Welcome to the Trilobite EOL

The Trilobita is an impressively diverse extinct clade, familiar to schoolchildren and scientists alike, that captures the imagination for both aesthetic and scientific reasons. Their 300 million year history, deployed across perhaps 10,000 species, combined with a complex anatomy that can be coded for a broad array of quantitative and qualitative characters, has made them model citizens for applying phylogenetic methods to fossil organisms. Indeed, trilobites figured prominently in some of the earliest forays into cladistic analysis on American shores. One noteworthy aspect of phylogenetic studies incorporating trilobites is that they have not only been used to adduce questions about the nature of macroevolutionary patterns. They have also figured in studies about the processes that may have motivated these patterns. For instance, punctuated equilibria was developed based on information from trilobite phylogenies. Trilobite phylogenies have been used to explore how rates of evolution vary throughout the history of life, and consider the meaning of disparity and how it varies over evolutionary time and during the Cambrian radiation. They have also served as the basis for studies of the mechanisms of evolutionary radiations and mass extinctions. In addition, phylogenetic analyses of trilobites have served as the essential component data of various paleobiogeographic studies. Finally, they have even played a role in testing hypotheses in the burgeoning new field of evo-devo.

 

During the last 20 years trilobite workers have made major strides towards reconstructing the trilobite “tree of life”. Prominent questions certainly still exist, paraphyletic groups like “Ptychopariida” continue to be a messy problem, and some orders are basically unknown from a phylogenetic perspective, yet significant progress has been made. We suspect that phylogenetic analyses of trilobites will continue apace. Some of these will take the form of studies that try to tease apart in greater detail ordinal and familial relationships. Others will focus on species-level phylogenies to gain insight not only into the nature of evolutionary patterns but also the processes that may generate these patterns. Moreover, detailed phylogenetic work at the species-level will be critical for unraveling the higher-level relationships among trilobites as many genera are undoubtedly not monophyletic. We find it encouraging and also salutary that such long extinct organisms can continue to capture the public’s imagination and spur scientific research. Once an evolutionary success story, they now serve as a potential model of how to integrate fossils and phylogeny in the service of shedding light on evolutionary patterns and processes.

 

Here we present a classification of trilobites with a focus on groups that have been treated in phylogenetic studies. We present the classification and phylogenies herein not as the final word on evolutionary relationships, and readers are of course referred to the references in the bibliography for greater details. Instead, they are offered as a framework to build on for future studies and a unified resource and synthesis of what is available regarding cladistic studies of trilobites.

 

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