Three major families have been described for the Nectrideans that accommodate all 26 species. Only one species has not yet been assigned to a family, Arizonerpeton wellsi, due to its unique anatomy amongst the Nectridea. For more information also see the Anatomy and Behaviour section.
Recent cladistic study (Ruta et al. 2003) suggests that the Nectrideans are a polyphyletic group, meaning those species included in the order Nectridea do not include all those species to which they share a common ancestor (see figure 1). All discussions on cladistics are based on the observation of present or absent traits which means that some authors would still describe Nectrideans as monophyletic, i.e. the group contains all the members that share a common ancestor (see Milner 1980).
Figure 1. Relationship between select Nectridean species. Where the lines split indicates a speciation event. Based on Ruta et al. 2003.
For information about the relationship of Nectrideans to other early amphibian orders and modern amphibians also see the Modern Forms section (and Anderson 2001).
Author: Iain McIntyre
Last updated: 19/11/2006
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