The Oracle Solaris threads API and the pthreads API are two solutions to the same problem: build parallelism into application software. Although each API is complete, you can safely mix Oracle Solaris threads functions and pthread functions in the same program.
The two APIs do not match exactly, however. Oracle Solaris threads support functions that are not found in pthreads, and pthreads include functions that are not supported in the Oracle Solaris interface. For those functions that do match, the associated arguments might not, although the information content is effectively the same.
By combining the two APIs, you can use features not found in one API to enhance the other API. Similarly, you can run applications that use Oracle Solaris threads exclusively with applications that use pthreads exclusively on the same system.