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Associations of fossil genera and species commonly display repeated and predictable patterns of change in stratigraphic sections. These changes exhibit some analogies with the phenomenon of ecological succession but are longer-term allogenic temporal changes, occurring over time scales of tens to hundreds of thousands of years, that should be referred to as biotic (faunal-floral) replacement. Habitat tracking is one of a suite of possible mechanisms of biotic replacement, but one that may be important in certain marine settings. Evidence of tracking includes (1) recurrence of similar replacement series of differing age; (2) mirroring of vertical replacement of faunas by lateral gradients of species associations along single time planes; (3) the occurrence of similar gradient transects at different time planes; (4) the correlatability of highs and lows of quantitative faunal curves (e.g., variations of detrended correspondence analysis scores) at different localities despite offsets of absolute scores; and (5) the high fidelity recurrence of stenotopic species in particular associations representing narrowly defined environments. The degree of ecological fidelity (e.g., similarity of species richness, guild structure) that is maintained during tracking is variable. Recurrent assemblages in different cycles, separated by as much as several million years, can be very similar in terms of species composition and trophic structure. Common species, however, may show significant differences in rank and relative abundances. This evidence indicates individualistic tracking of preferred habitat by various species. In shallow-shelf and ramp settings, sea-level fluctuations may produce approximately symmetrical patterns of biotic replacement where biofacies are arrayed typically in elongate belts parallel to depositional strike. Asymmetries, however, are common and may result from variations in sediment supply during sea-level fluctuation. Hence, the low siliciclastic input typical of transgressions predictably favors those organisms that require lower sedimentation or turbidity and perhaps firmgrounds to hardgrounds, whereas the regressive half cycle at analogous depths favors more eurytopic organisms that tolerate or prefer higher sedimentation or turbidity. The phenomenon of tracking may be of considerable importance in evolutionary paleoecology. Tracking implies that species commonly respond to long-term physical changes, not by adaptation, but primarily by migration of species to preferred habitats, if the rate or magnitude of environmental change is not too great. Provided that the same basic environment existed through time, despite lateral shifting by up to hundreds of kilometers, most species of benthic invertebrates were capable of surviving with little or no evolutionary change. Tracking may be the primary basis of patterns of morphological stasis, as well as relative stability in biofacies richness, composition, and trophic structure.
Analysis of the limestone-dominated Upper Ordovician (Chatfieldian-Edenian) Point Pleasant–Fulton interval provides detailed documentation of the internal composition of a transgressive systems tract within a mixed-carbonate siliciclastic foreland basin succession. This 14 m interval is divisible into 13 small-scale cycles that are widely traceable and record a lithofacies gradient from calcarenite-rich outcrops of central Kentucky into interbedded fine-grained grainstones and organic-rich shale in the subsurface of western Ohio. Hardgrounds and condensed beds are widespread and numerous and commonly cap the limestone hemicycle of small-scale cycles. Taphonomic- and faunal-gradient analyses of 233 bedding planes reveal both lateral and vertical gradients indicative of deepening from central Kentucky northward and a similar signature of deepening upward through the study interval. The deepening-upward trend within the Point Pleasant member is coincident with a decrease in the thickness of shale hemicycles, whereas the deepening-upward trend within the Fulton submember shows a slight increase in shale hemicycle thickness and quartz silt content. A polymictic intraformational conglomerate, the most complex discontinuity surface within the study interval, marks the contact of the Point Pleasant–Fulton members. The study interval, thus, is interpreted to represent distinctive early and late phases (Point Pleasant and Fulton members, respectively) of the transgressive systems tract separated by a maximum starvation surface. These patterns suggest that this widespread limestone-dominated interval formed primarily in response to basinwide, relative sea-level rise and siliciclastic sediment starvation, rather than simply through winnowing.
