ECOSYSTEMS ON A DIET: PROLIFERATION OF ISOETALEAN LYCOPHYTES EXPLAINS DELAYED RECOVERY PATTERNS OF TERRESTRIAL ECOSYSTEMS AFTER THE END-PERMIAN BIOTIC CRISIS

The slow recovery of the terrestrial biosphere following the end-Permian biotic crisis is poorly understood since the early Triassic terrestrial macrofossil record is notoriously incomplete. We used fossil plant distributions and morphology to reconstruct physiology, ecology and life-history properties of dominant floral elements. Combined with the more complete palynological record, this allows us to infer system-level aspects of the terrestrial ecosystems across all latitudes, for which these floras provided the trophic foundation. These system patterns have macroevolutionary implications in the long run.

In many macrofloral records across Pangea, once diverse floras were replaced by populations of enigmatic isoetalean lycophytes: Pleuromeia and its allies. Such impoverished floras persisted for millions of years. Our analysis of Pleuromeia’s biology reveals an unusual suite of traits, indicating that it was an excellent stress-tolerator, but also a slow-growing, weak competitor. This means that whenever Pleuromeia dominates the record (1) stress levels may have been high, (2) other plants were absent, and (3) productivity was low. Its ubiquitousness thus implies a widespread, unproductive and narrow base of the ecosystem’s food chain, which would have had difficulties to sustain complex and stable heterotroph foodwebs. In addition to the likely recurring episodes of environmental stress, the duration of the isoetalean interregnum meant a prevailing lack of recovery at the base of the food chain. Collapsed ecosystems, caused by extinction of prominent autotrophic ecosystem engineers, could not recover until they could be rebuilt from the bottom-up via re-diversification among autotrophs first. This delay prevented adaptive radiation among higher trophic levels, thus delaying recovery of animal lineages and complex ecosystems throughout Early Triassic.