Zhangpu Amber

A summary of the Zhangpu ant fauna has been provided by Wang et al. (2021, supplemental material).

Ants (Formicidae) are the most diverse eusocial insects in Zhangpu amber (Fig. 4A, B and figs. S7 and S8). They are among the most successful groups of insects and can comprise up to 15–20% of the animal biomass in tropical forests (59). Our preliminary investigation reveals at least 65 morphospecies in 41 genera and nine subfamilies, which outnumbers the ant diversity known from Dominican amber despite its much longer history of study (59). The increased percentage of ants in Zhangpu amber (12% of all insects) over Eocene ambers (Oise amber <2.5%, Baltic and Fushun ambers ~ 5%), and on a par with Mexican amber (9%), is in accordance with the general pattern of increasing ant abundance during the Cenozoic (59). Not surprisingly for a Miocene-aged ant assemblage, the ‘big four’ subfamilies that are dominant in ecosystems today are also prevalent in Zhangpu amber. Myrmicinae are largely dominant (65% of all ants, 16 genera identified), followed by Dolichoderinae (9%, four genera), Ponerinae (8%, seven genera), and Formicinae (6%, seven genera). So far, all Zhangpu ants belong to extant genera, in contrast with Dominican amber where 10% of the recorded genera (three out of 31) are extinct (60), perhaps owing to the insular nature of the latter paleofauna, and with Shanwang fossils where 35% of the genera (seven out of 20) have since vanished (61). Overall, the ant community of Zhangpu amber is most similar to that found today in tropical rainforests from Southeast Asia and eastern Australia, particularly the dipterocarp forests of Borneo, the Philippines, and Malaysia. The record of Leptomyrmex (Fig. 4A) is particularly remarkable, as the genus is now absent from this bioregion, but is distributed in Australasian mesic forests and Brazilian dry savannah (62). Our findings highlight a closely related geographical distribution of Leptomyrmex as well as other Australasian ant genera such as Lophomyrmex (Fig. 4B) and Myrmicaria, with that of the dipterocarp forests.

The remaining taxa comprise 1–3 genera in each of the subfamilies Amblyoponinae, Dorylinae, Ectatomminae, Proceratiinae, and Pseudomyrmecinae. Several genera found in Zhangpu amber also occur for the first time in the fossil record, namely: Aenictus (fig. S7A), Cerapachys (fig. S7B), Yunodorylus, Prionopelta (fig. S7C), Cardiocondyla (fig. S8A), Gauromyrmex, Lophomyrmex (Fig. 4B), Lordomyrma, Meranoplus (fig. S8C), Myrmicaria, Proatta (fig. S8D), Brachyponera, and Odontoponera. All these records will constitute important new points of fossil calibrations that will allow for a critical reevaluation of divergence time estimates in molecular phylogenetic analyses.

The two genera Pheidole (fig. S7B) and Carebara (fig. S8B) are the most abundant taxa by far (approximately 20% of all ant individuals each), and for both genera several amber pieces even revealed portions of colonies with up to 50 syninclusions of minor and major workers as well as pupae and larvae. Several of the Pheidole ‘colonies’ also contained one individual of Yunodorylus, suggesting a possible ecological association of these two genera nesting in soils. Other frequently encountered genera include Dolichoderus, Tapinoma, Crematogaster, Lophomyrmex (Fig. 4B), Tetramorium (fig. S8E), Brachyponera, Leptogenys (fig. S8D), and Tetraponera (fig. S7F). Arboreal and/or epigaeic taxa are dominant, as is commonly the case in amber deposits, but diverse hypogaeic taxa also occur, such as Cerapachys (fig. S7B), Yunodorylus, Carebara (fig. S8B), Strumigenys (fig. S8F), Hypoponera, Discothyrea (fig. S7E), and Proceratium. Interestingly, several specimens are fossilized in a behavioral position similar to that of their living relatives: some Crematogaster have their gaster curled forwards over the mesosoma, which is typically a repelling behavior (63); and some Cataulacus are huddled, which is a potential response to an aggressive signal after a glide or a dropping off (64). This suggests that these ecological behaviors are highly conservative through time.