Dinosaurs Without Bones (37 page)

However, a dinosaur coprolite turned to stone is not necessarily the end of its journey, in which it waits patiently for a well-trained paleontologist to recognize it for its true fecal nature. Coprolites are among the few dinosaur trace fossils—such as toothmarks in
bones, and gastroliths—that could have been transported before and after burial, or reburied. Before burial, a dinosaur turd could have fallen down a hill slope, had part of it rolled by a dung beetle, or carried some distance by flowing water. After burial, it could have been exhumed by wind, streams, tides, or waves, and moved to a new place before getting buried again, or not—in which case it might have weathered, eroded, and vanished from the fossil record millions of years before human consciousness. This also means a dinosaur coprolite, like dinosaur bones, feasibly could be reburied in geologically younger sediments. In short, where you find a dinosaur coprolite in the field should never be assumed as representing the same place or time where that dinosaur took a dump.

Given all of these special conditions for preserving dinosaur coprolites, they understandably are among the most precious of dinosaur trace fossils. They are also among the most difficult to attribute to a specific dinosaur. Once a coprolite is identified, paleontologists often limit themselves to saying it belongs to a carnivorous or herbivorous dinosaur based on its contents (bone or plant fragments, respectively). This is where bigger is better, in that large coprolites are easier to connect to dinosaurs big enough to have made them, whereas smaller ones could have been made by a wide range of small dinosaurs. Dinosaur body and trace fossils also help, in which paleontologists can play the much-cherished game of “match the defecator.”

Of dinosaur coprolites identified thus far, the best understood ones are attributed to the Late Cretaceous hadrosaur
Maiasaura
of Montana. Some of these coprolites are quite large; although most are broken into smaller pieces, some suggest original volumes of about 7 liters (1.8 gallons). (For perspective, a regulation U.S. basketball is about 8.5 liters.) Other large coprolites are credited to Late Cretaceous tyrannosaurids, such as
Tyrannosaurus rex
; one of these is more than twice the length of a 12-inch sub sandwich. Moreover, coprolites in Late Cretaceous rocks of India have been connected to sauropods.

Nonetheless, the vast majority of probable dinosaur coprolites fall into the aforementioned nebulous categories of “carnivore” or
“herbivore.” For example, Early Cretaceous coprolites from Belgium have bone fragments in them. Hence, these are allied with carnivorous theropods, but nothing more can be said about them. This vagueness is especially apparent once dinosaur ichnologists admit, with much embarrassment, that we have no idea how to distinguish whether therizinosaurs, ornithomimids, ankylosaurs, nodosaurs, stegosaurs, ceratopsians, pachycephalosaurs, or lots of other dinosaurs made some dinosaur coprolites. We also have not yet discovered a dinosaur coprolite showing any evidence of insect eating, nor of clear omnivory in which, say, a dinosaur had a salad with its steak.

Of these, coprolites showing that some dinosaurs ate insects as a regular part of their diet would qualify as fantastic finds. This seemingly un-dinosaur-like behavior, which is extremely common in modern birds, was proposed for the bizarre theropod
Mononykus
from the Late Cretaceous of Mongolia and a few other theropods. So if insect-bearing coprolites of the right size were found in strata of the right age, environment, and place as
Mononykus
bones, this would be one way to confirm an idea that is now mostly speculative.

Dinosaur Dung, Conifers, Insects, Bacteria, and Snails: A Love Story

Every day, I give thanks to dung beetles. My thanks is offered for what these insects do to keep our planet clean, because otherwise we would be up to our waists in waste. The tight relationship between large herbivores and their diligent insect cleanup crews is easy to witness today: wherever elephants, cows, horses, or other plant-eating mammals loosen their bowels, dung beetles are not far behind. Why are these beetles and other insects, such as dung flies, attracted to feces? Because it is irresistible as baby food. Some beetles roll balls of this nutritious stuff as take-out, which they push into burrows, lay eggs on them, and seal off the burrow. Other beetles burrow below these patties, or into a patty itself, and lay eggs there so that the beetle larvae are surrounded by food when they hatch, which they can then devour. (It’s dinner
and
a nursery!)

