99 Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts, Part I

This session is dedicated to studies that illuminate the biotic, geobiological, sedimentological, and geochemical contexts of the evolution of early animal life in the oceans, with a particular focus on the Cambrian Period.

Monday, 20 October 2025
8:00 AM - 12:00 PM
HBGCC, 304B

Dispersal and Evolutionary Dynamics of Cambrian Trilobites and Echinoderms

Paper number: 99-7
Program: Technical Sessions
Time: 9:40 AM – 9:55 AM

Authors

Abstract

The Cambrian was a time of massive environmental changes along with an explosion of biological diversity and morphology. This radiation was particularly significant in  trilobite and echinoderm clades. Recent work has developed robust phylogenetic hypotheses for these clades upon which to statistically test models of speciation and drivers of evolution during an early Phanerozoic Greenhouse World. Using phylogenetically-informed paleobiogeographic methods, we leverage Bayesian-constructed phylogenetic hypotheses for the earliest trilobites (olenellines and redlichiines,) and echinoderms (eocrinoids and rhombiferans). BioGeoBEARS was employed to reconstruct ancestral states, infer dominant speciation types within each group, and identify global dispersal pathways. BioGeoBEARS includes three historical biogeographic models, and within the package, each model is modified with an additional “+j” factor that accounts for founder-event speciation. The models that fit the phylogenies best include the “+j” component: DEC+j for the trilobites, and BAYAREALIKE+j for the echinoderms. This finding indicates that 1) founder event speciation was an important mechanism implemented by trilobites (mobile lifestyles) and echinoderms (sessile lifestyles), and/or 2) there are missing species from the fossil record and trees, thus masking true patterns of biogeography. The models with a “+j” component also fit best with other phylogenies of early Paleozoic marine invertebrates. Within the trilobite phylogeny, there is large uncertainty regarding the location where the earliest trilobites originated; our results  indicate peri-Gondwana as a  potential origin, although this might reflect the taxon sampling in our analysis. For the eocrinoids and rhombiferans, earliest ancestors were optimized to have first occurred in peri-Gondwana and Baltica, with further dispersal occurring into Laurentian basins and South China. As benthic marine invertebrates dispersed during their larval phases, we will discuss dispersal paths and potential drivers of evolution in the context of geochemical records and reconstructed surface ocean circulation patterns. Comparison of the dispersal paths utilized by the two clades will be compared to identify what common abiotic factors led to speciation and if either group responded to different paleoceanographic and paleoclimatic events. Inferring speciation and dispersal patterns of marine invertebrates during Hothouse worlds with increased atmospheric carbon dioxide levels provides context for how these processes may operate in the near future, with increased warming of the ocean and subsequent changes to ocean circulation.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412159

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