ABSTRACT
After a relatively low but stable phase of taxonomic diversity during the Lias (Lower Jurassic), echinoids diversity strongly increases from Aalenian (Dogger-Middle Jurassic) to Kimmeridgian (Malm-Late Jurassic) and a conspicuous crisis takes place in Tithonian times, before the end of the Jurassic. During these major phases, minor decreases in diversity correspond with regressions (sea-level falls), mainly in the Middle Bathonian (Middle Jurassic). From the Aalenian to Callovian, the average duration of irregular echinoid species is about 3.8 m.y. whereas it is about 5.5 m.y. for the regular ones; the later value corresponding to the average duration of a Jurassic. From Callovian to Tithonian, it is respectively 5.3 m.y. and 8.3 m.y. The longest durations corresponds to reefal species or species found in coarse bioclastic platform deposits (Holectypoids, 16 m.y.; Cidaridae, 18 m.y.) but also to taxa insufficiently studied. Conversely, in the restricted area investigated (Paris Basin, Aquitanian and South East Basins in France), some taxa have a very short time span (Centropygus petitclerci, 1 m.y.; Acropeltis aequituberculata, 1 m.y.) which is comparable to that of an ammonite biozone. So, if these data are also true for other countries, such taxa would be used as time markers in biostratigraphy.
