2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 338-3
Presentation Time: 2:00 PM


MITCHELL, Emily G., Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom, ek338@cam.ac.uk

Bedding-plane assemblages of Ediacaran fossils at Mistaken Point, Newfoundland (~565 Ma), are among the oldest known examples of macroscopic communities. The constituent organisms have few similarities with living forms, making their relationship with their environment difficult to assess. Their preservation in large in-situ bedding plane populations allows original spatial distributions to be analysed, shedding light on the interactions between taxa and their environment. Mistaken Point ‘D’ and ‘E’ surface communities were mapped using differentiated GPS to millimeter accuracy. For each surface, the spatial correlations between taxa were identified using Bayesian network inference (BNI). These spatial distributions were then described using pair correlation functions (PCF) and compared with different types of models, to deduce the most likely underlying ecological process.

BNI found eight ‘E’ surface inter-taxa relationships, but none on the ‘D’ surface. For the ‘E’ surface relationships, PCF analysis showed that five were the result of impact by four different environmental heterogeneities, and the remaining three relationships the result of interactions between living and dead organisms. The environmental interactions had a relatively small impact: segregations (reduced taxa density) of 25%, and aggregations (increased taxa density) of 56%, which is in stark contrast to individual taxa which had increased aggregation between 250% and 600%. Two interactions exhibited small scale aggregations with large segregations, for which shared source models fit the aggregations best, indicating environmental interactions, not biological ones such as facilitation. The presence of these environmental aggregations shows that the large scale segregation isn’t due to an inability to settle - it can only be due to competition between the taxa. The spatial distributions, scales and magnitudes of these competitive interactions were found to mirror extant communities where taxa reduced inter-specific competition by niche differentiation instead of trading off competitive advantages over colonization ability. My results show that while environmental interactions of Mistaken Point organisms only had a small impact on survival, it was still enough to drive competition and likely niche differentiation.