The late Paleozoic ice age (LPIA) began in the Mississippian period (Visean-Serpukhovian) and is associated with a global mass extinction. This study examines how the development of high-amplitude ...glacioeustasy at the start of the LPIA affected the taxonomic richness and ecologic diversity of tropical marine macro-invertebrate assemblages from the Illinois Basin (United States). Rarefaction and faunal turnover analyses reveal that regional taxonomic and guild richness persist nearly unchanged across the inferred onset of the LPIA and through multiple glacioeustatic cycles spanning a total duration of ∼3 to 5 myr. In addition, low levels of taxonomic turnover characterize the LPIA transition and subsequent glacioeustatic cycles; 76% to 92% of pre-LPIA taxa persist into the ice age where they account for 63% to 75% of LPIA taxa. Thus, the onset of high-amplitude glacioeustasy was not a main driver of regional biotic change or extinction in the Illinois Basin at the start of the LPIA. Potential mechanisms for the regional persistence of taxonomic diversity and faunal composition despite major glacioeustasy include habitat tracking and an immigration rescue effect, where localized extinction is balanced by immigration of taxa from nearby areas. Finally, evidence from this and other studies of late Paleozoic faunal turnover suggest that persistence, not change, is the normal biotic response to high-amplitude glacioeustasy during the LPIA.
The late Mesozoic through early Cenozoic is an interval of significant biologic turnover and ecologic reorganization within marine assemblages, but the timing and causes of these changes remain ...poorly understood. Here, we quantify the pattern and timing of shifts in the diversity (richness and evenness) and ecology of local (i.e., sample level) mollusk-dominated assemblages during this critical interval using field-collected and published data sets from the U.S. Gulf Coastal Plain. We test whether the biologic and ecologic patterns observed primarily at the global level during this time are also expressed at the local level, and whether the end-Cretaceous (K/Pg) mass extinction and recovery moderated these trends. To explore whether environment had any effect on these patterns, we examine data from shallow subtidal and offshore settings. Assemblages from both settings recovered to pre-extinction diversity levels rapidly, in less than 7 million years. Following initial recovery, diversity remained unchanged in both settings. The trajectory of ecological restructuring was distinct for each setting in the wake of the K/Pg extinction. In offshore assemblages, the abundance and number of predatory carnivorous taxa dramatically increased, and surficial sessile suspension feeders were replaced by more active suspension feeders. In contrast, shallow subtidal assemblages did not experience ecological reorganization following the K/Pg extinction. The distinct ecological patterns displayed in each environment follow onshore-offshore patterns of innovation, whereby evolutionary novelties first appear in onshore settings relative to offshore habitats. Increased predation pressure may explain the significant ecological restructuring of offshore assemblages, whereby the explosive radiation of predators drove changes in their prey. Habitat-specific ecological restructuring, and its occurrence solely during the recovery interval, implies that disturbance and incumbency were also key in mediating these ecological changes.
Understanding what drives global diversity requires knowledge of the processes that control diversity and turnover at a variety of geographic and temporal scales. This is of particular importance in ...the study of mass extinctions, which have disproportionate effects on the global ecosystem and have been shown to vary geographically in extinction magnitude and rate of recovery. Here, we analyze regional diversity and turnover patterns for the paleocontinents of Laurentia, Baltica, and Avalonia spanning the Late Ordovician mass extinction and Early Silurian recovery. Using a database of genus occurrences for inarticulate and articulate brachiopods, bivalves, anthozoans, and trilobites, we show that sampling-standardized diversity trends differ for the three regions. Diversity rebounded to pre-extinction levels within 5 Myr in the paleocontinent of Laurentia, compared with 15 Myr or longer for Baltica and Avalonia. This increased rate of recovery in Laurentia was due to both lower Late Ordovician extinction rates and higher Early Silurian origination rates relative to the other continents. Using brachiopod data, we dissected the Rhuddanian recovery into genus origination and invasion. This analysis revealed that standing diversity in the Rhuddanian consisted of a higher proportion of invading taxa in Laurentia than in either Baltica or Avalonia. Removing invading genera from diversity counts caused Rhuddanian diversity to fall in Laurentia. However, Laurentian diversity still rebounded to pre-extinction levels within 10 Myr of the extinction event, indicating that genus origination rates were also higher in Laurentia than in either Baltica or Avalonia. Though brachiopod diversity in Laurentia was lower than in the higher-latitude continents prior to the extinction, increased immigration and genus origination rates made it the most diverse continent following the extinction. Higher rates of origination in Laurentia may be explained by its large size, paleogeographic location, and vast epicontinental seas. It is possible that the tropical position of Laurentia buffered it somewhat from the intense climatic fluctuations associated with the extinction event, reducing extinction intensities and allowing for a more rapid rebound in this region. Hypotheses explaining the increased levels of invasion into Laurentia remain largely untested and require further scrutiny. Nevertheless, the Late Ordovician mass extinction joins the Late Permian and end-Cretaceous as global extinction events displaying an underlying spatial complexity.
