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

A New Soft-Bodied Halwaxiid-grade Metazoan from the Middle Cambrian Marjum Formation, Western Utah

Paper number: 99-6
Program: Technical Sessions
Time: 9:25 AM – 9:40 AM

Authors

Abstract

The Cambrian Marjum Formation of the House Range, western Utah, preserves a diverse assemblage of exceptionally preserved marine organisms within a deep-water, low-energy depositional environment. Within this succession, we identify an enigmatic new soft-bodied organism with possible affinities to halwaxiid-like lophotrochozoans. The new taxon possesses a dorsal covering of delicate scales arranged into discrete zones, but differs from conventional halwaxiids in lacking mineralized or even sclerotized elements instead bearing cuticular dorsal elements. The combination of a halwaxiid-like organization and scale metamerism, along with the soft-bodied nature of the dorsal scales, suggests a previously unrecognized early evolutionary stage for this clade before the development of hardened armor. The phylogenetic position of halwaxiids makes them crucial to discussions of molluscan origins, bridging the morphological and fossilization gap between typical Cambrian “small shelly” forms and more derived crown-group representatives. The discovery of new Marjum species adds complexity to our understanding of Cambrian lophotrochozoan diversity and body-plan evolution, reinforcing the significance of this exceptional deposit as a critical window into early animal evolution and paleoecology.

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

doi: 10.1130/abs/2025AM-413012

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