ratios of the isotopes to the parent element we can determine how long the argon has been in the ground and thus the age of the surrounding rock. But there's very little volcanic ash in the Judith River Formation.
In the absence of measurable argon, we must rely on less precise methods: comparing the sediments with other strata, in both the Judith River Formation and nearby formations, such as the Two Medicine, which have been dated, and studying the sediments themselves for clues to the kind of environment they represent. At this juncture in her comparative analysis, which she's conducting for her thesis project, Vicki tends to believe that the sediments, and thus the nesting horizons embedded within them, lay near the top of the formation, which is to say that they were deposited fairly late during the final expansion of the seaway. In fact, she estimates that the sea was no more than twenty miles away when the duckbills laid their eggs. But I'm not yet convinced this is the case.
One thing, however, is certain: Though the Milk River sandstone northeast of Havre unmistakably represents a lowland envi ronment, the strata that contain the nesting horizons, which are comprised of silty mudstone, resemble those of upland environments. They are bright gray, for instance, and in some places almost green, which means that they were well drained. This suggests to me that the nesting horizons were preserved because the surrounding sediments were somehow spared by the numerous, widely meandering rivers that we know scoured the eastern part of the coastal plain, erasing most traces of dinosaur existence. Given the swamplike condition of much of the region, it stands to reason that the Milk River duckbills would have favored any location that remained high and dry, so to speak, returning often to lay their eggs. And that's exactly the scenario that the evidence points to at the two major nesting horizons we discovered there.
The exploration of the Judith River Formation is still a work in progress. Since we haven't yet found a skull associated with the eggs, babies, and juveniles of the Milk River badlands, we don't know exactly which duckbills constructed the colonies there.
Even so, nothing we found during the study of dinosaur survival strategies in the Judith River Formation, or, for that matter, elsewhere in Montana during the early 1990s, nullified our original ideas about parent care and other forms of social behavior. Yet a striking new pattern had developed, one that I couldn't have predicted on the basis of our findings at the Willow Creek anticline alone. To be specific, after almost twenty years of exploration, the Maiasaura nest Bob Makela and I unearthed in 19 78 remains the only one that contained post-hatchling babies. In every other nesting horizon we discovered, from the colonies at Landslide Butte and Blacktail C reek North to the Milk River egg sites, we found plenty of baby and juvenile bones, in some instances thousands of them, but no skeletons inside a nest. Significantly, though, the immature skeletal remains were always confined to the colonies. By the time we left the Blackfeet Indian Reservation, this pattern—baby bones within horizons but outside nests—had become so firmly established that I began to think of it as the rule rather than the excep tion. Indeed, a little experiment I conducted in 1991 convinced me that I shouldn't expect anything else. It turns out that although our ideas about parental care have withstood the test of further research, the starting point for those ideas, the first Maiasaura nest, is an anomaly.
I'll resolve this apparent paradox, as well as describe the crucial experiment, in the next chapter. But first I wish to point out that the reinterpretation of one of the key specimens of the Willow Creek anticline exemplifies how the meaning of paleontological evidence shifts as additional evidence is gathered. As I said earlier, paleontology is a field for those who place a higher value on knowing what is real—practically speaking, a continuous process of revision—than on being right. As historical scientists we cannot conduct experiments on our subjects; they're dead and, to make matters more difficult, the environments they inhabited have vanished. Nor can we derive conclusions from first principles or perform calculations based on universal equations. Instead we examine the evidence, the fossils and the geological contexts within which they are found, then reconsider it, and reconsider it once more, going back again and again, revisiting previous stages in the history of paleontology while at the same time returning to earlier periods in the history of life.
I've already described this method in terms of retracing footsteps, those of other collectors, amateur and professional alike, as well as one's own. But one might also characterize it as an attempt to study something from all angles, a circling approach, very much like that of a hawk as it hovers above a field in search of prey or the downward spiral of a flock of pelicans landing on water. That's how I see our most recent expeditions in the Milk River badlands. I returned to the terrain where Bob Makela and I taught ourselves how to track down and dig up dinosaur fossils—returned, essentially, to my origins as a paleontologist—finding things the second time around that we never dreamed existed there. And given the cyclic nature of my work I suspect that someday I'll go back for a third look, with new eyes, and, if I'm lucky, leave with yet another version of what happened there.
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