If you need the full skull as well, you should threshold what you need to get that first.
Then what I do is use the “Editable Intensity Range” checkbox to Erase only those dark threshold values that will give you a nice gap between parts. It can be tedious but by setting that editable range, you can be messy when painting over the area and not worry too much about following the suture closely. Once you think you’ve separated it enough, you can create another segment for the temporal bone and use the Islands tool to add the temporal bone island to it. If it’s still connected, you can go back and erase the connections.
“Editable Intensity Range” can also be used in Paint if you need to go back and add something back in. I tend to go back and forth, but make sure you adjust the range accordingly. Also use the “Editable Area” option. Sometimes I use the Level Tracing tool as well.
Maybe there’s an easier way, but this is what I do when separating complex fragments in skeletal specimens!