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CYTH is a large protein superfamily that is conserved in all three domains of life with its unique triphosphate tunnel metalloenzyme (TTM) fold. Besides phosphatase functions, e.g. as RNA triphosphatase, inorganic polyphosphatase or thiamine triphosphatase, some CYTH orthologs cyclize nucleotide triphosphates to 3’,5’-cyclic nucleotides. So far, archaeal CYTH proteins are annotated as adenylyl cyclases although experimental evidence is lacking. To address this gap, we characterized a CYTH ortholog, SaTTM, from the crenarchaeote Sulfolobus acidocaldarius. Our initial in silico studies suggested a close relationship between archaeal CYTH enzymes and class IV adenylyl cyclases compared to the other CYTH-subclasses, but biochemical data showed no cyclic nucleotide production. Instead, our structural and functional analyses show a classical TTM behavior. The Ca2+-inhibited Michaelis complex indicates a two-metal ion reaction mechanism analogous to other TTMs. Different co-crystal structures of SaTTM further reveal conformational dynamics in SaTTM, let us to assume feedback inhibition in TTMs due to tunnel closure in the product state. Combining our structural insights with sequence-similarity network based in silico analysis, we further set out a firm molecular basis for distinguishing CYTH orthologs with phosphatase activities from class IV adenylyl cyclases.Major highlights-CyaB-like class IV adenylyl cyclase homologs in archaea are triphosphatases.-The co-crystal structure of SaTTM in sulfate and triphosphate bound state revealed conformational transition of the TTM tunnel during catalysis.-Atomic insights into TTM inhibition by calcium and pyrophosphate.-In silico and structure-function analysis revealed the molecular determinant for functional diversification among CYTH proteins.