We studied mollusk-dominated multispecies assemblies from the south Pyrenean foreland in Spain by using relative abundance data in a framework of high-frequency depositional sequences for an interval spanning 2 myr across the early Eocene climatic optimum. The sequences are part of the Figols (middle Ypresian) and Castigaleu (upper Ypresian) allogroups and together document environmental changes influenced by tectonics at the lower frequency and driven by sea level and climate at the high-frequency temporal scale. We applied ordination through multidimensional scaling and other techniques to explore the structure of the data set. Six Figols and eight Castigaleu communities, linked along onshore-offshore gradients, were interpreted on uniformitarian grounds. Paleoenvironments ranged from mangrove forest to tidal flat, tidal creek, estuary, delta front, shoreface, carbonate ramp, and inner shelf. Some habitats were represented throughout the interval, allowing examination of the effect on coastal ecosystems of environmental disturbances tied to sea-level lowstands, as deduced from the sequence stratigraphic analysis. These disturbances presumably amounted to extirpation by river floods, burial, increased turbidity, high river-derived nutrient input, and decreased ecospace availability. The major effect on the mangrove-estuary-delta gradient was coincident with the unconformity separating the two allogroups. Mangrove communities suffered a change in rank of dominating species and a turnover of rare species. Subtidal estuarine and delta communities were affected in their relative abundances, but we observed no change in rank or turnover. We suggest that the different niche breadth of organisms leads to different responses to perturbation of several scales. Many intertidal cerithioidean gastropods went extinct, whereas subtidal turritelline gastropods were unchanged, during the major environmental variation. Mangrove gastropods experienced increased originations in the upper Ypresian. The carbonate ramp heterotrophs did not change across the Figols-Castigaleu perturbation, although throughout the early Eocene the phototroph guild within the same ecosystem was undergoing frequent turnover events. All studied associations showed significant changes in the relative abundances of constituent species across unconformities of minor entity, proving that soft-bottom marine communities conform to an open-membership model of ecosystem recruitment, as suggested by studies of past open marine ecosystems. Persistence to a degree is suggested by mangrove communities, indicating a slightly more limited membership in low-nutrient estuarine habitats, a response more similar to that of coral reef tropical ecosystems. This study confirms the idea that different species and communities may experience opposite effects from the same events and shows that faunal distributions in estuarine and deltaic systems reflect more than just bathymetric change. High onshore origination also conforms to the theory of onshore-offshore faunal change.
Fine-scale paleocommunity dynamics were studied in a short (∼16 m) section in the Middle Miocene (Badenian Stage) of the Central Paratethys, which consists of siliciclastic, pelitic, and sandy-to-gravely shallow-water deposits. Two basal, coarsening- and shallowing-upward parasequences of a late highstand systems tract are separated by a third-order sequence boundary from the deepening-upward basal part of a transgressive systems tract at the top of the section. Benthic faunas in this succession are primarily autochthonous and storm-influenced, level-bottom assemblages, but a distinct oyster-vermetid boundstone occurs near the base of the transgressive systems tract. Additionally, three tempestitic shell beds were included, which were found out of sequence in an associated basinal setting; their faunal content relates them closely to the fine-grained deepest parts of the transgressive systems tract. Ordination of species and samples using detrended correspondence analysis and analysis of similarity suggest that two basic benthic assemblages can be distinguished. The oyster-vermetid boundstone is tied to a unique set of environmental conditions and indicates a major environmental change at the sequence boundary. The faunal assemblage in the boundstone shows a weak gradient into the pelitic (deeper and quiet-water) level-bottom assemblage, which in turn is characterized by strong overlaps with the fauna of sandy (shallower and more agitated) habitats. Therefore it is concluded that the benthic assemblages in the studied section belong to the same basic metacommunity, which was not seriously affected by the strong facies changes at the sequence boundary and at the flooding surfaces. Moreover, the species in the studied benthic assemblages reacted to changes in the environment by habitat tracking.
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.
