Ammonoids at the Triassic-Jurassic Transition: Pulling Back from the Edge of Extinction

Abstract

The mass extinction at the end of the Triassic is one of the "big five" extinctions in the Phanerozoic and one of four that directly affected the evolutionary history of the ammonoids. At the Triassic-Jurassic (T-J) transition, approximately 22 % of marine families, 53 % of genera and 80 % of species became extinct (Pálfy 2003;Sepkoski 1996) but the event is of particular significance to the ammonoids because of their near demise. Left hanging by a thread, the group managed to pull back from the brink of extinction and, in spectacular fashion, re-establish itself as a dominant presence throughout the rest of the Mesozoic. As a result of the exceptionally low eustatic sea-level at the time, a situation that almost certainly contributed to the dynamics and severity of the extinction, there are only a very small number of marine stratigraphic sections that are reasonably continuous across the T-J boundary. The list includes an exposure in Austria that was recently designated Global Stratotype Section and Point for the base of the Jurassic (Hillebrandt et al. 2013). Studying the precise details of the T-J event horizon is therefore difficult, but a clear picture of the scale of the ammonoid near extinction and a sense of the dynamics of recovery can be obtained by considering high level taxonomic changes, broad patterns of generic diversity and differences in morphospace occupation across the entire Late Triassic to Middle Jurassic interval. This is the purpose of our brief review.Interpreted environmental changes over the T-J interval include initial cooling, longer term global warming, widespread aridification, a global regression/transgression couplet, oceanic anoxia and a biocalcification crisis brought on by lower seawater pH

Stability–Maneuverability Tradeoffs Provided Diverse Functional Opportunities to Shelled Cephalopods

“…Ammonoid diversity and disparity generally decline during every survived mass extinction ( Smith et al 2014 ; Brayard and Bucher 2015 ; Landman et al 2015 ; Longridge and Smith 2015 ). After these events, ammonoids generally recover until they occupy the previous regions of their morphospace.…”

Impact of the Late Triassic mass extinction on functional diversity and composition of marine ecosystems

“…Some functional groups, such as pelagic, fast‐moving carnivores, suffered a significant reduction in relative richness at the LTE, but then quickly recovered to pre‐extinction levels by the late Hettangian, reinforcing the idea that both vertebrates (Thorne et al . ) (excluding conodonts which become extinct) and cephalopods (Longridge & Smith ) passed through an evolutionary bottleneck at the T–J boundary where rapid turnover rates aided rapid recovery. Despite this reduction in relative richness across the LTE, we see an increase in relative occurrences of pelagic, fast‐moving carnivores.…”

Geological duration of ammonoids controlled their geographical range of fossil distribution

“…There were mass extinction events at the end of the Permian and Triassic when drastic taxonomic turnovers of ammonoid fauna occurred (e.g., House, 1988 ; Monnet, Brayard & Brosse, 2015 ; Brayard & Bucher, 2015 ; Longridge & Smith, 2015 ). In the Early Triassic and Early Jurassic, origination rates of ammonoids were high (Ceratitida originated in the Triassic and Lytoceratina and Ammonitina originated in the Jurassic; Neige & Rouget, 2015 ; Yacobucci, 2015 ; Longridge & Smith, 2015 ), which probably indicate radiations into open niche followed mass extinction events. At these times, ammonoids appear to have originated and resulted in an increased diversity rather than an increased geographical spread of each taxa (with the exception of some long-ranged taxa).…”