Complex sentence: Zanno and Mackovicky (2013) using other megaraptoran taxa as a proxy, estimate the femur length of the holotype specimen FMNH PR 2716 at 11.9 meters long, and using a femur circumference regression, estimate its mass at roughly 4 tons.
Split into simpler sentences: Zanno and Mackovicky (2013), using other megaraptoran taxa as a proxy, estimated the length of the holotype specimen FMNH PR 2716 to be long, scaling upwards from the femur. Additionally, using a femur circumference regression, they estimated its mass at roughly 4 tons.