Complex sentence: Wendy Harris and Marvin White, two teen-age sidekicks of the Super Friends who, along with Marvin's pet dog Wonderdog, first appeared in the 1973 "Super Friends" cartoon.
Split into simpler sentences: Wendy Harris and Marvin White, two teen-age sidekicks of the Super Friends who, along with Marvin's pet dog Wonderdog. These fictional characters first appeared in the 1973 "Super Friends" cartoon.