The calcite rostra of belemnites, Mesozoic marine mobile cephalopods, has been extensively used for the reconstruction of palaeotemperatures and carbon cycle. Many studies now employ compilations of belemnite data as a backbone for interpreting and modeling climate trends in the Mesozoic. While belemnite rostra are undoubtedly of great value as fossil substrates for geochemical proxy work, it is also commonly acknowledged that their ecology and life cycle are incompletely understood and that vital effects may bias the geochemical information extracted from this archive.
Here, a systematic literature survey of Early Jurassic marine invertebrate shell calcite C and O isotope data is conducted to assess basinal signatures, compare benthic versus planktonic records, and test the uniformity of isotope trends across the Jurassic Laurasian Seaway.
The new data compilation suggests that Early Jurassic belemnites predominantly sought out open ocean environments of the Tethys, but that their migration range was too limited to allow belemnites from the Cleveland Basin (NE England, UK) to reach the open Tethys. Instead, Cleveland Basin belemnites recorded more local oceanographic signals of a somewhat restricted basin. The Toarcian Oceanic Anoxic Event (T-OAE) catalysed an evolutionary adaption in at least some belemnite taxa towards shallower and/or more proximal palaeoenvironments, causing divergence of O isotope data in basins after the T-OAE. 
Currently employed belemnite C and O isotope compilations are skewed towards the Cleveland Basin in the Toarcian stage, when restriction, possible freshening and enhanced productivity led to strong deviations from open ocean conditions. The above findings highlight that this skew causes artifically high Toarcian palaeotemperature estimates and too positive C isotope signatures and therefore an unrealistic scale of reconstructed lower latitude climate change during the T-OAE. The presently most consistent and reliable Toarcian temperature records with reasonable resolution instead derive from benthic invertebrates from Portugal and Spain.

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