Complexity of septal surfaces and suture lines in ammonoids - implications for the hydrostatic apparatus and palaeoecology using modern CT-techniques

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In the course of the 350 million years (Lower Devonian-Upper Cretaceous) of ammonoid evolution the iterative trend of development starting with simple to complexly, folded septa of the phragmocon is well documented. In the literature different approaches on the role of marginal folded septa (= suture) are discussed. These can be divided mainly into physiological and mechanical explanations. By the calculations of the ratio of chamber volume against the inner chamber surface the functional model as hydrostatic apparatus is compared against all other known functional interpretations. On the basis of complete ammonites in hollow preservation (no sediment, calcite or pyrite in the shell) changes in that ratio are detectable. They allow statements about different lifestyles in different ontogenetic stages, and comparisons between different taxa. The creation of digital models also allows the specific examination of ontogenetic development from simple to complex sutures and thus the possibility to test for the taxonomic relevant mode of lobe insertion during sutural ontogeny. As working hypothesis we assume that the most complex structures found in late Cretaceous tetragonitids including the septal lobe and a sixlobate primary suture, in conjunction with presumed pelagic lifestyles on the shelf slope, most likely support the idea that they improve the efficiency of the hydrostatic apparatus as a key selection factor for the design of the septa. It is the main goal to prove whether or not this hypothesis is correct.