Estudio sistemático, paleobiológico y paleobiogeográfico de los briozoos fenestrados devónicos

Resumo

This thesis embraces a systematic, palaeobiogeographical and palaeobiological study of the Devonian fenestrate bryozoans from NW Spain, as well as a comprehensive approach to the growth habits of fenestrates focused on the interpretation of zooidgenerated currents as a functional factor constraining disparity in colony shape. The main subjects of this study are identification of fenestrate taxa, comparison with faunas from other regions in order to recognize palaeobiogeographical affinities, and the analysis of symbiotic associations between fenestrate bryozoans and other organisms by means of the indirect evidence provided by bioclaustration. Furthermore, palaeobiological studies were carried out in order to describe and interpret the functional morphology of unusual growth habits, and hence to identify evolutionary trends in colony shape within the Order Fenestrata. Two sections of the Lower-Middle Devonian Moniello Formation (upper Emsian-lower Eifelian, Asturias, northern slope of the Cantabrian Zone) have been sampled for the study of fenestrate bryozoans. A highly diverse fauna has been identified, composed of 29 species belonging to 18 genera and 5 families. A new genus, Ernstipora, and 13 new species have been described; updated and detailed diagnoses have been provided for the genera Bigeyina and Pseudoisotrypa, and the latter has been restored as a valid taxon. The fenestrate fauna identified in the Lower-Middle Devonian of NW Spain shows affinities with Lower Devonian species and genera from Bohemia and the USA. Most of the genera present in Spain were cosmopolitan or had wide distributions, which implies a relatively reduced provincialism for the time. The occurrence of Paraseptopora, Cavernella and Kalvariella in the Emsian of Spain has led to a remarkable expansion of their geographical and stratigraphical distributions, and points to an eastward migration of these genera during the Carboniferous and Permian. The study of specimens from Belgium, Germany, Spain and the USA has allowed for the expansion of the stratigraphical and geographical distribution of the ichnogenus Caupokeras, which constitutes indirect evidence of the symbiotic Estudio sistemático, paleobiológico y paleobiogeográfico de los briozoos fenestrados devónicos ii association between fenestrate bryozoans and a soft-bodied modular organism. The Spanish fauna shows the highest diversity of fenestrate genera that hosted this symbiont, and the study and interpretation of particular cases has lead to the amendment of the diagnosis of Caupokeras. The Lower-Middle Devonian fenestrate fauna of Spain includes taxa with extremely unusual morphological features that have been described and interpreted in terms of functional morphology. Some colonies of Bigeyina ibera developed a composite tubular habit superficially resembling dendroid bryozoans such as trepostomes. The new species Hemitrypa adversa shows low, flat colonies with features that have been interpreted as adaptations for competition with encrusting forms; one of the most outstanding findings is a unique specimen with a secondary zooid-bearing meshwork within the Order Fenestrata. The new genus Ernstipora is also remarkable as it is the only fenestrate that developed encrusting colonies composed of subcolonies that probably acted as maculae producing bidirectional feeding currents. A survey of branch width/branch spacing morphospace has been undertaken to assess the interdependence of these characters. The results point to functional constraints related to feeding activity being a main control of fenestrate disparity. The analysis of the stratigraphical distribution of fenestrate growth habits shows both longlasting and short-lived growth forms; those habits having features that probably reduced feeding and cleaning efficiency, such as the encrusting and bifoliate ones, show a very short stratigraphical range. An increase in fenestrate disparity took place during the Early-Middle Devonian, producing unusual growth habits with very short stratigraphic distributions, some of which (e.g. helical growth form) were later developed successfully by unrelated taxa.