Daniele Scarponi

Stratigraphic paleoecology: Bathymetric signatures and sequence overprint of mollusk associations from upper Quaternary sequences of the Po plain, Italy

Upper Quaternary sequences of the Po Plain (Italy) were used to assess the informative strength and sequence-stratigraphic overprint of quantitative paleoecological patterns. Three densely sampled cores (89 samples, 98 genera, 23,280 specimens), dominated by extant mollusk species with known environmental distributions, were analyzed with detrended correspondence analysis (DCA). The DCA scores, calibrated using extant genera, provided outstanding estimates of bathymetry (±3 m) and related environmental parameters. Depth-related successions of mollusk associations delineated by using DCA were consistent with independent sequence-stratigraphic interpretations and yielded insights inaccessible via routine techniques (e.g., depth estimates for maximum flooding surfaces). The DCA ordination demonstrates the severity of the sequence-stratigraphic overprint: samples are highly uniform taxonomically during late transgressive systems tracts and highly variable during the following highstand systems tracts. When analyzed across comparable systems tracts, similar species associations repeat during the last and current interglacial cycles, suggesting that Po Plain mollusk associations have remained remarkably stable over the past 125 k.y. The results are consistent with the bathymetric interpretation of the DC axis 1 postulated previously for the Paleozoic fossil record, demonstrate the sequence-stratigraphic overprint of paleoecological patterns predicted by computer modeling, and illustrate the utility of quantitative paleoecological patterns in augmenting sequence-stratigraphic interpretations.

Ecological, taxonomic, and taphonomic components of the post-Paleozoic increase in sample-level species diversity of marine benthos

Abstract Biological veracity of the sharp diversity increase observed in many analyses of the post-Paleozoic marine fossil record has been debated vigorously in recent years. To assess this question for sample-level (“alpha”) diversity, we used bulk samples of shelly invertebrates, representing three major fossil groups (brachiopods, bivalves, and gastropods), to compare the Jurassic and late Cenozoic sample-level diversity of marine benthos. After restricting the data set to single-bed, whole-fauna, bulk samples (n ≥ 30 specimens) from comparable open marine siliciclastic facies, we were able to retain 427 samples (255 Jurassic and 172 late Cenozoic), with most of those samples originating from our own empirical work. Regardless of the diversity metric applied, the initial results suggest that standardized sample-level species (or genus) diversity, driven by evenness and/or richness of the most common taxa, increased between the Jurassic and late Cenozoic by at least a factor of 1.6. When the data are partitioned into the three dominant higher taxa, it becomes clear that (1) the bivalves, which dominated the samples for both time intervals, increased in sample-level diversity between the Jurassic and the late Cenozoic by a much smaller factor than the total fauna; (2) the removal of brachiopods, which were a noticeable component of the Jurassic samples, did not significantly affect standardized sample-level diversity estimates; and (3) the gastropods, which were rare in the Jurassic but common in many late Cenozoic samples, contributed notably to the increase in sample-level diversity observed between the two time intervals. Parallel to these changes, the samples revealed secular trends in ecological structure, including Jurassic to late Cenozoic increases in proportion of (1) infauna, (2) mobile forms, and (3) non-suspension-feeding organisms. These trends mostly persist when data are restricted to bivalves. Supplementary analyses indicate that these patterns cannot be attributed to sampling heterogeneities in paleolatitudinal range, lithology, or paleoenvironment of deposition. Likewise, when data are restricted to samples dominated by species with originally aragonitic shells, the observed temporal changes persist at a comparable magnitude, suggesting that the pervasive loss of aragonite in the older fossil record is unlikely to have been the primary cause of the observed patterns. The comparable ratio of identified to unidentified species and genera, observed when comparing the Jurassic and late Cenozoic samples, indicates that the relatively poorer (mold/cast) preservation of Jurassic aragonite species also is unlikely to have been responsible for the observed patterns. However, the diagenesis-related taphonomic and methodological artifacts cannot be ruled out as an at least partial contributor to the observed post-Paleozoic changes in diversity, taxonomic composition, and ecology (the outcomes of the three tests of the diagenetic bias available to us are incongruent). The study demonstrates that the post-Paleozoic trends in the sample-level diversity, ecology, and taxonomic structure of common taxa can be replicated across multiple studies. However, the diversity increase estimated here is much less prominent than suggested by many previous analyses. The results also narrow the list of causative explanations down to two testable hypotheses. The first is diagenetic bias—a spurious trend driven by either (a) increasing taphonomic loss of small specimens in the older fossil record or (b) a shift in sampling procedures between predominantly lithified rocks of the Mesozoic and predominately unlithified, and therefore sievable, sediments of the late Cenozoic. The second hypothesis is genuine biological changes—macroevolutionary trends in the structure of marine benthic associations through time, consistent with predictions of several related models such as evolutionary escalation, increased ecospace utilization, and the Mesozoic marine revolution. Future studies should focus on testing these two rival models, a key remaining challenge for identifying the primary causative mechanism for the long-term changes in sample-level diversity, ecology, and taxonomic structure observed in the Phanerozoic marine fossil record.

