The Herd

Only in our first year did we camp by the Teton River and drive to the Peebles ranch. In 1980, courtesy of the Peebles family, we moved to a spot right on their ranch, smack dab in the middle of the anticline. It was also in 1980 that the National Science Foundation took over the funding from Princeton and our grants began to grow steadily. More money meant a larger crew, more finds and more evidence of how rich the site was, thus better grant applications and still more money.

More money also meant construction. Bob, who had a passion for carpentry, was the builder. First he built a kitchen, a frame on which to put a tarp so that we could cook and eat our meals out of the sun and rain. Then he built a root cellar, to supplement our gas-fired refrigerators, and the coolers. Along the way, he built big wooden boxes to store food away from the ground squirrels and a loading platform for the huge, plaster-covered chunks of rock and fossil we brought out of the field and transferred from one truck to another. In the later years of the camp, he built a framework to hold a truck inner tube full of water so we could have warm showers through solar heating. This meant we were no longer limited to one shower a week as we had been before; we got that one at the campground in Choteau ($2 per shower) on our usual Friday trip to town for groceries, telephone calls and ice cream cones.

I found camp very comfortable, except when it rained; then the ground, which was mostly bentonite, turned to a heavy, sticky substance something like wet cement that clung to our boots and made us walk as if earth gravity had just gone up a notch. Rain also reminded us how isolated you can be in Montana, even when you're on somebody's ranch. Choteau was about 12 miles away on a good gravel road, but getting to that road meant driving on well-worn ruts through cattle range for about a quarter of a mile. This was fine in dry weather. In rain the ruts turned to muck, as did the rest of the range, but they were still the safest place to drive because they had been packed down. The surrounding ground was softer and even more likely to bog down a truck or car. We once had visitors who had brought along a one-year-old and ran out of diapers in the middle of a day and a half of hard rain. I drove one of them out in their rent-a-car, which, fortunately, was front-wheel drive, with my foot on the accelerator and the rear end fishtailing for the whole quarter-mile. If I had stopped, the car would have sunk in the mud. Nor would one of the four-wheel drive trucks or jeeps have helped me. In that kind of mud, all you get with four-wheel drive is four wheels digging down deeper, faster. You end up high-centered, with the drive train sitting on the mucky center of the road and four wheels spinning free.

Naturally, with each success at the anticline, my position at Princeton improved. I became less and less a preparator, except of my own fossils, and more and more a principal investigator (as the National Science Foundation terms it) in charge of a research project. Vertebrate paleontology was not a high priority for Princeton University, however; there was a small museum of natural history that was part of the geology department, and there was only one professor, Don Baird, with one or two assistants. In 1982, partly because of the limited future at Princeton and partly because I wanted to go back home, I moved to Montana State University, where I became curator of vertebrate paleontology at the university's Museum of the Rockies and an adjunct professor. The museum was flourishing, and it was particularly interested in supporting paleontological research. Here, in Boze-man, I knew I would be close to my field research. I even managed to get an advanced degree once I moved back. In 1986 the University of Montana, my one-time intended alma mater, gave me an honorary doctorate. That same year Princeton University shut down vertebrate paleontology, except for Don Baird, who stayed on until his retirement in the spring of 1988 and who is now a research associate with the Carnegie Museum in Pittsburgh. Princeton closed its small museum of natural history and gave its entire fossil collection to Yale, including the finds from the Willow Creek anticline. Yale has been kind enough to let me keep those fossils on loan in the research collection at the Museum of the Rockies.

One of the major features of that collection is a group of strangely battered adult maiasaur bones from the anticline. I have thousands of these bones, cataloged and arranged on floor-to-ceiling metal shelving. They all show the same pattern of breakage and wear; they're all the same gray-black color, which indicates similar conditions of fossilization; and, of course, they all came from the same site. They belong together, although this fact was not immediately apparent to us when we started finding them. Only in retrospect, after analysis of the finds, could we see the whole site clearly.

