In the latter half of the twentieth century, studies of ontogeny had a vital influence on trilobite systematics. Protaspid morphology especially has been regarded as one of the most significant criteria in classifying trilobites (Whittington 1957; Palmer 1962; Robison 1967; Fortey & Owens 1975; Chatterton 1980; Edgecombe et al. 1988; Fortey & Chatterton 1988; Speyer & Chatterton 1989; Fortey 1990, 2001; Edgecombe 1992; Chatterton et al. 1994, 1999; Lee & Chatterton 2003, 2007; Campbell & Chatterton 2009). Although it has been pointed out that a versatile morphological diversity of trilobite larvae could be independent of mature morphology (Bergstrom 1977; Lane & Thomas 1983; Thomas & Holloway 1988), protaspid morphology has been widely used for the higher-level classification of trilobites. For instance, the Order Phacopida was recognized by a morphologically distinctive protaspis with three prominent marginal spines in a characteristic disposition and a forwardly expanding glabella (Whittington 1957). Fortey & Owens (1975) claimed that one of the main characters of the Order Proetida was the protaspides having a preglabellar field. The protaspides of the members of the Order Asaphida were known to have a unique globular shape (Fortey & Chatterton 1988; Fortey 1990). This protaspis-based approach for suprageneric classification of trilobites has been proved to be successful so far and the morphological information from protaspides is expected to help elucidate many of unsolved conundrums in trilobite systematics (Fortey 2001). Whittington (1957) proposed that great emphasis should be given to protaspid morphology in trilobite classification, and the application of this idea to the study of post-Cambrian trilobites has been quite successful (Whittington 2007). This is in part attributable to already distant phylogenetic relationship between the post-Cambrian trilobite orders. One of the seemingly intractable problems in trilobite phylogeny is to find the Cambrian ‘roots’ of post-Cambrian trilobites, and Cambrian ptychoparioid ontogenies are expected to help elucidate the problem (Fortey 2001). However, even though many protaspides of the Cambrian ptychoparioids have been reported (see Chatterton & Speyer 1997 for full list and Lee & Chatterton 2007 and references therein), trilobite phylogeny still seems to be far from being resolved. The current state-of-play in trilobite systematics, in which protaspid morphology has played a significant role, is rooted in von Baer’s rule, and this rule has been applied well to trilobites. Hence, ontogenetic studies are expected to reveal the currently unresolved phylogenetic relationships. Chatterton & Speyer (1997) mentioned ‘as a rule, monophyletic groups based on characteristics of adult growth stages have similar larvae and life-history strategies so that larval morphology appears to be a useful indicator of relationship’, implying that protaspides are more informative for systematics than later growth stages. However, von Baer’s rule does not always apply well. Recently, Poe (2006) quantitatively tested von Baer’s rule and raised doubts about the rule per se. A similar conclusion has been drawn for the phylogenetic use of the crustacean nauplius. Dahms (2000) pointed out significant differences in the morphology of nauplii of closely related species and emphasized that, for phylogenetic use, characters of the whole ontogenetic sequence including the adults should be considered and evaluated. These studies raise the possibility that protaspid morphology alone is not as useful as traditionally expected. With the application of von Baer’s rule to trilobites aside, the validity of using protaspid morphology in trilobite classification has seldom been questioned from a logical perspective. Even if the hypothesis that trilobites with similar protaspides share a common ancestry is considered to have withstood the test of time rather well (Fortey 2001), there seems a logical pitfall that is derived from tradition rather than sound scientific reasoning.