Evolution of the earliest molluscs
Sammanfattning: The first helcionelloid molluscs appeared in the late Vendian and were forerunners ofthe sudden molluscan diversification of the Cambrian explosion. The diversification ofMollusca coincided with global changes in the lithosphere, hydrosphere, atmosphere,and biosphere coincident with the break-up of the Late Proterozoic supercontinentRodinia, a distinct shift in oceanic water geochemistry, and the sudden abundance ofshelled multicellular organisms. Some of these changes have left indicators in themorphology and chemical properties of molluscan shells that allow reconstruction ofglobal settings which are essential for understanding early molluscan evolution. Acombination of tectonic and climatic cyclic changes resulted in a major globaltransgression at the beginning of the Cambrian and the formation of extensive shallowwater habitats within which the first molluscs expanded.Diversification into new ecological niches has been recognised in twoevolutionary-adaptive lineages. The lineage from Oelandiella to Watsonella showsadaptation to a soft substrate and infaunal mode of life. In contrast, the Oelandiella-Helcionella lineage was an adaptation to a harder substrate. Appearance of largehelcionelloids with associated archaeocyathan reefs in the middle Tommotian maysuggest evolution of a browsing carnivorous habit capable of supporting a considerablylarger body. Despite a few such examples, most helcionelloids retained a small sizeuntil the Early Ordovician.The evolution of the earliest helcionelloids was also driven by increase inrespiratory efficiency that in some cases led to strong shell asymmetry. Hydrodynamicconstraints on the development of shell form and respiration favour the endogastricmodel of shell orientation in helcionelloids.Early Cambrian helcionelloids show considerable morphological diversityproviding a fertile ground for speculation about early molluscan evolution. While almostall molluscan classes are known from the Cambro-Ordovician boundary therelationship of these, including Tergomya, Gastropoda, Cephalopoda andRostroconchia, to helcionelloids remains highly problematic. Analysis of morphologicalvariation and evolutionary trends within the earliest molluscs may support derivation ofbivalves and paragastropods from helcionelloids, though there is currently a lack ofdirect intermediate forms.
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