The US was a common law country, then created the constitution based on that common law and now it is a constitutional republic. The constitution is the pillar now not "common law".
The constitution is the floor the stool sits on, and it supports common law both implicitly (cf, e.g., [1]), and explicitly (cf the 7th Amendment).
[1] https://constitutionalism.wordpress.com/2008/09/15/common-la...