  We report the discovery of the oldest stellar substructure in the Orion star
forming region (OSFR), the Orion relic filament. The relic filament is
physically associated with the OSFR as demonstrated by Gaia DR2 photometry and
astrometry, as well as targeted radial velocity follow-up observations of a
bright sub-sample of proper-motion selected candidate members. Gaia DR2
parallaxes place the Orion relic filament in the more distant part of the OSFR,
approx. 430pc from the Sun. Given its age, velocity dispersion, spatial extent,
and shape, it is not possible for the Orion relic filament to have formed as a
single stellar cluster, even taking into account residual gas expulsion. The
relic filament is also too young to be a tidal stream, since Galactic tides act
on much longer time scales of order 100 Myr. It therefore appears likely that
the structure formed from a molecular cloud filament similar to Orion A in the
OSFR and retained its morphology despite decoupling from its natal gas. Hence,
the Orion relic filament bears witness to the short-lived evolutionary phase
between gas removal and dispersion due to shears and tides, and provides
crucial new insights into how stars are formed in molecular clouds.
