Why the evolution and fossil record of life history traits of the group Euarthropda matters in modern-day biodiversity research

Life history traits describe aspects of the investment strategies of organisms. These traits have changed and diversified over the course of evolution, but they can also react to short-term changes of the environment. Under the current human-induced climate change, life history traits of many animals have already reacted, long before a species starts to decline or even go extinct. Changes in life history are, therefore, important early indicators for biodiversity assessments. Yet, for evaluating such changes and their significance, these need to be set in a sound frame of life history traits in an evolutionary context. Fossils can add to this reconstruction of the evolution of life history traits by adding now extinct states, including transitory ones. Yet, the challenge is assessing these traits. Even for many extant species, life history traits are only incompletely known or difficult to access entirely. For fossils, getting this access is even more challenging. Here I present examples from the group Euarthropoda (e.g. beetles, spiders, centipedes, shrimps). The advantage of using these animals for this approach is their discontinuous, pseudo-discrete growth pattern: they have to moult in order to grow, due to their non-stretchable chitin cuticle. Therefore, they offer an independent ontogenetic factor not present in other animals: stages separated by moulting. I will discuss in how far we can recognise differences in observable life history traits, such as egg size, hatching size, size at metamorphosis, in fossil and extant species, and in how far selective pressures act on these.