Complex sentence: There are many forms of aptychus, varying in shape and the sculpture of the inner and outer surfaces, but because they are so rarely found in position within the ammonite shell it is often unclear as to which species of ammonite many aptychi belong.
Split into simpler sentences: There are many forms of aptychus, varying in shape and in the sculpture of the inner and outer surfaces. However, because they are so rarely found in position within the ammonite shell, it is often unclear as to which species of ammonite many aptychi belong.