Arthropod Evolution

FIGURE 1.5. Branchinecta sp., a fairy shrimp. [From R. D. Barnes, 1968, Invertebrate Zoology, 2nd ed. By permission of the W. B. Saunders Co., Philadelphia.]

FIGURE 1.5. Branchinecta sp., a fairy shrimp. [From R. D. Barnes, 1968, Invertebrate Zoology, 2nd ed. By permission of the W. B. Saunders Co., Philadelphia.]

swimmeret (forward swimming) FIGURE 1.6. Crayfish. Ventral view of one side to show differentiation of appendages.

eyes, elongate trunk that bears many pairs of legs, articulation of the coxa with the sternum (rather than the pleuron as in hexapods), tracheal respiratory system, Malpighian tubules for excretion, absence of mesenteric ceca, and distinctive mechanism by which the animal exits the old cuticle during ecdysis. Further, they are found in similar habitats (e.g., leaf mold, loose soil, rotting logs).

For these reasons, they were traditionally placed in a single large taxon, the Myriapoda. The monophyletic nature of the myriapods has been supported by some, but not all, cladistic analyses of large data sets with a combination of morphological and molecular characters of living species (Wheeler et al., 1993; authors in Fortey and Thomas, 1998). Yet other morphological and molecular studies indicate that the myriapods constitute a paraphyletic or even polyphyletic group. Determination of the relationships within the Myriapoda has proved difficult because potentially homologous characters are shared by different pairs of groups. For example, Diplopoda and Pauropoda have the same number of head segments and one pair of maxillae; Diplopoda and Chilopoda have segmental tracheae; and Symphyla

FIGURE 1.7. Myriapoda. (A) Lithobius sp. (centipede), (B) Julus terrestris (millipede), (C) Pauropus silvaticus (pauropod), and (D) Scutigerella immaculata (symphylan). [From R. D. Barnes, 1968, Invertebrate Zoology, 2nd ed. By permission of the W. B. Saunders Co., Philadelphia.]

and Pauropoda develop embryonic ventral organs that become eversible vesicles, as well as contributing to the ventral ganglia. Boudreaux's (1979) overall conclusion was that the Pauropoda-Diplopoda and Chilopoda-Symphyla are the two sister groups within the taxon (Figure 1.8). An alternative view, based on cladistic analysis of morphological characters of living forms (Kraus, in Fortey and Thomas, 1998), is that the myriapods are paraphyletic: the Chilopoda is the sister group to the other three. Unfortunately, though there is a rich fossil record of myriapods extending back to the Upper Silurian, insufficient study has been done to clarify the monophyletic nature or otherwise of this group (see Shear, in Fortey and Thomas, 1998).

Some 3000 species of chilopods (centipedes) (Figure 1.7A) have been described (Lewis, 1981). They are typically active, nocturnal predators whose bodies are flattened dorsoven-trally. The first pair of trunk appendages (maxillipeds) are modified into poison claws that are used to catch prey. In most centipedes the legs increase in length from the anterior to the posterior of the animal to facilitate rapid movement. The earliest known fossil centipedes, from the Upper Silurian, are remarkably similar to some extant species, suggesting that the group may be considerably more ancient.

In contrast to the centipedes, the diplopods (millipedes) (Figure 1.7B) are slow-moving herbivorous animals. The distinguishing feature of the almost 10,000 species in the class is the presence of diplosegments, each bearing two pairs of legs, formed by fusion of two originally separate somites. It is believed that the diplosegmental condition enables the animal to exert a strong pushing force with its legs while retaining rigidity of the trunk region. As they cannot escape from would-be predators by speed, many millipedes have evolved such protective mechanisms as the ability to roll into a ball and the secretion of

FIGURE 1.8. Schemes for the possible monophyletic origin of the arthropods as proposed by Snodgrass (1938), Sharov (1966), and Boudreaux (1979). Note also the differing relationships of the Annelida, Onychophora, and Arthropoda.

defensive chemicals (Hopkins and Read, 1992). Fossil millipedes are known from the Lower Devonian.

Pauropoda (500 species) are minute arthropods (0.5-2 mm long) that live in soil and leaf mold. Superficially they resemble centipedes, but detailed examination reveals that they are likely the sister group to the millipedes. This affinity is confirmed by such common features as the position of the gonopore, the number of head segments, and the absence of appendages on the first trunk segment (Sharov, 1966). A characteristic feature are the large

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