Stable Isotope Ecology of Coastal Floodplain Communities
Using extant models to constrain predictions & test hypotheses of dinosaur palaeoecology
Similar to the above research using vertebrate microfossil bonebeds to understand Cretaceous community structure and its response to environmental changes on broad scales, I am interested in performing research at finer temporal and spatial scales in order to understand community structure, habitat preferences and niche-partitioning, as well as diet and trophic interactions. While my interests in these questions pertain to whole communities, and all the taxa within them, I have a particular interest in the ecology of dinosaurian fauna. Considerable research has been performed attempting to characterize the habitat preferences, diet, and niches of dinosaurs, and stable isotope analyses, by virtue of recording the original diets, habitats, and behaviours of these animals, offer a potential avenue for understanding those topics. However, prior to performing detailed analyses of ancient systems, we should understand the constraints that may exist on our inferences, and where possible frame our predictions based on data from similar systems today. To that end, a large component of my PhD research focused on characterizing the stable isotope ecology of vertebrate communities in both extant and ancient systems, particularly coastal floodplain forests.
(Contrasting images of Cretaceous sedimentary rocks preserving fluvial and coastal floodplain environments, and their modern near-equivalents from the Atchafalaya River Basin of Louisiana.)
First, stable isotope analyses were performed on vertebrate taxa sampled (both opportunistically and obtained through local collaborations and/or museum collections) from within the modern Atchafalaya River Basin of Louisiana, an analogue for the ecosystems and environments preserved across much of western North America during the Cretaceous (Cullen et al 2017 SVP abstract, Cullen et al 2019). One of the major constraints limiting palaeocommunity analyses are the lack of clear modern analogues against which to test broad-scale ecological and evolutionary hypotheses, such as mechanisms of community assemblage and patterns of biogeography, within the context of deep time. Our study of the Atchafalaya Basin identified patterns of ecological resource use and aquatic-terrestrial interchange in this system, facilitated the development of improved methods of bioapatite oxygen-temperature inference, and provided an isotopic baseline for use in testing hypotheses in the fossil record.
The Atchafalaya River Basin study made use primarily of fieldwork 'opportunistically collecting' specimens, that is to say – using roadkill and other carcasses. This was surprisingly effective, though obvious some taxa are less easily locatable in this manner. As a result we also partnered with the Louisiana Department of Wildlife & Fisheries, as well as local universities, naturalists, and private citizens to obtain our full sample. Having this broad sample, with excellent observational natural history data and multiple tissue types facilitated our isotopic research.
(Stable isotope analyses of Atchafalaya communities, Cullen et al 2019)
Nevertheless, even with well-established natural history data on ecological variables for our Atchafalaya River Basin taxa, such as trophic habits and microhabitat use, the subtleties involved in identifying these patterns in our isotopic data highlight the difficulties involved in detecting and interpreting ecological patterns in a complex C3-based system. It therefore stands to reason that for a palaeoecological system where the description of niches often requires significant inference (particularly among non-avian dinosaurs, for which there is no exact extant analogue), using isotopic data to describe fine-resolution details of niche partitioning should be done conservatively. These comparative data should assist in constraining predictions and limiting the degree to which palaeoecological-proxy data may be over-interpreted.
Having established the Atchafalaya Basin as an appropriate modern comparator for a variety of Cretaceous animal assemblages opens up the potential for a variety of phenomena to be investigated such as habitat use, niche partitioning, diet, and physiology (Cullen 2018 CSVP abstract, Cullen 2018 SVP abstract, Cullen et al In Review, Cullen et al Submitted). These fine-resolution questions are rarely accessible within a palaeontological context, and the framework developed through our extant comparisons will facilitate an expansive range of future research. Thus far, I have presented on some of the results of these fossil stable isotope analyses, where we have determined that isotopic baselines in the Cretaceous were shifted relative to the modern day (explaining why several previous studies, as well as our own, find that dinosaur carbon isotope ratios are higher than expected for animals in a C3-based system), that broad overlap exists in the isotopic distributions of large herbivorous dinosaurs (suggesting that at least in the Campanian of Alberta that these animals were not partitioning their niches through differences in microhabitat use), and that much like in extant coastal floodplain forests there was considerable aquatic-terrestrial resource interchange in these Cretaceous communities. These results, as well as some others, are not yet published, though they are detailed in several conference presentations and my PhD thesis, with two papers relating to these results submitted to or under peer-review for publication. Additional details will be posted soon as these papers are accepted for publication.