yeah i know; it just brought another question to my mind.
depends on a lot of factors. is your proboscidea being buried by an avalanche/mudslide, sinking into a bog/quicksand pit, swallowed by a sinkhole or standing in a pre-fabricated mastadon-preservation tank?
what's the input rate of flow and the confined volume? is there an outflow spigot? what's the average density and viscosity of the bog/quicksand and the mass of your diplodocid? are there mitigating factors such as struggle or initial downward velocity?