We performed sensitivity experiments using an ocean general circulation model at two stages of the Late Ordovician (Caradoc, ∼454 Ma; Ashgill, ∼446 Ma) under a range of atmospheric
pCO
2 values ...(8–18× PAL; pre-industrial atmospheric level) at high and low sea level.
The model results indicate that the long-term cooling trend during the Late Ordovician can be explained by progressive cooling of the global ocean in response to falling levels of atmospheric
pCO
2, sea level change, and paleogeographic change. These results also explain the occurrence of low latitude cool-water carbonates in North America.
In all simulations, a drop in sea level led to a reduction in poleward ocean heat transport. This indicates a possible positive feedback that could have enhanced global cooling in response to sea level drop during the Late Ordovician. Alterations in poleward ocean heat transport linked to changes of atmospheric
pCO
2 also indicate that there is a threshold of 10× PAL, above which ocean current change cannot be responsible for glaciation in the Late Ordovician. Continental drift could explain the observed global cooling trend in the Late Ordovician through a combined poleward ocean heat transport feedback and increased ice-albedo effect if atmospheric
pCO
2 was low during the entire Late Ordovician.
The model results further indicate that the response of meridional overturning to changes in paleogeography, atmospheric
pCO
2, and sea level is stronger than the response of surface circulation to these perturbations. Because the overturning circulation is so strong, meridional overturning was the dominant mechanism for described changes in heat transport in the Late Ordovician.
Linking biotic patterns across spatiotemporal scales provides a greater understanding of the processes that drive ecological and evolutionary change. Here, we examine how global patterns of biotic ...turnover are expressed in the structure of regional biotic gradients from the Illinois Basin (USA) during the late Paleozoic ice age (LPIA)— an interval noted for low global rates of taxonomic turnover. Results indicate that the pre-LPIA interval is characterized by well-defined biotic gradients; depositional environments are dominated by distinct sets of taxa, arrayed along a gradient of substrate composition, and are clearly differentiated by ordination and ANOSIM analyses. There is a marked shift in gradient structure across the onset of the LPIA; environments become weakly distinguished, are dominated by similar sets of taxa that are widespread across the ramp, and are arrayed along a nearshore-offshore gradient. Our results are consistent with findings from global-level studies, which indicate that broadly adapted taxa (eurytopes) increased in importance following the start of the LPIA. Unlike the global level, however, the regional increase in eurytopy is not linked to the extinction of narrowly adapted taxa in response to climate change. Instead, eurytopy increased as the geometry of the Illinois Basin shifted from a flat carbonate ramp, composed of shallower-water and higher-stress environments in the pre-LPIA interval, to a steeper ramp comprising deeper-water and more stable habitats in the LPIA interval. We suggest that increased eurytopy drove (1) a previously documented decrease in regional-level turnover during the late Paleozoic and (2) a perceived pattern of greater persistence in late versus early Paleozoic biotic gradients.
