 In 1999 Martill suggested that a climate change at the end of the Callovian led to the extinction of "Leedsichthys" in the northern seas, the southern Ocean offering a last refuge during the Oxfordian.However, in 2010 Liston pointed out that "Leedsichthys" during the later Kimmeridgian was still present in the north, as testified by Normandian finds.Liston did nevertheless consider in 2007 that the lack of any vertebrate suspension feeders as large as 0.5 metre (1.6 feet) prior to the Callovian stage of the Mesozoicum might indicate that the Callovian had seen a marked change in productivity as regarded zooplankton populations.