Phylogenetic Patterns in Trilobites and Macroevolution
Lieberman has used phylogenetic patterns in trilobites to consider several macroevolutionary phenomena. For instance, he has studied the nature of the Cambrian radiation and rates of evolution during this key time period in the history of life.
Spectacular Cambrian deposits in the Northwest Territories, Canada, packed with trilobite fossils.
Collecting Cretaceous Western Interior Seaway fossils from the Niobrara Chalk of western Kansas.
In the field, at the classic Monument Rocks locality of western Kansas, looking at fossils (image by Richard Bryant).
Lieberman was funded by NSF, through their RevSys program in Systematic Biology, to investigate phylogenetic patterns in a diverse clade of phacopid trilobites, the cheirurids. He used phylogenies to produce a stable, modern classification for the group, and to understand biogeographic patterns and macroevolution in this major invertebrate clade. This included a consideration of patterns of speciation and extinction during a key episode in the history of life, the Late Ordovician mass extinction. He worked closely with former students Curtis Congreve and Wes Gapp to produce a phylogenetic hypothesis for the Deiphonine trilobites, a subfamily of Cheiruridae and conducted a biogeographic analysis on the group. This work has been published in the Journal of Paleontology. They have also produced a phylogenetic hypothesis for the Sphaerexochinae and the Eccoptochilinae. We have also investigated phylogenetic patterns in a closely related group of trilobites, the Homalonotidae. These have a different life history strategy than many cheirurids and also occupy different environments. Thus they provide an excellent point of comparison to consider macroevolutionary patterns and processes and how these may vary among trilobite groups.
Along with former student Francine Abe he also considered tempo and mode of evolution during what is often termed an adaptive radiation of trilobites during the Devonian. They determined this radiation was not governed by adaptive factors but instead was driven by the complex geography of the region the trilobites occurred in. This work helped Lieberman to develop a macroevolutionary view of evolutionary radiations, which showed that adaptive radiations are only one type of radiation, abiotic factors rather than adaptation are more likely to have caused most radiations, and exaptation, species selection, and the Turnover Pulse hypothesis are all phenomena that need to be incorporated into the literature on evolutionary radiations. This work also culminated in a paper that appeared in Trends in Ecology & Evolution (TREE).
