Methane seeps were a common feature in the Late Cretaceous Western Interior Seaway of the United States. We document the occurrence of methane seep deposits in the Pierre Shale on the Cedar Creek Anticline in east‐central Montana for the first time. The seep deposits occur in the lowermost part of the Baculites baculus Zone (the Endocostea typica Zone), corresponding to the lowermost Maastrichtian. They are therefore the youngest seeps yet described from the Western Interior Seaway. We conducted a detailed faunal analysis of a single seep deposit, together with geochemical investigation of both seep carbonates and molluscan shell material to determine palaeoenvironmental conditions. Oxygen isotope analysis of well‐preserved molluscan shell material reveals water temperatures of between 19 and 27°C, while depleted carbon isotope values of seep carbonates are indicative of the anaerobic oxidation of methane. The morphology of the seep deposit suggests a strong advective flux of methane to the sediment–water interface. Comparison to a nearby contemporaneous non‐seep site reveals that similar groups of organisms occur in both settings, albeit with varying relative abundances – the seep is numerically dominated by the lucinid bivalve Nymphalucina occidentalis. Substrate appears to be the major control on the diversity and palaeoecological composition of both seep and non‐seep sites.

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[EMBARGOED UNTIL 8/1/2023] Holocene deposits of the Adriatic coastal plain are rich with clams parasitized by trematodes (complex life cycle parasites), as indicated by diagnostic pits on the interior of their valves. Surges in parasite prevalence associated with past sea level rises have been documented by previous studies in transgressive coastal deposits (TST) and may indicate changes in ecosystem health with ongoing climate change. Similarly, trematode prevalence in the modern northern Adriatic, parallel past biotic trends. In sediment starved, TST-like settings (north of the Po Delta) trematode prevalence was found higher than in those settings along and south of the Po Delta and characterized by high sedimentation rates. Previous studies have ruled out taphonomy, host availability, diversity, turnover, and community structure as drivers of this pattern, yet proxy data for environmental variables are lacking. Here we analyze the spatial and temporal distribution of pooled δ18O and δ13C (V-PDB) values of Chamelea gallina carbonate from modern death assemblages with known trematode prevalence values. We have analyzed 1,299 samples from 57 valves from 16 locations along the Adriatic coast. The negative shift in median δ18O values between Chioggia (T 22 ST-analog; NE of Po delta) and Goro (high-sediment accumulation setting; S of Po delta) reflects the introduction of sediment and colder freshwater from the Po and Apennine rivers. Similarly, there is a significant negative shift in median δ13C values when comparing Chioggia and Goro δ13C median values in the locations on and just south of the delta, which are negatively skewed and have significantly lower median values than Chioggia, potentially indicating higher input of freshwater due to higher river discharges. Ongoing work will produce data from 38 C. gallina valves retrieved in 14 thanatocoenoses sampled along the Northern and Central Adriatic coasts and 19 sub-fossil valves from transgressive and regressive Holocene coastal deposits of the Adriatic.

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