A CAUTIONARY TALE OF CONIFORM FOSSILS TAPHONOMY FROM THE CAMBRIAN HARKLESS FORMATION, NEVADA

The Cambrian Harkless Formation in the Gold Point, Nevada, region is probably best known for its well-exposed and documented archaeocyathan reefs. The underlying bioclastic limestone deposits are home to abundant yet poorly diverse silicified, glauconitic, and organophosphatic small shelly fossils. Among these SSFs are conical forms typified by Saltarella and indeterminate hyoliths. Due to the coarse silicification of many of these small shelly components, making clear distinctions between morphologically similar forms—such as Saltarella and the hyoliths—becomes compromised. Moreover, silicification may produce taphonomic morphotypes (i.e., “taphomorphs”) that resemble alternative modes of growth habit (e.g., “funnel-in-funnel” morphology). However, because of this replacive silicification, internal morphological features that can more readily distinguish these taxa can be observed via x-ray tomographic microscopy (microCT). Herein, we investigate the influence of preservational mode on the external and internal morphology of the coniform faunas and how this may introduce bias on the taxonomic identification of these taxa.

Samples studied were collected from a stratigraphic section below S1 of Hicks (2001), hereafter identified as Hicks Reef–S1. Samples were digested in a dilute 7% acetic acid solution and SSFs were manually picked and separated by general morphology. The taxa were categorized into three taphonomic grades. Select specimens from each grade were imaged using paired SEM and microCT, enabling direct comparison between the external and internal morphological features and the mechanisms by which silicification progressed. Preliminary results show that well-preserved coniform fossils were more prolific in the sample and identified with more confidence than the poorly preserved fossils. SEM images show that the “funnel-in-funnel" morphology is likely the result of repetitive cycles of silica deposition within some of the hyoliths. MicroCT images show that Saltarella specimens are exceptionally well-replicated, preserving the internal tube and septae. While identifying SSFs is a difficult task, this study helps bolster our understanding of their potential preservational biases and how taphonomy affects their morphology.