Tiny insects trapped in amber could tell us a great deal about their roles in past ecosystems: pollinators, parasites, predators, and prey. But how many of the insects preserved alongside each other reflect interactions during life, and how many are just unlucky coincidences? Scientists in Spain scrutinized six key samples which preserve now-extinct insects unusually well, to try to learn more about the ants that lived at the same time as the dinosaurs.
"Amber inclusions are representative of possible interactions between different organisms shaping the environment," explained Dr Jose de la Fuente of the Institute for Game and Wildlife Research, Spain, lead author of the article in Frontiers in Ecology and Evolution. "The identification and morphological characterization of fossil ants in amber with other inclusions of insects provides a snapshot of life on Earth millions of years ago."
Snapshots of the past
The scientists looked at six different pieces of amber which include multiple different organisms of different species, a rare phenomenon called syninclusion. They chose these pieces of amber because they include ants, which are considered particularly important to ecosystems. The earliest ants, which were first found in the Upper Cretaceous, are known as Stem ants and didn't leave modern descendants; all ants alive today evolved from Crown ants. Both species are found in the six pieces of amber studied by the scientists, as well as Hell ants, which evolved from Stem ants.
The study sample included four pieces of Cretaceous amber (around 99 million years old), one piece of Eocene amber (from approximately 56-34 million years ago), and one piece of Oligocene amber (from approximately 34-23 million years ago). The scientists used powerful microscopes to examine the amber, identify the different species found inside, and measure the distance between ants and other species.
In three of the six pieces of amber, the scientists found ants in close proximity to mites. In the first piece of amber, Case 1, the scientists found a Crown ant, wasp, and two mites so close to the ant that they may have been travelling on it. Similarly, Case 4 contained a Stem ant and a mite, about four millimeters apart. Case 5 also contained three different species of ant close to a mite and some termites, as well as poorly-preserved mosquitoes and a winged insect.
In Case 6 the scientists found a Stem ant alongside a probable parasitic wasp and a spider. The ant seems to have been feeding on something. It is resting against another insect inclusion, which could be a worm or a larva, but as there's no indication that the two were interacting, the scientists think this was a coincidence.
Finally, Case 2 contained a Stem ant and a spider, while Case 3 contained a Hell ant, a snail, a millipede, and some unidentifiable insects.
Wrong place, wrong time?
"The closest ant syninclusions are more likely to reflect behavior and interactions between these organisms," said de la Fuente. "The proposed ant-mite interactions in Case 4 may reflect two possible scenarios. First, a commensal specialized temporal relationship where mites attach to ants for free ride dispersal to new habitats. Second, a parasitism when mites feed on the ant host during transport."
Although pieces of amber that contain ants are rare, and pieces of amber that contain multiple species are rarer, there is some published evidence that points to interactions between mites and ants, sometimes mutually beneficial. Future research could help clarify this by using micro-CT scanning to look for attachment structures on mites which would have allowed them to clamber onboard ants for travel purposes. Similarly, the spider in Case 6 is a species which could camouflage itself as an ant and might have benefited from proximity to real ants.
The scientists say that smaller distances between insects in amber are more likely to reflect interactions during life, such as those between ants and mites. But they call for caution around inferred interactions: insects that aren't in contact could just be insects that got stuck in the same resin.
"To improve the analysis of interactions between different organisms in fossil amber inclusions, future research should use advanced imaging techniques," said de la Fuente. "Nevertheless, these results provide evidence of insect behavior and ecological habits."
