World Congress of Malacology Antwerp ... - Unitas Malacologica

alternative explanations. We found that the shell morphology of these thiarid gastropods actually
change throughout the Koobi Fora Formation. However, based on stratigraphic analyses taking the
revised geological data into account, this change appears to be more gradual and not as abruptly as
predicted in Williamson’s original model. Where abrupt changes of shell morphology do occur, they
are associated with significant changes in the sedimentary facies or hiatuses in the sedimentary
succession. Therefore, we suggest that these molluscs are no palaeontological documentation of
speciation and, thus, for the theory of "punctuated equilibrium". Alternatively, ecophenotypic
variation and migration of faunas due to changes in the drainage pattern through time could have
caused the changes in shell morphology observed by Williamson. This is documented in particular by
dwarfism of Melanoides associated with an increase in morphological variability possibly due to
environmental stress caused by shrinking of the palaeo-lake Turkana. Eventually, these morphotypes
disappeared and were replaced later by more typical morphotypes of Melanoides from the Omo
River. Thus, ecophenotypic variation followed by extinction and new immigration of these
gastropods mimic lacustrine speciation processes.
Germany’s next top model? Towards a morphological phylogeny and evolution of acochlidian
opisthobranch gastropods
Schrödl, Michael; Neusser, Timea
Zoologische Staatssammlung München, Münchhausenstr. 21, 81247 München, Germany,
Email: schroedl@zi.biologie.uni-muenchen.de; timea-neusser@gmx.de
Reconstructing evolutionary history requires knowledge on the relationship of units involved.
Phylogenetics, however, faces serious problems. Historic diversity has been lost by extinction,
samples and characters available for analysis are even more limited and selected, homology
assumptions are probabilistic, and primary data are often deficient, among others. Obtaining robust
and plausible phylogenetic hypotheses for at least some nodes is exception rather than rule, especially
in more ancient groups and those with “rampant parallel evolution” like opisthobranchs. Conclusions
on evolution, beyond population genetics and recent radiation level, are seldom more than
speculation.
Acochlidians are enigmatic shell-less opisthobranchs which have been widely ignored. Not a
surprise, since most species are tiny and worm-like, inhabiting interstitial spaces of coastal marine
sands. Others are macroscopic, but live hidden under stones in a few rivers on tropical islands. Why
should anybody seriously consider acochlidians as a suitable model group for phylogenetics, or for
studying evolutionary pathways and processes?
Our results indicate the Acochlida is 1) monophyletic, 2) quite ancient, 3) comprises 27 valid plus
several undescribed species worldwide, and 4) is extraordinarily diverse with regard to morphology
and (especially reproductive) biology. We would not know of other animal groups combing an
(early?) Mesozoic age with manageable numbers of recent species expressing an evenly fascinating
evolutionary history. Minimizing branch lengths and selection, we attempt for considering “all”
species and “all” structural characters discernable. High quantity and quality data is obtained by
applying histological and ultrastructural techniques. By computer-based 3D reconstructions, entire
specimens are analyzed. Organs are compared in unseen detail and homology assumptions are
improved. Such optimized morphological information is used for cladistic analyses. Can we obtain a
robust topology and reveal an ancient evolution, at least for a likewise ideal model group such as the
Acochlidia?
(Yes, some funding for supplementary molecular analyses would help!)
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