You might say that the course of the dig at the Willow Creek anticline was like the course of one of the braided streams that, 80 million years ago, in the Cretaceous, deposited the mud on the dinosaur bones to preserve them for us. Those streams consisted of several individual channels, really separate streams themselves, that wove back and forth between set banks, crossing each other, joining and separating. That's what work at the anticline was like—separate streams of investigation weaving in and out of each other, joining and separating.

IN 1979 THE BRANDVOLDS were still prospecting on the Peebles ranch, looking for a big, relatively intact dinosaur skeleton to reconstruct and put on display in their rock shop. And in July they thought they had found one. They had uncovered a big femur and an equally big humerus on a ridge about a half-mile from the part of the anticline where Bob and I and the crew were searching for maiasaur nests. The task of exhuming a full dinosaur skeleton in good enough condition to reconstruct was a major one, however, and they needed our help. In return, they agreed to map the site and keep track of each individual bone fragment so we would know what had come out of that deposit. They would have the bones, and we would have the information.

Before they even got started on any serious digging, I put my crew on the site for a week to take off some of the overburden. (In plain language overburden is dirt, the soil that lies on top of rock in which the best-preserved bones are likely to be embedded.) In that week, the crew found 65 bone fragments. These fossils were poking out of the rock, directly under the layer of dirt. The Brandvolds worked that site through the summer of 1979 and the following fall and winter, carefully mapping and photographing the site as we had requested. At the beginning of our 1980 field season, I looked over what they had found and saw that they were in for a disappointment. There were the remains of at least five individual dinosaurs, three of which were juveniles. That is to say, they were longer than 4 feet, which is as big as the babies got in the nest, but were less than 10 feet from tip to tail, making them less than half grown. The Brandvolds had uncovered a rich but messy deposit. There were probably well over a hundred individual fragments—pieces of crushed, distorted and badly broken bones. The odds of getting a whole composite dinosaur out of the site were very small, even if you worked on it for several years, but for us the dig was worth something because here were the remains of a group of adult and juvenile dinosaurs.

We could tell from the bones that these were hadrosaurs, and we hoped they might be maiasaurs and that we might have found not just a chance collection of bones but some kind of social grouping. Perhaps, we thought, these dinosaurs nested in colonies and lived in small family groupings. It was just speculation, but it's this kind of speculation that fuels the imagination and gives you the energy to dig, and dig, and dig, until you find out what you've actually got. The Brandvolds turned the site over to us to excavate as part of the dig, and from that point on it was our crew who did all the prospecting and digging at the anticline. As it turned out, we never got enough for even half a dinosaur from that particular spot, but then that wasn't what we wanted.

We wanted to know if the dinosaurs preserved, albeit badly preserved, in this spot had been together in life. If this was really a family unit at the Brandvold site, then all these dinosaurs had died together and been buried together. We had to think if that was possible, and if so how. We also had to consider the possibility that perhaps this was a random collection of bones of the sort that might accumulate in the bend of a river where sediment and detritus were deposited by currents. By the end of 1980 we had 200 bones from that site, representing at least eight individuals, all in very bad condition, embedded in mudstone with a lot of volcanic ash. I knew the dinosaurs were all hadrosaurs, and all the same kind of hadrosaur, but I couldn't make a more precise definition. There was certainly no indication that these bones had been brought together by accident. They were not in a river or stream bed. In fact, it wasn't clear what kind of deposit they were in. It was mudstone, all right, but we couldn't tell where it came from. John Lorenz, who was working on the dig in 1980, thought the animals might have been caught as a group in some kind of catastrophic mud flow—a flood, but of mud instead of plain water. Lorenz was a Ph.D. student at Princeton at the time, and he was studying the stratigraphy of the Two Medicine formation. (Stratigraphy is basically the study of the various layers of rock, and what has happened to them. A stratigrapher sets you straight about which beds are oldest, for instance. It's part of figuring out the geology of your site so you have a framework to fit the fossils into.) Lorenz' theory could help explain some odd facts about the site. The bones were in awful condition. Some even looked as if they had been sheared lengthwise. However, right next to a badly damaged bone would be one that was untouched. Furthermore, we found some of the bones standing upright. Bones caught in water or lying on the ground and buried by sediment don't stand up vertically. But if the creatures had been caught in a mud slide, they might have just been bashed to pieces and left in these odd positions as the mud settled.

