Chancelloriid sclerites from the Dyeran–Delamaran (‘Lower–Middle’ Cambrian) boundary interval of the Pioche–Caliente region, Nevada,USA

Abstract

Chancelloriids are a poorly understood group of phylogenetically problematic Cambrian metazoans; complete specimens show they were sessile, radially symmetrical, club‐shaped organisms covered with sclerites in the form of rosettes of spines. While isolated sclerites are common components of Cambrian shelly assemblages, they have been relatively little studied. We describe chancelloriid sclerites from a series of nine sections spanning the upper Dyeran to lower Delamaran stages (latest Stage 4 to perhaps basal Wuliuan) from the Pioche–Caliente region of east‐central Nevada, USA. Acid maceration of samples from the Combined Metals, Comet Shale and Susan Duster Limestone members of the Pioche Formation yielded more than 2000 sclerites. Based on careful examination of these sclerites and statistical analyses of co‐occurring sclerite types, we distinguish six species, each with a restricted stratigraphic range. Chancelloria impar Moore sp. nov. is the dominant species in most upper Dyeran samples. Archiasterella cometensis Moore sp. nov. and A. auriculata Moore sp. nov. are rare in the upper Dyeran but abundant in the lowest Delamaran; A. uncinata Moore sp. nov. and C. lilioides Moore sp. nov. replace these in younger samples. A. auriculata is noteworthy for sharing features with species of both Archiasterella and Chancelloria. These results provide further support for the taxonomic tractability and biostratigraphical utility of chancelloriid sclerites; large collections from single horizons allow intraspecific variability to be assessed and species to be distinguished. Our results document a taxonomic turnover in chancelloriids at the Dyeran–Delamaran boundary, showing that not only trilobites were affected at this time.

Cambrian Age 3 small shelly fossils from the Terrades inlier, southern Pyrenees, Spain: Biostratigraphic and paleobiogeographic implications

“…However, the present assemblage only consists of a limited collection of disarticulated sclerites, some of which are not known from articulated taxa (e.g., 4 + 0 sclerites in Allonnia Doré and Reid, 1965; Yun et al, 2019). Moreover, some species are represented only by a fraction of their known dominant sclerite types in the present assemblage (e.g., Chancelloria eros Walcott, 1920 is here represented only by 7 + 1 sclerites, but 6 + 1 sclerites should also be dominant in this taxon; Bengtson and Collins, 2015; Moore et al, 2021), whereas individual sclerite types can be produced by distinct scleritomes (e.g., 4–6 + 0 types from Chancelloria Walcott, 1920 or Allonnia ; Moore et al, 2014). We have therefore adopted a conservative approach by following traditional taxonomy (Moore et al, 2014) without assuming correspondence between sclerite-based and scleritome-based taxa.…”

“…Order Chancelloriida Walcott, 1920Family Chancelloriidae Walcott, 1920 Remarks.-The classification of chancelloriid sclerites has continuously been reviewed following the recovery of complete scleritomes and large sclerite populations (Yun et al, 2019;Moore et al, 2021). However, the present assemblage only consists of a limited collection of disarticulated sclerites, some of which are not known from articulated taxa (e.g., 4 + 0 sclerites in Allonnia Doré and Reid, 1965;Yun et al, 2019).…”

Nature and significance of intraspecific variation in the early Cambrian oryctocephalid trilobiteOryctocephalites palmeriSundberg and McCollum, 1997

“…1; Merriam, 1964; Eddy and McCollum, 1998; Palmer, 1998; Sundberg and McCollum, 2000; Webster, 2011a, b). Detailed study of the formation over the past two decades, involving extensive field campaigns, has yielded a wealth of Dyeran and Delamaran trilobites (Sundberg and McCollum, 1997, 2000, 2002, 2003a; Eddy and McCollum, 1998; Palmer, 1998; Webster, 2007a, b, 2009a, b, 2011b, c, 2015; Hopkins and Webster, 2008, 2009; Webster et al, 2008) and other invertebrates (Lieberman, 2003; Moore et al, accepted a, b). The abundance and diversity of trilobites within many intervals have contributed toward an improved resolution of Dyeran and Delamaran biostratigraphy (Fig.…”