The phylogenetic position of the Clitellata and the Echiura - on the problematic assessment of absent characters*
By G. Purschke, R. Hessling, W. Westheide. 2000
85 citations recorded by the source corpus.
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.
Subfields: Ecology, Global and Planetary Change, Oceanography
View Paper: https://www.semanticscholar.org/paper/509fff5aa0c98c2abddb354bf291965dc22e8d79
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Citations in the corpus
- Morphology of the nervous system of Polychaeta (Annelida) (2005, background): “In Clitellata, however, the central nervous system is always entirely subepidermal, even in the smallest oligochaete species (Purschke et al., 2000; Purschke, 2002).”
- The origin of annelids (2014, background): “These adaptations include the absence of parapodia and head appendages, a distinct neuroanatomy, and specialized reproductive and developmental characteristics (Purschke et al. 2000).”
- 18S rDNA phylogeny of Clitellata (Annelida) (2004, background): “Thus, the 18S rDNA gene provides no support for the hypothesis proposing that the first clitellates were terrestrial annelids (Westheide 1997; Purschke 1999, 2003; Purschke et al. 2000).”
- On the ground pattern of Annelida (2002, unknown): “As will be reviewed in detail, absence of characters and its phylogenetic evaluation as primary or secondary is the main reason for these contradictory hypotheses within Annelida (Purschke et al. 2000).”
- Macrofauna associated with echiuran burrows : A review with new observations of the innkeeper worm, Ochetostoma erythrogrammon Leuckart and Ruppel, in Venezuela (2005, unknown): “Until relatively recently, the phylogenetic placement of the phylum within the Metazoa was enigmatic (Purschke et al. 2000).”
- Sense organs in polychaetes (Annelida) (2005, background): “…comes from the reduc- tion or absence of these organs in terrestrial polychaetes, the lack of epidermal cilia in Clitellata, the backward displacement of the brain and the reduction of the prostomium (Hessling & Westheide, 1999; Purschke, 1999, 2000; Hessling et al., 1999; Purschke et al., 2000).”
- Phylogeny of oligochaetous Clitellata (2005, background): “The analysis thus refutes the proposal that the ancestor of Clitellata was adapted to terrestrial life (Westheide, 1997; Purschke, 1999, 2003; Purschke et al., 2000 ), and instead supports the traditional view that the first clitellates were aquatic.”