Matias do Nascimento Ritter

The fossil record of bioerosion on rudists is commonly restricted to brief mentions that mainly use general terms and do not constitute detailed ichnological studies. This contribution comprises a detailed study of the bioerosion structures present on the shells of different species of rudists from the Upper Cretaceous (Maastrichtian) of Cuba. In addition, paleobiological, paleoecological and taphonomic implications of these boring are inferred. Among the studied material, seventeen rudist shells exhibits bioerosion structures. Based on their morphological features the borings have been ascribed to Gastrochaenolites isp. and Entobia isp. Gastrochaenid bivalves and clionaid sponges, respectively, have been proposed as their more likely producers. The modes of occurrence, density, and position of such bioerosion structures rule out a syn-vivo relationship between rudists and boring bivalves and sponges, demonstrating that colonization mainly was postmortem. Furthermore, the combination of these data together with previous paleoecological interpretations and the fact that the most parts of the rudist shells are filled by sediment, provides enough evidence to propose a subsequent process of reworking and reburial of these shells in shallow marine settings.

Anomalocardia flexuosa is a bivalve that inhabits shallow, low hydrodynamics coastal environments of normal to brackish salinity, currently distributed from the Caribbean up to the state of Santa Catarina (∼28°S) in southern Brazil, but its fossil record extends along the southwestern Atlantic up to ∼40°S, in Argentina. Its absence in southern coasts today is attributed to ocean water cooling as a result of Middle-Late Holocene changes in relative influence of the warm waters of the Brazil Current and the cold waters of the Malvinas/Falklands Current, but geomorphologic and stratigraphic data suggest that coastal evolution controlled mainly by glacioeustatic-driven oscillations may have also played a role on the shifts of its distribution. Here we review the past and present distribution of A. flexuosa along southern Brazil, establishing a correlation with the Holocene geological history of this area. The Holocene post glacial marine transgression (PMT) produced a large complex of interconnected coastal lagoons landward of sandy barriers stretching from southern Brazil (state of Rio Grande do Sul) to Argentina, creating a corridor that allowed for the southward dispersion of A. flexuosa. The few available numerical ages indicate that A. flexuosa was established in the northern coastal plain of Rio Grande do Sul around ∼7.1 ka BP, and by ∼5.8 ka BP it had reached the southern plain, facilitated by warmer ocean waters than today and the sea-level highstand of 6–5 ka BP. The combination of cooling, sea-level fall that reduced marine influence, and fluvial inputs of freshwater and sediments, converted most of the lagoon complex into smaller isolated freshwater lakes after ∼4 ka BP, leading to the regional extinction of that species. The fossils of A. flexuosa and other tropical mollusks in middle and late Pleistocene interglacial barrier-lagoon coastal deposits along the southwestern Atlantic suggest that their latitudinal distribution shifted cyclically, driven by glacial-interglacial oscillations of sea-level and temperatures. The understanding of the coastal processes that affected the distribution of A. flexuosa may help assessing how mollusks and other marine species respond to environmental forcings related to sea-level oscillations and climate, thus contributing from a paleobiological perspective for conservation and management efforts under present and future scenarios of changes in coastal ecosystems.

At the beach, death assemblages are constantly reworked by wind and waves. One of the various consequences of this shore dynamic is the constant burial and exhumation of the shells, making them inappropriate for epibionts. However, some gastropod shells collected in the death assemblages arranged on the foreshore of the coastal plain of south Brazil were hardly encrusted. Olivancillaria urceus corresponds to 48.8% of all encrusted taxa, suggesting that some shell species may play a more striking role than others as available bioclasts. Therefore, the research aims to discuss the taphonomic implications for epibionts and bioerosion in gastropod shells. Abandoned gastropod shells were collected on 27 sites along a 150 km coastal strip in southernmost Brazil. Epibionts and bioerosion traces were identified, and their frequency was calculated considering their abundance, which taxa they occurred in, and their settlement on the different parts of the shells. At least 13 of 21 taxa were colonized by epibionts, of which 97% were by bryozoans. Other epibionts recognized were serpulid tubes, bivalves, and balanids. Fifteen taxa were bioeroded, showing traces made by worms (cf. Caulostrepsis), bryozoans (cf. Pennatichnus), balanids (cf. Rogerella), bivalves (cf. Gastrochaenolites), and sponges (cf. Entobia). The results reached in this survey suggest that the bryozoans have an advantage over other epibionts at colonizing the gastropod shells.

