  A recent dynamical formulation at derivative level $\ptl^{3}g$ for fluid
spacetime geometries $({\cal M}, {\bf g}, {\bf u})$, that employs the concept
of evolution systems in first-order symmetric hyperbolic format, implies the
existence in the Weyl curvature branch of a set of timelike characteristic
3-surfaces associated with propagation speed $|v| = \sfrac{1}{2}$ relative to
fluid-comoving observers. We show it is the physical role of the constraint
equations to prevent realisation of jump discontinuities in the derivatives of
the related initial data so that Weyl curvature modes propagating along these
3-surfaces cannot be activated. In addition we introduce a new, illustrative
first-order symmetric hyperbolic evolution system at derivative level
$\ptl^{2}g$ for baryotropic perfect fluid cosmological models that are
invariant under the transformations of an Abelian $G_{2}$ isometry group.
