Chordate evolution is built upon traits that evolved in invertebrates — bilateral symmetry, cephalisation, segmentation, coelom, gut tube.
Highest (most advanced) phylum among the 30 odd phyla in the animal kingdom.
All are multi-celled, bilateral and coelomate animals.
Traits are either conserved from ancestors or evolved independently.
Classification of Chordates
Phylum chordata is divided into 2 groups, on the basis of cranium:
a. Acraniata/Protochordata (lower chordates)
- It is divided into two sub phyla –
1. Urochordata (e.g- Herdmania)
2. Cephalochordata (e.g- Amphioxus)
b. Craniata/Eurochordata (higher chordata)
- Craniata includes the sub – phylum Vertebrata which is further categorised into 2 divisions,on the basis of jaws –
a. Agnatha (jawless vertebrates)
- It is further divided into two classes –
1. Ostracodermi (extinct)
2. Cyclostomata (e.g : Petromyzon, Myxine)
b. Gnatha (jawed vertebrates)
- It is further divided into two super classes –
1. Pisces (true fishes)
- It is further divided into three classes –
(i)Placodermi (fossils) – eg : Climatius
(ii)Chondrichthyes (cartilaginous fishes) – e.g : Scoliodon
(iii)Osteichthyes (bony fishes) – e.g : Labeo
2. Tetrapoda (usually having four limbs)
It is further divided into four classes:
(i) Amphibia: 4 Orders in this class: Order: Anura — frogs and toads; Order: Urodela — Salamanders; Order: Trachystoma — Mud eels; Order: Apoda — Caecilians
(ii) Reptilia: 4 orders in this class: Order: Testudines; Order: Squamata — lizards, snakes, amphisbaenids; Order: Crocodilia; Order: Rhynchocephalia
(iii) Aves/ Birds: 23 orders of birds: Passeriformes, Falconiformes, Trogoniformes, Strigiformes (Owls), Columbiformes (Pigeons), Piciformes (Wood peckers), Psittaciformes (Parrots)
(iv) Mammalia: 3 subclasses: Prototheria; Eutheria; Metatheria
4 major features of Chordates:
1) Strong but flexible Notochord — provides internal support. Beneath the nerve cord and just above the digestive canal. In adult vertebrates, it is surrounded or replaced by the vertebral column.
2) A nerve cord parallels the notochord and gut, whose anterior end develops into the brain. It is a fluid filled tube of nerve tissue that runs the length of the animal, dorsal to the notochord. It is derived from the dorsal ectodermal neural plate of the embryo and encloses a cavity called Neurocoel. The posterior part of the nerve cord becomes the Spinal Cord.
3) Embryos have gill slits on walls of pharynx — functions in both feeding and respiration. In reptiles, birds and mammals, gill slits are vestiges occurring only in the embryo. In fish, they develop into true gills.
4) Post-anal tail – Tail extends past anus as shown in the diagram.
Invertebrate Chordates: Notochord remains during the entire life — Cephalochordata and Urochordata (Proto Chordata)
Vertebrate Chordates: As the animal matures, rings of bones called the vertebrae replace the Notochord (Chordata)