Lorenz suggested that the mud slide could have been caused when a volcano erupted, spewing out ash. The ash found in the Brandvold site might have clogged a lake, turning it into thick, viscous mud. Had the lake breached some natural barrier, the resulting flood would have produced this kind of mud slide. We knew that there had been large lakes in the area at earlier time periods, and there could have been one at this time as well.

This was a speculative explanation, and both Lorenz and I recognized that it raised as many problems as it solved. In the Brandvold site, for example, there were hardly any small bones, such as toes and fingers. And there was only one skull, which we found much later. More of these kinds of bones would presumably have been there if the animals had been caught and buried alive. Furthermore, the damage was not really of the sort that could happen to living animals. How could any mud slide, no matter how catastrophic, have the force to take a two- or three-ton animal that had just died and smash it around so much that its femur—still embedded in the flesh of its thigh—split lengthwise? We left the problem unsolved, because we didn't have much choice. And, as happens with such problems, it got bigger.

ONE D^Y IN i98i MY SON JASON, who was eight years old at the time, took a small hike while the rest of us were working on the Brandvold site. Jason has made a number of paleontological and biological discoveries, including one I remember quite well that occurred when he was four and we were in the Judith River formation. He was some distance away, and he informed me that he had found a cute baby lobster. I told him not to touch it and hurried over to him. He just let the animal crawl around in the rock fort he had built around it until I got there. I saw exactly what I expected to see, the closest thing to a lobster that the badlands have to offer: a scorpion. I explained all about scorpions to Jason, and he took the information in stride.

He kept on exploring and discovering things—horned toads, Indian arrowheads and dinosaur fossils. On the walk he took in 1981, he found nice, big hadrosaur fossils. We named the site Nose Cone, for the simple reason that Jason, who found it, thought the rock there looked like the nose cone of a rocket. On my digs, whoever finds a site, names it. When we got around to excavating the Nose Cone site, we found adult and juvenile bones of the same kind found at the Brandvold site, and in the same condition and state of disarray. Even more intriguing, it was obvious that these two bone deposits were connected. You could walk from the Brandvold site to Nose Cone and follow exposed bones all the way. Both sites were on the side of a hill, and we poked into the hill here and there to see if there were more bones. There were. The two sites were clearly on the same fossil horizon. So it looked as if we had a bone bed that extended for at least an eighth of a mile. The family of hadrosaurs was growing.

We found Camposaur in 1981, too. Or Camposaur found us. It more or less stuck itself right in Wayne Cancro's back. One of our first steady volunteers, Wayne joined us in 1981 when we were in our second year of camping on the anticline itself. He arrived early and picked a choice spot for his tent, planned how to dig his trench to prevent accumulation of rainwater around his head, and proceeded to attempt to hammer in his stakes. He had a lot of trouble getting them into the ground because he kept running into bones. In itself, this wasn't such a big surprise—we were finding the odd bone fragment around camp all the time, and there were two fairly large bones on a little rise in camp that we were trying to get out intact. So Wayne kept working to get his tent set up.

When he finally did get his stakes in and went inside to lie down after the effort, he suffered the Princess and the Pea phenomenon. There were bones poking him everywhere. It was Wayne's extreme discomfort, which led him to move his tent, that led us to start digging seriously in camp. We figured if it was that bad, it had to be good. So we began to dig, and eventually we had a pit in the middle of camp about 20 feet by 30 feet and in places 3 feet deep. From that pit, between 1981 and 1984, we pulled out 4,500 maiasaur bone fragments. They were all like the bones from the Brandvold and Nose Cone sites—black, rock-hard but crumbly and often fragmented. That total represented about 30 individuals. The bones were of adults and half-grown juveniles, the same kinds of animals found at the Brandvold site. Wayne called this site Camposaur on the basis of its location—right in the middle of camp.

