Monophyly versus theCephalaspidea (Gastropoda, Opisthobranchia) with an analysis of traditional Cephalaspid characters

The opisthobranch order Cephalaspidea is well-recognized as an unnatural, paraphyletic
group characterized by «evolutionary trends» toward reduction and loss of many features. A
survey of 35 key classifications and published phylograms involving cephalaspids revealed a
general lack of morphological definition for the order and the tenacious use of traditional characters.
Of 49 frequently-used characters, 44 (90%) are problematic for use in modern phylogenetic (cladistic) analyses due to reductive nature, non-homology, incompleteness, or other
grounds. Claims of «rampant parallelism» involving a majority of these characters are based on a
priori decisions and are therefore presently unjustified. The few consistent family groups in
published phylograms are most strongly supported by characters correlated with diet, and may
therefore also be open to question. Successful resolution of the phylogeny of these and other
<<lower heterobranchs» will require critical reevaluation of cephalaspid morphology to determine
an improved set of taxonomically informative, homologous characters. New areas of investigation
are proposed.