13 results
Marine conservation palaeobiology: What does the late Quaternary fossil record tell us about modern-day extinctions and biodiversity threats?
- Michał Kowalewski, Rafał Nawrot, Daniele Scarponi, Adam Tomašových, Martin Zuschin
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- Journal:
- Cambridge Prisms: Extinction / Volume 1 / 2023
- Published online by Cambridge University Press:
- 08 November 2023, e24
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Near-time conservation palaeobiology uses palaeontological, archaeological and other geohistorical records to study the late Quaternary transition of the biosphere from its pristine past to its present-day, human-altered state. Given the scarcity of data on recent extinctions in the oceans, geohistorical records are critical for documenting human-driven extinctions and extinction threats in the marine realm. The historical perspective can provide two key insights. First, geohistorical records archive the state of pre-industrial oceans at local, regional and global scales, thus enabling the detection of recent extinctions and extirpations as well as shifts in species distribution, abundance, body size and ecosystem function. Second, we can untangle the contributions of natural and anthropogenic processes by documenting centennial-to-millennial changes in the composition and diversity of marine ecosystems before and after the onset of major human impacts. This long-term perspective identifies recently emerging patterns and processes that are unprecedented, thus allowing us to better assess human threats to marine biodiversity. Although global-scale extinctions are not well documented for brackish and marine invertebrates, geohistorical studies point to numerous extirpations, declines in ecosystem functions, increases in range fragmentation and dwindling abundance of previously widespread species, indicating that marine ecosystems are accumulating a human-driven extinction debt.
A downcore increase in time averaging is the null expectation from the transit of death assemblages through a mixed layer
- Adam Tomašových, Susan M. Kidwell, Ran Dai
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- Journal:
- Paleobiology / Volume 49 / Issue 3 / August 2023
- Published online by Cambridge University Press:
- 19 January 2023, pp. 527-562
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Understanding how time averaging changes during burial is essential for using Holocene and Anthropocene cores to analyze ecosystem change, given the many ways in which time averaging affects biodiversity measures. Here, we use transition-rate matrices to explore how the extent of time averaging changes downcore as shells transit through a taphonomically complex mixed layer into permanently buried historical layers: this is a null model, without any temporal changes in rates of sedimentation or bioturbation, to contrast with downcore patterns that might be produced by human activity. Assuming stochastic burial and exhumation movements of shells between increments within the mixed layer and stochastic disintegration within increments, we find that almost all combinations of net sedimentation, mixing, and disintegration produce a downcore increase in time averaging (interquartile range [IQR] of shell ages), this trend is typically associated with a decrease in kurtosis and skewness and by a shift from right-skewed to symmetrical age distributions. A downcore increase in time averaging is thus the null expectation wherever bioturbation generates an internally structured mixed layer (i.e., a surface, well-mixed layer is underlain by an incompletely mixed layer): under these conditions, shells are mixed throughout the entire mixed layer at a slower rate than they are buried below it by sedimentation. This downcore trend created by mixing is further amplified by the downcore decline in disintegration rate. We find that transition-rate matrices accurately reproduce the downcore changes in IQR, skewness, and kurtosis observed in bivalve assemblages from the southern California shelf. The right-skewed shell age-frequency distributions typical of surface death assemblages—the focus of most actualistic research—might be fossilized under exceptional conditions of episodic anoxia or sudden burial. However, such right-skewed assemblages will typically not survive transit through the surface mixed layer into subsurface historical layers: they are geologically transient. The deep-time fossil record will be dominated instead by the more time-averaged assemblages with weakly skewed age distributions that form in the lower parts of the mixed layer.
