323-5 Trends in Megafaunal Fruit Evolution Following the Cretaceous-Paleogene Extinction Event

Abstract:

Clear adaptations for endozoochory, in which disseminules are consumed and passed through an animal’s gut, appear in fossil angiosperms or flowering plants from the Early Cretaceous (132 to 112 Ma) onward. “Megafaunal dispersal syndrome,” which targets large herbivores that can facilitate long-distance seed dispersal, is a common strategy seen across various plant families. Many living species are adapted for primary dispersal by animals that became extinct during the Late Quaternary Extinctions (50,000 to 10,000 Ka), now depending on secondary dispersal mechanisms associated with decreased germination, range contractions, reduced gene flow, and other adverse consequences. Prior to the Late Quaternary Extinctions, the last major extinction event to wipe out large herbivores on a global scale was the Cretaceous-Paleogene (KPg) extinction event ~66 Ma. Characterizing how plants adapted for endozoochorous seed dispersal were affected by the KPg extinction event, before the evolution of megafaunal mammal dispersers ~50 Ma, could improve our ability to assess the ecological impacts of recent and ongoing megafaunal extinctions. This study uses several existing data sets to test the hypothesis that megafaunal fruits were adversely impacted by the KPg extinction event: phylogenetic data which reconstructs the timing of transitions in fruit size within the palm family (Arecaceae) and custard apple family (Annonaceae), and fossil data revealing trends in fruit size and other aspects of angiosperm seed dispersal from the Cretaceous through the Cenozoic. Contrary to expectations, megafaunal fruits appear to have diversified within extant lineages of the Arecaceae during the Paleocene (66-56 Ma), while fruit size increased both within extant lineages of the Annonaceae and across angiosperms in general. These findings suggest that plants adapted for megafaunal dispersal can survive and even diversify long-term in the absence of large herbivore dispersers, likely via recourse to secondary short-distance dispersal mechanisms within isolated refugia. Nonetheless, further study is needed to substantiate or refute the observed trends, fossil data being especially important to calibrate phylogenetic trees and account for the extinction of taxonomic groups with no living members. Moreover, the apparent diversification of disconnected populations of plants in the wake of mass extinction does not negate the adverse impacts of decreased overall abundance and restricted dispersal upon ecosystems, nor does it lessen the importance of conserving megafaunal fruit species amid the present biodiversity crisis.

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