The silvery rays of a bright moon silhouetted a small carnivorous dinosaur moving silently among the stalks of giant reeds bordering the forest. The night air was filled with the croaking of frogs, chirping of tree crickets, and rustling sounds of leaves in the wind. With large eyes adapted to low light intensities, the activities of a rodent-like mammal feasting on seeds in the undergrowth were easy to detect. The dinosaur crouched down and slowly began to stalk the unwary prey. When it heard the bending and snapping reeds, the quarry stopped chewing and immediately darted to the nearest tree. The predator followed in close pursuit and as the mammal began scurrying up the trunk, the dinosaur's grappling claws tore out some of the animal's hairs even as the prey managed to scamper high into the upper branches. The detached hairs drifted down into a blob of resin on the bark of the tree, leaving behind a lasting memento of this drama.
The commotion attracted the attention of squadrons of mosquitoes hovering in the vicinity of a nearby salt marsh. Most of the females had already laid one batch of eggs and now needed another blood meal for a subsequent one. As the dinosaur peered hopefully upward at what would have been a meal, a cloud of sanguinary mosquitoes alighted unobtrusively on its body and began probing the thin skin separating the tuberculate scales. Most settled on the back and shoulders, while a few selected areas around the eyes and ear openings. With surgeon-like precision they began piercing the capillaries and sucking up blood. The bites were ignored and after three or four minutes, the insects withdrew their stylets and laboriously flew to some lower branches to digest the meals. The on slaught was to continue throughout the night hours as others avidly took their places.
Mosquitoes are uncommon as fossils, even in recent amber from the Dominican Republic,190 and only a single uncontested specimen has been described from the entire Cretaceous191 (color plate 11D). Another possible representative from Early Cretaceous deposits of England35 suggests that these bloodsuckers occurred throughout the period.
By the Late Cretaceous, their hosts would have included mammals, birds, reptiles, and amphibians, as well as dinosaurs. Primitive species could have developed in salt or brackish water and preyed on vertebrates in the same habitat, similar to those that now attack sea turtles in Florida and South America.192193 The bloodsuckers seemingly arrive out of nowhere when the first turtles appear, then swoop down and eagerly feed on the hapless giants as they laboriously dig holes and deposit their eggs. The stoic turtles show no signs of irritation from the hundreds of ravenous insects simultaneously imbibing their blood. When satiated, the mosquitoes retreat to various dune plants and rest until their meal is digested. This same scenario probably was played out innumerable times along the ancient coastlines of epicontinental seas, including those near the Canadian amber site.
Just being attacked by massive numbers of them can be dangerous. When enormous swarms descend on humans, half of their blood can be removed in just two hours.194 A small dinosaur hatchling could have been exsanguinated in less time. At least five genera of mosquitoes attack lizards today. Most of these are opportunists, meaning that they also dine on man and other mammals, birds, and amphibians.195-197 But more important than removing bodily fluids are the microorganisms they add to the circulatory system of their victims. These hematophagous insects appear to have no problem penetrating the scaly surface of reptiles, and no trouble leaving and acquiring various microorganisms.
Malaria is the single most important disease carried by mosquitoes, not just to humans but to birds and reptiles as well. Some 1.6 billion people are at risk and about a million die each year from the disease.194 Could malaria have played a role in the disappearance of the dinosaurs? Mosquitoes currently transmit some 29 species of Plasmodium malaria to reptiles, but the infections appear to be tolerated.197-200 However, in the Cretaceous, when arthropod-borne malaria was a relatively new disease, the effects on dinosaurs could have been devastating until some degree of immunity was acquired.
I can personally vouch for the ability of mosquitoes to both torment and infect victims during the dead of night. When out searching for medically important insects in the Ivory Coast, West Africa, our research team spent many nights in the bush. We each carried a hammock equipped with mosquito netting on all sides. If, after securing both ends to tree trunks, you could enter quickly enough to leave the mosquitoes behind and you didn't mind the constant humming of the hungry females right next to your head all night, then perhaps you might get a few hours sleep. But if the oppressive heat and humidity made you toss and turn, and your sweaty arm touched the mosquito netting, you awoke the next morning with twenty to thirty bites, evidence of a mosquito banquet. That is exactly how I became infected with malaria and learned the drug I was taking (chloro-quine) was not very effective against newly mutated strains the mosquitoes were carrying. Enervating bouts of fever and shivering showed me how serious this disease was, and without further treatment I may have become one of the million or so fatalities this disease claims every year. With such a high mortality rate, it was understandable how the sickle-cell trait, which conveys some immunity against malaria, became incorporated into the African population. Just how many of our laboratory workers had this gene was impossible to know, but they certainly were able to survive their malaria attacks, which at most caused them to remain at home one or two days every fortnight.
