Sanitary Engineers of the Cretaceous

On an alluvial flood plain near the amber forest, a large or-nithopod dinosaur momentarily stopped grazing among the fern fronds, raised its tail, and defecated. Before the last of the waste even reached the ground, the air was crackling with the vibrating wings of thousands of dung beetles of all sizes and shapes. From large and round to small and oval, the beetles landed directly on or adjacent to the evacuated material. Not a moment was lost, and some began burrowing into the pile of waste even before the dinosaur had moved away.

In order to get a head start, a few small brown females began laying their eggs immediately in the mound of partially digested plant material. Their robust cream-colored larvae would dwell in the dung heap for several days, hopefully developing fast enough to avoid predacious rove beetles and slender parasitic wasps that would eventually be searching for them, along with pecking birds, scratching lizards, and other vertebrate predators.

A few dung beetle larvae ate some eggs of a parasitic nematode worm that had lived in the dinosaur's intestine. The nematodes would remain inside the beetles for life but could only finish their life cycle if another dinosaur came along and gobbled down the infested insects.

Some of the larger dung beetles were more protective of their progeny and after reaching the bottom of the pile, continued tunneling into the ground. No eggs were laid until the females packed the tunnels with dung fragments dragged in from above. The developing lavae would feed on these brood balls in relative safety since if the dung pile was broken apart by predators, the buried brood was hidden from view.

Other beetles never entered the dung mass, but broke off and molded small portions into balls that could be rolled far away from the original source. Using massive jaws to clip away a small portion of dung from the original pile and their front legs to shape the prize into a ball, the beetles turned around and used the two back pairs of legs to push the sphere along the ground. A latecomer suddenly appeared and attempted to steal a ball away from one of the females. She viciously defended her future brood cell, grabbing the opponent with her mandibles and flipping it over. After successfully fending off several additional marauders, the exhausted female dug a hole into which she rolled her prize, deposited an egg on the fecal mass, and buried them together by refilling the hole.

Soon mites and nematodes that had hitched rides on the adult beetles would begin multiplying in the remains and moving through and over the dung pile, encountering various stages of flies and beetles that were finishing their development and would provide transportation to yet another fecal deposit.

A group of immature garudimimids, looking like horny-crowned cassowaries, moved out of the forest and onto the plain under the watchful eyes of their parents. When they encountered the or-nithopod dung, they paused and began to scratch through the mass, using their hind legs, sending feces flying while pulling the pile apart. The strong claws at the end of each toe sliced through the soft fibrous material and exposed scurrying adult beetles and unwary grubs which they snatched up in their toothless beaks. After only a few minutes of feeding on the exposed plain, they rushed off to seek cover in a nearby patch of shrubs.

Deep in the forest, an old, infirm dinosaur took a final breath and slowly sank to the ground, leaving his body to the whims of nature. In less than a minute, several large carrion flies that had detected death in the air landed on the still body and began depositing masses of eggs on the face, especially around the mouth opening.

Within the day, white legless maggots hatched from those eggs and began squirming into the mouth cavity, creating a hot, fetid environment that other insects couldn't tolerate. Eventually, the wriggling larvae finished feeding and left the cadaver, crawling some distance away before burrowing into the soil to pupate.

As the corpse dried, small brown beetles began feeding on the skin, dried flesh, and even on the surface of some of the bones. The eggs these oval insects laid hatched into minute larvae with long, stiff hairs that served both as camouflage and defense from predators on the lookout for insect prey. These beetle larvae consumed all the remaining flesh over the next few days, even the desiccated strips flattened against the exposed bones. After a short pupation period, the adults emerged, mated, and the females sought out new cadavers.

The world in the Cretaceous would have been a fetid mess without insects. Can you imagine putrefying dinosaur corpses littering the landscape, heaps of dung remaining for months on end, and dead vegetation taking forever to be recycled? Eventually microbes, earthworms, and other scavengers would have decomposed this waste material, but certainly without the assistance of insects, parts of the prehistoric world would have been almost uninhabitable for many animals.

