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That is the correct spelling of "predecessors" (those who came before their successors).
The predecessors to the Romans were the Latins or Latinii, who lived in the area that became Rome.
Predecessors of who or what?
The Etruscans
Investigators .
nothing
Conventional cars.
cyanobacteria
The League of Nations
The Thule people
When using the terms "successors" a "predecessors", it is usually in reference to a highly-valued title, for example king or president. Successors are the ones who come after; predecessors are the ones who came before. Bush is Obama's predecessor and Obama is Bush's successor.