Ants, Mites, and Ancient Resin: Cretaceous Amber Preserves Evidence of 99-Million-Year-Old Relationships
Amber preserves the past with a specificity no other fossil medium can match. A piece of resin that hardened 99 million years ago can still hold the individual hairs of an insect's body, the iridescence of wing venation, and - in rare cases - multiple organisms caught together in moments that may have been the last moments of their lives. These multi-organism inclusions, called syninclusions, are rare. Interpreting them is harder still.
When two organisms appear side by side in amber, the obvious interpretation is that they were interacting when the resin engulfed them. But that is not necessarily correct. Two insects that happened to be in the same tree at the same time, or that stumbled into the same pool of resin independently, might end up preserved centimeters apart with no biological relationship between them at all. Distinguishing genuine interactions from coincidence requires a systematic approach, and that is what Dr. Jose de la Fuente of the Institute for Game and Wildlife Research in Spain and colleagues attempted in a study published in Frontiers in Ecology and Evolution.
Six Pieces of Amber, Three Geological Periods
The team selected six amber samples specifically because each contained multiple species - and specifically because each contained ants. Ants are considered keystone organisms in ecosystems: they redistribute nutrients, protect aphids, prey on other insects, and serve as prey themselves. Understanding how ants interacted with other species in ancient environments provides a window into how those ecosystems were organized.
The six samples spanned a substantial geological range. Four came from the Cretaceous period, approximately 99 million years old. One was Eocene amber, from roughly 56 to 34 million years ago. One was Oligocene, from approximately 34 to 23 million years ago. This span allowed the researchers to observe ant lineages from two distinct evolutionary groups: Stem ants, the earliest forms with no modern descendants, and Crown ants, the ancestors of all living ant species. A third group, Hell ants, which evolved from Stem ants and are known only from fossil evidence, also appeared.
Using powerful microscopes, the team identified each species present in the samples and measured the physical distances between organisms. The logic is straightforward: insects that are in direct contact or within a few millimeters of each other are more plausibly interacting than those preserved centimeters apart with other material between them.
Ants and Mites: The Strongest Signal
Three of the six amber pieces contained ants and mites in close proximity. In the first case - a Crown ant, a wasp, and two mites - the mites were positioned so close to the ant that they may have been travelling on it when the resin fell. In another piece, a Stem ant and a single mite were approximately four millimeters apart. A third sample held three ant species alongside a mite and termites, with poorly preserved mosquitoes also present.
The ant-mite proximity in multiple independent samples is not easily dismissed as coincidence. Published evidence from other amber sources supports the existence of relationships between mites and ants, sometimes mutually beneficial. One scenario is phoresy - a commensal relationship in which mites attach to ants for dispersal to new habitats, receiving a free ride without harming their carrier. Another is parasitism, in which mites feeding on the ant during transport would benefit at the ant's expense. The amber cannot definitively distinguish these, but advanced imaging techniques such as micro-CT scanning could reveal whether mites in these samples carry attachment structures designed for clinging to insect hosts.
Where Caution Is Required
The study also documents a case where apparent proximity is almost certainly coincidental. In one sample, a Stem ant rests against what might be a worm or larva, but the researchers find no evidence of interaction and interpret this as an accidental co-entrapment. A spider in another piece belongs to a species known to camouflage itself as an ant, which raises the question of whether its proximity to a real ant reflects predatory strategy or mere chance.
De la Fuente calls for explicit caution: insects that are not in contact could simply be unrelated organisms that fell into the same resin pool. The amber record captures moments, not relationships, and inferring biology from preserved proximity requires careful reasoning about what other evidence - morphology, phylogeny, known behavior of related living species - can support.
The six samples studied are exceptional for what they preserve. Amber pieces containing multiple species are rare. Amber pieces containing ants are rarer still. And samples containing ants alongside organisms that may have been interacting with them in life are rarer again. The study provides a methodological framework for analyzing what those rare specimens can tell us, and identifies the next imaging tools needed to go further.