Like all our other pits of any size, Camposaur was divided into a grid of squares about three feet on a side, with each square mapped to show the fossils as they lay in their original position. The actual digging proceeded in typical fashion. First we shoveled dirt, carefully, until we reached the level of the preserved fossils. Then, gradually, we worked on the pit. One or two people would take a small area and brush and

The fossil finds at each site were mapped on a grid. The grid reproduced here, of the Brandvold site, reflects the profusion of bones common to Camposaur and other parts of the big bone bed.

sweep dirt away with whisk brooms if a bone was already exposed, or gently poke and pry with an ice pick, carefully, if no fossils were sticking up from the ground.

When we found a fossil, a legbone, for instance, we used the ice pick, toothbrush and whisk broom to loosen the dirt on and around it. Then, as each section of the bone was exposed, we painted it with polyvinyl acetate to help hold it together. Once the top of the bone was exposed, we dug and poked around it with the ice pick, all the while cleaning and painting each section that we exposed. Eventually we would have the bone resting on a pedestal of dirt or rock, at which point we put a plaster cast on it.

We made the casts the old-fashioned way, by soaking burlap strips in wet plaster of Paris and wrapping them around the bone. When a cast had hardened, we took out the bone by breaking through the pedestal. What we ended up with in this process was a whole bone protected by a cast. Smaller fossils found close together would all be included in one cast. Some of the better-preserved nests were wrapped in plaster, with all the eggs and other fossil bones in one cast. In the winter, back in the laboratory, we would take off the casts and begin the preparation of the bone—cleaning it again, identifying it, putting it back together if it was broken and coating it with varnish to help preserve it. Then the bone would be cataloged and saved for study either then or later.

BY THE END OF THE SUMMER of 1981, we had a lot of similar bones to think about. The Brandvold site and Nose Cone were clearly part of one bone bed. They were in the middle of the anticline on the north side of the eroded center. If you crossed over the center, you came to camp. There you found Camposaur. And if you continued on, you found the children's dig, which we had created near the top of a steep ridge where Bob Makela liked to demonstrate how well his Toyota jeep handled inclines. He'd get a passenger, put the Yoda, as it was called, in four-wheel drive, and then shoot up the hill until he brought it to rest at a 45-degree angle. Kids loved this ride. We created the children's dig not because we wanted to employ child labor but because Jason was there for each field season, Bob's son Jay came down sometimes, and we had a lot of visitors who brought their children. Often they would stay for only a few days, but we needed a place for children to work, where they could actually do some good. The children's dig was a pit on the same horizon as Camposaur and so similar in the bones that came out of it that we could afford to take some chances with a bone or two getting smashed or lost. We did keep an eye on it and mapped the bones in their original locations before the kids started on them with ice picks and whisk brooms to liberate them from the mudstone.

It was fairly obvious that the Brandvold site and Nose Cone were part of the same deposit. And it was equally clear that Camposaur and the children's dig were part of one deposit. But was it all the same deposit? Late in 1981, we had our answer.

I had been sitting on the hill behind the kitchen one day, looking down on Camposaur and north toward the Brandvold site, when the thought came to me again, but this time more forcefully, that each of these deposits had the same black, battered bones of adult and juvenile maiasaurs.

