  The LHCb Collaboration has reported resonant activity in the channel $D^+
K^-$, identifying two components: $X_0(2900)$ with $J^P = 0^+$ at $2866 {\pm}
7$ MeV, $\Gamma_0=57{\pm} 13$ MeV and $X_1(2900)$ with $J^P = 1^-$ at $2904
{\pm} 7$ MeV, $\Gamma_1=110{\pm} 12$ MeV. We interpret the $X_0(2900)$
component as a $cs \bar u\bar d$ isosinglet compact tetraquark, calculating its
mass to be $2863 {\pm} 12$ MeV. This is the first exotic hadron with open heavy
flavor. The analogous $bs\bar u\bar d$ tetraquark is predicted at $6213 {\pm}
12$ MeV. We discuss possible interpretations of the heavier and wider
$X_1(2900)$ state and examine potential implications for other systems with two
heavy quarks.
