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

Convergent Taphonomies Mask Diversity in an early Cambrian Assemblage in southwestern Mongolia

Paper number: 139-4
Program: Technical Sessions
Time: 2:20 PM – 2:35 PM

Authors

Abstract

The end of the Ediacaran Period marks the first mass extinction of macroscopic multicellular life. In both animals and algae, there is a globally observed decrease in morphological complexity and diversity across this event. The relatively simple morphologies of many surviving taxa may be the characteristics of ecological generalists, which were better suited to survive the extinction event. Consequently, this proliferation of simple morphologies can make it difficult to diagnose different species, inhibiting our ability to effectively assess ecological changes over this interval. This challenge can be exacerbated by taphonomic effects that further mask biological characteristics of the organisms. When studying morphologically simple fossils, we must be considerate of how these biases can impact our interpretation of the fossil record. This is particularly true with fossils from the Precambrian and early Cambrian that may not always easily fit into the paradigm of modern living organisms.

Here we describe an array of filamentous fossils from the Fortunian (Cambrian) Bayan Gol Formation of southwestern Mongolia. These fossils are primarily preserved as iron oxide-rich, simple, ribbon shaped forms, and occur in abundance on multiple shale bedding surfaces. At first glance, they appear as a monotaxic assemblage, however rare carbonaceous compression fossils preserve greater biological detail and reveal morphological diversity suggestive of multiple different taxa. These forms include algae such as the tubular Vendotaenia and Sinocylindra, the ribbon-shaped Tyrasotaenia, and the branching Eoholynia. But not all of these filaments are algae; some annulated carbonaceous specimens are assigned to the tubular animal Zuunia, which is previously documented in the Bayan Gol Formation as phosphatic casts. Still other iron oxide filaments are likely Planolites trace fossils. Crucially, individual specimens of these various fossils preserved as both carbonaceous compressions and iron oxides record the taphonomic loss of morphological characters showing how these different taxa, spanning multiple kingdoms, can converge on a similar fossil morphology. This work emphasizes the importance of well-preserved specimens to accurately diagnose ecological diversity in this assemblage. In the wake of the extinction event at the end of the Ediacaran Period, the shallow ocean of southwestern Mongolia hosted a diversity of algal and animal taxa that laid the foundation for the Cambrian explosion.

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

doi: 10.1130/abs/2025AM-414738

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