Scale dependence of drilling predation in the Holocene of the northern Adriatic Sea across benthic habitats and nutrient regimes
- Martin Zuschin, Rafał Nawrot, Markus Dengg, Ivo Gallmetzer, Alexandra Haselmair, Sandra Wurzer, Adam Tomašových
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- Journal:
- Paleobiology / Volume 48 / Issue 3 / August 2022
- Published online by Cambridge University Press:
- 29 March 2022, pp. 462-479
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Predation has strongly shaped past and modern marine ecosystems, but the scale dependency of patterns in drilling predation, the most widely used proxy for predator–prey interactions in the fossil record, is a matter of debate. To assess the effects of spatial and taxonomic scale on temporal trends in the drilling frequencies (DFs), we analyzed Holocene molluscan assemblages of different benthic habitats and nutrient regimes from the northern Adriatic shelf in a sequence-stratigraphic context. Although it has been postulated that low predation pressures facilitated the development of high-biomass epifaunal communities in the eastern, relatively oligotrophic portion of the northern Adriatic shelf, DFs reaching up to 30%–40% in the studied assemblage show that drilling predation levels are comparable to those typical of late Cenozoic ecosystems. DFs tend to increase from the transgressive systems tract (TST) into the highstand systems tract (HST) at the local scale, reflecting an increase in water depth by 20–40 m and a shift from infralittoral to circalittoral habitats over the past 10,000 years. As transgressive deposits are thicker at shallower locations and highstand deposits are thicker at deeper locations, a regional increase in DFs from TST to HST is evident only when these differences are accounted for. The increase in DF toward the HST can be recognized at the level of total assemblages, classes, and few abundant and widespread families, but it disappears at the level of genera and species because of their specific environmental requirements, leading to uneven or patchy distribution in space and time.
Tracing the effects of eutrophication on molluscan communities in sediment cores: outbreaks of an opportunistic species coincide with reduced bioturbation and high frequency of hypoxia in the Adriatic Sea
- Adam Tomašových, Ivo Gallmetzer, Alexandra Haselmair, Darrell S. Kaufman, Martina Kralj, Daniele Cassin, Roberto Zonta, Martin Zuschin
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- Journal:
- Paleobiology / Volume 44 / Issue 4 / November 2018
- Published online by Cambridge University Press:
- 16 August 2018, pp. 575-602
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Estimating the effects and timing of anthropogenic impacts on the composition of macrobenthic communities is challenging, because early twentieth-century surveys are sparse and the corresponding intervals in sedimentary sequences are mixed by bioturbation. Here, to assess the effects of eutrophication on macrobenthic communities in the northern Adriatic Sea, we account for mixing with dating of the bivalve Corbula gibba at two stations with high accumulation (Po prodelta) and one station with moderate accumulation (Isonzo prodelta). We find that, first, pervasively bioturbated muds typical of highstand conditions deposited in the early twentieth century were replaced by muds with relicts of flood layers and high content of total organic carbon (TOC) deposited in the late twentieth century at the Po prodelta. The twentieth century shelly muds at the Isonzo prodelta are amalgamated but also show an upward increase in TOC. Second, dating of C. gibba shells shows that the shift from the early to the late twentieth century is characterized by a decrease in stratigraphic disorder and by an increase in temporal resolution of assemblages from ~25–50 years to ~10–20 years in both regions. This shift reflects a decline in the depth of the fully mixed layer from more than 20 cm to a few centimeters. Third, the increase in abundance of the opportunistic species C. gibba and the loss of formerly abundant, hypoxia-sensitive species coincided with the decline in bioturbation, higher preservation of organic matter, and higher frequency of seasonal hypoxia in both regions. This depositional and ecosystem regime shift occurred in ca. a.d. 1950. Therefore, the effects of enhanced food supply on macrobenthic communities were overwhelmed by oxygen depletion, even when hypoxic conditions were limited to few weeks per year in the northern Adriatic Sea. Preservation of trends in molluscan abundance and flood events in cores was enhanced by higher frequency of hypoxia that reduced bioturbation in the late twentieth century.
Taxonomic and numerical sufficiency in depth- and salinity-controlled marine paleocommunities
- Martin Zuschin, Rafał Nawrot, Mathias Harzhauser, Oleg Mandic, Adam Tomašových
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- Paleobiology / Volume 43 / Issue 3 / August 2017
- Published online by Cambridge University Press:
- 09 March 2017, pp. 463-478
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Numerical and taxonomic resolution of compositional data sets affects investigators’ abilities to detect and measure relationships between communities and environmental factors. We test whether varying numerical (untransformed, square-root- and fourth-root-transformed relative abundance and presence–absence data) and taxonomic (species, genera, families) resolutions reveals different insights into early to middle Miocene molluscan communities along bathymetric and salinity gradients. The marine subtidal has a more even species-abundance distribution, a higher number of rare species, and higher species:family and species:genus ratios than the three habitats—marine and estuarine intertidal, estuarine subtidal—with higher fluctuations in salinity and other physical parameters. Taxonomic aggregation and numerical transformation of data result in very different ordinations, although all habitats differ significantly from one another at all taxonomic and numerical levels. Rank correlations between species-level and higher-taxon, among-sample dissimilarities are very high for proportional abundance and decrease strongly with increasing numerical transformation, most notably in the two intertidal habitats. The proportion of variation explained by depth is highest for family-level data, decreases gradually with numerical transformation, and is higher in marine than in estuarine habitats. The proportion of variation explained by salinity is highest for species-level data, increases gradually with numerical transformation, and is higher in subtidal than in intertidal habitats. Therefore, there is no single best numerical and taxonomic resolution for the discrimination of communities along environmental gradients: the “best” resolution depends on the environmental factor considered and the nature of community response to it. Different numerical and taxonomic transformations capture unique aspects of metacommunity assembly along environmental gradients that are not detectable at a single level of resolution. We suggest that simultaneous analyses of community gradients at multiple taxonomic and numerical resolutions provide novel insights into processes responsible for spatial and temporal community stability.
