Arthropods are a diverse group of animals with over a million described species and an estimated 6 to 7 million in total. They have a hard exoskeleton, segmented bodies, and an open circulatory system. Arthropod phylogeny is a volatile scientific topic, and opinions keep changing. Arthropods are almost universally considered monophyletic. There are currently two main theories regarding the placement of arthropods within the Tree of Life. Our understanding of the relationship between arthropod groups is currently in a state of flux.
Arthropods are the largest of all animal phyla, with over a million species described, estimated at 6 to 7 million in total. Arthropods, whose name means “jointed feet,” are characterized by a hard exoskeleton, segmented bodies, and an open circulatory system. The group includes insects, crustaceans, myriapods (centipedes and millipedes), chelicerates (arachnids and horseshoe crabs), and several extinct groups including trilobites. Arthropod phylogeny is a volatile scientific topic, and opinions keep changing as new information arrives.
Arthropods are almost universally considered monophyletic, meaning they are descended from a common ancestor rather than arising multiple times. This is in contrast to the predominant view during the 1970s. A 2001 study of arthropods places the group alongside tardigrades, a phyla of microscopic aquatic animals. Both are in turn related to the velvet worms, a group of sophisticated terrestrial worms with fossil records dating to or earlier the Cambrian (~545 million years ago). Arthropods existed in the early Cambrian, about 530 million years ago, but whether they existed before this is up for debate. Hard cuticles, like the universal ones among arthropods, only appear in the fossil record about 545 million years ago.
There are currently two main theories regarding the placement of arthropods within the Tree of Life. Arthropods are joined by annelids (segmented worms), based on their shared segmentation. A more recent analysis places arthropods alongside nematodes and several small phyla such as penile worms, based on the common moulting feature. This group is called “Ecdysozoa” after “ecdysis”, which means moult. Shedding means the animal grows by shedding its exoskeleton, then growing larger until a new exoskeleton hardens.
Our understanding of the relationship between arthropod groups is currently in a state of flux. A 1970s classification scheme placed arthropods into mandibulates, including myriapods, crustaceans, and hexapods (insects), with hexapods and myriapods being the common clade, Atelocerata, and the rest of the arthropods in Chelicerata. This classification scheme has been progressively rejected as more recent studies point out that hexapods are actually nested within crustaceans (meaning that the first terrestrial arthropods evolved from crustaceans, not myriapods), with myriapods and chelicerates being they are actually a sister group called Myriochelata. This is called the Pancrustacea hypothesis. These classifications are sure to be refined as more molecular and fossil data arrives.
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