Changes in coiling direction, habitat depth and abundance in two menardellid species

We present a detailed study of menardellid speciation, abundance and coiling changes between 6.34 and 5.1 Ma (latest Miocene and earliest Pliocene). Menardellids are widely distributed in tropical to subtropical waters from Miocene to modern time, revealing a stable isotopic signature characteristic for the top of the thermocline and shallower rather than deeper habitat depth. Menardellid response to its environment reveals a tripartite pattern. The first interval from ∼6.34 to 5.75 Ma is marked by a dominance of Menardella limbata, frequent changes in coiling direction, and variable abundance with strongest reductions during intervals of dextral dominance. The onset of an interval of cooling and ice build-up at 5.82 Ma coincides with a weakening of the mixed layer leading to nearly 70 kyr of strongly reduced menardellid abundance. At 5.75 Ma, Menardella multicamerata replaces M. limbata as the prevailing menardellid species, at the same time as the overall abundance of menardellids stabilises. Initial stabilisation of sinistral coiling until the end of the cold interval at ∼5.49 Ma suggests the development of a temperature preference in sinistral (cold) and dextral (warm) M. multicamerata. A strong increase in menardellid δ13C values relative to the thermocline gradient (difference between the δ18O records of surface-dwelling Globigerinoides trilobus and Menardella spp.) following 5.77 Ma suggests a move to shallower habitat depths of both M. multicamerata morphotypes. Initiated by mixed layer destabilisation, both the development of a temperature preference and the move to shallower habitat depth in M. multicamerata appear to identify its adjustment to a new niche driving the replacement of M. limbata as the dominant menardellid species.