A mosquito’s last supper reminds us not to underestimate the fossil record

Years ago, I gave a conference paper in San Diego with one eye completely closed by a mosquito bite. Mosquitoes command attention, like other biting arthropods, because of the discomfort they cause and their role as disease vectors. Mosquitoes also played a critical role in the most famous dinosaur movie of modern times: the last meal of these blood suckers was the source of the dinosaur DNA that populated the fictional Jurassic Park 20 y ago. Therefore, the PNAS report of a 46-My-old mosquito with blood in its abdomen (1) is arresting, but its significance lies in emphasizing the importance of unusual preservations. We need to understand the taphonomy (the process of fossilization) of such fossil deposits to interpret the information they provide (Fig. 1) (2).

Fig. 1.

(A) Large insects succumb to decay and predation while tiny ones sink and are buried in the seasonal varves. (Prepared by J. Hall.) (B) The mosquito is flattened in the oil shale while the protein component of the blood hemoglobin decays, leaving the heme which is modified slightly during fossilization. (Live mosquito image from Centers for Disease Control and Prevention; hemoglobin image courtesy of Richard Wheeler, licensed under the GNU Free Documentation License.)

The chances of postprandial preservation of a delicate insect like a mosquito are vanishingly small. There are >3,500 living species of mosquito (Family Culicidae) (3). A recent molecular phylogeny calibrated by rare fossil occurrences indicates that mosquitoes radiated in the early Cretaceous, but probably first appeared much earlier (4). Nonetheless, just 25 fossil species are recognized. The oldest example is from mid-Cretaceous (93.9–100.5 Mya) amber from Burma; granular material in its gut may represent a blood meal, but it has not been analyzed (5). The second, somewhat younger (72.1–83.6 Mya), Cretaceous specimen is a male in amber from Alberta, Canada (6). Without these two occurrences, each single specimens, there would be no Mesozoic record of mosquitoes. Earlier examples are unknown, and they would be difficult to recognize, because the adults had yet to evolve the characteristic long proboscis (7). Although mosquitoes coexisted with dinosaurs, other ectoparasites such as the giant flea-like insects from the Jurassic and Cretaceous of China (8) are more likely to have bitten them. Modern mosquitoes appear after the Cretaceous, in the Eocene (7).

Eleven species of fossil mosquitoes are known from amber (3), but specimens are never abundant; mosquitoes were presumably rare in the forests of resin-producing trees. Most fossil mosquitoes are preserved in lake sediments, reflecting their reliance on water for reproduction. The newly discovered blood-engorged specimen is from Eocene sediments of the Kishenehn Formation that crop out along the Flathead River in Montana (9). Insects were first discovered there by Kurt Constenius, when he was a young geology undergraduate in the early 1980s (10). Most of the Constenius family collection was donated to the Department of Paleobiology at the Smithsonian Institution of Natural History, a generous gift that has started to yield dividends as the remarkable fossils are investigated (9). The insects occur in a laminated oil shale, the individual laminae, or varves, up to 0.25 mm thick, representing sediment deposited during a single season. The fossils, which are exposed by splitting the shale, are often so small (typically 1–5 mm in length) that they can only be detected with a lens or microscope, and they are best photographed immersed in ethanol, which increases the contrast.

Even though the climate in Montana was tropical to subtropical during the Eocene, the insects represented in the Kishenehn Formation are not particularly diverse (9). Nearly half of them are chironomids (nonbiting midges), and we can imagine clouds of them swarming over the surface of the lake. The second most abundant category is water boatmen (Corixidae), which lived in the lake or the marshy areas surrounding it. The familiar larger terrestrial forms, such as dragonflies, butterflies, and moths, are very poorly represented and only by fragments; presumably, their bodies rarely reached the lake, and those that did decayed or were eaten by fishes such as suckers (Catostomidae), which are the most common fossil fish in the unit (10). Small insects are trapped by surface tension, and their wings adhere to the surface of the water, where they normally decay and disarticulate (11) before sinking. It takes special circumstances to get tiny insects like those in the Kishenehn Formation to the floor of the lake intact. Sometimes rain or storms disturb the surface, but they also tend to damage delicate carcasses. Thus, the list of insects preserved, even in an exceptional fauna like that of the Kishenehn Formation, is biased by the various factors involved in the transition from living insect to fossil (Fig. 1).

The insects in the Kishenehn Formation occur in darker organic-rich layers, which accumulated during the summer months (9). Microbial mats may have formed as seasonal blooms on the surface of the water and trapped the insects, carrying them intact to the lake beds as the mats sank. Other examples of beautifully preserved insects in lake settings have been explained as a product of sinking mats, including those from the famous Florissant Fossil Beds of latest Eocene age in Colorado (12). Alternatively, insect carcasses may have adhered to mats that grew on the lake floor, as inferred for the beautiful Miocene insects of Rubielos de Mora near Teruel in Spain (13). Preservation was facilitated by quiet anoxic conditions, and although the deposit preserves remarkable morphological details, including color patterns, only the tiniest insects survived. A number of new species await description, but many of the fossils are strikingly similar to species living today (9).

Just as the preservation of tiny insects depends on exceptional conditions of fossilization, so does the survival of chemical components of their tissues. Beetle cuticles with relatively intact chemistry have been reported from 25-My-old lake sediments from Enspel, Germany (14). The cuticles of older fossil arthropods are composed largely of longer chain hydrocarbons, which form over time by a polymerization process (15). Thus, the cuticle of the 46-My-old Kishenehn Formation insects is likely to be altered, although synchrotron analyses of older arthropods indicate that traces of chitin and protein persist back into the Paleozoic. DNA is much more decay prone and does not survive over geological time scales. Early reports of DNA from amber proved overly optimistic. Subsequent investigations of the cuticle of Dominican amber beetles showed that even the chitin component is significantly altered (16). The idea that blood could be preserved in the dark-colored mosquito abdomen from the Kishenehn Formation seems fantastic.

Red blood cells consist mainly of hemoglobin. Hemoglobin combines a protein with heme, the porphyrin ring structure that accommodates the iron involved in oxygen exchange. Heme as been detected in extracts of late Cretaceous dinosaur bone, using a variety of analytical techniques including NMR and Raman spectroscopy, as well as with immunological methods (17), and blood cells may also survive (18). The protein component of the hemoglobin in the mosquito’s last meal was likely lost through decay, so the authors (1) used nondestructive methods (necessarily on a unique specimen) to test whether the heme survived. Much higher concentrations of iron are present in the female mosquito abdomen than elsewhere, as would be expected if heme were present. More importantly, time-of-flight secondary ion MS (ToF SIMS) yielded a trace very similar to that for heme-derived porphyrins in pig hemoglobin. Porphyrins commonly occur in oil shales, but they are usually sourced from plants and are preserved as metal complexes, which form at elevated temperatures. The preservation of heme is unlikely to be a common occurrence, but ToF SIMS provides a new method to search for it, where morphological evidence suggests it might be present.

New discoveries, like the well-fed mosquito from the Eocene of Montana (1), force us to revise our ideas about the limits of fossilization. Investigating the conditions of preservation indicate what new information might result from future excavation and collecting, and novel applications, like ToF SIMS in this case, emphasize how future technological advances will yield data that we cannot yet imagine. These new insights will come not only from newly discovered fossils but also from those that already reside in museum and other collections around the world.