Dinosaurs, like a majority of animals today, voided solid waste. Certainly "waste" is a poor term since animal dung actually contains enough nutrients to nurture a wide range of smaller creatures like beetles, flies, earthworms, nematodes, and even miniscule mites. Defecation is a normal, everyday physiological act, although the end product may serve other functions. For example, hippos, rhinos, and quite a few other animals will scatter their dung, sometimes even scraping it into the ground, as a territorial marker, and probably many dinosaurs did the same. Once on the ground, feces from terrestrial vertebrates is processed by insects and a variety of other organisms. Waste disposal by arthropods is quite a complex process, especially when large amounts are deposited. For example, in the case of cow-pats, a succession of insects are attracted to the material, with each group preferring dung at a particular stage of "ripening",105 just as there are people who prefer aged cheese over fresh.

Most coprophagous insects are beetles and flies, many of which are represented in Cretaceous amber. These insects often bring along mites, nematodes, and other creatures on their bodies, and sometimes parasites within. Further, some drawn to a heap are not interested in the product as such, but have come to prey on others feeding there. So dung is a microcosm containing quite a hodgepodge of creatures, each with their own agenda but all playing important roles in the breakdown of this resource.

To illustrate how important a balanced dung ecosystem is and why it must also have been functioning millions of years ago, one can look to Australia. Native dung beetles had become adapted to relatively small and specialized kangaroo and dingo feces. Everyone was surprised when a sanitary problem arose after introduced cattle populations reached commercially sustaining levels and began producing more solid waste than beetles could handle. The cowpats began to pile up, eventually paving over many of the pastures, and the situation became intolerable. This imbalance benefited the bush flies since they essentially had the feast to themselves, and they took full advantage of the situation. The result was huge fly populations that, curiously enough, became very attracted to people's faces.106

Needless to say, a considerable amount of money and manpower was invested to eliminate this problem by searching for dung beetles throughout the world—and not just any beetles, but only those that dined on cow waste. Finally, candidates were found in Africa and introduced into the southern continent. A portion of these adapted to Australia and settled in to do their business. Over time, the cowpats receded, the fly populations dropped, and the Australians were no longer bombarded by pesky flies. (In North America, some dung-breeding flies, like the face fly, are attracted to the heads of grazing animals. Certainly, dinosaurs also had their own types of face flies.)

It doesn't take much to imagine that similar beetles served as sanitary engineers for the large herds of herbivorous dinosaurs. And what a monumental task that must have been! Recognizable coprophagous beetle types had evolved at least by the Jurassic, since Geotrupoides occurred in Europe at that time and this genus was also recovered in Asia during the Cretaceous.35 107 Dung beetles in general are too large to be commonly entrapped in amber, and most of their fossils occur in sedimentary rocks.

A number of Cretaceous scarab beetles could have fed on dinosaur dung.108 The larvae of undetermined adult scarabs in Burmese amber (color plate 1A, 1B) may well have developed on dino feces. These adults were associated with fruiting bodies of a club mushroom also preserved in the amber,91 which suggests that they had come to this site for feeding, a typical behavior of mature dung beetles.106

Certainly quite a few coprophagous beetle species that escaped fossilization or have not yet been discovered were around then feasting on dinosaur droppings. Those that have been found represent the three methods of breeding behavior found in extant dung beetles, namely the dwellers, the tunnelers, and the rollers.106 The dweller types feed, oviposit, and develop in excrement, without bothering to make any nest at all. The tunnelers and rollers are the real engineers since they make dung chambers for their offspring. Tunnelers make shafts through the pile that extend to various depths into the ground below. These are then turned into nests when they are packed with feces fashioned into molded brood balls for larval development. The rollers are the showiest and most well known of the co-prophagous beetles. They take a portion of the dung, shape the feces into a sphere several times larger than themselves, laboriously roll the ball away from the source, dig a hole, and bury it with an egg. At any rate, beetle fossils such as Geotrupoides and Cretogeotrupes were probably tunnelers, while Proteroscarabeus and Cretaegialia could have been dwellers and Holocorobius possibly a roller.

