The Jurassic (∼201–145 Myr ago) was long considered a warm ‘greenhouse' period; more recently cool, even ‘icehouse' episodes have been postulated. However, the mechanisms governing transition between so-called Warm Modes and Cool Modes are poorly known. Here we present a new large high-quality oxygen-isotope dataset from an interval that includes previously suggested mode transitions. Our results show an especially abrupt earliest Middle Jurassic (∼174 Ma) mid-latitude cooling of seawater by as much as 10 °C in the north–south Laurasian Seaway, a marine passage that connected the equatorial Tethys Ocean to the Boreal Sea. Coincidence in timing with large-scale regional lithospheric updoming of the North Sea region is striking, and we hypothesize that northward oceanic heat transport was impeded by uplift, triggering Cool Mode conditions more widely. This extreme climate-mode transition provides a counter-example to other Mesozoic transitions linked to quantitative change in atmospheric greenhouse gas content.

Reconstructing ecological niche shifts during ontogeny in extinct animals with no living analogues is difficult without exceptional fossil collections. Here we demonstrate how a previously identified ontogenetic shift in the size and shape of the dentition in the early Toarcian ichthyosaur Stenopterygius quadriscissus accurately predicts a particular dietary shift. The smallest S. quadriscissus fed on small, burst‐swimming fishes, with a steady shift towards faster moving fish and cephalopods with increasing body size. Larger adult specimens appear to have been completely reliant on cephalopods, with fish completely absent from gut contents shortly after onset of sexual maturity. This is consistent with a previously proposed ontogenetic niche shift based on tooth shape and body size, corroborating the idea that dental ontogeny may be a useful predictor of dietary shifts in marine reptiles. Applying the theoretical framework used here to other extinct species will improve the resolution of palaeoecological reconstructions, where appropriate sample sizes exist.

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A Callovian erratic boulder from Pleistocene glacial drift deposits of NE Germany yielded a 'prosopid' crab, which is described here as a new species of the genus Planoprosopon SCHWEITZER FELDMANN & LAZAR, Planoprosopon quadratum. It represents the first unequivocal record of Planoprosopon from the Middle Jurassic and thus one of the stratigraphically oldest records of the genus. Another new species, Planoprosopon thiedeae, is described from the Kimmeridgian of SW Germany. Tanidromites lithuanicus n. sp., a member of Tanidromitidae, is recorded from the Callovian of Lithuania. It is compared with new well-preserved material of other representatives of this genus. Tanidromites richardsoni (WOODWARD) is recorded for the first time from the Bajocian of SW Germany. Tanidromites insignis (V. MEYER), Tanidromites sculpta (QUENSTEDT), and Tanidromites scheffnerae n. sp., all coming from the Kimmeridgian of SW Germany, are briefly described. Prosopon aequilatum V. MEYER is excluded from Tanidromites and now assigned to Eodromites PATRULIUS, and a neotype is designated for this species. Possibly, the early diversification of crabs took place during the Middle Jurassic in more temperate, Subboreal shallow water areas lacking reefal habitats and accelerated when reefs occupied these areas in the Late Jurassic.