Arthropod Evolution

(Onychophora-Myriapoda-Hexapoda). In some schemes the Trilobita are included as a sis- 19

ter group of the Chelicerata in the phylum Arachnomorpha; in others they are ranked as an independent phylum.

Manton, especially, argued that there are fundamental differences in the structure of the limbs in members of each phylum, related to the manner in which the animals move. In Crustacea the limb is biramous, bearing a branch (exopodite) on its second segment (basipodite); in Uniramia there is an unbranched limb; and in almost all Chelicerata there is a uniramous (unbranched) limb. However, in Limulus (and, incidentally, trilobites) the limb is biramous, though the branch originates on the first leg segment (the coxopodite), suggesting that chelicerates may have been initially biramous, losing the branch when the group became terrestrial. This view has been strongly disputed by Kukalova-Peck (1992, and in Fortey and Thomas, 1998) who sees the ancestral leg of all arthropods as being biramous and points out that many fossil insects have legs with several branches (i.e., they are "polyramous Uniramia"!). She has urged that the term Uniramia be abandoned. Manton (1973, 1977) made a strong case that the mandibles of the three major groups are not homologous. Based on their structure and mechanism of action, she suggested that the jaws of crustaceans and chelicerates are formed from the basal segment of the ancestral appendage (gnathobasic jaw), though in each group the mechanism of action is different. In Uniramia, however, Manton claimed that the mandible was formed from the entire ancestral appendage, and that members of this group bite with the tip of the limb. Manton pointed out that a segmented mandible is still evident in some myriapods, though the mandible of insects and onychophorans appears unsegmented. Again, this proposal has been severely criticized by Kukalova-Peck (1992, and in Fortey and Thomas, 1998) whose paleoentomological studies suggest the ancestral limb of all arthropods included 11 segments, 5 of which make up the jaw seen in extant species.

Anderson (1973, and in Gupta, 1979) drew evidence from embryology in support of the polyphyletic theory. He compared fate maps (figures indicating which embryonic cells give rise to which organs and structures) among the various groups and concluded that the pattern of development seen in Uniramia bears similarities to that in annelids, yet is very different from that of crustaceans (chelicerates show no generalized pattern, leading to speculation that they may themselves be polyphyletic). It should be noted that not all embryologists agree with Anderson's methods of analysis and, therefore, his conclusions (e.g., Weygoldt, in Gupta, 1979).

When the Mantonian viewpoint was initially presented, there was little supporting evidence from the arthropod fossil record. Within the last three decades, however, there has been considerable activity both in analyzing new species and in reinterpreting some specimens described earlier. Many of these fossils cannot be placed in extant arthropod groups or even along evolutionary lines leading to these groups (Whittington, 1985), indicating that arthropodization was experimented with many times and implying that arthropods had multiple origins. Most of the groups to which these Cambrian fossils belong rapidly became extinct. The arthropod groups seen today represent "successful attempts in applying a continuous, partially stiffened cuticle to a soft-bodied worm" (Willmer, 1990, p. 290).

Willmer (1990) drew attention to the very different methodology used by polyphyletic and monophyletic schools, by which they reach opposite conclusions regarding arthropod evolution. The approach taken by Manton and her supporters has been to search for differences among groups in the belief that they provided evidence for polyphyly. On the other hand, the modern monophyleticists, notably Boudreaux and Wheeler et al., have attempted to determine similarities and use these as proof of a common origin for all arthropods.

20 The debate as to arthropod relationships and evolution continues to be vigorous (and polarized!) (see Edgecombe, 1998; Fortey and Thomas, 1998). Overall, the balance currently rests in favor of monophyly, with the major groups having had a common origin from a primitive segmented wormlike animal.

3.3. The Uniramians

In agreement with the 19th century zoologists Haeckel and Moseley, Tiegs and Manton (1958) and Manton (1973) made a forceful case for uniting the Onychophora, Myriapoda, and Hexapoda in the arthropod group Uniramia. In their view the many structural similarities between onychophorans and myriapods (see Section 2.1) indicated true affinity and were not the result of convergence. This view received support from the fate map analyses made by Anderson (1973, and in Gupta, 1979) showing the similarity of embryonic development in the three groups. These authors envisaged the evolution of myriapods and insects from onychophoranlike ancestors as a process of progressive cephalization. To the original three-segmented head (seen in modern Onychophora) were added progressively mandibular, first maxillary, and second maxillary (labial) segments, giving rise to the so-called monognathous, dignathous, and trignathous conditions, respectively. Of the monognathous condition there has been found no trace. The dignathous condition occurs in the Pauropoda and Diplopoda, and the trignathous condition is seen in the Chilopoda (in which the second maxillae remain leglike) and the Symphyla and Hexapoda (in which the second maxillae fuse to form the labium).

