What Bugged the Dinosaurs?: Insects, Disease, and Death in the Cretaceous

Introduction

Introduction The surviving members of a herd of ornithopod dinosaurs grazed along the edge of a Cretaceous conifer forest. It had been a particularly hard and long dry season. A few individuals fed sporadically on the parched sedges and horsetails growing near the banks of a meandering river. The river had dwindled down to a trickling stream flowing between high-cut banks. Once numbering in the hundreds, disease had now reduced the herd to less than fifty—the very young and the aged were conspicuously absent. Many of the majestic animals appeared lethargic and even were oblivious to the pack of predatory theropod dinosaurs that followed . Those fierce carnivores, armed with sharp teeth and sicklelike claws, were fat and satiated because sick herbivores made easy prey. Ordinarily, these plant-feeding dinosaurs spent at least half of their waking hours chomping on the tender vegetation sprouting around streams or in open meadows. However, with waning appetites , the infected animals had not fed for days and stumbled along in a debilitated state. They normally avoided the direct rays of the afternoon sun, but now many stood motionless in its intense heat. Frequently they would shuffle down to the waters edge, laboriously bend down, and drink for long periods, apparently forgetful of the dangers posed by lurking crocodiles. Persistent diarrhea had dehydrated them and their thirst was almost insatiable. The surrounding terrain was discolored by bloody stools that attracted hordes of flies and beetles. One trembling ornithopod, with dry skin clinging to prominent ribs and vertebrae, staggered off to one side and began to vomit strands of bloodstained mucus filled with glistening, writhing roundworms . With eyes now reduced to narrow slits, the sick individual was too exhausted to dislodge the ravenous masses of annoying insects crawling over his thin scaly skin while seeking sites to engorge themselves. When the dying animal finally collapsed, a few members of the herd came over and nudged him, but there was no response and they moved off, giving way to the advancing theropods. The carnivores started tearing away at the carcass, not realizing that they were eating infected meat and being attacked by the same insects that had previously fed on the diseased dinosaurs . Several of them, however, were beginning to show the first signs of infection and withdrew from the feeding frenzy to lie down and rest after only a few mouthfuls. As the others were devouring the remains, a contingent of mites and ticks seized the opportunity to move their residence from the corpse onto the skin of the theropods. If an autopsy had been made on this ornithopod, it would have revealed many parasites and pathogens inhabiting the tissues. Some, like amoebic dysentery, malaria, and ascarid roundworms , would have caused lesions in the gut, liver abscesses, and distorted blood cells. But the actual cause of our dinosaur’s death would have been listed as leishmaniasis, a protozoan disease . Just like the other members of the herd, he was the victim of an emerging pathogen that was decimating the Cretaceous world. Some 100 million years ago, some of these microorganisms developed novel relationships with biting flies, when the flies’ previously harmless symbionts turned into deadly pathogens . In an unprecedented alliance, these insect-borne infections together with already long-established parasites became more than the dinosaurs’ immune systems could handle. Sweeping epidemics began changing the herbivore-carnivore dinosaur balance that had existed for millennia. Armed with their deadly weapons, biting insects were the top predators in the food chain and could now shape the destiny of the dinosaurs just as they shape our world today. Even as the remaining members of this herd succumbed to disease , insects were busy ensuring that the epidemic would spread. Biting flies, sucking the blood of the infirm, were collecting path2 – INTRODUCTION ogens to inject into other victims. Because of their ability to fly, they could disperse and infect other susceptible dinosaurs within their range. Flies, beetles, and cockroaches visiting the infested feces and cadavers picked up bacteria, protozoans, and nematodes that were then carried to contaminate other vertebrates . Dinosaurs that dined on cockroaches now carrying eggs of ascarids would end up with stomach lesions. On a larger scale, as the outbreak killed off the herbivorous dinosaurs , the balance of their ecosystem was destroyed. Carnivores may have initially benefited because the dead and dying were plentiful. The downside of this apparent bounty was that they too were becoming ill as their food supply was dwindling. In the following...