Biology subjects

Mitchell, E. G.

Publications and source records attributed to Mitchell, E. G..

9 recordsLinked to original sources

bioRxiv2026

Diversity in early animal communities of the Ediacaran, Avalon assemblage

Diversity is an emergent property of macroecological and macroevolutionary processes. The Avalon assemblage preserves near-census snapshots of Ediacaran communities, enabling tests of the drivers of diversity. However, it remains unclear how alpha diversity varied among surfaces or whether variation reflected uneven sampling, abiotic context or biotic processes. We analysed diversity for 11,342 specimens across 22 surfaces and examined abiotic and biotic correlates. Despite compositional variation, most surfaces fell into four community types dominated by Fractofusus andersoni, F. misrai and/or Pectinifrons, Bradgatia, or upright fronds, with potential substructure within the upright-frond type, whereas Clapham's Pigeon Cove and Shingle Head communities, from the oldest and youngest formations, respectively, may indicate greater community differentiation near the sampled stratigraphic limits. Sampling effort varied widely, yet taxon-area curves showed richness was well captured on 12 of 22 surfaces, and Shannon and Simpson diversity remained consistent across sampling completeness and taxonomic treatments. Diversity showed no clear associations with locality or stratigraphic affiliation, but differed among community types and declined with increasing vertical segregation. Overall, these results suggest that, among the predominantly Newfoundland communities sampled, Avalon alpha diversity may have been shaped more by local community assembly and organisation than by broader geographic, environmental and temporal context.

paleontology↗

bioRxiv2025

Community ecology dynamics on reefs in Fiji are dependent on soft coral density and depth

Some soft corals (order Alcyonacea) have the potential to increase in prevalence on tropical coral reefs as the severity of anthropogenic climate change increases. While soft corals are therefore an increasingly important component of reef ecosystems, little is known about their ecological role on coral reefs and their influence on community dynamics and diversity. We used Bayesian Network Inference to identify the relationships among benthic taxa across sites with varying degrees of soft coral dominance on the Great White Wall, Fiji, and then employed spatial point process analysis to reveal the ecological processes behind these associations. We found that depth was the dominant driver of community dynamics, and that white Nephtheidae soft corals were negatively associated with scleractinian corals and positively associated with algae due to a facilitative mutualism - possibly due to soft corals reducing grazing pressure. Our results show distinctions in reef benthos and ecological dynamics between scleractinian- and soft coral-dominated reefs. We found that diversity levels were significantly lower on soft coral than scleractinian reefs, potentially highlighting the risk of a loss in benthic diversity on reefs where soft corals replace scleractinians.

ecology↗

bioRxiv2025

Recolonisation strategies of early animals in the Avalon (Ediacaran 574 - 560 Ma)

The first geographically widespread metazoans are found in the Avalon assemblage (Ediacaran; 574 - 560 Ma). These early animals were regularly disturbed by sedimentation events such as ash flows and turbidites, leading to an apparent "resetting" of communities. However, it is not clear how biological legacies - remains or survivors of disturbance events - influenced community ecology in the Avalon. Here, we use spatial point process analysis on 19 Avalon palaeocommunities to test whether two forms of biological legacy (fragmentary remains of Fractofusus and surviving frondomorphs) impacted the recolonisation dynamics of Avalon palaeocommunities. We found that densities of Fractofusus were increased around the Fractofusus fragments, suggesting that they helped to recolonise the post-disturbance substrate, potentially contributing to the Fractofusus dominance found in 8 of the 19 palaeocommunities. However, we found no such effects for survivor fronds. Our results suggest that the evolution of height was for long-distance dispersal rather than local recolonisation. In modern deep-sea environments, there is a trade-off between local and long-distance dispersal, and our work demonstrates that this differentiation of reproductive strategies had already developed in the early animals of the Avalon.

ecology↗

bioRxiv2025

The influence of reproductive mode on resource competition and diversity patterns in Ediacaran early animal communities

The appearance of the oldest known animals during the late Ediacaran ([~]574 million years ago [Ma])1-4 was followed by a phase of little change5. This period, which lasted [~]14 million years, ended with a burst of rapid diversification known as the Ediacaran "Second Wave". The reasons for these diversity patterns are poorly understood. Here we investigate how reproductive mode mediated community dynamics, and in turn macroevolutionary change, in the Ediacaran. We show that widespread reproduction via stolon (namely via filaments connecting clones) in the first animals limited intra-specific competition among neighbours, leading to inter-specific competition acting at smaller-spatial scales than intra-specific competition, a phenomenon called heteromyopia6. Heteromyopia enables co-existence of sub-optimal competitors because the dispersal limitation of the dominant species means that they do not inhabit all the optimal habitat, so that lesser competitors can still exist within the same community, operating under reduced selection pressure. We explored the consequences of this dispersal limitation on community diversity using Approximate Bayesian Computation to estimate the posterior distributions of dispersal with a spatially explicit model fitted to the three Ediacaran assemblages and showed that the change from stoloniferous to sexual reproduction that coincided with the Second Wave could explain the sudden increase in alpha diversity observed in the fossil record. We conclude that widespread asexual reproduction via stolon likely constrained early animal evolution, limiting diversification until the onset of mobility and widespread sexual reproduction7-9.

