Proliferation of Isoëtalean Lycophytes During the Permo-Triassic Biotic Crises: A Proxy for the State of the Terrestrial Biosphere

Cindy V Looy,Ivo A P Duijnstee,Johanna H A Van Konijnenburg-Van Cittert

doi:10.3389/feart.2021.615370

Abstract

Throughout their 420-Ma-long history, Lycopodiopsida have played a subordinate role at the landscape level with very few exceptions. One being the arborescent Lepidodendrales that dominated Pennsylvanian peat swamps in equatorial regions. Another is the enigmatic world-wide proliferation of sub-arborescent Isoëtales during, and in the aftermath of the Permo-Triassic terrestrial biosphere crisis that extended deep into the Triassic. Palynological as well as megafossil data shows that in a great proportion of locations around the globe that produced a fossil record, the provincial floras characteristic for the latest Permian were replaced by communities dominated by Isoëtales such as Pleuromeia and its allies. Our analysis of the isoëtalean biology, especially of the genus Pleuromeia, reveals an unusual suite of physiological and life-history traits, all indicating that it was an excellent stress-tolerator, but also a slow-growing weak competitor. This enabled Pleuromeia to thrive during environmental crises and occupy diverse habitats following the decline of other plants groups. Given their unusual biology, Isoëtales’ repeated ubiquity throughout the Early Triassic implies prolonged and repeated environmental stress in localities worldwide. Additionally, it demonstrates that the cosmopolitan isoëtalean-dominated systems produced a low-productivity, low-diversity terrestrial trophic base of the food web that no longer provided the same level of ecological and evolutionary goods and services (energy source, niche construction, ecosystem engineering, etc.) as the communities they replaced.

Highlights

Paleobotanists study plant fossil morphology to retrieve information regarding the fossils’ botanical affinity, to build more comprehensive phylogenies, to record changes in their communities, or use plant remains as biostratigraphic markers
Moving away from anecdotal evidence for the increased presence of isoëtaleans, we set out to quantify the proportion of isoëtalean taxonomic occurrences at stage level, based on fossil identifications in the literature as compiled by Nowak et al (2019)
The frequency of isoëtaleans stayed relatively high throughout the Triassic in the microfossil data, but declined rapidly in the case of the megafossil record, in which they were no longer detected by the Norian

Summary

Introduction

Paleobotanists study plant fossil morphology to retrieve information regarding the fossils’ botanical affinity, to build more comprehensive phylogenies, to record changes in their communities, or use plant remains as biostratigraphic markers. Utilize the plant fossil record as a source of information for the functioning and dynamics of entire biotic systems, and to Significance of Permo-Triassic Isoëtaleans understand the macroecological and macroevolutionary force they represent. This is despite the fact that plants form the trophic basis of all terrestrial ecosystems, are prominent niche constructors, and generally comprise critical ecosystem engineers (Lawton, 1994). Autotrophs—which per definition lack the same level of trophic dependence on other ecosystem members—constitute a much better model for the exploration of terrestrial crises and mechanisms for heightened extinction rates than other biota

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