Due to the undeniable importance of the role that cyanobacteria play in different ecosystems, and the new discoveries and periodic changes in their taxonomic classification, the main objective of this research was a taxonomic update of the cyanobacteria recorded in an artificial, subtropical freshwater water body. This work also aimed at the knowledge of the geographic distribution and frequency of the species throughout the study period. The analyzed pond (called Basin 7) is located in an environmental preservation area called Braskem Environmental Station (29°51 0 57.3 00 S and 51°21 0 54.7 00 W), which is part of the ''green belt'' that surrounds the set of companies of the Southern Petrochemical Complex of Rio Grande do Sul state (Brazil). The studied samples were taken from May 2000 to December 2015, using a plankton net (30 lm), and floating thallus of cyanobacteria were collected directly by hand. Thirty-three species were identified and distributed in the orders Oscillatoriales (14), Synechococcales (12), Chroococcales (6) and Spirulinales (1). Phormidium Kützing ex Gomont presented the highest species richness, with five species identified, and P. tergestinum (Kützing) Anagnostidis & Komárek was the one with the highest frequency of occurrence (81.3%). Among the identified species, seven are especially important for being potentially toxic. Eucapsis pseudalpina Komárek & Hindák also stood out for its first recorded occurrence in Brazil, and Ancylothrix rivularis Martins & Branco, Arthrospira jenneri Stizenberger ex Gomont and Pannus cf. brasiliensis Malone et al. for their first recorded occurrence in Rio Grande do Sul state. For each species, diagnoses, basionyms and synonyms when appropriate, illustrations and comments are presented.

The Predebon Site, located in São João do Polêsine, Quarta Colônia region (Rio Grande do Sul, Brazil) has a Triassic fauna, including footprints and traces of small vertebrates and abundant remains of rhynchosaurs. This fauna is typical of the Hyperodapedon Assemblage Zone, which is correlated to the late Carnian Ischigualasto Formation (Ischigualasto-Villa Unión Basin) in Argentina. The present study records new fossil remains for the Predebon Site collected in 2001 that consists of two dorsal and two sacral vertebrae referred to the same individual (MCN-PV 10344), and two isolated, serrated blade-shaped teeth (MCN-PV 10425 and 10426). The trunk vertebrae were identified as between 11th and 15th. The sacral vertebrae are fully fused, with no indication of another fused vertebral element, not even signs suggestive of dorsosacral and caudosacral vertebrae, also because the ilia are not preserved. The general characteristics observed are: (i) lateral fossa in the vertebral body; (ii) concave ventral face of the vertebral body; (iii) neural spine high and wide at the top, subquadrangular in shape, in both dorsal and sacral vertebrae; (iv) infrapre- and infrapostzygapophyseal ridges on the dorsals; (v) presence of hyposphene and hypantrum; and (vi) first sacral vertebra larger than the second, with transverse processes and broad sacral ribs, triangular in shape in dorsal view. This set of features in the vertebral elements is similar to that present in Herrerasaurus ischigualastensis from the Ischigualasto Formation and allows us to consider MCN-PV 10344 as a member of the Herrerasauridae clade. It differs from Staurikosaurus pricei, which has three smaller and more slender sacral vertebrae and a low neural spine without subquadrangular platform at the top, and from Gnathovorax cabreirai, which has subrectangular platforms, slightly more elongated anteroposteriorly, suggesting the presence of a new herrerasaurid morphotype. The two isolated, serrated blade-shaped teeth were attributed to Archosauriformes indet., because this morphotype is observed in several groups within this clade. Keywords: Dinosauria, Hyperodapedon Assemblage Zone, late Carnian, Predebon Site, Rio Grande do Sul, Brazil.