I decided to try a simple test right then. What I used was a Jacob staff—a five-foot-long board with a Brunton compass attached. The staff is used to measure vertical distances between beds of rock. The Brunton compass, a common gadget in geological fieldwork, has a level and can be set to compensate for a slope in the ground so that you get a true reading of vertical distance. I started my measurements at the bottom of the anticline, in the first hadrosaur nesting ground. This nesting ground was the lowest fossil layer we identified in the anticline, and it clearly existed on both sides of the eroded scoop that separated the Brandvold/Nose Cone bones from the Camposaur/Children's bones. I measured from the first nesting ground to the other deposits on each side. And I found that the vertical distance was the same to the Brandvold/Nose Cone site as it was to the Camposaur/Children's site. By this fairly crude test (the Jacob staff is an instrument for quick estimates, not precise information), I confirmed my suspicions. All four sites were on the same fossil horizon.

We didn't get a final confirmation for this conclusion until the end of the summer season three years later in 1984, our last season at the dig. The season's work was just about over and I was walking around the anticline with Will Gavin, the graduate student who had done a study of the geology of the Willow Creek anticline. We were up on the ridge, where the children's dig had been, and he was showing me some of the peculiar geological features that he had found. In the hillside just above the bone deposit that the children had been working on, Will noticed an ash bed. This wasn't something he had found previously. He saw it as we were standing there. Ash itself was not unusual; we had found it at the Brandvold site. But this was a definite layer of ash sitting just above the bones, something we hadn't noticed at the other sites.

We immediately set out to check them. The first stop was camp, where we realized that we were standing on the ash layer. Bentonite, the stuff that turns to wet cement in the rain, is a mineral that is, in essence, volcanic ash. At the Camposaur pit we could see the layer nicely delineated, just as it had been at the children's dig. We spent all that day checking the other sites, and we found the same ash layer precisely the same distance—18 inches—above the layer of bones. This held for the Brandvold site, for Nose Cone, for two other pits of bones, which, up until that point we had not connected to the big deposit, and for some test pits we had been digging to see how far the deposit extended.

There was no question anymore. We had one huge bed of maiasaur bones—and nothing but maiasaur bones—stretching a mile and a quarter east to west and a quarter-mile north to south. Judging from the concentration of bones in various pits, there were up to 30 million fossil fragments in that area. At a conservative estimate, we had discovered the tomb of 10,000 dinosaurs.

I should point out that, although we suspected from the start that these were maiasaur fossils and we knew they were hadrosaurs, it was when we found parts of several skulls in Camposaur, in 1982, that we positively identified the animals buried in this bone bed as maiasaurs. At that time, in the early '80s, there was no other single deposit known with so many fossils of one kind of dinosaur. And it was just one kind. In all the years and all the pits we dug in that big bone bed, the only other things we found were carnosaurs' teeth and one small dinosaur of unknown variety that was rolled up in a fossilized mudball.

What could such a deposit represent? None of the bones we found had been chewed by predators. But most of the bones were in poor condition. They were either broken or damaged some other way, some broken in half, some apparently sheared lengthwise. They were all oriented from east to west, which was the long dimension of the deposit. Smaller bones, like hand and toe bones, skull elements, small ribs and neural arches of vertebrae, were rare in most of the deposit. At the easternmost edge of the deposit, however, these bones were the most common elements. All the bones were from individuals ranging from 9 feet long to 23 feet long. There wasn't one baby in the whole deposit. The bone bed was, without question, an extraordinary puzzle. First there was the terrible condition of the bones. As early as the first Brandvold site, we thought that a mud flow might have done this. However, on reflection, the condition of the bones argued for something other than animals just being buried alive, even in a vicious mud flow from a breached lake. As I mentioned before, it didn't make sense that even the most powerful flow of mud could break bones lengthwise when they were still padded in flesh and tied together by ligaments. Nor did it make sense that a herd of living animals buried in mud would end up with all their skeletons disarticulated, their bones almost all pointing in one direction and most of the small bones at one edge of the deposit. It seemed that there had to be a twofold event, the dinosaurs dying in one incident and the bones being swept away in another.

