An Early Triassic small shelly fossil-style assemblage from the Virgin Limestone Member, Moenkopi Formation, western United States

Abstract

Small shelly fossils (SSFs) are minute fossils moulded or replaced by apatite, and less commonly, other minerals like glauconite and iron oxides. This taphonomic mode is best known from Cambrian deposits, though some occurrences occur across geological time. Instances of small shelly‐style preservation were found in insoluble residues from the Lower Triassic Virgin Limestone Member exposed in southern Nevada, the western United States, a second such occurrence known from this unit. Fossil steinkerns of tiny brachiopods, echinoid spines and ophiuroids are fluorapatite, with scarce phosphatic internal moulds of bivalves and two replaced ostracods. In contrast, the crinoid ossicles, almost all of which are >1000 µm, are preserved as stereomic moulds of silica or dolomite. Though the style of preservation is similar to another Virgin Limestone small shelly fossils‐style assemblage, this assemblage preserves greater diversity, likely reflecting the variation in palaeocommunities across the shelf. The size selectivity of phosphatization is clear, as the majority of the fossils <1000 µm are phosphatized. Importantly, the original skeletal material does not exert the strongest control on style of preservation: crinoid ossicles are replaced or moulded by silica whilst ophiuroid and echinoid fragments are phosphatized. It is likely that the underlying phosphatization mechanisms are related to the small particle size of the skeletons or skeletal elements coupled with the local pore water environment. Early Triassic equatorial seas characterized by warm temperatures and lower oxygen levels likely fostered small shelly fossil‐style preservation across the shelf during this time.

A new phosphatized ophiuroid from the lower Triassic of Nevada and its position in the evolutionary history of the Ophiuroidea (Echinodermata)

“…There has been some debate surrounding the mechanisms behind phosphatization during this time period, in particular phosphogenesis driven by high-energy mixing events (Freeman et al 2013;Milam et al 2017) versus phosphatization occurring in beds with low sedimentation rates and/or sedimentary hiatuses (Freeman et al 2019,). Maxwell et al (2020) observed a link between small shell size and phosphatization, and suggest phosphatization resulted from the combination of warm pore water with low levels of oxygen and the small particle size of the ophiuroid fragments.…”

“…There is an abundance of unusual facies present in the Virgin Limestone Member, including microbialite facies, such as large microbial reef mounds that span the length of the Lost Cabin Springs outcrop area , wrinkle structures , and flat-pebble conglomerate facies (Pruss et al, 2005). Of eight fossiliferous samples, ophiuroids were only present in Bed LC-18-34 (Maxwell et al, 2020). Approximately 115 meters from the base, this bed is a thin 3.5 m-thick fossiliferous packstone with isolated crinoidal ossicles overlying a small microbial build-up.…”

Life on the Edge: The Cambrian Marine Realm and Oxygenation

“…Clarifying the link between paleoenvironment and fossilization is important to understand what closed the taphonomic window that fostered these styles of preservation in the later early and middle Cambrian globally (Porter 2004) (see Figure 3). The occurrence of phosphatized and/or glauconitized fossils at other intervals in the Paleozoic (Dattilo et al 2019, Dzik 1994 and Mesozoic (Maxwell et al 2021, Pruss et al 2018 suggests that recurring environmental factors led to these styles of preservation. In this case, the small size of skeletal material (and organismal volume) coupled with early oscillating porewater redox conditions must be critical factors in these modes of preservation and to marine authigenesis of the Cambrian more broadly.…”