Crabs are wonders of adaptation, with multiple crustacean evolutionary lines developing similar features independently through a process called carcinization. Crabs' squat, compact, hard-shelled bodies are perfect for defense, while their claws allow for combat and consuming food.

Crabs of the Macrophthalmus genus, also called sentinel crabs, are known to display complex social behaviors: cleaning of their environment, ritual displays, and fighting. Sentinel crab combat can be classified into two main categories: grasping fighting and claw-extending fighting. They appear across the Indo-Pacific, both in the prehistoric fossil record and still scuttling on beaches today.

This specimen is a one-of-a-kind fossil crab specimen of the Macrophthalmus genus. It dates back to around 2.58 million years ago during the Pliocene Epoch. It comes from Madagascar and measures 6.004"

The fossil ships in a sturdy carton along with a certificate of authenticity.

Crabs first appear in the fossil record during the early Jurassic Period 180 million years ago, breaking from the rest of the Crustacea subphylum. In the time since their appearance, crabs have evolved into many different forms. As a group that are polyphyletic, meaning that they do not all share common ancestry but are rather the work of convergent evolution. The independent evolution into the crab form is easy to understand: with their sharp claws and hard shells, crabs are well suited to fend off predators. Other advantages include a lower center of gravity, all the better for moving about the ocean floor.

Despite their hard shells, crabs have a limited fossil record, but more recent discoveries are shedding some light on the evolution of the animals. In 2021, scientists discovered a 100-million-year-old Cretapsara athanata crab, preserved in amber. The specimen dates to a turning point in crab evolution, when the marine creatures began to inhabit freshwater and dryland environments.

To that point, modern crab specimens only dated to around 50 to 75 million years ago, but this amber specimen supports the idea that crabs made the jump to land far earlier, around 130 million years ago. This period of crab development is dubbed the Cretaceous crab revolution, when crabs conquered new terrains and evolved into what today is considered the base crab shape, a process called carcinization. 

At the same time, other crabs diversified away from this form through decarcinization. About 80% of modern crabs appeared at this time, though the evolutionary boon was hampered by the K-Pg extinction at the end of the Cretaceous Period. Today, crabs appear in a wide diversity of forms, from the pea crab to the Japanese spider crab, which tops out at 12 feet in length across.