Barry Letts and Terrance Dicks had decided they wanted a sea-based story and asked Malcolm Hulke to write it. Rather than merely bringing back the Silurians, Hulke invented an aquatic version called the Sea Silurians. Unlike their earlier counterparts, these "Sea Devils" were given clothing, designed by Maggie Fletcher. To answer the insistence by fans that the Silurian era could not have spawned man-sized life, Hulke introduced a line in which the Doctor says they should correctly be called Eocenes. However, this period was still well in advance of humanoid life.