THE INFLUENCE OF DEHYDRATION AND GUT BIOTA ON PRESERVATION IN AMBER

The quality of preservation of insect inclusions in amber is highly variable, ranging from the retention of high fidelity anatomical details of soft tissues through to empty molds of cuticle. What controls this variation is not understood, and consequently biases in the amber record of fossil insects cannot be assessed. Here we use actualistic taphonomic experiments to test the influence of dehydration and gut biota on fossilization in amber. Fruit flies (Drosophila melanogaster) were embedded in resin from a Wollemi pine (Wollemia nobilis) for 14 days after which external and internal morphological decay was assessed through phase-contrast synchrotron tomography. We tested three states of dehydration: entombed without dehydration; partially embedded in resin, dried for three days at 50°C, and then covered in more resin; and dried in air for three days at 50°C prior to complete entombment. In addition we tested the role of gut microbiota through using fruit flies with “wild-type” microbiota, and antibiotic-treated fruit flies, with a severely reduced gut biota. We ran six replicates of each of the treatment groups. Synchrotron tomographic analyses revealed that dehydrated flies (both groups) were better preserved than fresh flies. Dehydration has been previously proposed as a mechanism of exceptional preservation in amber, and our results support this. Dehydration inhibits bacterial decay, therefore enhancing preservation. However, counterintuitively, flies with the wild-type gut biota were better preserved than flies treated with antibiotics to reduce their gut microbiota. This suggests that the antibiotic treatment, which ceased when the flies died, did not completely eradicate the gut biota. After death, any remnant bacterial populations quickly rebounded and decayed the fly. The rebound population may have been composed of different bacteria than the wild-type population, which could explain the enhanced decay rate. The variations in preservation we observed in these experiments matches some (although not all) of the variation seen in fossils in amber, suggesting that these experiments do replicate pathways of fossilization in amber and that dehydration and the composition of the gut biota are both important controls on the process of fossilization in amber.