Amgen Inc. v. Connecticut Retirement Plans and Trust Funds

In this securities law case, the Supreme Court considers the relationship between class certification and the merits. What does the plaintiff need to prove at the class certification stage? What are the arguments for considering the merits at this early stage of the litigation? What other motions might the defendant make and why did they insist on arguing this issue during certification rather than making other motions? (Hint: think about how a ruling in defendant's favor will impact future certification motions).