ABSTRACT: Multilocular benthic foraminifera are divisible into serial (Se) and spiral (Sp) morphogroups. Beneath oxic bottom waters, the Se morphogroup lives infaunally. Spiral species are typically epifaunal. Thus, the Se infauna react to different environmental factors from most Sp epifauna. Previous investigations of the Se:Sp ratio found that the serial infauna migrate toward the sediment/water interface with decreasing sediment oxygen levels, increasing the Se:Sp ratio in suboxic environments. We examine assemblage turnover within the Se and Sp morphogroups at two Brasso Formation exposures (planktonic foraminiferal Zones M4b-M6, deep middle bathyal palaeodepth). Ten samples were taken from Mayo Pond Side (MPS), and four from the stratigraphically younger Mayo Quarry Southside (MQSS). Across all samples, the Se morphogroup species richness was almost the same as that for the Sp morphogroup. The mean Shannon Function H did not differ between the exposures. At MPS, approxiimately 20% of the total assemblage belonged to the Se morphogroup, compared with approximately 36% at MQSS, giving a higher Se:Sp ratio at MQSS than at MPS. A sample-wise assemblage turnover index (ATIs) was calculated across the exposures. A peak in total assemblage ATIs reflected the MPS and MQSS samples being taken from different biofacies (MQSS = Cibicidoides crebbsi dominant; MPS = Anomalinoides mecatapenensis dominant). Mean ATIs (total assemblage) differed significantly between the exposures. Across all samples, mean Se ATIs was significantly different from and greater than mean Sp ATIs. ANOVA was used to compare means of Se ATIs and Sp ATIs at MPS and MQSS (four means in total). It showed a significant difference between at least two means. At both exposures the Se morphogroup's ATIs was greater than the Sp ATIs. This greater Se ATIs is apparently related to changing dissolved oxygen concentrations in sediment pore water associated with increasing proximity to an oxygen minimum zone over time.