Accounting for the effects of biological variability and temporal autocorrelation in assessing the preservation of species abundance
- Adam Tomaŝových, Susan M. Kidwell
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- Journal:
- Paleobiology / Volume 37 / Issue 2 / Spring 2011
- Published online by Cambridge University Press:
- 08 April 2016, pp. 332-354
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Quantifying the effects of taphonomic processes on species abundances in time-averaged death assemblages (DAs) is pivotal for paleoecological inference. However, fidelity estimates based on conventional “live-dead” comparisons are fundamentally ambiguous: (1) data on living assemblages (LAs) are based on a very short period of sampling and thus do not account for biological variability in the LA, (2) LAs are sampled at the same time as the DA and thus do not necessarily reflect past LAs that contributed to the DA, (3) compositions of LAs and DAs can be autocorrelated owing to shared cohorts, and (4) fidelity estimates are cross-scale estimates because DAs are time-averaged and LAs are not. Some portion of raw (total) live-dead (LD) variation in species composition thus arises from incomplete sampling of LAs and from biological temporal variation among LAs (together = premortem component of LD variation), as contrasted with new variation created by interspecific variation in population turnover and preservation rates and by the time-averaging of skeletal input (together = postmortem component of LD variation). To tackle these problems, we introduce a modified test for homogeneity of multivariate dispersions (HMD) in order to (1) account for temporal autocorrelation in composition between LAs and DAs and (2) decompose total LD compositional variation into premortem and postmortem components, and we use simulations to evaluate the contribution of within-habitat time-averaging on the postmortem component. Applying this approach to 31 marine molluscan data sets, each consisting of spatial replicates of LAs and DAs in a single habitat, we find that total LD variation is driven largely by variation among LAs. However, genuinely postmortem processes have significant effects on composition in 25–65% of data sets (depending on the metric) when the effects of temporal autocorrelation are taken into account using HMD. Had we ignored the effects of autocorrelation, the effects of postmortem processes would have been negligible, inflating the similarity between LAs and DAs. Simulations show that within-habitat time-averaging does not increase total LD variation to a large degree—it increases total LD variation mainly via increasing species richness, and decreases total LD variation by reducing dispersion among DAs. The postmortem component of LD variation thus arises from differential turnover and preservation and multi-habitat time-averaging. Moreover, postmortem processes have less effect on the compositions of DAs in habitats characterized by high variability among LAs than they have on DAs in temporally stable habitats, a previously unrecognized first-order factor in estimating postmortem sources of compositional variation in DAs.
Preservation of spatial and environmental gradients by death assemblages
- Adam Tomašových, Susan M. Kidwell
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- Journal:
- Paleobiology / Volume 35 / Issue 1 / Winter 2009
- Published online by Cambridge University Press:
- 08 April 2016, pp. 119-145
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Although only a few studies have explicitly evaluated live-dead agreement of species and community responses to environmental and spatial gradients, paleoecological analyses implicitly assume that death assemblages capture these gradients accurately. We use nine data sets from modern, relatively undisturbed coastal study areas to evaluate how the response of living molluscan assemblages to environmental gradients (water depth and seafloor type; “environmental component” of a gradient) and geographic separation (“spatial component”) is captured by their death assemblages. We find that:
1. Living assemblages vary in composition either in response to environmental gradients alone (consistent with a species-sorting model) or in response to a combination of environmental and spatial gradients (mass-effect model). None of the living assemblages support the neutral model (or the patch-dynamic model), in which variation in species abundance is related to the spatial configuration of stations alone. These findings also support assumptions that mollusk species consistently differ in responses to environmental gradients, and suggest that in the absence of postmortem bias, environmental gradients might be accurately captured by variation in species composition among death assemblages. Death assemblages do in fact respond uniquely to environmental gradients, and show a stronger response when abundances are square-root transformed to downplay the impact of numerically abundant species and increase the effect of rare species.
