Trilobites are extinct, but lived on Earth for about 290 million years, achieving incredible abundance during from the Cambrian to Devonian. The oldest trilobite fossils are about 540 million years old, but even earlier trilobites may have lacked the hard body parts that fossilize. Trilobites are one of the most diverse groups of extinct organisms known. Their variety of body forms were undoubtedly specialized to different modes of life. Imagine tiny (1 mm long) trilobites floating in the ocean and filter-feeding on suspended detritus, juxtaposed with a large (60 cm, or 2 feet long) predatory trilobite crawling on the seafloor and catching worms. In spite of their diversity, trilobites shared a three-lobed body plan consisting of a central axis from head to tail with lobes on each side. All three lobes were protected by a hard covering (exoskeleton) which, depending on the species, was ornamented with ridges, spines, and color. The last trilobite lineages disappeared during the huge end-Permian extinction event (about 252 million years ago).

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.