Morphoclines of larger Foraminifera from the West Pacific

Research project P 13613 Morphoclines of larger Forminifera from the West Pacific Johann HOHENEGGER 28.06.1999 Symbiont-bearing benthic foraminifers are extremely diverse in the tropical West Pacific and show strong depth dependence. According to their significant deviation from mean benthic foraminiferal test size they are called `larger foraminifers`, despite some `normal` sized representatives also bear symbiotic algae (Peneroplidae) and thus belong to this group. Depth dependence in living larger foraminifers of the Indo-Pacific region is expressed in coenoclines, which is a succession of communities that can also be regarded as a gradient (coenocline = gradient in community composition). Environmental gradients like depth can be estimated using species composition of such communities. Depth estimation using coenoclines strongly depends on species composition. Since larger foraminifers show high evolutionary rates, depth inference of fossil environments by comparison with taxonomically related, extant larger foraminifers are restricted to the Pliocene and Upper Miocene at least. But similar morphological characters of larger foraminifers developed in phylogenetically independent foraminiferal groups during earth history can be found from the Upper Paleozoic to the Recent. Starting from the Upper Cretaceous, similar trends in size increase - e.g., by a cyclic arrangement of chambers or chamberlets - led to gigantic tests, despite the phylogenetic roots were quite different. Characteristic traits of larger foraminifers can be mainly interpreted as functional, on the one hand to prevent irradiation in shallow waters, on the other to facilitate light penetration near the base of the euphotic zone. The second envirom-nental factor that influences test morphology is water energy, which is handled by wall thickening or the construction of fusiform and globular tests. The latter sometimes develop canalicular spines for attachment to hard organic or inorganic substrates. Considering these morphological characters independently from biologic systematics, qualitative characters can be treated in a similar manner like species or other taxa in coenoclines. This subject of `comparative ecology` may lead to the recognition of morphoclines, which is defined here as a succession of qualitative characters along an environmental gradient. Since these characters can be interpreted functionally, they are not restricted to geological time periods as species in coenoclines. Thus, estimation of water depth could be extended to the Upper Cretaceous at least using these successions of traits. Quantitative characters are more complex than qualitative traits in regarding relations and intensities between character states. Therefore, correspondence to depth can be measured independently from species definition by various kinds of regression analysis. These analyses must incorporate differences in regression form as well as the influences of extrinsic variables, like wall structures. Excluding these external factors, depth estimation will be possible using multiple regression analysis. Sometimes, partial multiple regression excluding intrinsic factor like the wall thickness often seems to be the appropriate statistical method. The investigation of qualitative morphological characters in form of morphoclines and the treatment of quantitative characters by general and - excluding extrinsic factors - partial multiple regression analyses may allow depth estimation using larger foraminifers also in fossil environments.

The study of relations between organisms and environmental factors is a key topic in ecological research. Frequency distributions of species along an environmental gradient, which is called a `coenocline`, characterize the correspondence of species to the ecological factor. Correlation between gradients and species expressed in a sequence of communities can be used to determine gradient values in case of missing factor measurements that is usually the case in historical as well as geological ages. This is an important tool for the study of palaeoenvironments. An estimation of palaeodepth in marine environments using larger foraminifers (single celled giant organisms with calcareous tests) is restricted to the Upper Neogene (~16 million years) due to the high evolutionary rates in species that adapt to environments. Since the main single factors influencing depth distribution - light availability, hydrodynamics, and substrate - are encountered by similar test construction within phylogenetically different species of larger foraminifera leading to similar responses to the environmental factors, the depth dependence of traits allows the estimation of palaeodepth based on single traits or a combination of characters instead of species. Coenoclines of living larger Foraminifera were quantitatively studied in the West Pacific allowing depth estimation. Qualitative (presence-absence) characters were treated like species in coenoclines. Such traits are the main features of `comparative ecology` and lead to the recognition of `combined morphoclines`, which are defined as a succession of characters along an environmental gradient. Since these characters can be interpreted functionally, they are not restricted to the youngest geological time as species in coenoclines, and estimation of water depth by larger foraminifers can be extended to the Upper Cretaceous (~100 million years) at least using this overlapping succession of traits. Quantitative characters are more complex than qualitative traits for detecting relations and intensities between character states and the environmental gradient. Correspondence to depth can thus be measured by various kinds of regression analysis that relate quantitative characters to the gradient. These analyses have to incorporate differences in regression form as well as the influence of extrinsic variables, like wall structures. Metrical parameters describing planispirally-coiled tests of larger foraminifera like the expansion rate of a logarithmic spiral, the thickness/diameter-ratio and the chamber number per whorl was used in a first step. All characters are linearly correlated with the depth gradient, thus allowing depth estimation. The combination of traits by multiple regression analyses in dependence to the depth gradient leads to more precise approximations of the depth gradient and can thus be used for estimation of palaeodepth in fossil species that are distinguished by similar quantitative characters.