The diverse, well-preserved fauna of the Middle Devonian Hamilton Group of western New York has become an exemplar of long-term taxonomic and paleoecological stability and habitat tracking in response to sea-level change. Recent detailed, quantitative studies have challenged this view, suggesting instead a relatively low proportion of persistent lineages and recurrent biofacies sharing only the most abundant species; however, most studies have considered only limited geographic areas. As a result of shifting basin-forebulge positions and sedimentation patterns, analogous facies do not occur in every cycle of single geographic areas but show complex migration within the Appalachian Basin. Consequently, similarity of biofacies recurrence can only be fairly assessed by considering the most analogous facies wherever they occur across a major cross section of the basin. This paper evaluates patterns of biofacies recurrence based on samples from subsymmetrical cycles of dark-gray shale, calcareous mudstone, and argillaceous limestone. Low-sedimentation, depth-related biofacies, identified quantitatively using cluster analysis, recur symmetrically in single third-order regressive-transgressive cycles throughout the 5–6 myr duration of the Givetian Hamilton Group and Tully Formation at different geographic locations. Detrended correspondence analysis was used to recognize gradients of species and sample distribution both within and among depositional cycles; depth-related biofacies range from basinal, low-diversity leiorhynchid brachiopod– dominated associations to highly diverse coral-brachiopod (shallow subtidal) assemblages. This pattern is also comparable to the order of species-biofacies appearances in single, small-scale shallowing-upward cycles. In addition to similarities of species richness and guild structure, given biofacies show strong similarities of species composition. Low-diversity, high-dominance associations typical of deeper water biofacies show lower similarities (60%–75% species overlap), suggesting that they represent loosely structured aggregations of eurytopic taxa. Similarities are greatest in the diverse coral and brachiopod biofacies, for which most pairwise comparisons of samples throughout the Hamilton–Tully interval show >80% overlap in species composition and very strong similarity of richness and guild structures but not necessarily rank or relative abundance of taxa. Overall, these data suggest that gradients of species distribution in relation to environmental gradients, especially depth-related factors, were quite stable over several million years and that biofacies shifted in response to transgressive-regressive cycles. Such biofacies stability need not imply persistence of tightly integrated communities. Nonetheless, the long range of many species and maintenance of biotic gradients have important evolutionary implications—under relatively stable conditions, a majority of species track shifting habitats rather than adapt to changing local conditions.
The Manutahi-1 core of Wanganui Basin, New Zealand, provides a special opportunity to investigate variations in depositional paleoenvironments by way of high-resolution paleoecologic and sedimentologic studies in a succession that accumulated under the influence of late Neogene rapid glacioeustatic sea-level oscillations. Thirty cyclothems of the Matemateaonga Formation (late Miocene–early Pliocene) have been identified through a 960 m interval of the core. Temporal patterns in both lithofacies distribution and macrofaunal paleoecology indicate a close link with environmental shifts that took place over the duration of these sixth-order sea-level cycles (∼41 kyr duration). Because foraminiferal assemblages have proven to be depauperate in the Manutahi-1 core, macrofossils, especially bivalves and gastropods, are used as the primary source of paleoecologic data. The paleoenvironmental analyses presented here have relied extensively on the transfer of ecologic data from the modern fauna to extant or closely related species observed in the core. Most notably, a relative sea-level curve has been constructed for the Matemateaonga Formation from analysis of bathymetric data for appropriate modern taxa. A range of exploratory statistical approaches, including cluster and ordination techniques, allow rapid, objective, and reproducible classification of macrofaunal associations and the elucidation of large-scale paleoenvironmental patterns. Several implications arise from this study. First, sequence boundaries and other key surfaces are confidently identified on the basis of integrated sedimentologic, taphonomic, and paleoecologic investigation. Second, changes in depositional environment and the responses of benthic communities are revealed at a range of temporal scales. The temporal pattern of paleoecologic change appears to be the result of lateral, facies-related shifting of biofacies (habitat tracking) in response to sea level, sediment flux, and other associated paleoenvironmental variables. Temporal patterns of biofacies occurrence and diversity are, however, strongly overprinted by stratigraphic and taphonomic processes.