Carbon and oxygen isotopic compositions of calcareous nekton (aragonite Kosmoceras and calcite belemnites) and benthos (aragonite nuculacean bivalves and calcite Gryphaea) from the Peterborough Member of the Oxford Clay Formation vary considerably at any one horizon but show no obvious trend with time. This suggests that the temperature and isotopic composition of ambient seawater may have varied considerably over short time intervals but did not change systematically during deposition of the unit. Thus, short-term fluctuations were superimposed on longer-term 'stability' in this shallow-shelf environment. The overlap in carbon isotopic compositions between calcareous nekton and benthos suggest a well-mixed water column. Oxygen isotopic palaeotemperatures suggest thermal stratification, provided that Kosmoceras (16–28 °C) inhabited near-surface waters and belemnites (12–19°C, indistinguishable from the range for benthic bivalves) were nektobenthic. Oxygen isotopic compositions of phosphate from both shallow- and possible deeper-dwelling vertebrates (reptiles, bony fish and sharks) yield the same range of palaeotemperatures (20–29 °C), suggesting that all vertebrate taxa may have lived predominantly in warm, shallow waters.