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 are the most successful group of organisms on Earth in terms of numbers, including almost half of all known species. The hallmark of their success is a body plan that makes them durable and adaptable. All arthropods have external skeletons made of hard material (chitin embedded in a protein matrix). The exoskeleton protects them from predators, weather conditions, and other threats. As they grow, arthropods typically shed the exoskeleton to reveal a bigger, fresh one underneath. The two lengthwise halves of an arthropod body are a mirror image of each other (bilateral symmetry) and typically divided into segments. Each segment has appendages that are specialized for the many activities of the arthropod. An arthropod may use appendages to feed (mouthparts), to breathe (gills, tracheae, book lungs), to reproduce (genitalia), and to move around (walking, swimming, flying). Having a modular body plan with multifunctional appendages has allowed arthropods to thrive in an impressive variety of environments.