Leather Cuirass Lorica Musculata, I used to think no way but

[Dan Diffendale]

The abridged and simple version is this…. Marius reformed the armies of Rome and the three disk armor disappeared and was replaced largely by chainmaile and chainmaile based armors.

Just to butt in here, the triple-disc cuirass disappeared probably a century or more before Marius, at least based on the lack of archaeological, representational, or literary attestation, and it's questionable whether or not it was ever worn by Romans.