Waltzing To The Rhythm Of The Western

By mid-June 1988, after almost five weeks of digging, the crew at Hillside Quarry had recovered three skeletons of Gryposaurus lati-dens—an adult and two juveniles. But the amount of overburden had increased rapidly as they dug into the steep slope, trying to expose more of the narrow sedimentary layer in which the bones were entombed, and it now represented too great an obstacle to further excavation. I closed the quarry and transferred everyone to our new digs. From West Hadrosaur Bone Bed came dozens upon dozens of skeleton fragments, representing at least nine half-grown maiasaurs. An intact, undamaged hypacrosaur skull, two and a half feet long, was soon unearthed at Jason's Giant Site. And at Jack's Birthday Site the fossils were so plentiful that it took a crew three days to record and collect what lay on the surface alone. They then started to excavate in earnest the green-gray mudstone that forms the staggered backbone of the ridge.

Once again I set out to survey more of the region. Badger Creek joins the Two Medicine River at a spot where the river turns northward. The creek, then, runs from west to east, just as the river does farther downstream. Judging from its stratigraphic features and the dinosaur remains it contained, Jack's Birthday Site is located within the upper three hundred feet of the Two Medicine Formation, the same period of time represented by the sediments of Landslide Butte. Further evidence that the two are contemporaneous lay immediately west of the site—Bearpaw Shale marine deposits, marking the top of the formation and the end of dinosaur hunting along Badger Creek. Having discovered this, my first inclination was to explore badlands that parallel the creek drainage—that is, run from west to east, due north of Jack's Birthday Site. And I thought I found what I was looking for along the river about three miles upstream of the mouth of Badger Creek, on land leased by Truman Hall, an Indian who raises bucking horses and Brahma bulls for rodeos. During my first visit to the Hall ranch I turned up a few specimens, including baby bones, and several potential egg sites, but nothing that got me excited.

Accompanied by Carrie and Jason, I next visited an area near Blacktail Creek, about ten miles to the south and, ironically, only a few miles from Four Horns Lake, where three years earlier Bob Makela and I had excavated the mosasaur. I chose the spot because in his field notes Barnum Brown had described collecting there, briefly, and retracing his hurried footsteps had been a fruitful strategy ever since I arrived at the Two Medicine River. The move to Blacktail Creek would prove to be no exception. On June 23, our first day in the area, Jason and I discovered a large accumulation of baby hypacrosaur bones, jumbled together in what appeared to be a stream-deposited sandstone. In other words, the bones had washed in from elsewhere, though the absence of scratched or broken fossils indicated that they hadn't been carried over a large distance. Upon further exploration we discovered why they hadn't traveled far—the ancient stream had meandered alongside a hypacrosaur nesting ground. At Blacktail Creek North, our name for the site, Jason and I

found several discrete collections of eggshell that almost certainly were the remains of nests. Then Carrie made an outstanding find—a clutch of eggs containing skeletal fragments of hypacrosaur embryos.

Roughly two hundred yards square, the Blacktail Creek North nesting ground is only about one-tenth the size of Datum Ash Layer, the rookery at Landslide Butte. Blacktail Creek North also differs from its northern counterpart in that the skeletal elements are concentrated in an area where the ancient stream abruptly widened, leaving behind a splayed deposit, whereas those at Landslide Butte are distributed randomly across the entire horizon. And at Landslide Butte, of course, the rookery lies beneath a layer of volcanic ash, an immense number of eggs and small nestlings having been buried alive. The cause of death at Blacktail Creek North is unknown, but given the preponderance of drought-induced die-offs recorded in the upper layers of the Two Medicine Formation, it wouldn't be surprising if that were the case here as well. Despite their differences, however, the two sites are similar in the ways that count most. For one thing, they are the same age. For another, they contain the remains of the same dinosaur, which Philip Currie, of the Royal Tyrrell Museum of Paleontology, and I eventually named Hypacrosaurus stebingeri, to honor the memory of Eugene Stebinger, the geologist who first described the Two Medicine Formation.

In the family drama we witnessed at the Willow Creek anticline, Maiasaura had held center stage, with Orodromeus and Troodon playing crucial supporting roles. But at both Landslide Butte and the Two Medicine River, in sediments laid down 2.7 million years after those of the anticline, Hypacrosaurus stebingeri stepped into the spotlight while Maiasaura, though still present and important, withdrew into the background. From that rainy August afternoon back in 1985 when I discovered the first bones of the new crested duck-billed dinosaur in a rain-soaked patch of ground I christened Lambeosite, we had encountered Hypacrosaurus stebingeri just about everywhere we looked in the top of the Two Medicine Formation—the Landslide Butte rookery, Jason's Giant Site, Jack's

Birthday Site, and now Blacktail Creek North. Not only had we found the new dinosaur in abundance, we had recovered complete skeletons representing the major stages of its life cycle—embryos, juveniles of several different ages, and adults. Rarely is so complete a sample ever assembled, never mind as quickly as this. And most exciting of all, we had found nesting colonies, two of them, in fact, which told us that Hypacrosaurus, like Maiasaura, Orodromeus, and Troddon, was a gregarious dinosaur, a social animal. It routinely gathered with its own kind to build nests and to lay its eggs. That now was indisputable.

