The claim by Raup and Sepkoski1, that an approximately 26-Myr mass extinction periodicity can be recognized on a global scale at family level over the past 250 Myr, has excited much attention and provoked speculation about an ultimate extraterrestrial cause2-10. While the purported periodicity can be challenged on grounds of both the inaccuracy of the geological timescale11 and the method of extinction analysis12, there can be no doubting the reality of at least some of the 12 proposed extinction events, such as those at or near the end of the Permian, Triassic and Cretaceous. Others are less widely acknowledged. Further analysis by Raup and Sepkoski13 confirms the importance of two of these events within the Jurassic, at the end of the Pliensbachian and Tithonian stages. I demonstrate here that marine invertebrate mass extinctions at approximately these times occurred on a regional, not a global scale. They can be related to severe reductions in habitat area provoked either by regression of epicontinental sea or by widespread anoxia.