Size changes in Jurassic liparoceratid ammonites and their stratigraphical and ecological significance

Abstract

Over 50 species of liparoceratid ammonites have previously been recognized from the Ibex and Davoei Zones (Lower Pliensbachian, Lower Jurassic) of England alone. Such a high specific diversity is an artifact caused by their stratigraphical importance and the complex nature of their shells. Simple bivariate analysis indicates that Aegoceras species are arbitrary subdivisions of an allometric growth trend. Principal component multivariate analysis shows that Liparoceras and Androgynoceras (sensu Callomon) are quite distinct from Aegoceras, and they display a more random pattern of variation. The existence of stratigraphically useful size changes in liparoceratids mirrors precisely size increases in benthic molluscan taxa, such as the bivalve ‘Astarte’ platymorpha and the archaeogastropod Eucyclus subimbricata. The ammonite size changes, at least, probably result from phenotypic variation analogous to that observed in the living cephalopod Sepia officinalis. This evidence taken in conjunction with the fact that kosmoceratid ammonite size changes correspond somewhat with benthic faunal changes suggests that ammonites maybe less useful for stratigraphy than is believed.