To start it you have to go to the Apartments Area in the Presidium Commons and search the right corridor for a Salarian.
When to start
After Priority: The Citadel 2
“A Salarian scientist needs a preserved kakliosaur specimen to help the krogan fight on toxic worlds.
Find one and return it to him in the Presidium Commons.”
Experience Points, War Asset Update
To finish this mission, return to Normandy, and using your Galaxy Map, locate the Argos Rho cluster and Phoenix system.
Travel to Intaisei and scan the planet to get the Kakliosaur Fossil, then take it back to the Salarian.
NOTE: If you cant find the Argos Rho cluster the first time, make sure you look for it after you advance in the game.
Next Citadel mission: Wounded Batarian.