2. Species' niche positions (position of maximum abundance) along bathymetric and sedimentary gradients in death assemblages show significantly positive rank correlations to species positions in living assemblages in seven of nine data sets (both square-root-transformed and presence-absence data).
3. The proportion of compositional variation explained by environmental gradients in death assemblages is similar to that of counterpart living assemblages. Death assemblages thus show the same ability to capture environmental gradients as do living assemblages. In some instances compositional dissimilarities in death assemblages show higher rank correlation with spatial distances than with environmental gradients, but spatial structure in community composition is mainly driven by spatially structured environmental gradients.
4. Death assemblages correctly identify the dominance of niche metacommunity models in mollusk communities, as revealed by counterpart living assemblages. This analysis of the environmental resolution of death assemblages thus supports fine-scale niche and paleoenvironmental analyses using molluscan fossil records. In spite of taphonomic processes and time-averaging effects that modify community composition, death assemblages largely capture the response of living communities to environmental gradients, partly because of redundancy in community structure that is inherently associated with multispecies assemblages. The molluscan data sets show some degree of redundancy as evidenced by the presence of at least two mutually exclusive subsets of species that replicate the community structure, and simple simulations show that between-sample relationships can be preserved and remain significant even when a large proportion of species is randomly removed from data sets.
Modeling shelliness and alteration in shell beds: variation in hardpart input and burial rates leads to opposing predictions
- Adam Tomašových, Franz T. Fürsich, Thomas D. Olszewski
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- Journal:
- Paleobiology / Volume 32 / Issue 2 / Spring 2006
- Published online by Cambridge University Press:
- 08 April 2016, pp. 278-298
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Distinguishing the differential roles of hardpart-input rates and burial rates in the formation of shell beds is important in paleobiologic and sedimentologic studies, because high shelliness can reflect either high population density of shell producers or lack of sediment. The modeling in this paper shows that differences in the relative importance of burial rates and hardpart-input rates lead to distinct patterns with respect to the degree of shelliness and taphonomic alteration in shell beds. Our approach substantially complements other models because it allows computation of both shelliness and assemblage-level alteration. To estimate shelliness, we dissected hardpart-input rates into dead-shell production and shell destruction rates. To estimate assemblage-level alteration, we computed an alteration rate that describes how rapidly shells accrue postmortem damage. Under decreasing burial rates but constant hardpart-input rates, a positive correlation between alteration and shelliness is expected (Kidwell's R-sediment model). In contrast, under decreased destruction rates and/or increased dead-shell production rates and constant burial rates (Kidwell's R-hardpart model), a negative correlation between shelliness and alteration is expected. The contrasting predictions thus provide a theoretical basis for distinguishing whether high shell density in shell beds reflects passive shell accumulation due to a lack of sediment dilution or whether it instead reflects high shell input from a life assemblage. This approach should be applicable for any fossil assemblages that vary in shell density and assemblage-level alteration. An example from the Lower Jurassic of Morocco, which has shell-rich samples less altered than shell-poor samples, suggests that the higher shelliness correlates with higher community-level abundance and lower proportion of juveniles of the main shell producer, supporting the driving role of hardpart-input rates in the origin of the shell-rich samples in this case. This is of significance in paleoecologic analyses because variations in shelliness can directly reflect fluctuations in population density of shell producers.
Effects of a high-risk environment on edge-drilling behavior: inference from Recent bivalves from the Red Sea
- Devapriya Chattopadhyay, Martin Zuschin, Adam Tomašových
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- Journal:
- Paleobiology / Volume 40 / Issue 1 / Winter 2014
- Published online by Cambridge University Press:
- 08 April 2016, pp. 34-49
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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.