Sequence stratigraphy and the resolution of the fossil record

Sequence stratigraphy, a major theoretical achievement of earth sciences, integrates facies associations and stratal architecture within a chronological framework of the geological record. The sequence stratigraphic model implies that sediment supply to the basin, preservation potential of individual horizons, and the resolution of paleontological data co-vary with base-level fluctuations, especially for siliciclastic depositional systems. Using Holocene transgressive-regressive successions of the Po Plain (Italy), we assessed the model’s hypotheses by analyzing 249 marine mollusk shells dated individually using 14 C-calibrated amino acid racemization methods. As postulated by the model, the temporal resolution of the fossil record, frequency of depositional events, and net accumulation rates decreased upward concomitantly through the transgressive systems tract reaching minima in the condensed section (maximum flooding zone). The reverse trend, with increasingly frequent, thicker, and less time-averaged beds, was observed throughout the overlying highstand systems tract. The results quantify the postulated sequence stratigraphic asymmetry in temporal resolution of the fossil and sedimentary records.

SEQUENCE STRATIGRAPHIC ANATOMY OF DIVERSITY PATTERNS: LATE QUATERNARY BENTHIC MOLLUSKS OF THE PO PLAIN, ITALY

Abstract The influence of sequence stratigraphic (base-level driven) processes on patterns derived from the fossil record is receiving increasing attention. This study explores the stratigraphic anatomy of diversity patterns across two late Quaternary fourth-order sequences deposited on the Po Plain (Italy) over the last 150 ky (i.e., the two most recent glacial-interglacial cycles). The rich mollusk fauna, dominated by extant forms, preserved as a part of well-understood eustatic cycles, offers a testing ground for exploring how climate-driven sea-level changes influence sample diversity, diversity turnover, and higher-order diversity patterns within and across systems tracts and sequences. These two fourth-order depositional sequences were densely sampled from three cores. The data (152 species and 22777 specimens from 29 Holocene and 19 Pleistocene samples) were analyzed using single-sample and multisample rarefaction techniques. In all three cores and for both cycles, sample-level diversity decreased upward within sequences: the late transgressive systems tract samples displayed the highest equitability and richness, and the highstand systems tract samples displayed the lowest diversity (the trend primarily reflects the increase in the dominance of most common species in highstand systems tract samples). This pattern is likely due to a combination of ecological, environmental, and taphonomic processes. Multisample rarefaction indicates that species turnover is more limited in transgressive phases of both depositional cycles. This trend may reflect increasing environmental heterogeneity of marginal habitats averaged within shallowing-upward successions or decreasing time averaging associated with increasing sedimentation rates during highstand systems tract phases of the cycles. The sequence and multisequence diversity levels are lower than those observed within individual late transgressive systems tracts, indicating that species turnover was minimal both within as well as across the last two glacial-interglacial cycles. This study shows that species richness and equitability patterns of the most common mollusk species track closely the sequence stratigraphic architecture of late Quaternary successions of the Po Plain.