While few giant herbivorous reptiles are around today, there are many large mammalian herbivores, and ecologically speaking, elephants certainly are the largest and probably the closest we have for a comparison to dinosaurs. Elephants and some large herbivorous dinosaurs probably shared a similar type of fermentative digestive system, with both feeding on a variety of plants including trees and shrubs, and producing large droppings composed of only partially digested wet plant material. A mature elephant, voiding up to 22 pounds 17 times a day, can produce over 300 pounds of feces in 24 hours.109 Up to 16,000 dung beetles have been reported to rush to just a small 3-pound dropping, scurrying this way and that in their haste to obtain their share.110

The ecology of dung removal was probably not that much different back in the Cretaceous but definitely operated on a considerably larger scale. Dinosaur herds had to be accompanied by hordes of beetles in the same way that elephants are today.111112 To extrapolate how much solid waste the largest known dinosaurs probably produced, compare sauropods to the African elephant. A mature male elephant weighs in at 6.5 tons, while a female may exceed 3 tons. Contrast that with estimated weights of 55 to 100 or more tons for the largest of the sauropods.113114 Taking the lower end of the scale still makes them 9 times larger than a male elephant, and as such they could have eliminated well over a ton and a half of dung per day. Then consider that sauropods, like elephants, traveled in groups some, if not all, of the time.

We know that elephant herd size varies seasonally with related units coming together in the wet season to form groups of 50 or more animals. It seems unlikely that such large assemblages would have been the habit of sauropods simply because of the limitations of available forage needed to sustain them. But even if the herd was only 10 individuals, that still means a minimum of 15 tons of feces scattered in their wake each and every day. The sheer numbers of just beetles processing such a prodigious output is almost unimaginable, possibly in the order of

200-250 million. Of course, many sauropods were only elephant size and as such probably formed larger herds. The elephants of the Tsavo game park produce 1,500 tons of dung per day, perhaps with an accompanying 24 billion beetles!

The largest dinosaur coprolite recorded was presumably that of the immense theropod Tyrannosaurus rex and measured over 17 inches (43 cm) long.115 Analysis showed that these theropods ingested bone fragments along with the flesh of their victims, but there was no evidence of insect associates. However, another large dinosaur coprolite, a foot in circumference and almost the same in height, had dung-beetle burrows in it.116 The fossilized scat was composed mainly of undigested conifers. This discovery shows that specialized coprophagy was well established during the reign of the dinosaurs.

All of those beetle species associated with dinosaurs are certainly now extinct. Today, most dung beetles prefer mammalian to reptilian feces, probably because there are very few herbivorous reptiles around, and they do not produce the amount of waste seen with herbivorous mammals. There are fewer co-prophagous beetles associated with the droppings of carnivores than of herbivores. Herbivore excrement is probably preferred because of the high moisture content and abundant partially undigested plant material, which supplies copious nutrients for the beetle grubs.

There are, however, a few coprophagous beetles that have an appetite for reptile feces, possibly carried over from the distant past. Some seek out the droppings of plant feeders like land tortoises and iguanas.117118 In Turkey, one such insect lives only on the dung of the land tortoise,119 and similar associations occur with the beetles Copris and Onthophagus in Alabama, Mississippi, Florida, and South Carolina.120' 121 The adults search for the burrows and nests of gopher tortoises and lay their eggs in the soil beneath them. After hatching, the young feed on the tortoise dung.122 It seems likely that such an interaction could have taken place under the nests of hadrosaurs and other dinosaurs. Since there is data suggesting that some dinosaur parents fed their young at the nest,114 their accumulated waste would have been a source of food for beetles.

Scarab beetles are attracted to carnivorous reptilian dung. Two of these search out feces of the boa constrictor in tropical Amer-ica,117 and fresh lizard droppings attract others in Costa Rica.123 So it's easy to imagine that beetles back in the Mesozoic were also breeding in the waste of carnivorous dinosaurs such as T. rex.

