  MoS$_2$, a member of transition metal dichalcogenides (TMDs), recently
emerged as one of the fastest growing two-dimensional materials due to its
fascinating mechanical, thermal, electronic and optical properties. Unlike
graphene which possesses massless Dirac fermions with ultra-high electron
mobility, monolayer MoS$_2$ is a direct band gap semiconductor. An interesting
question arises: Can monolayer MoS$_2$ also possess massless Dirac fermions
with ultra-high electron mobility? Here, using first-principles calculations,
we show that a monolayer MoS$_2$ allotrope, which consists of repeated
square-octagon rings (abbreviated as so-MoS$_2$ to distinguish from the normal
hexagonal lattice, h-MoS$_2$) possesses both massless Dirac fermions and heavy
fermions. Distinct from the $p$-orbital Dirac fermions of graphene, the Dirac
fermions of so-MoS$_2$ are $d$-electrons and possess Fermi velocity comparable
to that of graphene. The Dirac cone structure in so-MoS$_2$ demonstrated here
greatly enriches our understanding on the physical properties of TMDs and opens
up new possibilities for developing novel electronic/spintronic devices.
