Polar ecosystems are highly vulnerable to ongoing climate change, and rapidly melting ice-sheets and changes in oceanography and in marine ecosystems are expressed on all levels of the food web1,2,3. Antarctica is arguably the most susceptible polar region to climate, evidenced in the fact that West Antarctica has warmed 2.4?±?1.2?°C between 1958 and 2010, making it one of the fastest-warming regions globally4. Understanding how Southern Ocean organisms respond to climate variability, including throughout past climate shifts, is thus of key importance to predict how the Antarctic marine ecosystem will evolve in the near future.