Structure & Dynamics of Cretaceous Communities
Using VMB data to test long-term biotic responses to environmental perturbations in dinosaur-dominated systems
The Late Cretaceous was a time of high biodiversity, relatively high global temperatures, and considerable fluctuation in regional sea levels. By characterizing the sedimentary environments in these systems, and quantifying the structure of Late Cretaceous ecological communities, as well as spatial and temporal changes in these systems, we can gain a greater understanding of how ecosystems respond to major environmental shifts, and should provide further insight into predicting extant ecological responses to climate change.
One method for estimating Cretaceous vertebrate community structure is through the study of vertebrate microfossil bonebeds (VMBs), which are mass accumulations of small teeth, bones, and scales deposited over geologically short timescales in ancient wetland/lake/river environments, and these assemblages are thought to be representative of the average relative abundance of different clades within their contemporaneous ecological communities. A major component of my PhD thesis focused on using data from Late Cretaceous VMBs to test hypotheses of community responses to environmental perturbation.
One method for estimating Cretaceous vertebrate community structure is through the study of vertebrate microfossil bonebeds (VMBs), which are mass accumulations of small teeth, bones, and scales deposited over geologically short timescales in ancient wetland/lake/river environments, and these assemblages are thought to be representative of the average relative abundance of different clades within their contemporaneous ecological communities. A major component of my PhD thesis focused on using data from Late Cretaceous VMBs to test hypotheses of community responses to environmental perturbation.
(how a vertebrate microfossil bonebed appears at the surface, with multiple weathering fossils)
(example of some of the many fossils that can be discovered within a microfossil bonebed)
The first of these investigations focused on describing a new VMB assemblage from the lower Belly River Group, during the regressive phase of sea level drop near the boundary between the Foremost and Oldman formations (specifically in the upper Foremost Formation, just below the Taber Coal Zone). The site preserved a transitional fauna containing a mix of marine taxa (like sharks and other chondrichthyans), and taxa with terrestrial affinities or no strong environmental affinity (turtles, dinosaurs, eusuchians, etc). In addition, the site preserved the first record of a large hybodont shark cephalic spine from the Cretaceous of Alberta, and the first record of the ratfish Elasmodus from the Foremost Formation. Comparisons of the site with other sites from the Foremost and Oldman suggested that certain groups (such as lissamphibians and shark) were integral for palaeoenvironmental inference from microsite assemblage data.
This paper was published in the February 2016 issue of the journal Palaeogeography, Palaeoclimatology, Palaeoecology.
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In the next stage of this project, we combined the data collected above with those from dozens of other VMBs (48 sites total), sampled from two regions approximately 150 km apart and spanning ~5 million years over the stratigraphic extent of the Belly River Group in Alberta, in order to analyze changes in community structure (via relative abundance among different taxa), and how these changes correlated with environmental changes related to transgressive-regressive cycles of regional sea level (as well as other factors). Specifically, we quantified changes in species composition and relative abundance between sites, and performed a series of analyses (including R- vs Q-mode cluster analysis, redundancy analysis, and pairwise relative abundance and similarity analyses) comparing the community trends against abiotic factors such as site sedimentology/deposition, sampling location, stratigraphic position, and palaeoenvironmental setting. A specific hypothesis of interest in these analyses was to test if dinosaurs were particularly sensitive to microhabitat changes driven by shifting altitudinal gradients (i.e. sea level changes), as has been hypothesized by a number of previous studies. Our analyses found that the strongest abiotic factor controlling community structure was palaeoenvironment, with site sedimentology, stratigraphic position within the Belly River Group, and sampling location having lesser impacts. On the broader community scale, the most severe changes in community structure occurred between chondrichthyans and lissamphibians, which showed a strong inverse relationship in relative abundance, particularly during shifts from marine-to-terrestrial or terrestrial-to-marine settings. While that result in of itself is not particularly surprising, it is important in establishing that community responses to environmental perturbations are detectable, and serves as a useful comparison to the dinosaur components of the community, which were largely unaffected in relative abundance by those changes (with the exception of periods at the very bottom and top of the Belly River Group where the system becomes fully marine and consequently almost all terrestrial fossils disappear). The results so far suggest that dinosaurs may not be particularly sensitive to small-scale environmental perturbations or changes in microhabitat.
This paper was published in November 2016 in the journal BMC Ecology, It was an Editor's Pick and BMC Ecology top 10 highlight for 2016, and was the subject of a BMC Series blog.
This paper was published in November 2016 in the journal BMC Ecology, It was an Editor's Pick and BMC Ecology top 10 highlight for 2016, and was the subject of a BMC Series blog.
(Temporal & spatial community relative-abundance comparisons, Cullen and Evans 2016)
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(Redundancy analysis of community & environmental data, Cullen and Evans 2016)
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We are currently expanding these comparisons through the sampling, description, and analysis of additional VMB assemblages. The first step of this process involves sampling sites from a currently unsampled interval from the uppermost Oldman Formation in the Milk River / Manyberries region of southernmost Alberta and several locations in the Judith River Fm of Montana, also collecting more detailed stratigraphic data throughout the sequence. Going forward, we will be expanding sampling both spatially and temporally to create a broad dataset for testing macroecological hypotheses. Of particular interest is the further quantification of trends in community structure across environmental gradients, the integration of functional trait components and non-traditional stable isotope systems into the analyses to gain even finer resolution of environmental and biotic factors, and also testing of how these systems respond to more abrupt, larger-scale perturbations such as were seen in the K-Pg mass extinction.
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Another area of recent interest has been in using the Belly River Group VMB data to examine sources of preservational and analytical bias in palaeo-macroecological datasets. This work was done in collaboration with Dr. Karma Nanglu (for his part examining his Burgess Shale palaeocommunity data). Together, we took our examples of datasets with high spatiotemporal resolution, large sample sizes, and good stratigraphic/environmental data, and examined the biases that impact them, how they compare to more 'standard' data quality in palaeo, and the impacts that can have on analytical outcomes and interpretive abilities (artificially downgrading the quality of our datasets to demonstrate this quantitatively). This work is current submitted and under review.