Among social insects, army ants are exceptional in their voracious coordinated predation and nomadic life history: the synthesis of these remarkable ecological traits is referred to as the army ant syndrome. Despite molecular evidence that the army ant syndrome evolved twice during the mid-Cenozoic, once in the Neotropics and once in the Afrotropics, fossil army ants are markedly scarce, comprising a single known species from the Caribbean 16 Ma. Here we report the oldest army ant fossil and the first from the Eastern Hemisphere (EH), Dissimulodorylus perseus, preserved in Baltic amber dated to the Eocene. Using a combined morphological and molecular Ultra Conserved Elements dataset spanning doryline lineages, we find that D. perseus is nested among extant EH army ants with affinities to Dorylus “driver ants.” Army ants are characterized by limited extant diversification throughout most of the Cenozoic; the discovery of D. perseus suggests an unexpected diversity of now-extinct army ant lineages in the Cenozoic, some of which were present in Continental Europe. 

Specimen

Type material is preserved within a ~1cm x 0.8cm piece of transparent Baltic amber with crazing (surface cracks). It is possible that a previously applied resin meant to affix the specimen to a microscope slide contributed to intense crazing (Sadowski et al. 2021). Specimen PALE-8463 is deposited within the Museum of Comparative Zoology (MCZ) at Harvard University. The precise collection date is unknown, however, the specimen number is recorded in a ledger titled ‘Catalogue of Fossil Insects in the Museum of Comparative Zoölogy’ on the same page (98) as a specimen received in 1937; the following page (99) also contains two entries dated 1937. In addition, the method of storage (the specimen was initially mounted to a microscope slide, a practice no longer followed) and aging on the specimen label are in line with this approximate date. The age of Baltic amber is not precise and likely varies depending on outcrop, however, an Eocene age (~34 Ma) has been suggested through multiple lines of evidence (Seyfullah 2018; Wolfe 2009).

Photomicrograph and microCT Imaging

The specimen was not prepared further to avoid damage due to crazing or chemical reactions with previously applied polymers. Microphotographs were taken using a Nikon SMZ25 stereomicroscope and DS-Ri2 camera with NIS Elements software, which generates extended focus composites of several individual focal plane images. X-ray micro-computed tomography (microCT) scanning was performed at the New Jersey Institute of Technology Otto H. York Center for Environmental Engineering and Science using a Bruker SkyScan 1275 microCT scanner. The type specimen was scanned at a voltage of 34kV and current of 160μA for 110ms exposure times averaged of 5 frames per rotation with a voxel size of 5.94μm. Z-stacks were generated using NRecon (Micro Photonics, Allentown, PA) and segmented using 3D Slicer v5.1 (Federov et al. 2012). An .stl file of the segmented 3D model was rendered using Blender v3.2.1 (Blender 2008).