Should the Project Manager also assume the role of Change Manager for their project or are these two separate and distinct roles within a project ?

They could be different roles within a project but I see that the project manager is accountable for the success of the change management and its integration (with accompanied risks) in the project plan.

Project Management and Change Management are two DISTINCT roles. The people handling these roles should NEVER be the same, If we are talking about the same project. 

Change Manager OR Change Management is driven by a steering committee (ideally stakeholders who have keen interest on the project, the sponsor, etc) who decide on whether a change should go to the production, given the facts and figures by the Project manager. 

EXAMPLE - On a construction site, the project manager observed the material used is of low quality, which does not meet the expected quality requirements. To implement a corrective action, a Change request need to be issued. 

Note - this change will directly affect the Time & Cost (additional time / cost). This need to be approved ideally by the Sponsor / higher stakeholders. Project Manager cannot take a decision on this. He can give his inputs / thoughts. 

Hope this helps.

Best Regards, Ananth

Depends on the expectations of Managemnt from the Project Manager. In few smaller Organizations PM manages the responsibilities of a Change Manager as well..

On a broader scale, Change Management is a specialized skill and is managed by skilled experience person only.

The decision here will be based on the size and complexity of the project  in hand, it is also possible for the project manager to assume the role of Change Manager on smaller  value and less risky projects.