Harvard University and Museum of Comparative Zoology, United States
Background/Question/Methods
The Cambrian Explosion is one of the most significant biotic events in the history of the Earth. During this time, the complexity of interactions between animals as well as with their environments increased rapidly, in turn leading to more complex community structures than the preceding Ediacaran period. Thus, a clear picture of the structure of Cambrian animal communities is integral to understanding the origins of modern ecosystems. However, relatively few Cambrian fossil sites preserve the total animal community, including the most soft-bodied taxa. Additionally, datasets with high stratigraphic resolution, which are necessary to understand fine scale spatiotemporal gradients, are rare. As a result, fundamental aspects of Cambrian community ecology, such as trophic structure, remain cryptic. In this study, we integrate four Cambrian datasets from the Canadian Burgess Shale and Chinese Chengjiang fossil site to quantify patterns of ecological change in some of the earliest complex animal systems. This data is partitioned into sequential 10cm bedding assemblages, which allows for uncommonly high resolution spatiotemporal pattern inference. We analyze these patterns principally through the lens of functional diversity and community structure, informed by trait data such as trophic mode, ecologically relevant characters such as defensive structures, and biovolume.
Results/Conclusions
Our analyses lead to two primary conclusions. The first is that Cambrian animal communities were highly variable in their structure over relatively short ecological timescales. All four localities show marked fluctuations in three measures of functional diversity over time, rather than maintaining a consistent structural state. Secondly, there is high differentiation across communities, with each of the localities having a fundamentally different structure than the others, particularly as exemplified by the distributions of biovolume across trophic groups. Together, these results reinforce the ecological complexity of Cambrian ecosystems, during which time marine animal communities began to resemble modern ones. Further, they highlight how little is generally known about Cambrian community ecology, as the vast majority of studies employ only very simple metrics of species richness or presence/absence based analyses. Moving forward, integrating more rigorously sampled datasets with time series information and functional traits is necessary to fully understand the ecological dynamics of the earliest complex ecosystems.
