  Using Gaia DR2, we trace the Anticenter Stream (ACS) in various stellar
populations across the sky and find that it is kinematically and spatially
decoupled from the Monoceros Ring. Using stars from {\sc lamost} and {\sc
segue}, we show that the ACS is systematically more metal-poor than Monoceros
by $0.1$ dex with indications of a narrower metallicity spread. Furthermore,
the ACS is predominantly populated of old stars ($\sim 10\,\rm{Gyr}$), whereas
Monoceros has a pronounced tail of younger stars ($6-10\, \rm{Gyr}$) as
revealed by their cumulative age distributions. Put togehter, all of this
evidence support predictions from simulations of the interaction of the
Sagittarius dwarf with the Milky Way, which argue that the Anticenter Stream
(ACS) is the remains of a tidal tail of the Galaxy excited during Sgr's first
pericentric passage after it crossed the virial radius, whereas Monoceros
consists of the composite stellar populations excited during the more extended
phases of the interaction. We suggest that the ACS can be used to constrain the
Galactic potential, particularly its flattening, setting strong limits on the
existence of a dark disc. Importantly, the ACS can be viewed as a stand-alone
fossil of the chemical enrichment history of the Galactic disc.
