Spatiotemporal variability of the Cenomanian macrobenthic fauna in Northeast Algeria: Species versus community level variations

Abstract

Environmental changes can impact benthic fauna at multiple levels. To better understand these changes at both the species and community levels, the Cenomanian macrofossils from two regions in northeastern Algeria, Tébessa and Batna, were analyzed. The results revealed that while changes at the species level (morphology) and community level (ecology) followed similar patterns, they were not identical. Non-metric multidimensional scaling (NMDS) and permutational multivariate analysis of variance (PERMANOVA) tests of the morphological variations of 114 complete shells of the suspension-feeding trigonoid bivalve Scabrotrigonia scabra (Lamarck, 1819) showed significant differences (P < 0.001) between the two regions. Two distinct echophenotypes were identified, with the first dominating in Tébessa and the second in Batna. The Batna echophenotype is characterized by smaller, flattened shells with numerous ribs, an adaptive strategy for stabilizing in soft substrates. Additionally, the smaller shells in Batna were linked to the oligotrophic conditions of the outer ramp setting, in contrast to the deeper outer ramp setting in Tébessa (mesotrophic). To further investigate environmental variations, the macrofaunal associations of the Cenomanian were examined. The NPMANOVA test indicated significant differences in faunal associations between the regions, with Batna exhibiting less diversity and a higher proportion of deposit-feeders, suggesting soft substrate conditions. In contrast, no significant differences were found among the faunal associations of the different Cenomanian substages, though ecological attributes showed minor, gradual changes. The early and middle Cenomanian fauna were dominated by epifaunal byssate taxa, while deposit-feeder taxa increased in the late Cenomanian, likely due to the global sea-level rise toward the end of the period. Overall, the paleogeographic setting (middle versus outer ramp) appears to have played a major role in shaping the ecological and morphological variations between Tébessa and Batna.