The Lion and Sun have been important symbols in Iran for millennia.
One interpretation is the Sun in the constellation of Leo.
However one can also trace the lion symbol back to the Lamassu at Persepolis.
The everlasting sun also harks back to Mithra (see the Mithraic representations at Taq-e Bostan).
Angels are shown above the lions as well (also similar to those at Taq-e Bostan).