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Oysters, whose inner shell layer contains chambers, vesicles, and sometimes chalky deposits, often have extraordinarily thick shells of large size, prompting the idea that there is something unusual about the process of shell fPormation in these and similarly structured bivalves with the oyster syndrome. I propose the hypothesis that calcifying microbes, especially sulfate-reducing bacteria growing on organic substrates in fluid-filled shell-wall chambers, are responsible for shell calcification away from the shell-secreting mantle of the host bivalve. Other phenomena, including the formation of cameral deposits in fossil cephalopods, the cementation of molluscs and barnacles to hard substrata, the formation of a calcified intriticalx on the shell's exterior, and cementation of objects by gastropods on the shell for camouflage, may also involve calcifying bacteria. Several lines of inquiry are suggested to test these hypotheses.
Recent observations indicate that shell fragmentation can be a useful tool in assessing crushing predation in marine communities. However, criteria for recognizing shell breakage caused by durophagous predators versus physical factors are still not well established. Here, we provide data from tumbling and aquarium experiments to argue that physical and biotic processes lead to different patterns of shell damage, specifically that angular shell fragments are good indicators of durophagous predation. Using such angular shell fragments as a predation proxy, we analyze data from 57 European Paleozoic localities spanning the Ordovician through the Mississippian. Our results reveal a significant increase in angular shell fragments (either occurring as isolated valves or present in regurgitalites) in the Mississippian. The timing of this increase is coincident with the increased diversity of crushing predators as well as marked anti-predatory changes in the architecture and mode of life of invertebrate prey observed after the end-Devonian Hangenberg extinction (359 Ma). More specifically, the observed trend in shell fragmentation constitutes strong and independent confirmation of a recently suggested end-Devonian changeover in the primary method of fish predation from shearing to crushing. These results also highlight the important effect of extinction events, not only on taxonomic diversity, but also on the nature of predator-prey interactions.
The late Maastrichtian sediments of the Mullinax-1 and Mullinax-3 boreholes from Brazos River, Texas, offer pristine material. These cores are prime candidates for providing an extraordinary window into the ecology of Guembelitria, a key genus in the K/Pg mass extinction event, as well as information on the habitats of other neritic species. Stable oxygen and carbon isotope analyses were performed on six planktic species (Guembelitria cretacea, Globigerinelloides asper, Heterohelix globulosa, Paraspiroplecta navarroensis, Pseudoguembelina costulata, Rugoglobigerina rugosa) and three benthic genera (Gavelinella, Cibicides, and Lenticulina). Our records support the contention that Guembelitria was fully planktic, as indicated by its δ18O values, which overlap the other planktic species, despite its possible origin from a tychopelagic benthic ancestor. However, Guembelitria is distinctly ranked very low in δ13C values, which overlap the benthic records. The anomalously low δ13C values of Guembelitria may represent an isotopic disequilibrium due to fast shell growth, like in its modern analogue Gallitellia vivans. Another explanation may be that these values are attributable to a neustonic life mode in the uppermost part of the oceans, where photosynthesis is inhibited by high UV and the near absence of nutrients. Because these waters are not photosynthetically depleted, calcification using carbon directly from these waters should yield δ13C values consistent with those found in Guembelitria. The ecological strategy that Guembelitria species used to deal with the nutrient-poor surface-water environments was an opportunistic blooming during stressful times of Maastrichtian global warming events and later during the K-Pg catastrophe.
Edge-drilling is an unusual predation pattern in which a predatory gastropod drills a hole on the commissure between the valves of a bivalve. Although it is faster than wall drilling, it involves the potential risk of amputating the drilling organ. We therefore hypothesize that this risky strategy is advantageous only in environments where predators face high competition or predation pressure while feeding. The high frequency of edge-drilling (EDF, relative to the total number of drilled valves) in a diverse Recent bivalve assemblage from the Red Sea enables us to test this hypothesis, predicting (1) a low EDF in infaunal groups, (2) a high EDF in bivalves with elongated shape, (3) high incidence of edge-drilling in groups showing a high wall-drilling frequency, and (4) high EDF in shallow habitats. We evaluate these predictions based on >15,000 bivalve specimens. Among ecological attributes, we found substrate affinity and predation intensity of a species to be good predictors of edge-drilling incidence. Infaunal taxa with high length/width ratio have a low EDF, in accordance with our predictions. Predation intensity is also a significant predictor of edge-drilling; groups with high predation intensity show higher incidence of edge-drilling, confirming our prediction. Although water depth fails to show any significant effect on EDF, this analysis generally supports the high-risk hypothesis of edge-drilling incidence because shallow depths have considerable microhabitat variability. Classically the drill hole site selection has often been linked to predatory behavior. Our study indicates that prey attributes are also crucial in dictating the behavioral traits of a driller such as site selection. This calls for considering such details of the prey to fully understand predation in modern and fossil habitats. Moreover, this perspective is important for tackling the longstanding riddle of the limited temporal and spatial distribution of edge-drilling.
