When we think about the distributions and community ecology of terrestrial vertebrates today, we generally find that large-bodied species (e.g. deer, bison, caribou, wolves) tend to be found across a very large area. White-tailed deer, for example, are found from Central America to southern Canada. Another example is the grey wolf, which historically ranged throughout most of North America, Asia, and Europe, from the high arctic to the sub-tropics. These broad distributions suggest a tolerance to environmental differences on the part of large-bodied species.
An interesting and perplexing pattern in the palaeoecological record is that large-bodied dinosaurs, at least in the Cretaceous of North America, do not appear to have followed this trend of very broad distributions. During this time, North America was often bisected by a large shallow sea (the Western Interior Seaway), and dinosaurs in western North America lived on a relatively narrow strip of land (sometimes referred to as Laramidia).
A long-standing explanation for the high diversity of dinosaurs across this relatively small landmass is that they were highly sensitive to environmental differences (particularly distance from the sea), and as a result different large-bodied species would share non-overlapping ranges despite sometimes being found less than a few hundred kilometres apart.
Our goal was to test this hypothesis of dinosaur environmental sensitivity. We wanted to see if in two environmentally different, but time equivalent settings, dinosaurs showed ecologically distinct trends in occurrence and abundance. To do this, we needed to show what the overall pattern in vertebrate community structure was in these two settings, and in the broader context of environmental change over time in this region.
We collected data from dozens of vertebrate microfossil sites (or microsites for short). Microsites are concentrations of small vertebrate fossils representing a palaeocommunity from a short time interval, and are thought to have formed in low energy ponds and rivers, acting as sink for material from the region. The relative abundance of different groups can be counted and compared between sites to identify how the community is changing temporally, geographically, and environmentally.
All of the microsites we analyzed were collected from the Late Cretaceous of Alberta, in two geographically distinct sampling areas, and all from a geological unit called the Belly River Group. Sediments of this unit record a transition from marine to terrestrial setting, which is followed by a mostly terrestrial component (reconstructed primarily as an upper coastal plain, like inland Louisiana today), and finally a shift back to more marine influence (a lower coastal plain, similar to the Mississippi delta of southern Louisiana).
Due to the depositional history of the area, there is a time-equivalent interval where one of the sampling regions records a lower coastal plain environment, and the other sampling region records an upper coastal plain environment. This provides the temporally equivalent but environmentally distinct interval in which to test our hypotheses.
We found that the relative abundances of vertebrates in the palaeocommunities preserved in these microsites changed in response to fluctuations in sea level. The largest change was an inverse shift in abundance between sharks/rays and amphibians, with the former dominant in marine-influenced intervals and the latter dominant in terrestrial-influenced intervals. While not surprising, that result confirms that these communities are responding to environmental changes, and that our analyses can detect that.
The next step was to analyze sub-sets of the data, specifically the time-equivalent but environmentally distinct interval in the two sampling areas. We found that dinosaurs, and particularly large-bodied groups, did not vary to any noticeable degree in their relative abundances through this time-equivalent interval. In fact they were highly similar in abundances in both environments.
Our results indicate that this long held notion of dinosaurs being highly sensitive to environment and sea-level may not be correct. But then what explains the high diversity of large-bodied dinosaur species present across the Cretaceous of North America? Some of the distant regions of western North America may not be as geologically time-equivalent as we currently think, or were isolated geographically in some way that prevented migration.
It’s also possible that despite not being particularly sensitive environmentally, that dinosaurs were partitioning their ecological niches in other ways that allowed for this diversity, such as a restriction on feeding heights or a dietary preference for particular plants. Perhaps primary productivity during this time was sufficiently high that competition was lower and strict niche partitioning was not required.
Our study addresses a long-standing question in dinosaur ecology, though it alone cannot provide the full answer. Many dinosaur teeth can only be diagnosed to the family or subfamily, not the species. So while we focused on trends in major dinosaur groups, additional work must be done to locate more of the much rarer dinosaur macrofossils in order to completely address what is driving dinosaur species distributions.