Jeff Hooker, an MSU graduate student, had worked on the fossils from the big bone bed and was the first to question seriously the idea of a herd dying in a mud flow. He was studying the bones from Camposaur—4,500 bones, representing 27 individual dinosaurs—and he began to notice certain things about the damage they had suffered.

First of all, the ones that had broken showed clean breaks, not jagged, splintery breaks. Fossil bones break this way, cleanly, like rocks. Fresh, or dry but unfossilized bones splinter. The Camposaur bones looked like they had been broken after they had been fossilized.

Furthermore, Hooker thought the bones that appeared to have been sheared lengthwise had not been broken at all. He suggested that the bones had lain on the ground, as they would have if the dinosaurs had died and rotted aboveground, and that because this was a volcanic environment, which the presence of the volcanic ash suggested and which is consistent with the known geology of the area, the ground-water would have been very acid. That groundwater could well have eaten away or dissolved parts of these bones, leaving them looking as if they had been neatly sliced lengthwise. Perhaps before or perhaps after the acid had partly dissolved these bones, fossilization had begun. Fossilization can occur before burial. That same groundwater could have been rich in minerals, starting the fossilization when the bones absorbed these minerals. It was after fossilization that the bones were swept along, in something like a mud flow, and deposited in their current location.

The layer of volcanic ash resting just a foot and a half above the bones was the key to how all these events could have occurred. You may remember the devastation and widespread ashfall caused by Mount Saint Helens. That was a little volcano. Volcanoes like that were a dime a dozen in the Rockies back in the late Cretaceous. There were much bigger volcanoes, in the Rockies, to the west of the site, and also south of the site, in what are now the Elk Horn Mountains near Great Falls. What Hooker suggested was that the dinosaurs, a herd of Maiasaura, were killed by the gases, smoke and ash of a volcanic eruption. And, if a huge eruption killed them all at once, then it might have also killed everything else around. That would explain the lack of evidence of scavengers or predators gnawing on the bones. They would have been dead, too, or perhaps they would have fled the heat, gases and fires of the volcanic eruption, not returning until the corpses had rotted. Without carnivores what we would have seen from a helicopter would have been a huge killing field, with the rotting corpses of 10,000 dinosaurs. The smell would have been overpowering. And the flies would have been there in the millions (there were flies in the Cretaceous). It must have been one hell of a mess.

Over time, of course, the stench disappeared and the killing field turned into a boneyard. Perhaps beetles were there to clean the bones. The bones lay in the ash and dirt. Some fossilization occurred, as well as some acid destruction of the bones. Then there was a flood.

This was no ordinary spring flood from one of the streams in the area, but a catastrophic inundation. Perhaps, as John Lorenz thought, a lake was breached, turning the field of death—now covered with partially fossilized, partially dissolved skeletons, unconnected by ligaments, flesh and skin—into a huge slurry as the water floated the bones, mud and volcanic ash into churning fossil soup. The bones of the maiasaurs would have been carried to a new location and left there as the floodwaters or mud settled. Had this occurred, the bones would have acquired their uniform orientation, and the smallest pieces, weighing the least, would have been carried the farthest. Finally the ash, being light, would have risen to the top in this slurry, as it settled, just as the bones sank to the bottom. And over this vast collection of buried, fossilized dinosaur bones would have been left what we now find—a thin but unmistakable layer of volcanic ash.

That's our best explanation. It seems to make the most sense, and on the basis of it we believe that this was a living, breathing group of dinosaurs destroyed in one catastrophic moment. The destruction is, of course, astonishing. But to be amazed by that is really to skip over something much more startling, and that is the herd of maiasaurs.

The notion of a herd of dinosaurs is not a new one. The paleontologist Roland T. Bird suggested that the sauropods might have been herd animals.1 There are footprints that suggest dinosaurs moved in groups, and there are numerous cases of a group of fossils of one sort of dinosaur found together.2 But there was nothing of this order-no pile, in one spot, of 10,000 dinosaurs.

