Because if they died together, they're part of the same group. If some are small skulls (ie. children) and some adults (a family that died in a natural disaster

Because if they died together, they're part of the same group. If some are small skulls (ie. children) and some adults (a family that died in a natural disaster, landslide, ... say), then they obviously interbreeded, at which point they would be re-classified as only one species, and maybe something other than genetical differences account for the differences in skulls.