Contemporary and contemporaneous both describe two or more things originating, existing, or happening during the same period, but they differ slightly in usage. People or groups that are active during the same time are contemporary with each other. For example, the Beatles and the Beach Boys were contemporaries (the word also works as a noun) because they were active at the same time. Contemporaneous describes events, movements, and trends that happen at the same time. For example, we could say that the financial crisis of 2008 was contemporaneous with the 2008 U.S. presidential campaign (because they both took place primarily in the latter half of 2008).
The definition of contemporaneous is narrow, confined to this one sense. Contemporary is broader; sometimes it means current or modern, and it’s also a noun denoting a person who exists at the same time as another.
The gallery began collecting contemporary works, mainly through Britain, which understandably remained our window on to the world of art. [Australian]
The change in the holiday was contemporaneous with a larger change in attitude among many American toward their government. [Taunton Daily Gazette]
Vermeer and Metsu were contemporaries, but Metsu was the star in the Golden Age of Dutch painting during the 17th century. [NPR]
This point is underlined at the exhibition by the inclusion of contemporaneous works of rivals such as Kitagawa Utamaro. [Japan Times]
The three-way race nicely illustrates the tensions within the contemporary GOP. [Telegraph]
They judge the past not by contemporaneous standards but by their own politically correct notions. [Washington Examiner]