The rise and macro-evolution of crabs (Crustacea, Brachyura) during the Mesozoic
Authors:   , van Bakel Barry Wilhelmus Martinus 
Published: November 2024

The present thesis provides new data on the evolution and palaeoecology of brachyuran crabs during the Cretaceous, with the emphasis on the Mesozoic Marine Revolution. A guild-based analysis, based on a detailed study of functional morphology, contributes to a broader understanding of macroevolutionary patterns within this group of decapod crustaceans during the MMR. By examining the evolutionary and ecological roles of brachyuran crabs within Mesozoic marine ecosystems, the current research features new insights into the drivers of diversification and specialisation.
Here, I address these new data and insights through exploration of the research questions proposed in Chapter 1 (Introduction), and by using the results and interpretations presented in Chapters 2–10.

1. Can the ecological guild concept constitute an effective framework for understanding evolutionary and ecological adaptations of brachyuran crabs during the Mesozoic?
My research concludes that the ecological guild concept constitutes a valuable and robust framework for understanding evolutionary and ecological adaptations of brachyuran crabs. By categorising crabs into guilds based on functional roles within the marine ecosystem and morphological traits, a better understanding of how brachyuran crabs interacted with their environment may be gained. The guild approach has proved particularly useful in identifying adaptive strategies and ecological niches, offering insights into the drivers of diversification and specialisation. However, the present study also acknowledges that there are limitations, such as the potential for oversimplification and the challenge of accounting for intra-guild variability. Overall, the guild concept may serve as a useful tool for the study of extinct brachyuran crabs, contributing significantly to the broader understanding of Mesozoic marine ecosystems and macroevolutionary processes.

2. How did the evolution and diversification of Mesozoic brachyuran crabs correlate with environmental changes and the development of new ecological niches?
The diversification of Mesozoic brachyuran crabs was closely linked to environmental changes, such as nutrient availability, proliferation of planktic organisms and shifts in sediment types. These changes provided new ecological niches, which crabs exploited through development of adaptive morphological traits. The significance of environmental factors in shaping evolutionary trajectories is pronounced in particular for benthic marine invertebrates such as brachyuran crab, as they inhabit a wide range of variable environments, in addition to their trophic position.

3. What role did morphological innovations play in the evolutionary success of specific crab guilds, such as the back-burying guild and the swimming guild, during the Mesozoic?
This research demonstrates that morphological innovations, specifically those related to the back-burying and swimming lifestyle, were crucial for the emergence, survival and success of these specific crab guilds. These adaptations, such as modifications in carapace shape (elongated for back-burying, or hydrodynamically streamlined for swimming), cuticle microstructure (emergence of burying structures, and keels for structural reinforcement and a more effective swimming behaviour) and pereiopod morphology (paddle-shaped dactyli at the posterior pereiopods, and chelipeds optimised for predation or scooping sand), enabled brachyuran crabs to exploit different substrates and avoid predation, contributing significantly to their evolutionary success. These successful functional innovations played a particularly significant role in the evolutionary history of brachyuran crabs during the Mesozoic Marine Revolution (MMR).

4. How do findings on crab guild evolution and diversification contribute to our broader understanding of Mesozoic marine ecosystems and macroevolutionary patterns?
This thesis illustrates how brachyuran crab guilds played a major role in the trophic structure of the ecosystem and to overall diversity. The results highlight the significance of guild-based analyses for understanding macroevolutionary patterns, offering a framework for further studies of decapod crustaceans and other marine invertebrates alike. This research attempts to understand better how macroevolutionary processes at the guild level are correlated with large-scale ecosystem changes.