The Mesozoic Lacustrine Revolution

Abstract

The Mesozoic Lacustrine Revolution (MLR) represents a significant evolutionary event for continental ecosystems. Evidence from the ichnologic and body-fossil records yields major insights into the timing and nature of this evolutionary breakthrough. Lacustrine ichnofaunas preceding the MLR, although present in lake-margin and permanent subaqueous settings, tend to be dominated by very shallow-tier structures reflecting an empty to underutilized infaunal ecospace. Pre-MLR freshwater biotas had poorly developed food webs; detritivores and top predators as near-exclusive consumers and herbivory largely was absent.

Lower Triassic ichnofaunas are poorly documented and appear similar to those of the late Paleozoic. Ichnologic data suggest that the Middle to Late Triassic was a time of significant evolutionary innovations in lacustrine communities that were established in lake-margin settings whose trace-fossil assemblages would typify these environments during the rest of the Phanerozoic. The picture in fully lacustrine settings is slightly different because profundal lake deposits display some ichnofaunas reminiscent of the late Paleozoic, supplemented by more penetrative burrows typical of the rest of the Mesozoic and Cenozoic. Triassic aquatic biotas after the end-Permian crisis are taxonomically depauperate and dominated by planktonic- and substrate-attached algal groups and generalist feeding insects. The diversity and abundance of plants, insects, and vertebrates increased significantly during the Middle and especially Late Triassic, as insect taxa functionally expanded their repertoire of feeding styles, and in particular evolved structures for active predation. Minimal aquatic herbivory was incorporated as microvory, and conspicuous herbivory was virtually nonexistent, except for a few interactions with aquatic macrophytes.

The ichnotaxonomic composition of Jurassic lake-margin invertebrate ichnofaunas is generally similar to those of the Middle to Upper Triassic, and no major evolutionary novelties or innovations are apparent. However, the appearance of extensive trampled surfaces produced by dinosaurs significantly altered lake-margin sediments. Trace-fossil information from Jurassic fully lacustrine environments provides a different picture of ichnodiversity levels similar to those of the Middle to Upper Triassic and even to those of the late Paleozoic. However, the principal difference is the degree of infaunalization, as revealed by widespread occupation of mid-tiers in deep-lacustrine sediments and exemplified by Jurassic lacustrine insects that consist of similar groups of today but with different proportional abundances and ecological tolerances. Jurassic aquatic-insect diets were overwhelmingly detritivorous (including microvory), omnivorous, and carnivorous. As in the Triassic, insect herbivory was minimal as was evidence for aquatic plants, except phytoplankton and charophytes. The only significant macrophytes were ferns, which colonized Jurassic lake-margin habitats. Augmenting the Jurassic insect-predator component were teleost fish, frogs, crocodilians and, late in the period, birds. Nevertheless, as in the Triassic, Jurassic body-fossils suggest continental aquatic biotas in hypotrophic settings still expressed similar, detritivore-driven, trophically simplified food-webs, albeit taxonomic diversity was higher.

Sparse ichnologic information available for the Cretaceous suggests continuation at lake-margin environments of the same trends occurring in earlier settings. Fully lacustrine deposits include intensely bioturbated deposits, suggesting establishment of a lacustrine mixed layer. Predatory Cretaceous lacustrine insect faunas expanded in a variety of habitats. The basic insect feeding types remained the same compared to the Late Jurassic, although there are significant proportional changes in taxa. A conspicuous exception is emergence of the herbivore feeding guild of aquatic insects along lake margins that paralleled expansion of aquatic angiosperms enriched in ferns and overshadowing long-standing phytoplankton and charophyte floras. A significant but modestly developed, Early Cretaceous lacustrine trophic chain appeared in the mesolimnion and epilimnion that linked diverse aquatic plants, herbivorous insects, and their arthropod and vertebrate predators. This trophic chain included links to adjacent terrestrial communities resulting in ecological adjustments key toward launching the MLR. Later global events, such as the mid-Cretaceous eutrophication crisis, end-Cretaceous ecological crisis, and repeated, transitory hyperthermal events during the Paleogene, modified long-term effects of the MLR. Combined body- and trace-fossil data indicate that the MLR was a protracted process that began during the later Triassic in the hypolimnion and benthos, and culminated during the Late Jurassic to Early Cretaceous in the mesolimnion and epilimnion.