Differential responses of marine communities to natural and anthropogenic changes

Responses of ecosystems to environmental changes vary greatly across habitats, organisms and observational scales. The Quaternary fossil record of the Po Basin demonstrates that marine communities of the northern Adriatic re-emerged unchanged following the most recent glaciation, which lasted approximately 100 000 years. The Late Pleistocene and Holocene interglacial ecosystems were both dominated by the same species, species turnover rates approximated predictions of resampling models of a homogeneous system, and comparable bathymetric gradients in species composition, sample-level diversity, dominance and specimen abundance were observed in both time intervals. The interglacial Adriatic ecosystems appear to have been impervious to natural climate change either owing to their persistence during those long-term perturbations or their resilient recovery during interglacial phases of climate oscillations. By contrast, present-day communities of the northern Adriatic differ notably from their Holocene counterparts. The recent ecosystem shift stands in contrast to the long-term endurance of interglacial communities in face of climate-driven environmental changes.

Quantitative Bathymetric Models for Late Quaternary Transgressive-Regressive Cycles of the Po Plain, Italy

In marine settings, quantitative bathymetric models can be developed using various water depth proxies, including epibiont distribution, sedimentologic features, and the distribution of benthic taxa in time and space. Here, the late Quaternary bathymetric history of the Po coastal plain (Italy) has been reconstructed using mollusk samples from a network of 16 cores. Multiple analytical approaches have been applied in a comparative fashion. A direct ordination approach was used to estimate sample bathymetry using weighted averaging of genera with known preferred depth. Weighted averaging carries an advantage of analytical simplicity and produces direct ordination models expressed in environmentally meaningful units. Indirect ordination methods, based on depth estimates developed using posteriori-calibrated ordination strategies (correspondence and detrended correspondence analysis calibrated against present-day bathymetric data), yielded results consistent with weighted averaging. Regardless of the choice of analytical methods, mollusk assemblages yielded bathymetric proxies congruent with independent sequence stratigraphic interpretations derived previously for both Late Pleistocene and Holocene transgressive-regressive cycles. The mollusk-derived proxies quantify spatial bathymetric gradients across the basin and local trends in absolute water depth in response to relative changes in sea level. However, for cores located in the most proximal part of the basin, mollusk-based ordinations failed to provide viable estimates due to inclusion of mixed marine and nonmarine mollusk faunas and scarcity of fossiliferous horizons necessary for adequate quantitative sampling. The multiple analytical approaches cross evaluated in this study consistently suggest that high-resolution quantitative bathymetric estimates can be derived for mollusk samples independent of stratigraphy for fully marine settings. When applied simultaneously to both samples and taxa, these approaches provide a viable strategy for quantifying stratigraphic and paleontological patterns and enhancing interpretations of basin-scale depositional systems.

Evolutionary and ecological implications of trematode parasitism of modern and fossil northern Adriatic bivalves

Abstract The role of antagonistic organismal interactions in the production of long-term macroevolutionary trends has been debated for decades. Some evidence seems to suggest that temporal trends in predation frequency share a common causative mechanism with genus-level diversity, whereas studies on the role of parasites in “shaping” the evolutionary process are rare indeed. Digenean trematodes (Phylum Platyhelminthes) infest molluscs in at least one stage of their complex life cycle. Trematodes leave characteristic oval-shaped pits with raised rims on the interior of their bivalve hosts, and these pits are preserved in the fossil record. Here we survey 11,785 valves from the Pleistocene–Holocene deposits of the Po Plain and from nearby modern coastal environments on the northeast Adriatic coast of Italy. Of these, 205 valves exhibited trematode-induced pits. Trematodes were selective parasites in terms of host taxonomy and host body size. Infestation was restricted to lower shoreface/transition-to-platform paleoenvironments. During the Holocene, individuals from the transgressive systems tract were significantly more likely to be infested than those from highstand systems tracts. Temporal trends in infestation frequency cannot be explained as an ecological/evolutionary phenomenon (e.g., the hypothesis of escalation); instead the trend seems controlled by environmental variation induced by glacio-eustatic sea-level changes and inadequate sampling. Because this interaction appears to be ephemeral, both temporally and spatially, it is not likely that any selective pressure would be continuous over geologic time in this region. Furthermore, these results support the hypothesis that antagonistic interactions are lower in the northern Adriatic Sea in comparison to other midlatitude shallow marine settings.

