Enormously numerous and ecologically dominant, ants today attract thousands of insect, plant, and fungal species which partner with them to receive protection or food. But this myrmecophily (Greek for “ant love”) is rarely evident in the fossil record, so its evolution is poorly understood. Scientists from Columbia University and the American Museum of Natural History recently discovered the oldest known myrmecophile, an amber-encased beetle in the supertribe Clavigeritae, which lived in the early Eocene Epoch when ants were rare but poised to proliferate.
Modern clavigerite beetles, found worldwide, are social parasites highly specialized for infiltrating ants’ well-guarded nests. Brushes on their abdomens, called trichomes, ooze a yet-unidentified chemical that ants love to eat. In return, they are fed regurgitated liquid, carried around within the nest, and not prevented from eating eggs and larval secretions. Other adaptations include rigid few-segmented antennae and rigidly-fused abdomens to reduce injury while being carried and tiny mouthparts recessed within the buccal cavity for easy mouth-to-mouth liquid transfer. Many are eyeless and wingless, able to live nowhere else.
Called Pseudoclaviger trichodens, the newfound species marks a transition between modern clavigerite beetles and their less-specialized relatives. It has eyes, trichomes, small but slightly-protruding mouthparts, no wings, a flexible-looking abdomen, and antennae with fewer segments than its ancestors but more than its probable dependents. Along with hundreds of other arthropod species, it was encased in roughly 52-million-year-old Cambay amber within a lignite coal mine in western India, the remnants of an Eocene rainforest. Genetic analysis of modern species, in the same study, indicates that this region may be Clavigeritae’s center of origin.
Ants appeared on Earth by the mid-Cretaceous Period (roughly 100 million years ago), but first became diverse and abundant in the mid- to late-Eocene Epoch (56-41 million years ago), as forests spread poleward in a rapidly-warming climate. P. trichodens provides evidence that specialized myrmecophiles had already evolved by then, exploiting them as they rose to dominate the lands of the world.
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