The calcareous nannoplankton underwent a major diversification between 60 Ma and 61 Ma with the appearance of two important Cenozoic genera, Fasciculithus and Sphenolithus. We have studied the ...evolution of these genera at four deep-sea sites using a combination of biostratigraphic and assemblage analyses. Our data show that the first appearances of both genera are time transgressive between sites by up to 1 myr, suggesting that these taxa were highly specialized or that ocean environments were heterogeneous. The interval of diversification coincided with a major turnover among nannoplankton assemblages involving an increase in taxa adapted to oligotrophic environments and a decrease in those suited to mesotrophic settings. The opening of niche space for oligotrophs such as Fasciculithus and Sphenolithus coincides with the radiation of oligotrophic planktonic foraminifers, including the genus Morozovella. The appearance of both groups occurred about 1 myr after the recovery of the biologic pump following its collapse at the Cretaceous–Paleocene mass extinction. This suggests that increased export efficiency stripped nutrients from surface waters, causing a major upheaval among marine food webs.
It has previously been thought that there was a steep Cretaceous and Cenozoic radiation of marine invertebrates. This pattern can be replicated with a new data set of fossil occurrences representing ...3.5 million specimens, but only when older analytical protocols are used. Moreover, analyses that employ sampling standardization and more robust counting methods show a modest rise in diversity with no clear trend after the mid-Cretaceous. Globally, locally, and at both high and low latitudes, diversity was less than twice as high in the Neogene as in the mid-Paleozoic. The ratio of global to local richness has changed little, and a latitudinal diversity gradient was present in the early Paleozoic.
Cluster analysis reveals eight biofacies during a time of faunal turnover and regional oceanographic change in the middle Upper Ordovician of central Kentucky. These biofacies are arranged along a ...deep-water to shallow-water gradient, with the dalmanellid and the Sowerbyella biofacies in offshore facies, the Rafinesquina, the atrypid, and the ramose trepostome biofacies in deep-subtidal to shallow-subtidal facies, the Constellaria-Cyclonema and the Rhynchotrema biofacies in the shallow-subtidal facies, and the Solenopora-Hebertella biofacies in sand-shoal facies. These biofacies are not discrete, but rather they share a large number of taxa suggesting that they are arbitrary subdivisions of a depth-related ecological gradient. Nonetheless, they are useful as a point of comparison with other studies of this time interval in the eastern United States. Previous lower and middle Upper Ordovician biofacies studies portray a similar pattern of plectorthines and rhynchonellaceans in the shallow subtidal, strophomenids in the deep subtidal, and dalmanellids and plectambonitaceans in the offshore, indicating an overall temporal consistency to many of these biofacies at suprageneric taxonomic levels. Similarly, values of preferred depth, depth tolerance, and peak abundance of most taxa are generally conserved, and this is particularly true for abundant taxa. Nonetheless, this 2-m.y. record also indicates temporal changes in the occurrence of some biofacies, in the relative abundances of taxa in some biofacies, in the species membership within biofacies, and in values of preferred depth, depth tolerance, and peak abundance for several taxa. Collectively these indicate a dynamic aspect to an ecosystem that otherwise displays several attributes of stability and underscore that ecologic stability should not be viewed as a simple dichotomy between stability and instability. The dynamic aspects of this ecosystem suggest that these biofacies were not governed by extreme degrees of species interactions that resulted in near constancy of biofacies structure. Continuous changes in water temperature, nutrient supply, and turbidity during this time may have been a contributing factor to changes within this ecosystem.
With the exception of the Neogene, it is difficult in much of the fossil record to measure range offset; that is, the difference in age between the first or last occurrence of a species in a local ...section and its time of origination or extinction within the sedimentary basin. A coupled simulation that incorporates a model of sedimentary basin fill, a random-branching model of evolution, and a model of the ecological characteristics of species is used here to explore stratigraphic and ecologic controls on range offset. Median values of range offset in much of the fossil record are predicted to range from several hundred k.y. to a few m.y., for a wide variety of stratigraphic architectures and species ecologies. Higher than average values of range offset are favored by unconformities of long duration, rapid facies changes of large magnitude, persistent monotonic trends in facies change, increased facies specificity of species, and decreased species abundance. These model predictions can be used as a guide for interpreting field data on first and last occurrences, such as evaluating zones of likely high or low biostratigraphic precision. Similarly, these results can be used to evaluate the support for paleobiological interpretations of local radiations, migrations, and extinction episodes.
