Ants (Hymenoptera: Formicidae) preserved in amber shed light on faunal turnover and ecological roles before the end-Cretaceous extinction

Substantial gaps in the ant fossil record surrounding the end-Cretaceous mass extinction have obscured the timing of early crown ant radiations and their palaeoecological diversity. Here, we describe two partial fossil ants (Formicidae: Pseudomyrmecinae; Formicidae: Aneuretinae) from Late Cretaceous Big Muddy amber (∼67.04 Ma). These are the oldest definitive representatives of Aneuretinae and Pseudomyrmecinae, extending their fossil records back into the Late Cretaceous and expanding their known spatiotemporal ranges. We used a broad extant ecomorphological dataset to evaluate the accuracy of Random Forest machine-learning classification for predicting the palaeoecology of partial ant fossils and to compare the inferred ecological niches of the Big Muddy ants and their respective extinct Cenozoic consubfamilials with those of their extant close relatives. We show that even with a limited subset of morphological traits, Random Forest models can generate accurate ecological predictions; however, model reliability is significantly impacted by the exclusion of sensory traits and the presence of flexible ecology. The Big Muddy pseudomyrmecine was recovered as terricolous, in contrast to the arboreal ecology of its Cenozoic and extant relatives, suggesting the transition to arboreal habitats may have occurred after the end-Cretaceous mass extinction. While the Big Muddy aneruetine results were not robust, its Cenozoic relatives were also reconstructed as terricolous and were more ecologically diverse than their sole living relative, the relictual Aneuretus simoni Emery, 1893. The presence of these subfamilies in Big Muddy amber supports the hypothesized timing of a major radiation of ecologically and taxonomically diverse crown ant subfamilies in the Late Cretaceous alongside flowering plants.