Amazingly, dung beetles have been performing this essential ecological service virtually unchanged since at least the Late Cretaceous
Period, and we know this because of dinosaur coprolites. Thanks to the careful and insightful collaboration of paleontologist Karen Chin and entomologist Bruce Gill, they convincingly showed how the Late Cretaceous hadrosaur
Maiasaura
, dung beetles, and conifer trees interacted with one another as part of a food web about 75
mya
.

As is often the case in ichnology, body parts had little to do with this discovery, which Chin and Gill documented in 1996. Plenty of dinosaur bones and eggs were in the same area, as this was the same place near Choteau, Montana where
Maiasaura
and
Troodon
nested, preserved in the Two Medicine Formation. Yet as of this writing, not one body fossil of a dung beetle—legs, wings, abdomens, antennae, or anything else—has been recovered from the rocks of that area. The only body fossils involved in this research came from the conifers, which were represented as blackened bits and pieces in calcite-cemented coprolites.

As mentioned before, some of these coprolites were big, spanning about 34 cm (13 in) wide, although others were mere cobbles. They were also plentiful, showing up in sixteen spots within a square kilometer. Such a coprolitic concentration might imply that the area was a Cretaceous latrine. However, a more likely scenario is that dinosaur feces were nearly everywhere on dry land then, but a few places—like river floodplains—were better at burying them rapidly, which aided in their fossilization.

To have such fine fossil feces in the same field area as dinosaur eggs, babies, adults, and nests was a very lucky find and thrilling enough in itself. Yet when Chin and others also realized these coprolites had burrows in them, their paleontological importance skyrocketed: a dreamy ichnological two-for-one deal. Some of the burrows were open, but in others the insects had actively filled them, having packed a mixture of sediment and dung behind them and leaving distinctly visual “plugs.” The burrows also varied considerably in size, from about a millimeter to 3 cm (1.2 in) wide, all of which were insect-sized and with circular outlines. This variation implied that more than one species of insect made them.

Which insects made these burrows? First of all, they had to be ones that loved tunneling into dinosaur manure. This narrowed down likely candidates to two major groups, dung flies and dung beetles. Dung flies are relatively small and normally just lay their eggs on feces; their larvae then hatch on this food supply and start chowing down, which they continue doing until they pupate. In their life cycles, dung flies do not dig wide and lengthy burrows into the dung, let alone backfill them. But dung beetles do. Accordingly, Chin and Gill focused on these insects as the most likely suspects for these trace fossils.

Direct observations of many modern dung-beetle species and their traces served as guides for figuring out how these Cretaceous beetles’ lives depended on dinosaur waste. Dung beetles today employ three different strategies in handling feces: tunneling, dwelling, or rolling. Tunnelers burrow into and below a patty, making and storing a brooding chamber with dung and eggs. Dwellers make themselves at home in the patty itself, digging out brooding chambers so that the larvae emerge in the dung-beetle equivalent of a candy store. Rollers scrape dung off the surface of a pile, shape it into a big ball, and leave the neighborhood with their prizes, evoking Sisyphus as they roll dung balls larger than themselves. They later stuff these dung balls in burrows dug elsewhere. Given the three choices, Chin and Gill figured these Cretaceous burrows were from tunnelers, which had burrowed into the dinosaur feces while it was still gooey, gathered some of this organic goodness, and placed it into nearby burrows.

The most exciting conclusion drawn from this discovery was how
Maiasaura
interacted with and affected plants and insects in its surroundings, which in turn provided a sketch of how a Late Cretaceous ecosystem might have functioned, and with a dinosaur as a possible keystone species. As a large herbivore,
Maiasaura
may have had an impact comparable to elephants in savannah ecosystems today, in which dung beetles played an important role in the flux and flow of elements consumed by such herbivores.

However, another mystery about the Two Medicine coprolites was how the pieces of conifer wood had become so blackened. The
answer came from within, as in fossil bacteria that originally lived in
Maiasaura
guts. In a paper published in 2001, geochemist Thomas Hollocher, Karen Chin, and two other colleagues detected both abundant body fossils and chemical signatures of anaerobic bacteria in the coprolites. These bacteria invaded vascular tissues in the wood and left distinctive black organic residue called
kerogen
, the same mix of organic compounds in oil shales. The simplest explanation for how these bacteria got into the plant tissues is that they were in the dinosaurs’ intestinal tracts. This made sense, as any modern herbivores likewise have gut microflora that aid in breaking down cellulose and other compounds in consumed plants.

