Sedimentological and paleontological studies carried out on the Upper Triassic Dolomia Principale and on the Lower Liassic Calcare Massiccio carbonate platform successions of the Gran Sasso area (Central Apennine, Italy) allowed to recognize several microfacies types arranged in three different types of high frequency cycles, subtidal, peritidal and diagenetic. The cycles can be attributed to high frequency sea-level oscillations on a fifth and fourth order scale. Five algal assemblages have been identified: 1. Gyroporella vesiculifera, 2. Griphoporella curvata, 3. Palaeodasycladus spp., 4. Palaeodasycladus and Thaumatoporella, and 5. Tersella genoti and Fanesella sokaci assemblage. In this latter assemblage two new species (Tersella genoti n.sp. and Fanesella sokaci n.sp.) are described, both from Liassic beds. Tersella genoti differs from the other Tersella species for a less 'differentiated' club shaped thallus, the shape of secondary branches and size. Fanesella sokaci differs from other species of the same genus in shape and size of the branches, size of the thallus and degree of calcification. A re-evaluation of the genus Tersella MORELLET considers Morellet's original diagnosis to be apter than the subsequent emendations. A discussion on several ''Terselliform'' taxa (Dissocladella crefica, Linoporella lucasi, Dissocladella iberica and Dissocladella ebroensis) is supplied. Some comments on the genus Fanesella CROS & LEMOINE and Palaeodasycladus gracilis CROS & LEMOINE are given. Several facies units have been identified on the basis of frequency and distribution of algal assemblages and type of cycle, each unit recording a different depositional paleoenvironment. In the Dolomia Principale Formation two main depositional environments have been recognized: -a shelf lagoon facies characterized by subtidal and peritidal cycles dominated by the Gyroporella vesiculifera algal assemblage; - a backmargin facies marked by peritidal and diagenetic cycles in wihich the Griphoporella curvata algal assemblage is widespread. In the Calcare Massiccio Formation the following depositional environments have been recognized: -a back-margin facies characterized by diagenetic cycles with high energy subtidal units and the Palaeodasycladus spp. algal assemblage. The presence of the Palaeodasycladus and Thaumatoporella algal assemblage indicates a local transition to a more protected open lagoon environment. -a margin facies characterized by oolites and bioclastic sandy shoals, with minor contributions of reef-building organisms, and by a very poor cyclothemic organization. This facies is marked by the widespread occurrence of the Tersella genoti and Fanesella sokaci algal assemblage. The environmental distribution of subtidal, peritidal and diagenetic cycles within the facies units were probably controlled more by the relative position to the margin than by the amplitude of the high frequency sea level fluctuations. The distribution of facies units allowed the reconstruction of the paleoenvironmental evolution of the shallow water facies in this area during the Late Triassic-Early Liassic interval. In the Late Triassic a shallow water carbonate facies (from lagoonal to backmargin) extended in the western and southern parts of the Gran Sasso area. In the Early Liassic the platform facies retrograded, and the margin moved back to the west and south of the Gran Sasso area. In the Middle Liassic, extensional tectonics fragmented the Calcare Massiccio platform. Large platform areas were drowned, and shallow water sedimentation retreated to the South of the Gran Sasso range.
