Merostomes include two groups of arthropods, one of which has survived to the present. The eurypterids, or “sea scorpions,” were common from 450 to 390 million years ago, but extinct by the end of the Middle Permian (about 270 million years ago). During their heyday, they lived in habitats ranging from shallow marine to estuarine to freshwater. Sea scorpions would have been hard to miss, with some species growing to almost 2.5 meters long (8 feet) with spiny tails and five pairs of walking legs, one of them often modified into large pincers for gathering food. Their relatives, the horseshoe crabs, still occur. Their fossil record begins in the Ordovician (about 450 million years ago). Unlike eurypterids, early horseshoe crabs were small, evolving to be larger over time. Their fossils are rare, perhaps because horseshoe crabs were never very abundant, or because their exoskeletons do not fossilize well. With a body plan that has changed little over time, horseshoe crabs been called “living fossils.”

Arthropods have been on Earth for more than 540 million years, and were diverse almost from the beginning. Different lineages of arthropods, such as crustaceans, diverged as early as 525 million years ago. The evolution of an external body covering (the exoskeleton), and the presence of body segments and paired appendages (mouthparts, legs, claws, antennae) signaled the transition from early worm-like precursors to arthropods. While modern arthropods live in nearly every habitat, the earliest arthropods were probably tiny, bottom-dwellers scavenging detritus at the bottom of warm seas. The enormous success of arthropods is at least partly due to their appendages. Located on all body regions, their appendages became specialized especially for feeding through the mouthparts, but also for getting oxygen through respiration (gills), reproducing (elaborate external genitalia), and moving around including walking, swimming, and/or flying. The gradual adoption of a modular body plan with multifunctional appendages has allowed arthropods to thrive in an impressive variety of environments.