CHANGES IN FOOD WEB STRUCTURE IN THE WESTERN TETHYS ASSOCIATED WITH THE MESOZOIC MARINE REVOLUTION

Increasing predation pressure during the Mesozoic Marine Revolution (MMR) is thought to have resulted in abrupt ecosystem reorganization due to changes in ecospace utilization, richness, and new modes of life. Ecological restructuring would include new evolutionary innovations and major changes in energy pathways, corresponding to changes in food web structure and functioning. However, current assessments rely predominantly on species diversity, and changes in food web structure have not yet been directly evaluated.

Here we compare the network topology of shallow marine food webs of the western Tethys from the Anisian (M. Triassic), Bathonian (M. Jurassic), Aptian (E. Cretaceous), and Maastrichtian (L. Cretaceous). Despite topological similarities, such as the number and density of interactions, modularity and connectance, several structural changes occurred. Although richness peaked in the Aptian, functional diversity did not. The number of guilds was consistent across stages, however, guild turnover was high, guilds absent in previous stages were replaced by functional equivalents. Many of these new guilds arose due to increases in body size, and not ecological innovations (new ways of utilizing resources), and mean species body size increased across all clades. As few functional guilds arose between the M. Triassic and E. Cretaceous, novel predator adaptations and diversification of predator clades was likely rapid, occurring prior to the Anisian in the E. Triassic. Functional diversity was concentrated at lower trophic positions (~2), and trophic chains became progressively shorter. The shortening of trophic chains and increased body sizes at higher trophic levels is consistent with the hypothesis that Mesozoic dinoflagellate radiations facilitated the delivery of more energy and biomass to predators at higher trophic levels.

Here we provide the first direct test of escalation and associated changes in trophic structure and energy transfer across the MMR using reconstructed trophic interactions. These findings support the hypothesis that metabolic and physiological escalation during the MMR prompted significant ecosystem restructuring and changes in energy transfer pathways, facilitated by the radiation of phytoplankton.