You might be interested in past posts on this topic:

• In the late Roman Empire why did they move towards a rounded shield rather than the rectangular Scutum?
  ^(24 Nov 2015 | 4 comments)
  ^(/u/HatMaster12 gives an in depth explanation of what is known and unknown about Roman use of rectangular shields.)

• Why did the Scutum fall out of use?
  ^(28 Sep 2014 | 5 comments)
  ^(/u/anchist describes the paucity of evidence both for and against standardized rectangular shields in the Roman military.)

You can ask followup questions by posting your questions in the comments with experts tagged to direct questions to them.