Distinguishing core and flank facies based on shell fabrics in Lower Jurassic lithiotid shell beds

Brandolese, Valentina; Posenato, Renato; Nebelsick, James H.; Bassi, Davide

doi:10.1016/j.palaeo.2019.04.010

Cited by 4 publications

(1 citation statement)

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“…1) shows a spectacular outcrop of Chondrodonta-bearing limestones which have been documented since the early 20 th century (Schubert, 1903), Here, C. joannae developed extremely aberrant shells, with a typical club-like morphology, reaching ~50 cm in height. This morphology is very similar to that of the best known extinct stick-like bivalves, such as the Lower Jurassic Lithiotis (e.g., Chinzei, 1982;Posenato and Masetti, 2012;Brandolese et al, 2019), the Cretaceous oyster Konbostrea, and the elevator rudists (e.g., Chinzei, 1986Chinzei, , 2013Skelton and Gili, 2002). These taxa, which belong to different evolutionary lineages, developed strongly elongated shells adaptive for occupying soft-bottom substrates subjected to high rates of sedimentation To avoid gill suffocation, the body cavity raised from the sediment-bottom interface, abandoning the dorsal/umbonal shell, which was filled by chalky carbonates or chambered.…”

Section: Introductionsupporting

confidence: 73%

Paleoecology and proliferation of the bivalve Chondrodonta joannae (Choffat) in the upper Cenomanian (Upper Cretaceous) Adriatic Carbonate Platform of Istria (Croatia)

Posenato

Frijia

Morsilli

et al. 2020

Palaeogeography, Palaeoclimatology, Palaeoecology

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Chondrodonta joannae (Choffat) is a morphologically variable oyster-like bivalve with a predominately calcitic shell. An exceptional exposure of C. joannae-bearing strata of late Cenomanian age crops out along the seaside in northern Istria (Croatia) and permits a taphonomical and functional analysis in order to define the life habit and growth strategies of this bivalve. The C. joannae population from the studied succession is characterised by highly-elongated, large and curved shells, reaching about 50 cm in height and 5 cm in length. This shell shape is typical of the club-like bivalve morphotype, which was adapted to soft-bottom substrates with high sediment accumulation. The shell is slightly inequivalve and characterized by a reduced body cavity, a few centimetres high, and a dorsal region up to 10 times longer. The shell opening mechanism was mostly based on the resilium located between the chondrophores which protrude in the body cavity.The abandoned dorsal cavity is filled by a calcite hinge plate, the ventral edge of which acted as fulcrum for the valve flexibility. In the hinge plate, the function of chondrophores changed. They acted as a hinge to keep tightly interlocked the valves, which considerably emerged above the sediment-water interface. The individuals were arranged in low shrub-type congregations, which produced low-relief mounds. The functional morphology and taphonomic signature suggest that C. joannae individuals collected food at a greater distance from the bottom with respect to the cooccurring rudists. We speculate that the C. joannae proliferation could be related to a late Cenomanian phase of environmental instability predating the OAE2 with fluctuating climatic conditions and ocean fertility.

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