  Symmetry principles play a critical role in formulating the fundamental laws
of nature, with a large number of symmetry-protected topological states
identified in recent studies of quantum materials. As compelling examples,
massless Dirac fermions are jointly protected by the space inversion symmetry
$P$ and time reversal symmetry $T$ supplemented by additional crystalline
symmetry, while evolving into Weyl fermions when either $P$ or $T$ is broken.
Here, based on first-principles calculations, we reveal that massless Dirac
fermions are present in a layered FeSn crystal containing antiferromagnetically
coupled ferromagnetic Fe kagome layers, where each of the $P$ and $T$
symmetries is individually broken but the combined $PT$ symmetry is preserved.
These stable Dirac fermions protected by the combined $PT$ symmetry with
additional non-symmorphic $S_{\rm{2z}}$ symmetry can be transformed to either
massless/massive Weyl or massive Dirac fermions by breaking the $PT$ or
$S_{\rm{2z}}$ symmetry. Our angle-resolved photoemission spectroscopy
experiments indeed observed the Dirac states in the bulk and two-dimensional
Weyl-like states at the surface. The present study substantially enriches our
fundamental understanding of the intricate connections between symmetries and
topologies of matter, especially with the spin degree of freedom playing a
vital role.
