Re: when did the group of prawns develop in the arthropoda phyla and how?

Hi Jibin,

A simple enough question, but not such a simple answer! First of all, in 
biological terms, shrimps and prawns are the same ('prawns' are just large 
edible shrimps). Second, there are 3 separate unrelated groups of shrimps, 
although they are all within the Crustacean order Decapoda. The groups are 
the infraorders Penaeidea, Stenopodidea and Caridea (which contains the 
most species).

Crustaceans are known to have existed since the Cambrian period, and on 
the basis of comparative anatomy, ancestral crustaceans were likely to 
have been small, swimming epibenthic animals with segmented bodies, two 
pairs of antennae, a pair of mandibles, two pairs of maxillae, a pair of 
stalked compound eyes, and a nauplius eye. The mouth was directed 
backward, and the various bodily appendages were numerous and similar to 
maxillae, being used for locomotion, gas exchange and feeding. Among 
living crustaceans, the cephalocarids and branchiopods are most like this 
hypothetical ancestor.

However, the fossil record is very patchy, and the origin of crustaceans, 
and their relationship to other arthropod groups is not well known. The 
most likely process is that the ancestral stock divided early on into 3 
main lines. One of these formed the malacostracans, which in turn formed a 
number of branches including the Decapoda which contains the shrimps. 
Exactly how and when this occurred is not known.

For more information (as there is much detail I have not included), try a 
good general text on Evolution, plus 'Invertebrate Zoology' by R. D. 
Barnes.

Hope that helps,

Dr David Hubble, UK