Lagerstätten from the Precambrian–Cambrian transition have traditionally been a relatively untapped resource for understanding the paleoecology of the “Cambrian explosion.” This quantitative paleoecological study is based on 10,238 fossil specimens belonging to 100 animal species, 11 phyla, and 15 ecological categories from the lower Cambrian (Series 2, Stage 3) Chengjiang biota (Mafang locality near Haikou, Yunnan Province, China). Fossils were systematically collected within a 2.5-meter-thick sequence divided into ten stratigraphic intervals. Each interval represents an induced time-averaged assemblage of various event (obrution) beds of unknown duration. Overall, the different fossil assemblages are taxonomically and ecologically similar, suggesting the presence of a single community type recurring throughout the Mafang section. The Mafang community is dominated by epibenthic vagile hunters or scavengers, sessile suspension feeders, and infaunal vagile hunters or scavengers represented primarily by arthropods, brachiopods, and priapulids, respectively. Most species have low abundance and low occurrence frequencies, whereas a few species are numerically abundant and occur frequently. Overall, in structure and ecology the Mafang community is comparable to the Middle Cambrian (Series 3, Stage 5) Burgess Shale biota (Walcott Quarry, Yoho National Park, British Columbia, Canada). This suggests that, despite variations in species identity within taxonomic and ecological groups, the structure and ecology of Cambrian Burgess Shale-type communities remained relatively stable until at least the Middle Cambrian (Series 3, Stage 5) in subtidal to relatively deep-water offshore settings in siliciclastic soft-substrate environments.
The set of environmental conditions under which a taxon can survive and maintain viable populations, known as the ecological niche, is a fundamental determinant of a taxon's distribution. Because of the central importance of ecological niches, they have been assumed to remain relatively stable during intervals of morphological stasis. However, the assumption of niche stability has rarely been tested directly with fossil data spanning multiple temporal intervals. Thus, the conditions under which this assumption is likely to be accurate are not well understood. In this study, we use ecological niche modeling (ENM) to reconstruct the ecological niche for 11 genera of marine benthos (crinoids, trilobites, molluscs, bryozoans, and corals) from the Type Cincinnatian Series (Late Ordovician, Katian Stage) across nine temporal intervals spanning approximately three million years. This interval includes both abiotic environmental change (gradual sea-level fall) and biotic change (rapid pulses of the Richmondian Invasion), thus allowing the relative effect of different environmental perturbations to be constrained. A previous symmetrical analysis of niche stability of brachiopod species recovered an increase in niche evolution following the Richmondian Invasion. Herein we test the generality of the brachiopod pattern within the community. Niche stability was evaluated in geographic space, ecological space, and niche parameter space. Niche stability varied through time; during the Pre-Invasion interval, taxa exhibited niche stability during gradual shallowing of sea level in the basin, whereas niche evolution became more common during the Richmondian Invasion. Taxa adjusted to the increased competition by altering aspects of their niche. Notably, surviving taxa contracted their niche into a subset of their previous niche parameters. This represents an adaptive response to increased competition for resources with the newly established invader taxa, and it was employed most successfully by generalist taxa. Patterns of niche evolution were congruent between clades, among feeding styles, and across taxonomic levels.
