A dramatic shift from carbonate-rich to chert-rich marine strata occurred during the Permian and is frequently attributed to the increased activity of siliceous sponges and their biosiliceous sedimentation. The first-order ecological consequences of this transition, if any, remain opaque. We analyze fossil occurrence data from the Phosphoria Basin (western North America) to test whether the presence of siliceous sponges, which are correlated with basin-wide chert strata, influenced the recruitment of benthic fauna. We digitized occurrence data from Yochelson and Van Sickle (1968), with particular attention to 228 fossil collections with detailed lithologic descriptions. We categorized fossil collections by formation, facies, and lithology, and used these data to code DCA and NMDS ordinations. We also analyzed the clustering of taxa into faunal units termed biofacies.

Results from these analyses indicate that fossil collections occurring in chert and carbonate are closely associated in faunal composition and community structure. These collections preferentially occur in the inner to mid ramp facies, in agreement with previous studies. Although largely similar in composition, changes in the frequency and abundance of accessory brachiopod taxa (e.g., Composita and Hustedia) reflect greater biosiliceous sedimentary input.

We assessed the relationship between paleocommunity and lithology using fossil collections digitized from Yochelson and Van Sickle (1968), which comprises over 1,500 individual collections made throughout the Phosphoria Basin. Major benthic faunal groups present in the Phosphoria Basin include: brachiopods, bivalves, bryozoans, and gastropods. Minor contributions from anthozoans are also present. Fossils were collected from both outcrops and trenches in Wyoming, Montana, and Idaho (Supplementary Materials 1). Yochelson and Van Sickle (1968) report that no consistent method for sampling abundance was implemented between collectors, however, the authors note that relative abundances of fossils remains constant from locality to locality. We filtered the digitized data to include only fossil collections with detailed lithologic descriptions (Sheldon 1963; Cressman and Swanson 1964). Facies assignments were made using these lithologic descriptions. Filtering the data by these criteria produced a subset of 228 fossil collections and 69 taxa from 35 localities (Fig. 1; Table 1 and 2; Supplementary Materials 1).

Faunal charts from Yochelson and Van Sickle (1968) tabulate fossil collections using a semiquantitative abundance scheme. This is: A- abundant, greater than 16 individuals; C- common, 6 to 15 individuals; R- rare, 1 to 5 individuals; X-  present, used to refer to remains which are disarticulated and difficult to count (e.g., disarticulate bryozoan branches or crinoid columnals); and, NA- absent. For statistical analyses, these have been converted to the following numerical values: A- 4; C- 3; R- 2; X-  1; absent- 0 (Supplementary Materials 1). Because of this semiquantitative scheme, when presenting our results, we use the terms frequent or occurrence in reference to the total collections with a given taxon, and abundance in reference to the taxon’s semi-quantitative code (See Faunal Composition).

When possible, taxa were identified to species level (Yochelson and Van Sickle 1968). For statistical analyses, we aggregate any species level taxa to genus with the exception of any dubious taxon entries (typically denoted with a question mark) in Yochelson and Van Sickle’s (1968) faunal charts. These entries were removed from this study. Semiquantitative abundances were aggregated by taking the highest abundance category of the aggregate fossil entry at a particular generic level. For example, if a particular sample includes a genus with three species with abundance categories of C, R, and R, then the aggregate abundance category for that genus would be C.

A fossil collection’s lithology was defined based upon the dominant lithology in its lithologic description (Supplementary Materials 1). Lithologic descriptions containing chert, either as nodular in a host rock or as bedded chert, were classified as chert. This inclusive definition of chert is appropriate as we seek to identify how biosiliceous sedimentation, preserved as chert, contributes to classically dominant lithologies (e.g., siliciclastics and carbonates). See Facies categorization for a comparison of nodular cherts vs. bedded cherts.

A composite cool-water carbonate ramp and glass ramp model was used to categorize fossil collections by detailed lithologic description (Table 2; James 1997; Gates et al. 2004; Blomeier et al. 2013). Fossil collections were categorized into three facies: inner ramp, mid ramp, and outer ramp. The inner ramp depositional environment is defined as coarse to fine sand size clasts composed of bioclasts, quartz, or peloids with thick to indeterminate bedding, and light-colored, nodule to massive chert. The mid ramp depositional environment is defined as fine sand to silt size clasts composed of bioclasts, quartz, or peloids with indeterminate to medium bedding, and light-gray to medium gray, nodular to massive cherts. The outer ramp depositional environment is defined as silty to argillaceous, with limited bioclasts and quartz, thin to medium or irregular bedding, and medium-gray to dark-gray, nodular to bedded chert. Total fossil collections from each depositional environment were: inner ramp- 55; mid ramp- 102; and, outer ramp- 71 (Table 3). The two most sampled units in this analysis are the Franson Member (64 collections) and the Retort Phosphatic Shale member (70 collections). These two units’ account for approximately half of all fossil collections analyzed in this study.

The community matrix was filtered to include only taxa that occur in at least two different collections and collections that have at least two different taxa. This further reduced the data to 105 fossil collections and 49 taxa. This procedure filters all collections of the Lower Chert, Rex Chert, and Cherty Shale from the community matrix. The Lower Chert and Cherty Shale are under-sampled as compared to the other units in the Phosphoria Rock Complex. The Grandeur contains many singletons. For some multivariate analyses, we subset the data by depositional cycle to explore how community structure changed through time. In these analyses, we use the same filtering procedure as for the entire dataset. This resulted in 49 collections and 32 taxa in the Franson cycle, and 56 fossil collections and 32 taxa in the Ervay cycle

This dataset includes the unfiltered faunal charts from USGS Professional Paper 313-D that correspond with stratigraphic sections reported in USGS Professional Papers 313-B and 313-C. Faunal charts were digitized following the procedure in methods. Data from stratigraphic sections and faunal charts should be joined using the collection ID number. This file also contains the results of ordination analyses presented in Wistort and Ritterbush (2022).