The fossil record of the Crustacea is exceptionally good, stretching back to the Cambrian. However, they don’t appear in any abundance until the Carboniferous.
The first undisputed crustaceans are Canadaspis and Perspicaris which belong to the Subclass Phyllocarida and were found in the Middle Cambrian Burgess Shale.
By the Carboniferous, all the other major groups of crustaceans are present except for the Eucarida. Most of these fossils are found within shallow marine sediments because these offered good potential for preservation. The Eucarida appeared in the Mesozoic, but they had their origins in the Devonian/Carboniferous. The Eucarida, especially the Decapoda, underwent an adaptive radiation during the Jurassic, with the appearence of crabs and modern shrimps.
The crabs and lobsters carried on diversifing to become one of the major groups of marine organisms. Hermit crabs are an unusual group that seek protection inside discarded mollusc shells. Hermit crabs are known at least from the Cretaceous Speeton Clay where hermit crabs inhabited empty ammonite shells.
The ostracods, which have a fossil record going back to the Cambrian, are commonly found in sediments of all ages. They are so abundant that they can be used in biostratigraphy and show such a range of environmentally-dependent morphologies that past ocean temperatures can be calulated from them
Fossil Crabs, Reefs Hint at the Future of Earth’s Seas
Paleontology is often viewed as a science of the dead. The goal of the fossil expert is to find, restore, and understand life that no longer exists, filling out the long backstory of the modern world. But paleobiologists with an ecological bent are challenging this scientific stereotype. The past is not merely a graveyard of strange species, but a record of how life responds to environmental changes that our species is now having an ever-greater role in spurring. Given how greatly past climate change has altered the pattern of life, especially, researchers are scouring the past for hints about how catastrophes triggered by human activity might unfold. Ancient crustaceans, and the reefs they lived upon, may hold such clues.
In a new Geology study, Kent State University paleontologist Adiël Klompmaker and coauthors tracked the history of lobsters, shrimp, true crabs, and squat lobsters in the 252 to 66 million year window of time called the Mesozoic. This was the timespan when these major groups, still present today, evolved and spread. And as Klompmaker and colleagues found when they looked at the habitats the fossil crustaceans were found in and fluctuations in prehistoric sea level, the fate of crabs and their kin has been closely tied to reefs.
The evolutionary story of crustacean lineages is one of ups and downs. Outlined in the new study, the rough version of the story goes something like this.
During the 252 to 201 million year old stretch of Triassic time, when weird crocodile cousins ruled the terrestrial realm, shrimp and lobsters were the dominant forms of crustaceans in the seas. True crabs didn’t start to become a prominent presence in the seas until about 175 million years ago, but their diversity quickly ramped up and outpaced that of the lobsters and shrimp. Crustaceans in general took a major hit during the mass extinction that closed the Jurassic, about 145 million years ago, but lobsters, shrimp, true crabs, and squat lobsters picked up where they left off in the Cretaceous, with true crabs and squat lobsters remaining dominant. As far as crustaceans go, the seas just prior to 66 million years ago may have looked familiar – Klompmaker and coauthors estimate that about 65% of decapod species were crabs and squat lobsters, similar to the count today.