From the appendix, it sounds like amphibians at least may still outweigh humans. They guess 1 amphibian per square meter over the entire non-ice covered terres

From the appendix, it sounds like amphibians at least may still outweigh humans. They guess 1 amphibian per square meter over the entire non-ice covered terrestrial surface to get .1 Gt C. Which puts the other weights into perspective... For reptiles, different estimates range from 0.00005 Gt C to .5 Gt C. They say their best guess is .003 Gt C (compared to .007 Gt C for wild mammals). Edit: I wonder if they might be somewhat underestimating the mass of rodents.