• Thalacians (e.g. salps)
Barrel shaped, see through, and supported by internal
o Sometimes parasitic crustaceans will lay eggs and
incubate them through two molts.
o Pelagic tunicates e.g. salps
o Derived from adult ascidian form
o Solitary forms produce chains of salps asexually
The aggregate phase is when salps form a chain to
o Salps Sequester Carbon
• Swarms covering several km often last for months and
are capable of sequestering several thousand tons of
carbon a day
• Antarctic salps have become increasingly common as sea
ice declines, may outcompete krill
o Pyrosoma- a bioluminescent colonial pelagic Thaliacean
• Appendicularia (Larvaceans)
Tubular organism secrets gelatinous house to live in.
They use their tail region to create a feeding current
o They have a filter to prevent entry of large particles.
o When they reverse movement of the tail, and thus the
current, they blow out particles from the screen.
o They may leave and create new house if the screen
becomes too difficult to clean.
Bathochordaeus- produces gelatinous house that is 1
meter wide while organism itself is 3 cm long. Cephalochordates — 22 species
• Amphioxus- a coastal marine filter feeder
Branchiostoma sp. 22 species (official genus)
o Chordate characters are well represented in amphioxus
• Notochord (with muscles that contract to stiffen),
Endostyle, Pharyngeal gill slits, and nerve cord are
• Extreme rostral extension of notochord
o Locomotion of Cephalochordates- myomeres, myoseptae
are shared characters with vertebrates. When myomeres
contract, notochord resists compression and body bends for
locomotion and burrowing
o Amphioxus Pharynx vs. Vertebrate Pharynx
• Pharynx functions only in filter feeding, not gas exchange.
• Cilary action brings food to mouth, not muscular pumping.
• Epipharyngeal groove collects mucus strands plus food;
endostyle produces mucus
o Circulatory System
• Dorsal and ventral aorta, one-chambered heart
Early Chordates from the Cambrian
• Pikaia is a stem chordate- Myomeres, notochord
• Recent Pikaia Study- 114 Specimens:
Conclusion: Pikaia's places as a stem chordate is strongly
• Conclusion: Pikaia is specialized and amphioxus is more basal
Chengjiang fauna 522 mya
• Vetulicolians Stem Deuterostomes
• No apparent notochord
• Pharyngeal gill slits