Taphonomy of an Ordovician Crinoid Lagerstätte From Kentucky

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.

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“…2013; Milam et al . 2017), which is argued to enhance phosphogenesis. As the organic matter decays, organic‐bound phosphorus is rapidly released.…”

Section: Discussionmentioning

confidence: 99%

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

Maxwell

Thuy

Pruss

2021

LET

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“…Thanks to phosphatization, the ophiuroid ossicles were left unscathed by the dissolution of the hard, fossiliferous limestone. 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.…”

Section: Discussionmentioning

confidence: 99%

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

Thuy

Maxwell

Pruss

2021

Zootaxa

335

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The Lower Triassic fossil record of brittle stars is relatively rich, yet most records published to date are based on poorly preserved or insufficiently known fossils. This hampers exhaustive morphological analyses, comparison with recent relatives or inclusion of Early Triassic ophiuroid taxa in phylogenetic estimates. Here, we describe a new ophiuroid from the Lower Triassic of Nevada, preserved as phosphatized skeletal parts and assigned to the new taxon Ophiosuperstes praeparvus gen. et sp. nov Maxwell, V. & Pruss. S.B. This unusual preservation of the fossils allowed for acid-extraction of an entire suite of dissociated skeletal parts, including lateral arm plates, ventral arm plates, vertebrae and various disk plates, thus unlocking sufficient morphological information to explore the phylogenetic position of the new taxon. Bayesian phylogenetic inference suggests a basalmost position of O. praeparvus within the Ophintegrida, sister to all other sampled members of that superorder. The existence of coeval but more derived ophiuroids suggests that O. praeparvus probably represents a member of a more ancient stem ophintegrid group persisting into the Early Triassic.

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“…It seems unlikely that this represents relaxation of coiling as a result of post-mortem decay because of the articulated state of such specimens. Perhaps unmineralized substances such as algae were encrusted, as suggested for Glyptocrinus (see Milam et al 2017), or certain individuals simply developed a looser coil during ontogeny. Regardless, I. subcrassus columns have not been documented previously as substrata for encrustation by juveniles.…”

Section: Implications For Iocrinus Palaeoecologymentioning

confidence: 97%

A noteworthy accumulation of disparid crinoids from the type Cincinnatian (Upper Ordovician) of southwestern Ohio, USA: implications for the palaeoecology and taphonomy of crinoid “logjam” assemblages

et al. 2018

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Crinoids are a common and well-studied faunal component of the Upper Ordovician (Katian; Edenian) Kope Formation in the greater Cincinnati Arch region, USA. However, a relatively fresh outcrop exposing the Southgate and McMicken members of the Kope Formation at Cleves, Hamilton County, southwestern Ohio, has yielded a crinoid specimen worthy of description and comment. The specimen is a ''logjam'' of numerous articulated columns of Iocrinus subcrassus displaying parallel alignment, reflecting the influence of storm-generated currents. Iocrinus is not typically found in such a state; the genera Ectenocrinus and Cincinnaticrinus are generally associated with ''logjams'' in the type Cincinnatian, making this an unusual occurrence. At least one of the columns has the coiled dististele of another, smaller I. subcrassus tightly wrapped around it. Although I. subcrassus is known to employ a coiled dististele as an attachment strategy, ramose bryozoans are generally utilised as substrates and tight coiling around larger I. subcrassus columns has not previously been reported. Preservation of coiled dististeles, in general, is a feature not previously documented in Cincinnatian crinoid ''logjams.'' This specimen illustrates that unusual, noteworthy and/or rare material representing relatively common organisms continue to be discovered even within extensively studied units in the type Cincinnatian.

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Taphonomy of an Ordovician Crinoid Lagerstätte From Kentucky

Cited by 8 publications

References 58 publications

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

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

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

A noteworthy accumulation of disparid crinoids from the type Cincinnatian (Upper Ordovician) of southwestern Ohio, USA: implications for the palaeoecology and taphonomy of crinoid “logjam” assemblages

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