Roman Archer shields?

[Caius Titius Verus]

Hello,

In our group there are going to be a couple of men portraying roman archers. We do early second century reenactment.

Now we have a question left about archers, did they use shields? And if so what kind of shields? Or did they work in pairs, ie one with a shield, the other one with a bow?

On Trajan's collum it looks as if they have shields...or do we observe this wrong?

Thanks,