Facilitating corals in an early Silurian deep‐water assemblage

Corals are powerful ecosystem engineers and can form reef communities with extraordinary biodiversity through time. Understanding the processes underlying the spatial distribution of corals allows us to identify the key biological and physical processes that structure coral communities and how these processes and interactions have evolved. However, few spatial ecology studies have been conducted on coral assemblages in the fossil record. Here we use spatial point process analysis (SPPA) to investigate the ecological interactions of an in situ tabulate and rugose coral community (n = 199), preserved under volcanic ash in the Silurian of Ireland. SPPA is able to identify many different sorts of interactions including dispersal limitation and competition within and between taxa. Our SPPA found that the spatial distribution of rugose corals were best modelled by Thomas clusters (pd = 0.834), indicating a single dispersal episode and that the tabulate corals were best modelled by double Thomas clusters (pd = 0.820), indicating two dispersal episodes. Further, the bivariate distribution was best modelled by linked double clusters (pd = 0.970), giving significant evidence of facilitation between the tabulate and rugose populations, and identifying the facilitators in this community to be the tabulate corals. This interaction could be an important ecological driver for enabling the aggregation of sessile organisms over long temporal periods and facilitation may help to explain trends in reef diversity and abundance during the Ordovician biodiversification and in the early Silurian

Ecological change and convergence; morphospace of suspension feeding tentaculate metazoans through deep time

A diverse set of taxa such as brachiopods, bryozoans, annelids, echinoderms, hemichordates and phoronids, have sub-cylindrical, often ciliated, suspension-feeding structures, here referred to as tentacles. Theoretical models and simulations of these tentacles imply they may be optimised either to maximise flow or interception with suspended food particles. However, no quantitative studies have compared tentacles across phyla, explored how their morphology may be influenced by ecological niche, or tracked how these structures have changed through deep time in different phylogenetic lineages. This study demonstrates the morphological changes in suspension feeders resulting from different ecological conditions in the Cambrian and the Recent. I show that the tentacular morphology of different ecological categories (motility, tiering, feeding, coloniality and phyla) do overlap in places, but may also segregate in distinct regions, suggesting the influence of these factors on the tentacular morphology. Further, the tentacular structures of Cambrian brachiopods, phoronids, entoprocts and hemichordates are more similar to one another than to the tentacles of extant representatives of those phyla. I suggest that different aspects of the striking cross-phylum convergence are due to changes in the constitution of phytoplankton, energy availability and ecological changes through deep time

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 (SPPA) to examine the community structure of seven late Ediacaran (558–550 Ma) bedding-plane assemblages 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 (nearshore) palaeocommunities were heavily influenced by local habitat heterogeneities, in contrast to their deeper-water counterparts. The local patchiness within shallow-water communities may have been further accentuated by the presence of grazers and detritivores, whose behaviours potentially initiated a propagation of increasing habitat heterogeneity of benthic communities from shallow to deep-marine depositional environments. Higher species richness in shallow-water Ediacaran assemblages compared to deep-water counterparts across the studied time-interval could have been driven by this environmental patchiness, because habitat heterogeneities increase species richness in modern marine 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