  In his celebrated article, Aronson established Gaussian bounds for the
fundamental solution to the Cauchy problem governed by a second order
divergence form operator with uniformly elliptic coefficients. We extend
Aronson's proof of upper heat kernel estimates to nonlocal operators whose
jumping kernel satisfies a pointwise upper bound and whose energy form is
coercive. A detailed proof is given in the Euclidean space and extensions to
doubling metric measure spaces are discussed.
