Evolution of ammonoid morphospace during the Early Jurassic radiation

Abstract

The morphologic radiation of Early Jurassic ammonites following the near extinction at the end of the Triassic is analyzed from 436 species of 156 genera that form a representative sample of morphs occurring worldwide in the first three stages of the Jurassic (Hettangian, Sinemurian, Pliensbachian: 36 subzones, 24 m.y.). Morphologic diversity is analyzed independently of taxonomy by processing 18 shape parameters using multivariate analysis and clustering techniques. The morphospace thus defined indicates that morphs fall readily into two groups made up of four and five adjacent morpho-subsets. The temporal pattern of morphospace occupation in the 36 Lower Jurassic subzones displays diversification, depletion (sometimes total), and displacement of successive parts of the morphospace, reflecting a complex history in which morphologic radiation appears to be more than a process of diffusion. The history of the morphologic evolution is tentatively related to sea-level changes and there is a suggestion that morphologic diversity increases during second-order transgressive periods.

Evolutionary Trends within Jurassic Ammonoids

“…Both the Early Jurassic and Middle Jurassic radiations observed in the present analysis concern drastic and profound changes in ammonoid diversity (i.e. changes in dominant clades), which has also been documented by cladistic and macroevolutionary studies (see Dommergues et al 1996Dommergues et al , 2001Neige 2004, 2007;Neige et al 2013).…”

Morphostructural constraints and phylogenetic overprint on sutural frilling in Late Jurassic ammonites

“…Alternatively, there were no major differences in habitat depth for epicontinental and epioceanic ammonites during the Late Jurassic. A similar picture can be assumed for the Early Jurassic ammonites according to data in Dommergues et al (1996) indicating that shells with more complex sutures were not especially favoured during higher sea levels, nor were they affected by sea-level uctuations. Thus, deeper habitats cannot be directly inferred for epioceanic ammonites from complex sutures.…”

Ammonite paleobiogeography during the Pliensbachian–Toarcian crisis (Early Jurassic) reflecting paleoclimate, eustasy, and extinctions

“…This biological crisisis mainly reflected in sharp drops in diversity levels and high turnover rates in Tethyan, Arctic, and Panthalassan seas during the Early Toarcian (Hallam, 1987;Aberhan, 1993;Hori, 1997;Nikitenko and Mickey, 2004;Ruban and Tyszka, 2005;Wignall et al, 2006;Zakharov et al, 2006). Moreover, this event coincides with bottlenecks in the morphological and scalar disparity of ammonites (Dommergues et al, 1996;Dera et al, 2010), and size reductions in microplankton and molluscan shells Pittet, 2002, 2004;Morten and Twitchett, 2009). Recently, studies aiming at specifying its dynamics have revealed a longer-term process spanning about 6 Myr, with successive extinction pulses ranging from the Late Pliensbachian (Margaritatus Zone) to the Late Toarcian (Dispansum Zone) (Fig.…”