Dung beetles are not the only decomposers that would have been associated with dinosaur waste. We know that they presently compete with flies. While less conspicuous than the beetles, dung flies represent a widely diverse group capable of breeding in all types of animal feces.105 Some cow-dung forms represented in Cretaceous amber are phorid flies (color plate 1D), moth flies, midges, and fungus gnats. Since the larvae characteristically breed in excrement with a high moisture content, they do not make tunnels that persist and can be later fossilized. Aside from finding cast skins of the larvae, which is difficult enough in recent droppings, it is almost impossible to find evidence of fly activity in coprolites. Although the highly chitinized pupal cases would be good candidates for fossilization, the fly larvae crawl away from the dung pile when they are ready to pupate. Thus, their pupal cases would not be preserved with the co-prolite sample. This leaves amber as the main source for verification of coprophagous flies in the Cretaceous.

A parade of other invertebrates visit dung. Many are non-specialized, such as springtails, earthworms, termites, millipedes, mites, and sow bugs, which prefer theirs dry and aged. To complicate matters, arthropod guilds come to parasitize or feed on the feeders. These include parasitic wasps, predatory dance flies, soldier flies, flower flies, rove beetles (color plate 1C), and others, all forming one active conglomerate in the dung world.

Furthermore, besides having competitors and parasites to worry about, there are vertebrate predators that relish dung insects. After all, their sluggish nature would make them easy prey, especially when they are competing with others at a fresh drop ping. Today, various mammals and birds eat beetles in elephant droppings. Certainly in the Cretaceous, there were dinosaurs that dined upon similar insects.

Aside from disposing of dung, insects play an important role in ridding the world of dead animals. Just turn over a gull on the beach or a squirrel on the side of the road and you will see insects working their way through the dead animal, transforming decaying flesh into a mass of aggressive, squirming maggots. Various insects are sequentially attracted to carcasses at various stages of decomposition. In fact, the time that insects arrive at a corpse is correlated so precisely with the state of decay that forensic entomologists are often called upon to use insect stages to determine the time of death.124 The flies are the first to arrive. Their young are particularly tolerant of a putrid, partially liquid diet found in the early stages of decomposition, while the beetles usually arrive much later after the carcass has dried out.

What would have been the sequence of events following the mortal attack of an injured ceratopsian by Tyrannosaurus rex? The giant herbivore, weighing 3 tons, would probably have provided meals for a variety of smaller dinosaurs after T. rex had finished with the carcass. But even as T. rex was feeding, the insects were on the scene and beginning their job. They would have swarmed over the raw flesh and blood, buzzing away as the tearing teeth of T. rex approached, only to resettle the moment his head withdrew. The flies probably alighted on the blood-drenched faces of the theropods and sought out fleshy fragments of the meal. Certainly, dinosaur bodies attracted large numbers of flies and beetles, many of which were the ancestors of those found today on dead vertebrates, including reptiles.

In a study of necrophagous insects attracted to a dead leather-back turtle in French Guyana,125 vultures set the stage by first removing the eyes of the dead reptile. The resulting open wounds attracted flesh flies (Sarcophagidae) and blowflies (Calliphori-dae). Additional damage by vultures resulted in more wounds that this time attracted anthomyid and muscoid flies. Cockroaches arrived at night and scavenged on the carcass (color plate 6D). These same insect types undoubtedly were drawn to dead and dying dinosaurs. We know that blowflies, which are one of the most common necrophagous insects today, occurred in the Mesozoic,35,126 and as such, they must have deposited eggs around the eyes, nostrils, and mouths of dead or dying dinosaurs.

Moth flies (Psychodidae) are another group that breed in decomposing bodies, especially under warm, moist conditions. These flies commonly occur in Cretaceous amber and probably bred in dinosaur corpses along with other fly groups such as phorids and sphaerocerids.124 Developing fly larvae would have attracted predatory insects such as ants that decreased the fly population by taking maggots back to their nests to feed their young.

