Phylum Arthropoda

Arthropoda is the most diverse animal phylum and its members are abundant in many environments. Living arthropods are split into two groups, Chelicerata and Mandibulata. Chelicerates include taxa like spiders, mites, and scorpions, and are defined by the presence of chelicerae—for example, the fangs found in spiders. Mandibulates include taxa like crabs, barnacles, and insects, and are defined by the presence of mandibles—for example, the enlarged mouthparts found in many beetles. Collectively, arthropods are defined by several features, including jointed appendages (arthropod means “joint leg”), an exoskeleton (often made of chitin), and bilateral symmetry (like us). Watch the video below to see some of these features in living specimens.

The fossil record of arthropods began in the Early Cambrian, approximately 537 million years ago, with the occurrence of arthropod-made trace fossils.

Arthropod body fossils are not known until 521 million years ago, when trilobites first appeared in the fossil record—and from there would go on to be an important member of Paleozoic ocean-bottom communities. 

Trilobites are generally accepted as crown-group arthropods, which makes it interesting that they are the first arthropods in the record. The first stem-group arthropods—animals that are phylogenetically basal to modern arthropods (e.g., Anomalocaris)—appear 518 million years ago. Considering the trace fossils, and the seemingly out-of-order appearance of stem- and crown-groups in the fossil record, it is highly likely that the earliest arthropod record has not yet been found. Indeed, arthropods likely evolved by at least 550 million years ago, at the very end of the Precambrian.

From the Cambrian onward, arthropods colonized, and have been abundant in, essentially all of Earth’s ecosystems, from the deep ocean to the skies.

Arthropods with mineralized exoskeletons (e.g., trilobites and some crustaceans) have left behind an excellent fossil record. Most arthropods, however, lack mineralized exoskeletons and are are known as fossils primarily from lagerstätten deposits, wherein exceptional conditions (e.g., rapid burial, no oxygen) led to the preservation of the often-fragile arthropod bodies. Some of the most exceptional arthropod fossils are found encased in amber (i.e., fossilized tree resin), enabling highly detailed study (see figure below).

Phylogenetic Relationships

Arthropods belong to a broader group of organisms called Ecdysozoa, which share the characteristic of molting during growth. Because arthropods have a hardened exoskeleton (whether mineralized, or not), they must shed their exoskeleton in order to grow. This process is called molting.

As reviewed by Giribet and Edgecombe (2019), within Ecdysozoa, arthropods are now often grouped with two other phyla, Tardigrada and Onychophora, and form the clade Panarthropoda (see figure below). These groups are united by the presence of paired, segmented appendages and by partitioning of the body into at least two regions, the head and trunk.

Understanding the relationships between arthropod groups has been a considerable challenge, in no small part because of the vast diversity of fossil and living arthropods. The advent of modern molecular techniques has improved our understanding of these relationships, as many arthropod groups previously had been defined based solely on morphological characters. The complementary information provided by molecular and morphological data has led to greater resolution in the arthropod phylogeny. As shown in the phylogeny above, the familiar arthropods (i.e., Chelicerata, Mandibulata) of today represent only a portion of the arthropod diversity that has existed during the history of life on Earth. On the next page, we will take a closer look at several of the stem-group taxa before during to the Trilobita, Chelicerata, and Mandibulata on the pages that follow.