The merged-mining family (AuxPoW chains)
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Chains that piggy-backed Bitcoin's proof-of-work via AuxPoW: Namecoin (2011-09), Ixcoin / i0coin / Devcoin / Groupcoin (2011-12 → 2012), Huntercoin (2014-02), Myriadcoin (2015-09), RSK (2018-01), Elastos (2018).
An umbrella entry for the AuxPoW ecosystem that Namecoin's 2011 merged mining set off. First merge-mining dates: Namecoin 2011-09-19 (the first, visible on the Bitcoin side from block 148,744); Ixcoin, i0coin, Devcoin and Groupcoin as those chains launched between late 2011 and early 2012; Huntercoin 2014-02; Myriadcoin 2015-09, which merge-mines against several parents including Litecoin; RSK/Rootstock 2018-01-02; Elastos 2018. Only two of these are identifiable from the Bitcoin coinbase itself — Namecoin's first marker, and RSK's 'RSKBLOCK:' ASCII tag. Every other chain shares the same opaque fabe6d6d marker carrying a 32-byte root with no chain name attached, so their dates come from each project's own launch records rather than from Bitcoin.