Scores of arboviruses are presently carried by these bloodsuck ing pests,194 and some of them have a great capacity for mutating and forming new virulent strains. West Nile virus was introduced into the New York area in 1999, and by 2004 had spread across North American, killing incalculable numbers of birds, mammals, and reptiles along the way.201202 Birds, who appear to be especially susceptible to the virus, are considered to be the major dispersal agents. Symptoms in humans include high fever, internal bleeding, encephalitis, and death. To make matters worse, once established, infections can be spread by animals eating virus-contaminated meat. In one case, over a thousand captive alligators died after eating meat tainted with West Nile virus.202 Considering the potential mutability of the West Nile virus and both the insect and oral transmission routes, we can expect a still-wider range of vertebrates to be infected. Especially when so many opportunistic mosquitoes that feed on a smorgasbord of reptiles, amphibians, birds, and mammals could help spread the disease. Fortunately, the West Nile virus is not presently highly virulent for humans, since if it was, the number of deaths could equal those of the plague during the Middle Ages.
One of the most notorious arthropod-transmitted viruses causes yellow fever. This pathogen is so lethal that if we were unable to control the vectors or vaccinate the population, there would be large uninhabitable areas throughout the globe. Yellow fever virus is restricted to primates and apparently was acquired from African monkeys, which incidentally do not show symp-toms.194 When untreated, mortality reaches 80% in some areas in Africa, and even with a vaccine, it is estimated that up to 200,000 new cases occur each year. Similar types of arboviruses undoubtedly wreaked havoc with dinosaurs, causing epidemics that debilitated entire populations.
Cretaceous mosquitoes also theoretically transported parasitic nematodes, especially those causing diseases known as filariasis. As with similar nematodes in lizards and crocodiles today, the parasites probably entered the dinosaur's connective tissues, muscles, lungs, heart, blood vessels, or skin. An impressive num ber of filarial nematodes carried by mosquitoes now infect birds and mammals.173 One that lives in humans collects in the lymphatic system and can cause enormous swelling of the limbs and other body parts, resulting in a disease called elephantiasis.194 The best-known filarial nematode in the temperate zone is the dog heartworm, which lives in the pulmonary artery and heart. The wide range of animals that are parasitized by mosquito-borne nematodes suggests that in the past dinosaurs were similarly infected. Although it is difficult to say just what symptoms they would have had, filarial worms can seriously damage the viscera of reptiles.203 If dinosaurs suffered as much as modern mammals do, the resulting illnesses were quite incapacitating, leaving the victims easy prey for carnivores.
Presently, mosquitoes occur throughout the entire world (except Antarctica) and survive under a broad range of climatic regimes and habitats. While the great majority require a source of standing water for the development of their larvae, these habitats can range from large lakes, ponds, and small enclosed pools in tree holes and rock formations to miniscule amounts of water in the bases of pitcher plants, epiphytic bromeliads, and even leaves. It seems that if there is any water around, there is a mosquito that can use it as a breeding source.194
Most engorge at night or around dawn and dusk; only a few are diurnal. They locate their prey by following carbon dioxide emissions or the release of oactic acid and octenol. Visual detection is particularly important when foraging in daylight, especially for those that attack in open areas. Some species feed in many different habitats, while others select wooded or unobstructed terrain or remain close to breeding sites. A few seek specific hosts while certain groups are opportunistic and attack victims belonging to three or more vertebrate classes. Within the amber forests, mosquitoes subsisted in all strata. There would have been no relief for dinosaurs since under the tropical and subtropical conditions of the Cretaceous, these bloodsuckers in all likelihood were active throughout the year.
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