The question is, was this just a bunch of maiasaurs eating together or did they have some social structure? Did they stay in this kind of herd all the time? What was the relation of the herd to the nesting ground? In other words, what do all these bones say about how these dinosaurs lived their lives? I can't say I have the answers to these questions. The physical data we can be absolutely certain of. Some forms of behavior, like colonial nesting and parental care of the young, are as certain as they can be for 80-million-year-old animals. When we get to the social structure of the maiasaur herd, we are suddenly on much shakier ground. I have clues, and I can offer them, but speculations and guesses are all I can build on them.

Jeff Hooker did some preliminary sorting of the bones from the Camposaur pit. On the basis of his work, it seems that the dinosaurs in the herd ranged from 13 feet long to 24 feet long. This means the youngest dinosaurs are missing. We know that the young grew to 4 feet long in the nest. A reasonable guess is that they reached something like 8 or 9 feet in a year, and we have found fossil remnants of 9-foot-long maiasaurs. Why didn't we find them in the herd?

Perhaps the babies of that year were still in the egg or in nests when the volcano erupted, or perhaps nesting had not even begun. This bone bed is not on the same layer as any of the hadrosaur nesting grounds, so we have no hope of finding those nests, eggs or babies. However, even if we were to assume this catastrophe caught a herd at nesting time, that still would not account for the previous year's young, animals who would have been nine feet long at the time. Where were they?

One answer is that they may actually have been in the herd. There is conflicting evidence on this point. Camposaur, the site that Hooker looked at, contains none of these nine-footers; however, at three other pits, we have found nine-footers. One pit, called Fire Ridge, is a mile away from the end of the big bone bed but may still be part of it. It has the telltale ash bed. (So the herd may have been much bigger than we realize.) The other two pits, Sacred Slump and Worthless Wash (I said we had weird names for our sites), both are clearly part of the bone bed. Fire Ridge yielded an adult and four nine-footers, and the other two each yielded an adult and three nine-footers. Now, Sacred Slump and Worthless Wash are on the edges of the big bone bed. And invariably the pits dug along the edges of this deposit show bones that are better preserved than those in the middle. It may be that the mud flow was something like a professional football game, and that the middle of it was like the point where the defensive and offensive lines meet and bones (human or dinosaur) are shattered. Small bones wouldn't have a chance. On what we might call the sidelines, however, the smaller bones would be less subject to breakage because the flow might be less forceful or because the concentration of bones would be lower. Or perhaps, because of vagaries of current in this slurry, some smaller bones might migrate to the sides of the flow.

That's a guess. Another guess, an even bigger one, is that the nine-footers stayed with their mothers in small family groups and some were joining the big herd when the volcano struck. And as long as we're involved in speculation right now, I should point out that nobody knows for sure that these dinosaurs would have produced young each year.

I hope in future field seasons to explore the edges of the big bone bed more thoroughly and get a bigger sample of bones. Until then, we can only talk about what we know. So far the story of maiasaur growth and social life seems to be hatching and growth in the nest, followed by a period of mystery, followed by joining a large herd or at least an aggregation. In biology a "herd" has a clear social structure; it's not just a random aggregation. In some species the males form the

Like the American bison millions of years later, herds of thousands of maiasaurs
traveled the late Cretaceous plains in search of forage.

perimeter of the herd and the females and young stay in the center. We have no evidence that this was true for the maiasaurs, because we don't know how to tell a male from a female, for one thing, and because the collection of fossils tells us nothing about how the dinosaurs arranged themselves in life. They were together, I'm sure of that, but I have no idea who had what place in line.

How did they behave in their aggregation, then? How did they mate? How did they defend themselves? Because these were social animals, and herbivores, I suspect that some of their behavior might have been parallel, in at least rudimentary form, to what we see in social herbivores today—namely, competition among males for female mates. Perhaps the problems that large herbivores must solve in terms of finding food, reproducing, and defending against predators are the same for any large herbivore and call forth similar solutions. We know at least that the maiasaurs had these huge aggregations, throwing all the males and females together and opening up the possibility for sexual competition. We also know that some duckbills had bodily characteristics such as frills of skin and cartilage running down the spine. Others, the lambeosaurs, had the elaborate skeletal crests. Why?

