Sedimentary context and palaeoecology of Gigantoproductus shell beds in the Mississippian Eyam Limestone Formation, Derbyshire carbonate platform, central England

Abstract

A sedimentological study was conducted at two localities exposing the Mississippian Eyam Limestone Formation of the Derbyshire carbonate platform, UK. Ricklow Quarry comprises seven facies with diverse skeletal assemblages, representing deposition on the inner to middle ramp within open marine waters. Once-a-Week Quarry comprises four facies, dominated by crinoidal debris representing deposition on the inner ramp. Both localities expose Gigantoproductus shell beds. Palaeoecological analysis of a single shell bed from each locality enabled investigation of the rapid colonization and success of this taxon on the platform. At Ricklow Quarry, on the eastern side of a localized mud mound, both life (>72% of thin and thick-shelled brachiopods in life position) and neighbourhood assemblages are present. A low-moderate diversity community (<1.37 and <0.8 Shannon diversity index) rapidly established over relict Brigantian mud mounds. Shell beds are preluded by intervals of decreased energy that allowed larvae to settle. Once established, the dominance of thick-shelled individuals enabled baffling, potentially providing localized shelter for larvae and nearby individuals. At Once-a-Week Quarry, where no mud mound is present, only thick-shelled Gigantoproductus species and a low diversity community (<1.07 Shannon diversity index) exclusively comprising neighbourhood assemblages (37% in life position) is present. The presence of inactive mud mounds at Ricklow Quarry appears to have been the key to the success of Gigantoproductus species enabling the onset of stable communities in the shelter provided by the relict mound. Once the first palaeocommunities were established, larvae dispersed and colonized higher energy settings, such as at Once-a-Week Quarry.

The giants of the phylum Brachiopoda: a matter of diet?

“…They all reach a gigantic size and have the same lifestyle, but in two different shallow‐water settings: high‐energy crinoidal shoals in inner ramp settings at Once‐a‐Week Quarry and within the shelter provided by relict mud mounds in middle ramp settings at Ricklow Quarry (Nolan et al . ).…”

“…() and Nolan et al . ()). Sixteen specimens from the Once‐a‐Week and Ricklow quarries were used for petrographic and cathodoluminescence analyses, and ten specimens were selected for ultrastructural (SEM, TEM, EBSD), oxygen and carbon isotope analyses of calcite, and carbon and nitrogen isotope analyses of occluded organic material.…”

“…Finally, as shown in detail by Nolan et al . (), the Once‐a‐Week Quarry gigantoproductids were deposited in a high‐energy setting. The gigantoproductid bed overlies cross‐bedded crinoidal grainstone–rudstone accumulated in an inner ramp swept by currents.…”

“…Geological sketch map (from Nolan et al . ). A, location of the Derbyshire carbonate platform ( DCP ) within mainland Great Britain.…”

“…To discriminate between a suspension‐feeding lifestyle vs a symbiotic lifestyle, we compared different species of Gigantoproductus occurring together in different palaeoenvironmental settings (Nolan et al . ).…”