After a cadaver has been stripped of the soft tissues by the first arrivals, a second group of insects enters the scene to demolish the remains. Especially obvious are the skin beetles or der-mestids. Both the adults and larvae relish dried skin and tissues, and leave their signature in the bones that appeal to them as a substrate for their pupal chambers. Bones of Allosaurus fragilis from the late Jurassic of Wyoming contain pits considered to be the work of skin beetles that excavated pupal chambers after filling up on dried flesh.127,128 The damage was extensive, appearing on at least 12% of the recovered skeleton.

Skin beetles were also considered to be the culprits that made pits in two bones of an Upper Cretaceous Prosaurolophus from Montana,129 as well as circular-to-elliptical borings in Upper Jurassic sauropod and therapod bones at Dinosaur National Monument, Utah. These latter borings occurred on nearly 40% of the skeletal remains and were again interpreted to be pupal chambers. Scanning electron microscope examination of the bones revealed scratch marks attributed to the mandibles of the larvae. Apparently, the pits were formed some 4 to 9 months after the dinosaurs died.127 It would appear that no object is too hard for the strong mandibles of skin beetles. These instruments, coupled with their strong desire to form pupation burrows, allow them to make tunnels in a number of dense materials, in cluding horns, hoofs, mortar, stone work, and even lead.130 A skin beetle in Burmese amber131 could well have played a role in the recycling of dinosaurs. These ancient dermestids may have transported juvenile stages of tapeworm and nematode parasites to dinosaurs, just as they do to birds today.

The Food Chain

The food chain in any ecosystem has two nutritional pathways based on what consumers eat. There are those that subsist on living organic material. This is the most well-known division, where energy is shunted through herbivores. The other division involves those organisms that consume detritus, dead or dying organic matter, where the energy moves through saprophagous organisms, also known as detritivores, decomposers, saprophages, and scavengers. In almost all terrestrial habitats the detrital food chain is dominant. Estimates indicate that 11% of the biota is comprised of saprophagous insects.132

The world remains green because herbivores do not consume all or even a major portion of living plants. The amount of floral production removed by plant predators varies between ecosystems, with an estimated low of 2% in a poplar forest, 10% in salt marshes, and between 33% and 66% in overgrazed dry savannahs. Overall, the total amount worldwide is probably around 10%, and about half of this is returned to the environment as fe-ces.66 So dropping leaves, withering grasses, and falling trees enter the detrital food web, along with animal waste and bodies, where they are broken down by saprophagous organisms. Insects share this nutritional bonanza with fungi, bacteria, and other invertebrates such as mites, snails, millipedes, nematodes, earthworms, and springtails.

Next to plants, detritivores process the largest amount of energy in food webs, much more than any herbivores past or present, including dinosaurs. In so doing, they return nutrients to the soil for plants to utilize. The majority of saprophages, including insects, become part of the food chain as prey for carnivores.

The importance of this predominant pathway in the food chain is frequently overlooked because it is not obvious. Large grazing animals and predators are conspicuous, but small organisms, especially insects, are primarily responsible for the everyday functioning of the biome. While some dinosaurs undoubtedly were carcass scavengers and others may have consumed herbivore feces like pigs and rabbits do now, they certainly were not a significant factor in the detrital food web. They were then, just as we are now, dependent on insects and others for processing detritus. Without saprophagous organisms functioning to remove waste, the world would suffocate under the accumulating masses of dead plant matter.

Perhaps this is why termites are such a successful group today. As probably the most significant insect detritivores on the planet, their more than 2,300 species, aided by internal symbionts, break down cellulose in wood and plant tissues. Their numbers are astonishing and individuals in a single nest can reach 20 million or more, while in some areas of the tropics, termite mounds occupy almost 30% of the soil surface.66 Their efficiency in destroying human dwellings can be judged by the billions of dollars in damage they cause each year just in the United States.

So dinosaurs benefited directly from saprophagous insects because they were important nutritional items for their young and for smaller species, and also indirectly because they supplied a source of food for insectivorous mammals, reptiles, and birds that they fed upon. And most importantly, dinosaurs profited because saprophagous insects cleaned up and maintained their environment.

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