Few modern authors would support the idea of the onychophorans having common ancestry with the myriapods and insects, preferring to believe that the similarities are due to convergence. Indeed, some authors do not accept that the myriapods and hexapods are sister groups. For example, Friedrich and Tautz (1995) concluded from their comparison of ribosomal nuclear genes that the myriapods were the sister group to the chelicerates, while the crustaceans were the sister group to the hexapods. Unfortunately, the term Uniramia is still used in some texts (e.g., Barnes et al., 1993; Barnes, 1994) to include only the Myriapoda and Hexapoda (i.e., as a synonym of the Atelocerata). As noted earlier, Kukalova-Peck (1992, and in Fortey and Thomas, 1998) has recommended that use of this word be discontinued as the group includes organisms with polyramous legs.

3.3.1. Myriapoda-Hexapoda Relationships

The sharing of features such as one pair of antennae, Malpighian tubules (though these may be secondarily reduced or lost in both groups), anterior tentorial arms, and a tracheal system gave rise to the traditional view that the Myriapoda and Hexapoda are sister groups, collectively forming the Atelocerata (Tracheata), with a common multilegged ancestor (Sharov, 1966; Boudreaux, 1979). Indeed, the existence of several shared features in Symphyla and Hexapoda (Section 2.4) led in the 1930s to the development of the Symphylan Theory for the origin of the hexapods. Within the last decade, however, a major change in opinion has occurred with respect to the relationship between myriapods and hexapods. It is now believed that their common features are the result of convergence or, at best, parallel evolution from a distant common ancestor. Much recent research, in molecular biology, neurobiology, and comparative morphology, often combined in extensive cladistic analyses, supports the hypothesis that hexapods are more closely related to crustaceans than to myriapods. Equally, the data suggest that myriapods are allied with the chelicerates.

Comparisons of mitochondrial and nuclear gene sequences and large hemolymph proteins, examination of eye and brain structure, and studies of nerve development have come out strongly in favor of insects and modern crustaceans as sister groups (Dohle, in Fortey and Thomas, 1998; Shultz and Regier, 2000; Giribet et al., 2001; Hwang et al., 2001; Cook et al., 2001; Burmester, 2002). Some data even suggest that insects arose from the same crustacean lineage as the Malacostraca (crabs, lobsters, etc.), an idea for which Sharov (1966) had been criticized almost 40 years ago.

4. Summary

The arthropods are a very diverse group of organisms whose evolution and interrelationships have been vigorously debated for more than a century. Supporters of a monophyletic origin for the group rely heavily on the existence of numerous common features in the arthropod body plan. Their opponents, who must account for the extraordinary degree of convergent evolution inherent in any polyphyletic theory, argue that all of these features are essentially the result of a single phenomenon, the evolution of a hard exoskeleton, and that arthropodization could easily have been repeated several times among the various ancestral groups. In the polyphyletic theory, therefore, the four dominant groups of arthropods (Trilobita, Crustacea, Chelicerata, and Insecta), as well as several smaller groups both fossil and extant, originated from distinct, unrelated ancestors. The proponents of poly-phyly use evidence from comparative morphology (notably studies of limb and mandible structure), comparative embryology (fate maps), and more recently the fossil record (which shows an abundance of arthropod types not easily assignable to already known groups). The monophyleticists claim, in turn, that these comparative embryological and morphological studies are of doubtful value because of the methodology employed and assumptions made. Overall, the current balance seems in favor of a monophyletic origin for the arthropods.

The uniting of Onychophora, Myriapoda, and Hexapoda as the clade Uniramiais highly questionable. Most modern authors agree that apparent similarities between onychophorans and members of the other two groups are due to convergent evolution. The Myriapoda, although including four rather distinct groups (Diplopoda, Chilopoda, Pauropoda, and Sym-phyla), are widely thoughtto be monophyletic. For many years, myriapods were considered the sister group to the Hexapoda. However, recent research indicates that myriapods may be allied more closely to the chelicerates, and hexapods to crustaceans. Five distinct groups of hexapods occur: collembolans, proturans, diplurans, thysanurans, and winged insects. On the basis of their entognathous mouthparts and other synapomorphies the first three groups are placed in the Entognatha and are distinct from the thysanurans and pterygotes which form the true Insecta.

5. Literature

Numerous general textbooks on invertebrates, as well as specialized treatises provide information on the biology of arthropods. Tiegs and Manton (1958) give a detailed historical account of schemes for the evolutionary relationships of arthropods. Other major contributors to this fascinating debate include Manton (1973, 1977), Sharov (1966), Anderson

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