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A morphological approach to the evolutionary history of basal heterobranch families (Gastropoda: Ectobranchia)

Abstracts of the 18th WCM, Ponta Delgada, Portugal (A.M. de Frias Martins, A.C. Costa, R.T. da Cunha, S. Ávila, S.C. Monteiro & P. Raposeiro, eds.). Açoreana Supplemento, 8: 67f

Authors/Editors: Haszprunar G
Publication Date: 2013

Whereas molecular data and analyses have substantially improved our understanding of phylogenetic relationships, morphology still is valuable (a) as an independent data-set for direct phylogenetic analysis and (b) to “tell the tree”, i.e. to describe the evolutionary novelties and processes during diversificatiy of a taxon and (c) to infer the characters of its stem species. Accordingly, we investigated several families of basal heterobranch gastropods mostly based on semi-thin section series and computer-aided 3D-reconstructions.

Based on current knowledge the Ectobranchia (Valvatoidea) include Hyalogyrinidae, Xylodisculidae, Cornirostridae (all marine forms), and Valvatidae (the only family in freshwater habitats). For a better understanding of not only ectobranch but also basal heterobranch relationships we consulted characters of further basic “allogastropods”, namely Architectonicidae, Mathilidae, Omalogyrinidae, and Orbitestellidae. Some preliminary results of these studies are presented:

All Ectobranchia are diagnostically characterized by their extendable bipectinate gill, which is a secondary structure rather than a primitive type of ctenidium as formerly believed and thus considered as a synapomorphy. Accordingly, the Orbitestellidae are to be excluded from the Ectobranchia, a result, also being supported by recent molecular studies: they entirely lack a gill and show a radically different genital system with a pedally innervated penis. The rhipidoglossate radula of Hyalogyrinidae probably is a retained plesiomorphic condition, thus the taenioglossate radulae of several heterobranch taxa is a matter of convergence to Caenogastropoda, and the Ectobranchia do probably represent the first extant offshoot of Heterobranchia.

It is quite likely that some more principal extant clades of basal heterobranchs do exist, many species based on shell descriptions alone need anatomical investigations. Accordingly, our current main goal is to widen the data-base and to correlate the morphological data with the results from the various molecular analyses towards an integrative approach of phylogeny.