(CNN) -- Scott Forstall, one of the most visible faces of Apple in the wake of Steve Jobs' death, is leaving the company in a surprise shakeup at the head of Apple' ranks.
Forstall, a senior vice president in charge of Apple's mobile operating system, iOS, will act as an adviser to CEO Tim Cook until departing some time next year, according to a news release from the company.
As the man behind the system that ran Apple's industry-changing iPhone and iPad, Forstall had been considered a favorite to replace Jobs as CEO after Jobs' death last October. More recently, Apple observers pegged Forstall, 43, as the next in line behind Cook.
Along with marketing chief Phil Schiller and design guru Jonathan Ive, Forstall anchored the company's top tier of executives, frequently speaking at Apple's heralded product unveilings to showcase the ins and outs of operating-system upgrades.
Many felt he was the most logical choice to continue Jobs' legacy of innovation at Apple, where 70% of revenue now comes from the mobile devices that iOS powers.
"He was as close to Steve as anybody at the company," Andy Miller, former head of Apple's iAd group, told Businessweek last year. "When he says stuff, people listen."
The Apple press release did not say what Forstall plans to do when he leaves the company. Apple did not immediately respond to a message seeking further details.
The release Monday came as Wall Street was virtually shut down in advance of Hurricane Sandy, and more than a million customers along the East Coast were already without power because of the storm's onslaught.
John Browett, Apple's head of retail, is also leaving, according to the release. The company is launching a search for his replacement, whereas it appears Forstall's duties will be divvied up for now among other Apple heavy hitters.
Ive, Bob Mansfield, Eddy Cue and Craig Federighi "will add more responsibilities to their roles," according to the announcement.
Cue, the senior vice president for Internet software and services, will take over voice-controlled assistant Siri and Maps, two of iOS's most prominent features. Federighi, the vice president of Mac software, will now oversee both Mac and iOS.
Almost universally lauded for its smooth, easily navigated design, iOS was dinged by many last month for a wonky Maps app and other problems. Replacing Google Maps for the first time as the system's default tool, users found the map product incomplete and sometimes comically wrong in many locations throughout the world.
Some critics said iOS 6 didn't offer many significant advancements over its predecessors, while Google's Android system continues to evolve more rapidly.
Monday's changes, particularly Forstall's departure, came as a major surprise. The only comment from Cook in the announcement didn't shed much light on them, either.
"We are in one of the most prolific periods of innovation and new products in Apple's history," Cook said in a written statement. "The amazing products that we've introduced in September and October, iPhone 5, iOS 6, iPad mini, iPad, iMac, MacBook Pro, iPod touch, iPod nano and many of our applications, could only have been created at Apple and are the direct result of our relentless focus on tightly integrating world-class hardware, software and services."
Forstall has been with Apple since 1997 and, in addition to guiding iOS, is one of the original architects of the company's desktop and laptop software, Mac OS X. He was responsible for several versions of the software, most notably Mac OS X Leopard in 2007.
Before Apple, he worked with Jobs at his short-lived NeXT, the computer company he ran between stints at Apple.