ORIGIN OF CHORDATES:
- Chordates is a group of animals having three important (salient) features.
- Presence of Notochord,
- Dorsal hollow nerve chord,
- Presents of pharyngeal gill slits.
- Basically they are identified by the primitive stick or notochord.
- Among 30 phylum the chordate is the last phylum and also well developed in animal kingdom.
- The chordates are comprised about 49,000 species approximately founded.
- These includes the Amphioxus (Cephalochordates) ,Balanoglossus, Hemichordata, Ascidian , tunicates (urochordata)also developed animals such as fish (pisces), Frog (Amphibian), Lizard(Reptiles), Aves (Birds).
TIME OF ORIGIN:
- It is believed that the first chordates are originate in the sea( PLACE OF ORIGIN).
- The fossils of vertebrates animal from the chordates have been collected from the cambrian beds of paleozoic era.Hence it is believed that the chordate originate prior to the cambrian period which is around 600 million years ago.
FIRST CHORDATE:
- It is believed that the ''Fixed Ascidians'' are the first chordates in the earth .The larvae of ascidians are developed ino amphioxus and fishes through Neoteny.
- Neoteny refers to that maturity of adult animal features are present in juvenile stage itself.
PROBABLE ANCESTOR:
- Though there is a several theories of ancestor of chordates, the commonly believed that the Lophophorata animals or arm feeding animals is an ancestor of chordates which is invertebrate deutrosome.
- The deutrosome animals have developed anus before the growth of mouth in embryonic stage.
[PUBLISHED AND CREATED BY : S.V. AVINASH, M.Sc. ZOOLOGY]
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