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The shallow northern Adriatic Sea has a long history of anthropogenic impacts that reaches back many centuries. While the effects of eutrophication, overfishing, pollution, and trawling over recent decades have been extensively studied, the major ecological turnovers during the Holocene as a whole remain poorly explored. In this study, we reconstruct ecological baselines defining benthic ecosystem composition prior to major anthropogenic changes at four stations characterized by low sedimentation and millennial-scale time averaging of molluscan assemblages. We discriminate between natural and anthropogenic drivers based on (1) stratigraphic changes in the composition of molluscan communities observed in sediment cores and (2) changes in concentrations of heavy metals, pollutants, and organic enrichment. The four 1.5-m long sediment cores reach back to the Pleistocene–Holocene boundary, allowing for a stratigraphic distinction of the major sea-level phases of the Holocene. During the transgressive phase and maximum flooding, sea-level and establishment of the modern circulation pattern determined the development of benthic communities in shallow-water, vegetated habitats with epifaunal biostromes and, in deeper waters, with bryozoan meadows. After sea-level stabilization, the composition of these baseline communities remained relatively uniform and started to change markedly only with the intensification of human impacts in the late highstand, leading to a dominance of infauna and a decline of epifauna at all sites. This profound ecological change reduced species richness, increased the abundance of infaunal suspension feeders, and led to a decline of grazers and deposit feeders. We suggest that modern soft-bottom benthic communities in the northern Adriatic Sea today do not show the high geographic heterogeneity in composition characteristic of benthos prior to anthropogenic influences.
Edrioasteroids are an extinct clade of echinoderm that are rarely preserved in the fossil record due to rapid post-mortem disarticulation of their multi-element skeletons. Here we investigate two well-preserved limestone slabs from the Mississippian (Chesterian) Kinkaid Formation of southern Illinois bearing 242 specimens of the edrioasteroid Neoisorophusella lanei attached to internal molds of Promytilus bivalves. To assess edrioasteroid paleoecology we examined edrioasteroid age structure, thecal orientation, spatial utilization, and degree of postmortem disarticulation. Size-frequency analysis, used as a proxy for age, shows right-skewed, bimodal distributions of thecal diameter on both slabs suggesting high juvenile mortality and two spatfall events. Thecal orientation, using an arbitrarily assigned “North” direction, suggests that no preferential orientation exists. When examined at large scale using Nearest Neighbor Analysis and Ripley's K statistics, edrioasteroids on the slabs exhibit clustering nearing random distributions across the encrusted surfaces. Clusters of edrioasteroids examined at a smaller areal scale show evidence of dispersion. The 242 edrioasteroids cover a combined surface area of 104.41 cm2, yielding an overall density of 23,177 edrioasteroids per m2, an order of magnitude greater than the highest densities previously reported in other studies. No thecal disarticulation occurs in 85% of the population of 242 edrioasteroids suggesting that the edrioasteroids were alive when catastrophically buried. Thecal deformation occurs frequently as a result of competition for space. A striking example of edrioasteroids superimposed onto a larger edrioasteroid substrate may be due to (1) conspecific overgrowth or (2) transport, and subsequent dissolution, of a shell with edrioasteroid epigrowth.
During the early Cambrian, organisms with robust skeletons began to integrate with microbialite reef structures. Specifically, archaeocyathan sponges were among the first metazoan reef-builders. Here we investigate the transition from microbial-dominated reef environments to metazoan-based reefs from strata in the western Basin and Range of California and Nevada. This study integrates point count data from petrographic thin sections with stable carbon isotopic and elemental composition of carbonates. From the earliest reef bearing formations to the latest, metazoan framework contribution increases from zero to 29.7%. This increase is linked to the addition of new framework-building organisms, namely coralomorphs, as well as an increase in archaeocyath body size. Correspondingly, Shannon's diversity increases from 0.652 to 1.492. However, skeletal contributions from additional organisms within the reefs (e.g., trilobites, echinoderms) appear unchanged and their diversity is not correlated with framework-builder diversity. A positive carbon isotopic excursion within the Lower Poleta Formation correlates with decreases in the abundances of uranium and molybdenum that suggest a global change in organic carbon burial as opposed to localized or diagenetic factors. This allows for chemostratigraphic correlation to published carbon isotopic data and provides a proposed regional age constraint of roughly 517 million years. Overall, early Cambrian reefs at this location exhibit a pattern of increasing metazoan contribution during the transition from microbial- to metazoan-based reef support, however, diversity remained low until additional organisms evolved to inhabit these ecosystems.
The Discussion of our paper by Martínez is very welcome because it supports our finding that bivalves may colonize wet-interdune settings and thus, trace fossils produced by them may record environmental changes—this is the main point of the contribution by Carmona et al. (2018). However, there are some comments in the Discussion by Martínez about the interpretations proposed in that paper that need to be clarified here.
Biologically mediated fabrics are disturbed sediment by organisms that resemble primary physical structures. In this aquaria-based study, the sand-sifting goby, Valenciennea puellaris, produced biogenic sedimentary structures resembling planar lamina and ripple cross lamina with grain sizes ranging from fine sand to gravel. Valenciennea puellaris moved and re-deposited fine- and coarse sand and gravel used in this study, but dug only in fine- to coarse sand. Gravel-sized particles were too large to pass through its gills and therefore the goby moved them individually. Through the bioresuspension behaviors of feeding, digging, and resting, the V. puellaris produces Piscichnus-like craters and moves about nine mouthfuls of sediment a minute, i.e., 0.18 cm3. The biogenic fabrics produced by V. puellaris in this study are similar to primary sedimentary fabrics produced by hydrologic flow. Similar behaviors and feeding styles are widespread and found in larger fish and marine mammals. While V. puellaris has only been around since the Eocene, burrowing Actinopterygians date back to 400 Ma, suggesting that similar biogenic sedimentary structures may have a long history in the geological record.