Remarkable preservation of shell microstructures from the Late Ordovician of the Cincinnati Arch region, USA, and the success of nacre among Ordovician mollusks

Abstract

Exceptional sub-micrometer details of shell microstructure are preserved in phosphatic micro-steinkerns representing several phyla from shell beds of the Upper Ordovician of the Cincinnati Arch region, USA. These fossils provide the most detailed record of Ordovician mollusk shell microstructures, as well as exceptional details on the earliest cases of undisputed nacre. The trend towards nacre in the Mollusca is one aspect of the surge in escalation between mollusks and their predators during the Great Ordovician Biodiversification Event.

Deep origin of the crossed‐lamellar microstructure in early Cambrian molluscs

“…Independent and multiple origins of particular shell microstructures, especially the CL and nacreous microstructures, seem to be a paradigm in studies of molluscan biomineralization on high taxonomic levels (McDougall & Degnan 2018; Vendrasco et al . 2019). This hypothesis is strongly supported by molecular evidence, as the organic matrix of both the CL and nacre differs markedly across various molluscan lineages, and many shell‐forming macromolecules (e.g.…”

“…Some authors have proposed that the LF microstructure might have given rise to the CL microstructure seen in a variety of post‐Cambrian molluscs (Vendrasco et al . 2017, 2019). The LF microstructure does share many similarities with the CL microstructure, but aragonitic crystallites of the former are apparently arranged parallel to the depositional surface and lack a hierarchical ordering (Feng & Sun 2003).…”

“…To date, no conclusive fossil evidence of nacre was found among Cambrian molluscs, but some shell microstructures described as calcitic semi‐nacre have been documented from some Miaolingian mollusc shells (Vendrasco et al . 2013, 2019). The laminar microstructure of the Australian pelagiellid exhibit some tooth‐like crystallites, similar to that of foliated calcite, but on a much larger scale, their characteristic straight growth fronts indicate a FOA microstructure as seen in P. madianensis , indicative of aragonite mineralogy (Li et al .…”

“…In recent years, there has been growing evidence that nacre seemed to emerge independently in various molluscan groups in response to increasing predatory pressure during the Great Ordovician Biodiversification Event (Vendrasco et al . 2019). Possibly, nacre derived from the foliated aragonite structure, a more common shell microstructure in early Cambrian molluscs and a modern relic in monoplacophorans (Vendrasco et al .…”

Tightly shut: flexible valve margins and microstructural asymmetry in pterioid bivalves

“…Kakisawa and Sumitomo 2012;Waqar et al 2018). But despite it being a very early microstructure (Taylor 1973;Vendrasco et al 2019), many extant molluscan groups do not secrete nacre and even where it does occur, it is always accompanied by at least one other microstructure (usually external to it) (Cartwright and Checa 2007;Bieler et al 2014). In the bivalves, which show an Responsible Editor: J. Grassle.…”