Animals having arthropod characteristics first appeared about 545 million years ago. Because all arthropods have a true coelom, an exoskeleton, and jointed appendages, biologists infer that all arthropods evolved from a common ancestor. However, biologists are still uncertain about the order in which subgroups of arthropods evolved and the exact relationships between the subgroups. The phylogenetic diagram in Figure 36-3 shows possible evolutionary relationships between the highly diverse arthropod subgroups.
The similar characteristics of many modern subgroups of arthropods may be the result of convergent evolution. For example, ancient and extinct arthropods, such as trilobites, had many body segments and one pair of appendages on each segment. However, most living arthropod species have some segments that lack appendages and some segments that are fused into a larger structure called a tagma (plural, tagmata). The tagmata tend to be specialized for functions such as feeding, locomotion, and reproduction.
Arthropods are usually divided into five subphyla on the basis of differences in development and in the structure of appendages, such as mouthparts. The two major types of mouthparts are mandibles, which are jawlike, and chelicerae (singular, chelicera), which are pin-cerlike. The five main subphyla are Trilobita, Crustacea, Chelicerata, Myriapoda, and Hexapoda, as Figure 36-3 shows.
This phylogenetic diagram shows hypotheses of the evolutionary relationships between the highly diverse arthropod subgroups. Dashed lines indicate relationships that are poorly understood or that are heavily debated. For updates on phylogenetic information, visit go.hrw.com and enter the keyword HM6 Phylo.
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