The preservation of the marine organisms is explained, but how did the land animals get into the lagoon?
(It is obvious how the marine organisms got into the lagoon, they were washed or carried into the lagoon by the ocean, or swam in and died due to the lagoons hypersalinity or anoxic bottom waters.)
There are a few theories for the preservation of the land organisms here, which may include:
The organism either lived near the shore and was washed there by a storm after death.
There was a river near-by and the land organisms were carried down stream to its final resting place, in the lagoon.
Animals like the Archaeopteryx fed on small fish near the surface of the water and got caught in the water and drowned.
The archaeopteryx may have been carried into the lagoon as the river is doing with this branch (modified from
Barthel et. al., 1990