With species found throughout both marine and fresh waters, the diatom order Thalassiosirales is one of the most phylogenetically and ecologically diverse lineages of planktonic diatoms. A clear understanding of the timescale of Thalassiosirales evolution would provide novel insights into the rates and patterns of species diversification associated with major habitat shifts, as well as provide valuable context for understanding the age and evolutionary history of the model species, Cyclotella nana (= Thalassiosira pseudonana). The freshwater fossil record for Thalassiosirales is extensive, well characterized, and generally supportive of a Miocene origin for the major freshwater lineages. The marine record is, by comparison, more sparse and in many cases, unverified. The discovery of freshwater thalassiosiroids in Eocene sediments pushed the freshwater fossil record considerably further back in time, highlighting an apparent gap of some 30 million years. An alternative interpretation is that the Miocene and Eocene reports represent competing hypotheses. In the absence of additional independent and decisive fossil data, I explored the relative plausibility of these two scenarios with Bayesian relaxed molecular clock methods under a range of fossil calibration schemes. Although I found no support for the Eocene fossil dates, the two major freshwater colonization events probably occurred much earlier than previously thought—as early as the Paleocene for Cyclotella, followed by an Eocene origin for the cyclostephanoid lineage. Much of the extant freshwater diversity in both lineages traces back to the Miocene, however, giving the impression of a single Miocene origin. Efforts to infer the timescale of Thalassiosirales evolution more accurately would benefit from a systematic reevaluation of the marine fossil record and formal integration of fossil species into existing phylogenetic hypotheses.
An enigma of deep-sea biodiversity research is that the abyss with its low productivity and densities appears to have a biodiversity similar to that of shallower depths. This conceptualization of similarity is based mainly on per-sample estimates (point diversity, within-habitat, or α-diversity). Here, we use a measure of between-sample within-community diversity (β1H) to examine benthic foraminiferal diversity between 333 stations within 49 communties from New Zealand, the South Atlantic, the Gulf of Mexico, the Norwegian Sea, and the Arctic. The communities are grouped into two depth categories: 200–1500 m and >1500 m. β1H diversity exhibits no evidence of regional differences. Instead, higher values at shallower depths are observed worldwide. At depths of >1500 m the average β1H is zero, indicating stasis or no biodiversity gradient. The difference in β1H-diversity explains why, despite species richness often being greater per sample at deeper depths, the total number of species is greater at shallower depths. The greater number of communities and higher rate of evolution resulting in shorter species durations at shallower depths is also consistent with higher β1H values.
Studies of the end-Permian mass extinction have suggested a variety of patterns from a single catastrophic event to multiple phases. But most of these analyses have been based on fossil distributions from single localities. Although single sections may simplify the interpretation of species diversity, they are susceptible to bias from stratigraphic incompleteness and facies control of preservation. Here we use a data set of 1450 species from 18 fossiliferous sections in different paleoenvironmental settings across South China and the northern peri-Gondwanan region, and integrate it with high-precision geochronologic data to evaluate the rapidity of the largest Phanerozoic mass extinction. To reduce the Signor-Lipps effect, we applied constrained optimization (CONOP) to search for an optimal sequence of first and last occurrence datums for all species and generate a composite biodiversity pattern based on multiple sections. This analysis indicates that an abrupt extinction of 62% of species took place within 200 Kyr. The onset of the sudden extinction is around 252.3 Ma, just below Bed 25 at the Meishan section. Taxon turnover and diversification rates suggest a deterioration of the living conditions nearly 1.2 Myr before the sudden extinction. The magnitude of the extinction was such that there was no immediate biotic recovery. Prior suggestions of highly variable, multi-phased extinction patterns reflect the impact of the Signor-Lipps effect and facies-dependent occurrences, and are not supported following appropriate statistical treatment of this larger data set.
Marine planktonic microfossils have provided some of the best examples of evolutionary rates and patterns on multi-million-year time scales, including many instances of gradual evolution. Lineage splitting as a result of speciation has also been claimed, but all such studies have used subjective visual species discrimination, and interpretation has often been complicated by relatively small sample sizes and oceanographic complexity at the study sites. Here we analyze measurements on a collection of 10,200 individual tests of the Eocene planktonic foraminifer Turborotalia in 51 stratigraphically ordered samples from a site within the oceanographically stable tropical North Pacific gyre. We use novel multivariate statistical clustering methods to test the hypothesis that a single evolutionary species was present from 45 Ma to its extinction ca. 34 Ma. After identification of a set of biologically relevant traits, the protocol we apply does not require a prior assignment of individuals to species. We find that for most of the record, contemporaneous specimens form one morphological cluster, which we interpret as an evolving species that shows quasi-continuous but non-directional gradual evolutionary change (anagenesis). However, in the upper Eocene from ca. 36 to ca. 34 Ma there are two clusters that persistently occupy distinct areas of morphospace, from which we infer that speciation (cladogenesis) must have occurred.