But what about other aspects of parental behavior? Did hypacrosaur adults brood? Did hatchlings remain in the nest, their parents feeding and protecting them until they grew large enough and strong enough to fend for themselves? With respect to these questions, the available evidence, though copious, remained

Hypacrosaurus

Skull of a juvenile Hypacrosaurus stebingeri from the Blacktail Creek nesting site. Note that the juvenile does not have the extensive nasal crest seen in the adult. (Bruce Selyem, reproduced courtesy of the Museum of the Rockies.)

ambivalent. The specimens present in our hypacrosaur excavations constituted the greatest volume of egg, nest, embryo, and juvenile material of any species of duck-billed dinosaur in the world. Yet we hadn't found a single hatched baby within a nest, as we had at the anticline, the breakthrough discovery that had convinced me that Maiasaura tended to the needs of its young following their birth. That in itself was puzzling. But, using the Null Hypothesis approach to prove my own ideas wrong, was it reason enough to abandon the parental care hypotheses altogether, at least insofar as Hypacrosaurus was concerned?

No, it wasn't. As I say, the evidence was ambivalent. Equally puzzling, and possibly contradictory, was the limited range of ages represented by the skeletons in the Landslide Butte rookery and at Blacktail Creek North. They consist exclusively of embryos, confined largely to nests, and juveniles up to about four feet in length, all found within the boundaries of the nesting ground. At neither location did we find larger juveniles, subadults, or adults. By the same token, Lambeosite and Jack's Birthday Site contain no hypacrosaur skeletons of nestling size; the smallest juveniles in those bone beds are considerably larger than the largest juveniles in the nesting grounds. Merged into a single, coherent picture, the two lines of evidence would seem to suggest that hypocrosaurs left their nests upon hatching but remained within the nesting ground until they reached a certain size, after which they departed the colony, joining herds comprised of subadults and adults. If they ever returned, it was only after they had reached breeding age themselves and instinctively found their way back to their native nesting grounds, a scenario that certainly is consistent with the overlapping colonies at Landslide Butte.

I had a hunch why only small juveniles were present in the colonies but I required additional information before I could be reasonably sure. Some of what I needed to know could come only from examining the bones of the embryos and nestlings much more closely, using the computer-aided imaging devices and microscopes back at the museum laboratory, a project that would have to wait till the winter, or perhaps the winter after that, depending on how quickly the specimens were cleaned and prepared. At the moment, a more pressing task presented itself—getting the specimens out of the ground. Although I'd assembled the largest field crew ever, all of them were occupied at our other sites, leaving only Carrie, Jason, and me to excavate the hypacrosaur nesting colony. There would be no more scouting trips for me in 1988. It took the three of us three full weeks to unearth the accessible fossils, representing at least eighteen juvenile dinosaurs, which we packaged in three five-hundred-pound plaster jackets. By that time, the last of the adult hypacrosaur bones had been removed from Jason's Giant Site and the excavation was complete. At the end of the month we closed down the rest of the quarries for the season and returned to Bozeman, but not before one last, haunting discovery.

Driven less by the expectation of finding something important than by a stubborn reluctance to quit looking, I returned to another site on Blacktail Creek on the final day of July, and there I located and collected a nearly complete Troddon skeleton, the first ever from North America. Troddon is a curious little theropod. From head to tail it was about six feet long. Rearing back on its hind legs—Troddon was bipedal—it stood no more than four feet high, if that. But like its distant cousin, Deinonychus, it was an aggressive carnivore, though not nearly as fierce or effective, probably preying on lizards, small mammals, and young dinosaurs such as the Orodromeus juveniles that lived at Egg Mountain, where Troddon teeth are plentiful. Troddon had a relatively large brain and, perhaps most interesting, large, forward-facing eyes, leading some paleontologists to believe that it possessed stereoscopic vision. Indeed, only a few years before I found the skeleton, Dale Russell, then with the Canadian National Museum of Nature, in Ottawa, speculated that had Troddon survived, it might have given rise eventually to a sapient, two-legged, upright creature that resembles human beings, one that could be sharing the planet with us today— or, more sobering, might have superceded us.

That, of course, is the sort of imaginative exercise that is great

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