Adoption by Roman legionaries of rectangle shields - how early?

[Daivid]

How early did the Romans start equipping legionaries with rectangle shields and how long was the transition?

More specifically - would do  a refight of Pharsalus with some rectangle shield troops and some with oval shields
be clearly wrong?

[aligern]

You could never say for certain, but the evidence seen for that period would say clearly wrong.

The typology for shield shape has them first adopt a chopped oval (top and bottom flattened) and that looks to take place in the reigns of Augustus/tiberius and then later it moves to the rectangular shape.
Roy

[Duncan Head]

OTOH there are clear rectangular shields on the Palestrina Nile Mosaic, which may date around 100 BC. Even if you think these are Ptolemaic troops and have not been altered by the Roman mosaicist, they suggest the shield type was around earlier than one might have thought. 

[aligern]

Indeed Duncan, I recall that Donnington make a figure with it for the Ptolemaic army.
Roy

[Daivid]

OTOH there are clear rectangular shields on the Palestrina Nile Mosaic, which may date around 100 BC. Even if you think these are Ptolemaic troops and have not been altered by the Roman mosaicist, they suggest the shield type was around earlier than one might have thought.

Can I take that as a not clearly wrong? 
I have already searched the internet and  while it is clear that most people think it more likely that rectangle shields came in later
there seems to be huge uncertainty.

Because Pharsalus is a Roman versus Roman battle I don't quite have enough figures to do the battle with only oval shields -
hence clearly wrong would mean me doing another battle.

Thanks to both of you for your replies