Stable isotope sclerochronology of rudist bivalves: Growth rates and Late Cretaceous seasonality

The almost complete translation of shell accretion to vertical growth was a successful morphogenetic adaptation of rudist bivalves, which replaced coral-algal-hydrozoan communities in most Tethyan shallow-marine environments during the Cretaceous. Growth rates of rudist shells are, therefore, important in reconstructions of Cretaceous shallow-marine benthic communities. Carbon and oxygen isotopic compositions were analyzed in sclerochronological profiles through the shells of two Late Cretaceous rudist bivalves from Greece and Turkey. Values of δ¹³C vary between +0.8‰ and +2.2‰ in the hippuritid Vaccinites ultimus and between +0.8‰ and +2.6‰ in the radiolitid Gorjanovicia cf. costata. Ranges in δ¹⁸O are -5.3‰ to -2.9‰ and -4.6‰ to -2.1‰, respectively. A distinct cyclicity of δ¹³C and δ¹⁸O in both shells reflects Late Cretaceous seasonality and indicates annual vertical-growth increments of 45 mm in V. ultimus and 54 mm in Gorjanovicia. The concentrations of minor elements are similar to those of extant bivalves with low-Mg calcite shells.