The answer I hold to is that the purpose of these characteristics was to attract mates.3 And I would say the same for the horns of Triceratops and its kin and the clubs of the ankylosaurs. I believe sexual attraction, not defense, was the reason these characteristics evolved. This is, in fact, one of my pet peeves. To me there is no subject on which so much nonsense has been heard as defense among dinosaurs. It's not just the paintings of Triceratops battling Tyrannosaurus made for popular consumption. Scientists, too, assume that horns and clubs were weapons. This is seldom the case among animals today, and I doubt things were different in the Cretaceous. All the existing herbivores with great antlers developed them to attract mates and conduct sexual combat between males. Occasionally, very occasionally, the horns may be used, opportunistically, to fend off a predator, but I doubt that this usually succeeds. An elk is better off using its feet rather than its horns to attack wolves. Bighorn sheep use their magnificent horns to butt each other. Elk and moose do the same. And so, I bet, did Triceratops.

I see no sign of combat among the maiasaurs, but I can imagine females making nesting sites and waiting there for dominant males to come to them. Who knows how the males might have established dominance—by vocalization, perhaps, or some kind of display of their frills, to seem larger. As for defense, I think the hadrosaurs and the lambeosaurs, all of them, and certainly the maiasaurs solved that problem by gathering in the herds that we now know some of them formed. This is the common defense in large mammalian herbivores today. The dinosaurs were also large terrestrial herbivores, and they may have come up with this solution first. Of course, predators still took their share. It doesn't pay to think of evolutionary adaptations for defense as if the animals themselves were deciding how to fend off those nasty meat-eaters. The process would go more like this:

Imagine early herbivorous dinosaurs with no innate instinct to gather together. Predators chase and kill whichever ones they can, finding them alone among the berry bushes. Suppose that some of these dinosaurs are born, through random shuffling of the genes, with the desire to stay in groups. These social dinosaurs do better in the struggle for survival, perhaps because it seems easier to a predator to attack a lone animal, or because in a group there are a number of eyes and ears to warn of attack. Consequently, the social dinosaurs produce more offspring, all of whom share the genetically programmed instinct to stick together. Over the course of time, the loners disappear. Predators adapt to following the herd, picking off the weak, the old, and the young who stray—in effect culling out the poorer specimens. The result is a kind of balance that is achieved, a state of equilibrium, and the word "defense" may be somewhat misleading.

I wonder a bit about how these kinds of herds affected the environment. Certainly the herds had to keep on the move. They must have stripped one area and then moved on to the next. I don't think Maiasaura migrated down to the sea and back, since her fossils have never been found in lowland areas. I suspect, although I have no positive evidence for this, that the maiasaur herds migrated seasonally on a north/south pathway. Perhaps it's hard to imagine dinosaurs in great herds surviving on the sedges and berry bushes that were prevalent in the upper coastal plains during the late Cretaceous. But think of the bison. One estimate has it that in North America, at the start of the nineteenth century, there were 60 million of them. All they ate was grass.

Given the fossils we have, I think that when all the scientific papers are written, when the studies of the bones are completed, Maiasaura may end up being the best-documented dinosaur paleontology has seen. Because of the wealth of bones of this one species we have the chance to learn more about her, from basic morphological structure, to evolution, to social behavior, than we know about any other dinosaur. This wealth of information that she has presented us with is a remarkable yield for one dig. But she is not the only dinosaur to come out of the Willow Creek anticline. The dig produced fossils of another dinosaur as well. And I don't mean just a few bones. I mean eggs, skeletons and nesting grounds—another entire dinosaur world.

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  • Senait
    Why do dinodaur babies stay in herds?
    7 years ago

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