Atelocerata Versus Pancrustacea

Within the Mandibulata we are faced with a similar problem concerning the relationships of lineages. As discussed, the mandibulate arthropods include the Crustacea, the Myriapoda, and the Hexapoda. Traditionally, myriapods and hexapods have been considered sister groups (e.g., Snod-grass, 1938) and together called either Tracheata or, more widely, Atelocerata (Figure 3.22). Familiar traits defining the Atelocerata include the loss of the second antennal pair, presence of a tentorium (internal head skeleton), a respiratory system involving a system of fine tubules or tracheae, and Malpighian tubules. However, based mostly on recent molecular studies, the monophyly of Atelocerata has been challenged, and an alternative relationship between Crustacea and Hexapoda put forward (e.g., Zrzavy and Stys, 1997). This alternative grouping, called Pancrustacea (= Tetraconata), has gained considerable support among molecular and developmental biologists (e.g., Zrzavy et al., 1998b). The Pancrustacea is supported by some molecular analyses (e.g., Field et al., 1988; Turbeville et al., 1991; Ballard et al., 1992; Boore et al., 1995, 1998; Friedrich and Tautz, 1995; Giribet et al., 1996; Giribet and Ribera, 1998), while morphological traits are not outwardly apparent and are poorly understood across a variety of taxa: for example, suppression of distal mandibular segments (Popadic et al., 1996, 1998; Deutsch, 2001), neurogenic pattern-formation processes (Whitington et al., 1991; Osorio et al., 1995; Dohle, 1998, 2001), and ultrastructure of the compound eye (Paulus, 1979; Osorio and Bacon, 1994; Osorio et al., 1995; Dohle, 1998, 2001). This grouping also indicates that several complex morphological features, specifically the tentorium, tracheae, and Malpighian tubules, were independently evolved. An extensive study of both morphological and molecular data by Edgecombe et al. (2000), however, supported Atelocerata monophyly. For the time being Atelocerata will be adopted pending the accumulation of more evidence to the contrary.

3.22. Phylogeny of Mandibulata showing alternative relationships of the Atelocerata (A) and Pancrustacea (B).
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