The paleoecological evolution of a peat bog in the Campos region, Rio Grande do Sul State, Brazil, was determined based on an integrated study of stratigraphy, palynology, and geochronology. The peat bog is constituted of silty-clay material, with high levels of ash (residue on ignition) and organic matter content that was deposited on a sandy substrate. Palynological analysis show that grassland taxa dominated from the base of the core at the early deglacial (∼17.4 cal kyr B.P.) to the Present. Four pollen zones were identified: CCQ I Zone (3.20 m to 2.50 m), covering the deglaciation (Upper Pleistocene), corresponding to dry climate conditions; CCQ II Zone (2.50 m to 1.60 m, latest deglaciation and Early Holocene), associated with a slightly humid climate; CCQ III Zone (1.60 m to 0.50 m, mid to late Holocene), established under very humid climatic conditions; and CCQ IV Zone (0.50 m to 0.00 m, last ∼500 years), representative of a low-humidity period. The floristic composition of grassland communities observed throughout the drill core is very similar to that found in the region where the municipality of Cacequi is located, in which Poaceae, Asteraceae, Fabaceae, and Rubiaceae were identified as the most abundant families. Consequently, although in the last ∼17.4 cal kyr B.P. the humidity oscillated, the grassland vegetation remained predominant, although not being represented by the same families, as indicated by changes in the floristic composition among the four pollen zones. Palynological and geochronological data, when compared with other sectors of the Río de la Plata Grasslands, show a predominance of dry conditions over the studied interval. From the early deglacial until the Present, climatic fluctuations shaped the diversity of plant communities and affected the particularities of each sector of the Río de la Plata Grasslands, including the Campos region. High ash content was detected along the core, similar to what occurs with other peatlands already studied in Brazil. In the peat bog analyzed, Poaceae is the predominant family, being known as a major producer of biomineralized structures, which would explain the high ash content recorded.

Fjords are considered biodiversity hotspots and aquatic critical zones, being extremely sensitive to climate change due to close oceanic, terrestrial, and glacial interactions. These ecosystems have received a great deal of attention in research on current and future anthropic impacts. Despite this, there is no analog in the geological record that presents icehouse-greenhouse biological and climatic changes. Here we present an analog, through a detailed taphonomic survey of the Lontras Shale Lagerstätte (Itararé Group, Paraná Basin, Brazil), related to a climatic optimum of the end of the Late Paleozoic Ice Age (Late Pennsylvanian), in which a temperate outer paleofjord with a rich well-preserved biota was installed. In the monotone layers of black shale, we find subtle variations of the dominant skeletal type, rates of fragmentation and disarticulation, and other taphonomic aspects that define distinct taphofacies. Each of them is the result of distinct time-averaging related to mass mortality events, turbidity, and depositional hiatus periods at different scales and intensities, mixing the ecologic census with short-term and long-term within-habitat assemblages. In addition, the rich paleobiota was reconstructed with autochthonous and allochthonous benthic fauna, many marine nektonic organisms, and intense continental contribution of terrestrial bioclasts, that proliferated and were exceptionally preserved by the establishment of an anoxic temperate outer fjord. The taphofacies show an evolution in a high-frequency sequence within a highstand systems tract, linked to climatic improvement. Furthermore, taphonomic detailing can be used as a comparison of deep marine and deep lacustrine taphofacies, in addition to serving as an analog for the short-time scale biological, biogeochemical, climatic, and stratigraphic changes associated with the icehouse–greenhouse transition in the past, present, and future.

{Surficial shell accumulations from shallow marine settings are typically averaged over centennial-to-millennial time scales and dominated by specimens that died in the most recent centuries, resulting in strongly right-skewed age-frequency distributions (AFDs). However, AFDs from modern offshore settings (outer shelf and uppermost continental slope) still need to be explored. Using individually dated shells (14C-calibrated amino acid racemization), we compared AFDs along an onshore-offshore gradient across the southern Brazilian shelf, with sites ranging from the inner shelf, shallow-water (\< 40 m) to offshore, deep-water (ġt; 100 m) settings. The duration of time averaging is slightly higher in deeper water environments, and the AFD shapes change along the depositional profile. The inner shelf AFDs are strongly right-skewed due to the dominance of shells from the most recent millennia (median age range: 0–3 ka). In contrast, on the outer shelf and the uppermost continental slope, AFDs are symmetrical to left-skewed and dominated by specimens that died following the Last Glacial Maximum (median age range: 15–18 ka). The onshore-offshore changes in the observed properties of AFDs—increased median age and decreased skewness, but only slightly increased temporal mixing—likely reflect changes in sea level and concurrent water depth-related changes in biological productivity. These results suggest that on a passive continental margin subject to post-glacial sea-level changes, the magnitude of time-averaging of shell assemblages is less variable along the depositional profile than shell assemblage ages and the shapes of AFDs.}