In order to complete Mario Kart 7 for release on 3DS this side of Christmas, Nintendo drafted in the assistance of Metroid Prime developer Retro Studios, the game's producer Hideki Konno revealed in a new Iwata Asks (translated via NeoGAF).
Development of Mario Kart 7 began with a team of just eight people, revealed Konno, but several designers were moved over to help complete nintendogs + cats in time for the 3DS launch. Eventually, Nintendo had to bring in Metroid Prime and Donkey Kong Country Returns developer Retro Studios to ensure Mario Kart 7 made it to stores this year.
Nintendo has previously said that Retro was used to help create a Donkey Kong Country-themed course for the handheld racer, but it now appears the studio was utilised to create the 16 revamps of retro classic courses. This left Nintendo's internal team free to focus on creating the 16 new tracks.
Mario Kart 7 will be released December 2.