Predicting the effects of increasing temporal scale on species composition, diversity, and rank-abundance distributions
- Adam Tomašových, Susan M. Kidwell
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- Paleobiology / Volume 36 / Issue 4 / Fall 2010
- Published online by Cambridge University Press:
- 08 April 2016, pp. 672-695
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Paleoecological analyses that test for spatial or temporal variation in diversity must consider not only sampling and preservation bias, but also the effects of temporal scale (i.e., time-averaging). The species-time relationship (STR) describes how species diversity increases with the elapsed time of observation, but its consequences for assessing the effects of time-averaging on diversity of fossil assemblages remain poorly explored. Here, we use a neutral, dispersal-limited model of metacommunity dynamics, with parameters estimated from living assemblages of 31 molluscan data sets, to model the effects of within-habitat time-averaging on the mean composition and multivariate dispersion of assemblages, on diversity at point (single station) and habitat scales (pooled multiple stations), and on beta diversity. We hold sample size constant in STRs to isolate the effects of time-averaging from sampling effects. With increasing within-habitat time-averaging, stochastic switching in the identity of species in living (dispersal-limited) assemblages (1) decreases the proportional abundance of abundant species, reducing the steepness of the rank-abundance distribution, and (2) increases the proportional richness of rare, temporally short-lived species that immigrate from the neutral metacommunity with many rare species. These two effects together (1) can shift the mean composition away from the non-averaged (dispersal-limited) assemblages toward averaged assemblages that are less limited by dispersal, resembling that of the metacommunity; (2) allow the point and habitat diversity to increase toward metacommunity diversity under a given sample size (i.e., the diversity in averaged assemblages is inflated relative to non-averaged assemblages); and (3) reduce beta diversity because species unique to individual stations become shared by other stations when limited by a larger but static species pool. Surprisingly, these scale-dependent changes occur at fixed sample sizes and can become significant after only a few decades or centuries of time-averaging, and are accomplished without invoking ecological succession, environmental changes, or selective postmortem preservation. Time-averaging results in less inflation of diversity at habitat than at point scales; paleoecological studies should thus analyze data at multiple spatial scales, including that of the habitat where multiple bulk samples have been pooled in order to minimize time-averaging effects. The diversity of assemblages that have accumulated over 1000 years at point and habitat scales is expected to be inflated by an average of 2.1 and 1.6, respectively. This degree of inflation is slightly higher than that observed in molluscan death assemblages at these same spatial scales (1.8 and 1.3). Thus, neutral metacommunity models provide useful quantitative constraints on directional but predictable effects of time-averaging. They provide minimal estimates for the rate of increase in diversity with time-averaging because they assume no change in environmental conditions and in the composition of the metacommunity within the window of averaging.
Fidelity of variation in species composition and diversity partitioning by death assemblages: time-averaging transfers diversity from beta to alpha levels
- Adam Tomašových, Susan M. Kidwell
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- Journal:
- Paleobiology / Volume 35 / Issue 1 / Winter 2009
- Published online by Cambridge University Press:
- 08 April 2016, pp. 94-118
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Despite extensive paleoecological analyses of spatial and temporal turnover in species composition, the fidelity with which time-averaged death assemblages capture variation in species composition and diversity partitioning of living communities remains unexplored. Do death assemblages vary in composition between sites to a lesser degree than do living assemblages, as would be predicted from time-averaging? And is the higher number of species observed in death relative to living assemblages reduced with increasing spatial scale? We quantify the preservation of spatial and temporal variation in species composition using 11 regional data sets based on samples of living molluscan communities and their co-occurring time-averaged death assemblages. (1) Compositional dissimilarities among living assemblages (LA) within data sets are significantly positively rank-correlated to dissimilarities among counterpart pairs of death assemblages (DA), demonstrating that pairwise dissimilarity within a study area has a good preservation potential in the fossil record. Dissimilarity indices that downplay the abundance of dominant species return the highest live-dead agreement of variation in species composition. (2) The average variation in species composition (average dissimilarity) is consistently smaller in DAs than in LAs (9 of 11 data sets). This damping of variation might arise from DAs generally having a larger sample size, but the reduction by ∼10–20% mostly persists even in size-standardized analyses (4 to 7 of 11 data sets, depending on metric). Beta diversity expressed by the number of compositionally distinct communities is also significantly reduced in death assemblages in size-standardized analyses (by ∼25%). This damping of variation and reduction in beta diversity is in accord with the loss of temporal resolution expected from time-averaging, without invoking taphonomic bias (from differential preservation or postmortem transportation) or sample-size effects. The loss of temporal resolution should directly reduce temporal variation, and assuming time-for-space substitution owing to random walk within one habitat and/or temporal habitat shifting, it also decreases spatial variation in species composition. (3) DAs are more diverse than LAs at the alpha scale, but the difference is reduced at gamma scales because partitioning of alpha and beta components differs significantly between LAs and DAs. This indicates that the effects of time-averaging are reduced with increasing spatial scale. Thus, overall, time-averaged molluscan DAs do capture variation among samples of the living assemblage, but they tend to damp the magnitude of variation, making them a conservative means of inferring change over time or variation among regions in species composition and diversity. Rates of temporal and spatial species turnover documented in the fossil record are thus expected to be depressed relative to the turnover rates that are predicted by models of community dynamics, which assume higher temporal resolution. Finally, the capture by DAs of underlying variation in the LA implies little variation in the net preservation potential of death assemblages across environments, despite the different taphonomic pathways suggested by taphofacies studies.