paleontology↗

bioRxiv2021

Orientations of Mistaken Point fronds indicate morphology impacted ability to survive turbulence

The Ediacaran organisms of the Mistaken Point E surface have provided crucial insight into early animal communities, including how they reproduced, the importance of Ediacaran height and what the most important factors were to their community dynamics. Here, we use this iconic community to investigate how morphological variation between eight taxa affected their ability to withstand different flow conditions. For each of Beothukis, Bradgatia, Charniodiscus procerus, Charniodiscus spinosus, Plumeropriscum, Primocandelabrum and Fractofusus we measured the orientation and length of their stems (if present) and their fronds. We statistically tested each taxons stem and frond orientation distributions to see whether they displayed a uniform or multimodal distribution. Where multimodal distributions were identified, the stem/frond length of each cohort was tested to identify if there were differences in size between different orientation groups. We find that Bradgatia and Thectardis show a bimodal felling direction, and infer that they were felled by the turbulent head of the felling flow. In contrast, the frondose rangeomorphs including Beothukis, Plumeropriscum, Primocandelabrum, and the arboreomorphs were felled in a single direction, indicating that they were upright in the water column, and were likely felled by the laminar tail of the felling flow. These differences in directionality suggests that an elongate habit, and particularly possession of a stem, lent greater resilience to frondose taxa against turbulent flows, suggesting that such taxa would have had improved survivability in conditions with higher background turbulence than taxa like Bradgatia and Thectardis, which lacked a stem and which had a higher centre of mass, which may have fared better in quieter water conditions.

paleontology↗

bioRxiv2021

Metacommunity analyses show increase in ecological specialisation throughout the Ediacaran

The first animals appear during the late Ediacaran (572 - 541 Ma); an initial diversity increase was followed by a drop, interpreted as catastrophic mass extinction. We investigate the processes underlying these changes using the "Elements of Metacommunity Structure" framework. The oldest metacommunity was characterized by taxa with wide environmental tolerances, and limited specialisation and inter-taxa interactions. Structuring increased in the middle metacommunity, with groups of taxa sharing synchronous responses to environmental gradients, aggregating into distinct communities. This pattern strengthened in the youngest metacommunity, with communities showing strong environmental segregation and depth structure. Thus, metacommunity structure increased in complexity, with increased specialisation and resulting competitive exclusion, not a catastrophic environmental disaster, leading to diversity loss in the terminal Ediacaran, revealing that the complex eco-evolutionary dynamics associated with Cambrian diversification were established in the Ediacaran.

paleontology↗

bioRxiv2019

The influence of environmental setting on the community ecology of Ediacaran organisms

The broad-scale environment plays a substantial role in shaping modern marine ecosystems, but the degree to which palaeocommunities were influenced by their environment is unclear. To investigate how broad-scale environment influenced the community ecology of early animal ecosystems we employed spatial point process analyses to examine the community structure of seven bedding-plane assemblages of late Ediacaran age (558–550 Ma), drawn from a range of environmental settings and global localities. The studied palaeocommunities exhibit marked differences in the response of their component taxa to sub-metre-scale habitat heterogeneities on the seafloor. Shallow-marine palaeocommunities were heavily influenced by local habitat heterogeneities, in contrast to their deep-water counterparts. Lower species richness in deep-water Ediacaran assemblages compared to shallow-water counterparts across the studied time-interval could have been driven by this environmental patchiness, because habitat heterogeneities correspond to higher diversity in modern marine environments. The presence of grazers and detritivores within shallow-water communities may have promoted local patchiness, potentially initiating a chain of increasing heterogeneity of benthic communities from shallow to deep-marine depositional environments. Our results provide quantitative support for the “Savannah” hypothesis for early animal diversification – whereby Ediacaran diversification was driven by patchiness in the local benthic environment.Author Contributions E. Mitchell conceived this paper and wrote the first draft. N. Bobkov, A. Kolesnikov, N. Sozonov and D. Grazhdankin collected the data for DS surface. N. Bobkov and N. Sozonov performed the analyses on DS surface. N. Bykova, S. Xiao, and D. Grazhdankin collected the data for WS, KH1 and KH2 surfaces and E. Mitchell performed the analyses. A. Dhungana and A. Liu collected the data for FUN4 and FUN5 surfaces and A. Dhungana performed the analyses. T. Mustill and D. Grazhdankin collected the data for KS and T. Mustill and E. Mitchell performed the analyses. I. Hogarth developed the software for preliminary KS surface analyses. E. Mitchell, N. Bobkov, N. Bykova, A. Dhungana, A. Kolesnikov, A. Liu, S. Xiao and D. Grazhdankin discussed the results and prepared the manuscript.View Full Text