Geographic variation of parasitic and predatory traces on mollusks in the northern Adriatic Sea, Italy: implications for the stratigraphic paleobiology of biotic interactions

Abstract. Parasitic trematode worms leave characteristic pits in their bivalve mollusk hosts and represent an ideal system for analyzing parasite-host interactions through space and time with statistically meaningful sample sizes. Previous work in Late Pleistocene—Holocene sequences from the Po plain revealed significant long-term fluctuations in trematode prevalence values: higher prevalence in retrogradational environments (TST) and negligible prevalence in progradational environments (HST). Here we expand upon this work by investigating traces of parasitism, kleptoparasitism, and predation on mollusk death assemblages from two domains along the northern Adriatic coastline. The domain north of the Po delta (TST-like) and the southern domain (including the Po delta; HST-like) comprise environments comparable to those recovered in late Holocene (<6 Kyr) subsurface progradational deposits. We collected 17,299 specimens representing 111 species from 11 locations on the northern Adriatic coast of Italy. Our results reveal high predation pressure, a high diversity of host taxa, and widespread presence of trematode infestation in starved, oligotrophic, environmentally more stable (i.e., TST-like) settings north of the Po delta. Immediately south of the Po delta, in settings with strong and variable sedimentary input, almost no infestation is recorded. The reappearance of infestation is evident in the southern portion of the study area (i.e., Cattolica-Montemarciano), relatively far from the highly stressed environments south of the Po River. There is no significant difference in trematode prevalence values between fossil and modern samples. The distribution of spionid traces (an indicator of stressed environments) was nearly the opposite of that displayed by trematodes. Drilling frequency is highest in TST-like environments and is not correlated with diversity indices. These results suggest that temporal trends of trematode prevalence (and possibly also other biotic interactions) in sedimentary successions are controlled by environmental changes driven by glacio-eustatic dynamics, and reaffirm the importance of interpreting temporal trends in the context of spatial variation.

Surges in trematode prevalence linked to centennial-scale flooding events in the Adriatic

The forecasts of increasing global temperature and sea level rise have led to concern about the response of parasites to anthropogenic climate change. Whereas ecological studies of parasite response to environmental shifts are necessarily limited to short time scales, the fossil record can potentially provide a quantitative archive of long-term ecological responses to past climate transitions. Here, we document multi-centennial scale changes in prevalence of trematodes infesting the bivalve host Abra segmentum through multiple sea-level fluctuations preserved in brackish Holocene deposits of the Po Plain, Italy. Prevalence values were significantly elevated (p < 0.01) in samples associated with flooding surfaces, yet the temporal trends of parasite prevalence and host shell length, cannot be explained by Waltherian facies change, host availability, salinity, diversity, turnover, or community structure. The observed surges in parasite prevalence during past flooding events indicate that the ongoing global warming and sea-level rise will lead to significant intensification of trematode parasitism, suppressed fecundity of common benthic organisms, and negative impacts on marine ecosystems, ecosystem services, and, eventually, to human well-being.

Early-Middle Pleistocene benthic turnover and oxygen isotope stratigraphy from the Central Mediterranean (Valle di Manche, Crotone Basin, Italy): Data and trends

Ostracod faunal turnover and oxygen isotope data (foraminifera) along the Valle di Manche (VdM) section are herein compiled. Specifically, the material reported in this work includes quantitative palaeoecological data and patterns of ostracod fauna framed within a high-resolution oxygen isotope stratigraphy (δ18O) from Uvigerina peregrina. In addition, the multivariate ostracod faunal stratigraphic trend (nMDS axis-1 sample score) is calibrated using bathymetric distributions of extant molluscs sampled from the same stratigraphic intervals along the VdM section. Data and analyses support the research article “Dynamics of benthic marine communities across the Early-Middle Pleistocene boundary in the Mediterranean region (Valle di Manche, Southern Italy): biotic and stratigraphic implications” Rossi et al. [1].