Inferring skeletal production from time-averaged assemblages: skeletal loss pulls the timing of production pulses towards the modern period
- Adam Tomašových, Susan M. Kidwell, Rina Foygel Barber
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- Journal:
- Paleobiology / Volume 42 / Issue 1 / February 2016
- Published online by Cambridge University Press:
- 30 October 2015, pp. 54-76
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Age-frequency distributions of dead skeletal material on the landscape or seabed—information on the time that has elapsed since the death of individuals—provide decadal- to millennial-scale perspectives both on the history of production and on the processes that lead to skeletal disintegration and burial. So far, however, models quantifying the dynamics of skeletal loss have assumed that skeletal production is constant during time-averaged accumulation. Here, to improve inferences in conservation paleobiology and historical ecology, we evaluate the joint effects of temporally variable production and skeletal loss on postmortem age-frequency distributions (AFDs) to determine how to detect fluctuations in production over the recent past from AFDs. We show that, relative to the true timing of past production pulses, the modes of AFDs will be shifted to younger age cohorts, causing the true age of past pulses to be underestimated. This shift in the apparent timing of a past pulse in production will be stronger where loss rates are high and/or the rate of decline in production is slow; also, a single pulse coupled with a declining loss rate can, under some circumstances, generate a bimodal distribution. We apply these models to death assemblages of the bivalve Nuculana taphria from the Southern California continental shelf, finding that: (1) an onshore-offshore gradient in time averaging is dominated by a gradient in the timing of production, reflecting the tracking of shallow-water habitats under a sea-level rise, rather than by a gradient in disintegration and sequestration rates, which remain constant with water depth; and (2) loss-corrected model-based estimates of the timing of past production are in good agreement with likely past changes in local production based on an independent sea-level curve.
A new Early Jurassic rhynchonellid brachiopod from the western Tethys and implications for systematics of rhynchonellids from the Triassic–Jurassic boundary
- Adam Tomašových
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- Journal of Paleontology / Volume 80 / Issue 2 / March 2006
- Published online by Cambridge University Press:
- 14 July 2015, pp. 212-228
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A new multicostate rhynchonellid brachiopod Jakubirhynchia occurs in the Hettangian (Early Jurassic) carbonate deposits of the West Carpathians. A comparison with externally similar multicostate rhynchonellids from the Hettangian–Sinemurian carbonate deposits of the Alps shows their congeneric and conspecific status, indicating relatively wide regional distribution in the Early Jurassic of the western Tethys.
Diagnostic characters include conjunct deltidial plates, divided, vertically inclined inner hinge plates forming sessile septalium, and subfalciform crura with relatively complex, serrate terminations and broad outline in lateral view. Although the shape of hinge plates and crura may vary according to shell convexity and shell orientation during serial sectioning, two-dimensional restorations of cardinalia and crura show their distinctive morphology. It is important to note that some of the cardinalia details and crural shape can be detected only with very short between-section distances. Based on external characters, Jakubirhynchia is a homeomorph to another multicostate rhynchonellid, Calcirhynchia, typical of the Hettangian and Early Sinemurian of the northwestern Europe province and reported also from the Tethyan province. This suggests that original distribution of Calcirhynchia might not be so wide and postextinction multicostate rhynchonellids were taxonomically more diverse as was previously supposed. Jakubirhynchia consistently occupies offshore, low-energy, micritic-rich habitats below storm wave base. It is the first reported Hettangian member of the family Basiliolidae and may bridge the gap between the Carnian Veghirhynchia and Early Jurassic basiliolids. Preextinction, Norian and Rhaetian brachiopods that are close morphologic relatives of Jakubirhynchia are known from outside the western Tethyan area only.