This discovery of bacteria that lived inside a dinosaur was important enough. But the bacteria also did paleontologists a 75-million-year-old favor by helping to fossilize the coprolites. Once these researchers examined thin sections of the coprolites under microscopes, they realized that the calcite in the coprolites was probably precipitated in two stages: inside the vascular tissues of the fragments, then in the areas between the fragments, including the dung-beetle burrows. They proposed that bacteria could have initiated this precipitation, starting with live bacterial colonies in the original feces hardening these droppings. Once these proto-coprolites were buried, calcification would have continued, turning what was originally dark, mushy, and smelly into just dark and rocky.

Yet the story of these coprolites does not end with these two studies. As often happens in paleontology and other sciences, this research raised more questions. For instance, as Chin looked more closely at thin sections of the fossilized wood, she realized that something was rotten in the Cretaceous. The hadrosaurs had not been masochistically masticating hard, fresh, living conifers. Instead, they went for long-dead and already-decayed wood. The fossilized wood lacked lignin, a connective tissue that holds wood fibers together. Without this “glue,” wood falls apart. In modern forests, fungi aid in this disintegration, which rots the wood throughout. Then wood-boring insects—such as termites, beetles, and ants—break it down further. Although the
fossil wood was too ground-up to tell whether insects had bored into it, Chin found some evidence of fungal damage in the wood fragments.

So Chin asked the best question of all: Why? As in, why would a dinosaur eat decayed wood? The nutritional value of the wood fiber itself would have been negligible, hardly worth the effort involved in chewing and digesting it. So there had to be something more behind this behavior than just exercising jaw muscles. The fungi on and in the wood must have provided some sustenance, but probably not enough to keep a hadrosaur going. Also recall that
Maiasaura
is the “good mother” dinosaur, with a scientifically earned reputation for its child-raising skills. Think about the disappointment (not to mention hunger) baby dinosaurs would have felt if their parents simply brought them degraded wood to eat.

This is when Chin thought about both woodpeckers and vomiting. The first part of her reasoning—woodpeckers—was prompted by how these birds feed. As most people know, woodpeckers drill into dead wood not to eat it but to gain access to yummy and protein-rich larvae of wood-boring insects living just under wood surfaces. Woodpecker parents also do this for their chicks, flying back to nest cavities with insect treats for their offspring before drumming up their own meals. So perhaps these hadrosaurs were also breaking up wood to get insects, and carrying these back to their nests.

However, this same strategy would not have worked very well for a 6-to-7-ton
Maiasaura
trying to feed more than a dozen ravenous hatchlings, especially as their appetites grew with them. The image of a hadrosaur mother or father wearily carrying one beetle grub at a time in its mouth to a nest, dropping it into one of many competing maws, then going back for another, is too absurd to consider. It also did not explain why their coprolites contained wood, meaning the hadrosaurs didn’t just break wood, but swallowed it. So one solution that neatly explained such wholesale consumption of woody tissues is that these dinosaurs were eating large volumes of this decomposing wood for the insects (and perhaps
some fungi). These caring dinosaur parents then transported these MREs (meals-ready-to-eat) in their stomachs back to their nests, where they obligingly regurgitated them into the waiting mouths of their hatchlings. Admittedly, this is a difficult hypothesis to test more directly, unless paleontologists some day find juvenile
Maiasaura
skeletons with enterolites matching the content of the adult’s coprolites.

What more could be learned from these coprolites, the gifts that kept on giving? It turned out that Cretaceous dung beetles were not the only ones taking advantage of bountiful supplies of nutritious dinosaur dung: snails got into the act, too. I remember these gastropods surprising me in 2009 while on a field trip with about thirty other paleontologists, led by Dave Varricchio, Frankie Jackson, and Jack Horner. Most participants would have admitted that the
Maiasaura
and
Troodon
nest sites were the main draw of the field trip for them. Yet I was equally excited to know that we would also be strolling through the same area with the hadrosaur coprolites I had seen nine years before when studying fossil insect cocoons and burrows there. Would I notice anything different about the coprolites this time?