Adaptive Radiation in Socially Advanced Stem-Group Ants from the Cretaceous

Abstract

Across terrestrial ecosystems, modern ants are ubiquitous. As many as 94 out of every 100 individual arthropods in rainforests are ants, and they constitute up to 15% of animal biomass in the Amazon. Moreover, ants are pervasive agents of natural selection as over 10,000 arthropod species are specialized inquilines or myrmecomorphs living among ants or defending themselves through mimicry. Such impact is traditionally explained by sociality: ants are the first major group of ground-dwelling predatory insects to become eusocial, increasing efficiency of tasks and establishing competitive superiority over solitary species. A wealth of specimens from rich deposits of 99 million-year-old Burmese amber resolves ambiguity regarding sociality and diversity in the earliest ants. The stem-group genus Gerontoformica maintained distinct reproductive castes including morphotypes unknown in solitary aculeate (stinging) wasps, providing insight into early behavior. We present rare aggregations of workers, indicating group recruitment as well as an instance of interspecific combat; such aggression is a social feature of modern ants. Two species and an unusual new genus are described, further expanding the remarkable diversity of early ants. Stem-group ants are recovered as a paraphyletic assemblage at the base of modern lineages varying greatly in size, form, and mouthpart structure, interpreted here as an adaptive radiation. Though Cretaceous stem-group ants were eusocial and adaptively diverse, we hypothesize that their extinction resulted from the rise of competitively superior crown-group taxa that today form massive colonies, consistent with Wilson and Hölldobler's concept of "dynastic succession."

New Evanioid Wasps from the Cenomanian of Myanmar (Hymenoptera: Othniodellithidae, Aulacidae), with a Summary of Family-Group Names Among Evanioidea

“…Burmese amber is currently the most abundant source of Mesozoic amber inclusions, and includes the greatest diversity of arthropods discovered to date (Grimaldi et al, 2002;Ross et al, 2010). The significance of Burmese amber for understanding Cretaceous biotic evolution cannot be overestimated, and even recently these mines have revealed the earliest Palpigradi (Engel et al, 2016b), Neophasmatodea (Engel et al, 2016c), diverse ants and termites (Barden and Grimaldi, 2016;Engel et al, 2016d;Perrichot et al, 2016), as well as amber-preserved vertebrates within Squamata (Daza et al, 2016).…”

Caste development and evolution in ants: it's all about size

“…The last common ancestor of extant ants likely possessed morphologically distinct worker and queen castes, but many species have subsequently gained or lost castes (Molet et al, 2012;Keller et al, 2014;Barden and Grimaldi, 2016). Additionally, the degree of queen−worker dimorphism has increased in some lineages of ants (Keller et al, 2014).…”

“…The ant fossil record contains multiple extinct lineages of early ants with winged queens and wingless workers, so it is likely that the most recent common ancestor of modern ants also possessed these two castes (Barden and Grimaldi, 2016). Many researchers have discussed how various groups of ants depart from this ancestral system, for example via the loss of the worker or queen caste or the evolution of novel castes from workers, queens or intercastes (worker-queen intermediates) (Emery, 1894;Wilson, 1953;Oster and Wilson, 1978;Bourke and Franks, 1991;Wheeler, 1991;Nonacs and Tobin, 1992;Urbani and Passera, 1996;Ward, 1997;Urbani, 1998;Molet et al, 2012;Peeters, 2012;Rajakumar et al, 2012).…”

Evidence for social parasitism of early insect societies by Cretaceous rove beetles

“…Yet, gauging whether this type of symbiosis was a bona fide feature of early ant and termite ecology has proven challenging for several reasons. First, although termites are believed to have evolved in the Late Jurassic2021, and ants somewhat later in the Early Cretaceous22232425, their frequency in fossil deposits implies that both taxa remained rare for much of their early evolution. Each group comprises <1% of all insect fossils in any given Cretaceous locality202224, and only in the Cenozoic do both groups increase dramatically towards their modern abundances.…”