The affix ṭa comes after the verb sṛ 'to move' when the words in composition with it are puraḥ , agrataḥ and agre , all meaning 'in front or before'.,
As पुरः सरति = पुरःसरः 'who goes in front, a harbinger'. So also अग्रतः सरः and अग्रसरः 'going in front, a leader'.,