139 Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts, Part II

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
1:30 PM - 5:30 PM
HBGCC, 304B

Microevolutionary trajectories in the late Cambrian complex trilobite species Dikelocephalus minnesotensis

Paper number: 139-12
Program: Technical Sessions
Time: 4:10 PM – 4:25 PM

Authors

Abstract

For nearly two centuries, the late Cambrian trilobite Dikelocephalus has been split and lumped from a peak of twenty-eight species to, presently, two species. Recent geometric morphometric analyses of Dikelocephalus minnesotensis cranidial and pygidial shape from the St. Lawrence and Jordan formations show a continuous morphological landscape with four common morphotypes emergent. Morphological variation among these Dikelocephalus is that which Darwin predicted in a widespread, populous and enduring lineage: a morphological landscape that likely represented an evolving adaptive landscape. When treated as a single species, variation among these Dikelocephalus shows no significant temporal trend throughout its approximately one-million-year regional representation. However, analysis of morphotype subsamples reveals microevolutionary nuances. Ontogenetic allometric trajectories of individual morphotypes vary, and interpretations as sibling species or polymorphs of various kinds are plausible. Further, geographic variation is apparent among morphotypes collected from single beds. Directional change of traits is apparent only when analyzing subsets of closely related morphotypes. Depending on the morphotype subset analyzed, directional evolution best fits traits related to eye shape of the cranidium or shape of the pygidial outline. Yet, when morphotypes are lumped together temporal trends of traits generally appear as net morphological stasis thus blurring possible microevolutionary patterns. Dikelocephalus provides an example of a notably morphologically variable lineage showing complex patterns of microevolutionary change. Future work will examine to what extent this intraspecific variation may represent incipient speciation.

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

doi: 10.1130/abs/2025AM-413561

© Copyright 2025 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.