The origin and ancestory of modern chordates is not very clear and remains obscure. Because their early ancestors most liably were soft bodied and left no definite fossils.
The earliest known fossils of truly vertebrate animals were, freshwater forms abundant during the late Silurian and middle devonian periods.
Their fossils are collectively known as ostracoderms. They are placed in the group of jawless animals called Agnatha.
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The oldest fragmentary fossils belonging to ostracoderms occur in the late cambrian and middle ordovician.
About the place of origin of chordates an American geologist Chamberlain gave the idea of fresh-water origin of chordates in 1900. This idea was supported by Romer and Homer Smith also.
Theories of Invertebrate ancestory of chordates:
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The invertebrate ancestory of chordates is now undoubtedly accepted by most of the zoologists.
Several theories have been advanced to explain the origin of chordates either directly from some invertebrate groups or through the intervention of some protochordates.
Almost each invertebrate phylum – Coelenterata, Nemertean Phoronida, Annelida, Arthropoda and Echinodermata, has been suggested for the invertebrate ancestory of chordates. But only the echinoderm theory has received some acceptance.
Echinoderm lineage of chordate evolution:
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Since the earlier chordate ancestors were all soft bodied forms, they left no fossil remains to give us clues about their origin.
Therefore on the basis of anatomical, embryological, palaeontological, biochemical and serological evidences, various workers had tried to establish that the chordates probably had originated directly from some primitive echinoderms or some echinoderm larvae.
Tornaria larva of hemichordates is strikingly similar to the bipinnaria or dipleurula larva of echinoderm.
Both have similar ciliated bands in loops, dorsal pore sensory cilia at the anterior end and a complete digestive system.
Garstang and de Beer proposed the neotenous larval theory suggesting that probably the auricularia larva of echinoderms became sexually mature and later this neotenic larva gave rise to the chordates. Nowadays this view is no longer accepted.
In present days, it is widely accepted that living echinoderms and the living chordates had in common an immediate ancestor.