Fossil Crabs

THE STORY

These well-preserved little “pea crabs” are found in the fine diatom-rich shale of the Monterey Formation of Carmel Valley, California. Despite their size, they are a true crab. Pinnixa is a rather specialized little animal that lived like modern pea crabs in symbiotic relationship with other marine invertebrates, particularly bivalves (clams). They are significant because they give us a window into the past environments that were thriving in the area during the Miocene.

These little crabs lived in association with clams on the seafloor, The crab is small enough to live inside the clam and we have even found fossil clams with crabs inside. The clam has two valves to its shell, with its soft body and mantle lining the inside. The crab moves into the space between the mantle and the shell itself. It does not bore a hole to get in and it doesn’t appear to hurt the clam While there, however, the crab exploits the clam’s food-gathering ability and helps itself to some of the catch. There’s evidence that some crabs provide benefits to the clam – such as cleaning the host and consuming waste material that would otherwise accumulate.

As happens along much of the west coast today, cold, nutrient-rich water would rise from the deep regions offshore toward the surface, ocassionally producing enormous blooms of diatoms. When the diatoms died, their silica skeletons accumulated on the seafloor in massive layers (diatomaceous earth) which had little oxygen. Sometimes the accumulation was too quick for the animals to escape. And since the oxygen-poor bottom environment limited decomposition, that’s why the Monterey Formation contains beautifully preserved fossils in finely laminated shale.

Name: Pinnixa gallehiri

Age: Miocene Epoch, 10 million years
Unit: Monterey Formation (Aguajito Member)
Site: Carmel Valley, California
Date: 2016