Controls on the sinking of hymenopterans and orthopterans in water of varying salinity: a taphonomic study

Insect fossils are numerous and widespread across the Phanerozoic in various states of preservation that can include colour, fully articulated specimens, and three-dimensional preservation. Due to the naturally buoyant nature of most terrestrial insects, it is surprising that so many become fossilized in lacustrine and coastal settings, and it is unclear how various processes involved in sinking differentially bias fossil insect collections. As informed by the fossil record, a series of experiments were designed to examine how salinity, disarticulation, and surface tension impact the likelihood of sinking for Teleogryllus oceanicus (oceanic house cricket), Bombus spp. (bumblebee), and their individual hindlegs and forewings. Most specimens sank readily in freshwater and saltwater when forcibly submerged; however, only cricket hindlegs sank in hypersaline water. Individual hindlegs and wings sank within hours, while whole bodies sank after days of decay. When left floating undisturbed on water surfaces, only hindlegs and cricket bodies in freshwater sank at some point within 75 days of decay. Photographed orthopteran fossils from Green River, Colorado, were categorized by body part to compare the biases in fossil collections with our experiments: isolated hindlegs comprised 20% of the categorized fossils. These results show a bias in the fossil insect record where very few isolated legs are found compared to bodies even though hindlegs sank readily in water of all salinities.