The phylogenetic position of the Clitellata and the Echiura ‐ on the problematic assessment of absent characters*

Abstract

Absent characters (negative characters) are difficult to assess and their correct interpretation as symplesiomorphies, synapomorphies or convergencies (homoplasies) is one of the greatest challenges in phylogenetic systematics. Different phylogenetic assessments often result in contradictory phylogenetic hypotheses, in which the direction of evolutionary changes is diametrically opposed. Especially in deciding between primary (plesiomorphic) and secondary (apomorphic) absence, false conclusions may be reached if only the outgroup comparison and the principle of parsimony are employed without attempting any biological evaluation or interpretation of characters. For example, in the higher‐level systematization of the Annelida and related taxa different assessments of absent characters have led to conflicting hypotheses about the phylogenetic relationships and the ground pattern of the annelid stem species. Varying phylogenetic interpretations regarding the absence of the chemosensory nuchal organs in the clitellates and their presence in polychaetes initiated a controversy that produced two alternative phylogenetic hypotheses: (1) the Clitellata are highly derived Annelida related to a subtaxon within the, in this case, paraphyletic ‘Polychaeta’ or (2) the Clitellata are comparatively primitive Annelida representing the sister group of a monophyletic taxon Polychaeta. In the former, the absence of nuchal organs in the Clitellata is regarded as a secondary character, in the latter as primary. As most Clitellata are either limnetic or terrestrial, we must ask which characters are plesiomorphies, taken from their marine stem species without changes. In addition to a thorough investigation and evaluation of clitellate characters, a promising approach to these questions is to look for such characters in limnetic and terrestrial annelids clearly not belonging to the Clitellata. A similar problem applies to the evaluation of the position of the Echiura, which lack both segmentation and nuchal organs. Evidence is presented that in both taxa these absent characters represent derived, apomorphic character states. The consequences for their phylogenetic position and the questionable monophyly of the Polychaeta are discussed. The conclusion drawn from morphological character assessments is in accordance with recently published hypotheses based on molecular data.

Polychaeta, Annelida, and Articulata are not monophyletic: articulating the Metameria (Metazoa, Coelomata)

“…Other evidence is indirect. Similar losses occur in the terrestrial polychaetes (PURSCHKE 1997(PURSCHKE , 1999PURSCHKE et al 2000). And when the taxon Questidae is accepted as closely related to the clitellate lineage based on the presence of a clitellum, it may be deduced that the polychaete-like ancestor of the Clitellata had nuchal organs.…”

“…This inervation becomes completely lost in the Clitellata. PURSCHKE et al (1993) recognize that these losses of the prostomial appendages represented an autapomorphy for the Clitellata (PURSCHKE et al,2000). Stout and articulated palps become a nested synapomorphy of the Nereididae Johnston, 1865 and Pilargidae Saint-Joseph, 1899 (Figs 4 and 5).…”

“…To minimize cases of "pseudoplesiomorphy" (WILEY 1981), two alternative coding strategies were adopted. In the "negative series", following the criteria of MIKKELSEN (1998) and PURSCHKE et al (2000), absences for which there are clear evidences of secondary reductions were considered as further valid character states. Cases in which it can be shown that there is structure reduction (because of marks, sutures, or whatever positive evidences) cannot be coded as absences (coded as zeros).…”

“…Being objective does not mean closing ones eyes for subtle evidences of secondary reductions (see PURSCHKE et al 2000). Such oversights frequently result in the misplacement of apical taxa to basal positions.…”

“…Evolutionary changes in the biology of many groups may dramatically affect them in parallel ways. For example, colonization of terrestrial environments has occurred in some specialized polychaetes, such as Hrabeiella periglandulata, and the clitellates (PURSCHKE 1999;HESSLING & WESTHEIDE 1999;PURSCHKE et al 2000).…”