paleontology↗

bioRxiv2019

Reconstructing the ecology of a Jurassic pseudoplanktonic megaraft colony

Pseudoplanktonic crinoid megaraft colonies are an enigma of the Jurassic. They are among the largest in-situ invertebrate accumulations ever to exist in the Phanerozoic fossil record. These megaraft colonies and are thought to have developed as floating filter-feeding communities due to an exceptionally rich relatively predator free oceanic niche, high in the water column enabling them to reach high densities on these log rafts. However, this pseudoplanktonic hypothesis has never actually been quantitatively tested and some researchers have cast doubt that this mode of life was even possible. The ecological structure of the crinoid colony is resolved using spatial point process techniques and its longevity using moisture diffusion models. Using spatial analysis we found that the crinoids would have trailed preferentially positioned at the back of migrating structures in the regions of least resistance, consistent with a floating, not benthic ecology. Additionally, we found using a series of moisture diffusion models at different log densities and sizes that ecosystem collapse did not take place solely due to colonies becoming overladen as previously assumed. We have found that these crinoid colonies studied could have existed for greater than 10 years, even up to 20 years exceeding the life expectancy of modern documented megaraft systems with implications for the role of modern raft communities in the biotic colonisation of oceanic islands and intercontinental dispersal of marine and terrestrial species.Significance statement Transoceanic rafting is the principle mechanism for the biotic colonisation of oceanic island ecosystems. However, no historic records exist of how long such biotic systems lasted. Here, we use a deep-time example from the Early Jurassic to test the viability of these pseudoplanktonic systems, resolving for the first time whether these systems were truly free floating planktonic and viable for long enough to allow its inhabitants to grow to maturity. Using spatial methods we show that these colonies have a comparable structure to modern marine pesudoplankton on maritime structures, whilst the application of methods normally used in commercial logging is used to demonstrate the viability of the system which was capable of lasting up to 20 years.View Full Text

paleontology↗

bioRxiv2018

The relative influence of niche versus neutral processes on Ediacaran communities

A fundamental question in community ecology is the relative influence of niche versus neutral processes in determining ecosystem dynamics. The extent to which these processes structured early animal communities is yet to be explored. Here we use spatial point process analyses (SPPA) to determine the influence of niche versus neutral processes on early total-group metazoan paleocommunities from the Ediacaran Period [~]565 million years in age. Preservation of these sessile organisms in large in-situ populations on exposed bedding planes enables inference of the most likely underlying processes governing their spatial distributions by SPPA. We conducted comprehensive spatial mapping of six of the largest Ediacaran paleocommunities in Newfoundland, Canada and Charnwood Forest, UK using LiDAR, photogrammetry and a laser-line probe. For each paleocommunity we determined the best-fit spatial model for each univariate and bivariate species distribution, comparing four sets of spatial models (complete spatial randomness, dispersal, habitat, and combined dispersal with habitat) using goodness-of-fit tests. Random and dispersal models are considered neutral processes while habitat and combined models are considered niche processes. We find the dynamics of these paleocommunities to be dominated by neutral processes, with limited influence from niche processes. Our findings are consistent with community model predictions of when neutral dynamics dominate, but are in stark contrast to the niche-dominated communities of the modern marine realm. Thus, while the underlying processes determining metazoan community assembly appear to have been in place since the appearance of the first macroscopic, complex animals, the dynamics of these early metazoan communities were fundamentally different to those of extant communities.\n\nSignificance statementThe extent to which habitat and dispersal processes structure the earliest animal communities found during the Ediacaran Period [~]565 million years ago, is unknown. In this study we analyse six of the largest and most diverse fossil assemblages from Newfoundland, Canada and Charnwood Forest, UK, using spatial point processes analyses to determine the relative influence of habitat processes (niche) and dispersal processes (neutral) on the spatial distribution of taxa. The vast majority of Ediacaran taxon distributions were controlled by neutral processes, in striking contrast to niche-dominated modern marine ecosystems, but consistent with model predictions of when neutral dynamics dominate communities. Thus the underlying processes determining metazoan community assembly have been in place since the appearance of the first macroscopic, complex animals.

ecology↗