The fossils appear in four distinct layers, suggesting that the mass die-offs occurred repeatedly around 6-9 million years ago. Due to their belly-up position, the researchers assumed that they had died offshore and washed into shallow waters, where they were eventually buried by the sand.
According to the scientists, algal blooms seem to be the likely cause to the mass die-offs. Such event may occur when a high level of nutrients flow excessively, either from the depth of the sea or agricultural fertilizer run-offs, boosting the growth of certain planktons.
These planktons may produce chemical compounds that can be lethal to other organisms when inhaled in larger doses, effectively killing the animals in the region.
This hypothesis is supported by the orange splotches in the surrounding rock that could have been the chemical traces left by the planktons. When viewed under the microscope, the rocks contain tiny spheres similar to today’s dinoflagellates, the plankton often seen causing similar events today.
Original findings published in Live Science.