Board Thread:Warhammer 40k General Discussion/@comment-71.91.63.134-20150920063835/@comment-26516936-20150929095827

There have been Tyranids in the galaxy for eons for example in the Ciaphas Cain novel The Last Ditch Cain discovered a number of Tyranids frozen in ice, survivors of a hive ship that crashed many thousands of years earlier perhaps predating the formation of the empire. Also in the Bood Drinkers/Nova Marines novel Death of Integrity the 2 chapters joined forces to clean out a space hulk infested with Gene Stealers, this story took place about the 39th millennium & even then Gene Stealers were a long recognized threat. And finally the Necron overlord The Silent King had become aware of the threat of the Tyranids before the great crusade & apparently intended to form an alliance with Sanguinious to combat them until the Heresy intervened.

In conclusion the Tyranids in some form have been a presence in the galaxy for at least 10,000 years but just haven't appeared in force until recently. So it's not beyond the bounds of credibility that the Emperor would be aware of their existence But he would also see that they are a threat for later & put them on the back burner. Though it is possible he could have sent a legion to combat them but it would have been the survivors of the 2 lost legions after their primarchs were removed for cause as the Emperor wouldn't have wiped them from all records otherwise.