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Digitate shell morphologies have evolved repeatedly in planktonic foraminifera throughout the Cretaceous and Cenozoic. Digitate species are usually rare in fossil and modern assemblages but show increased abundance and diversity at times during the Cretaceous and middle Eocene. In this paper we discuss the morphology and stratigraphic distribution of digitate planktonic foraminifera and establish the isotopic depth ecology of fossil ones to draw parallels with modern counterparts. δ18O and δ13C values of six extinct and two modern digitate species, from six time slices (Cenomanian, Turonian, Eocene, Miocene, Pleistocene and Holocene) have similar isotopic depth ecologies, consistently registering the most negative δ13C and usually the most positive δ18O compared to coexisting species. These results indicate a similar deep, subthermocline (>150 m) habitat, characterized by lower temperatures, reduced oxygen, and enrichment of dissolved inorganic carbon. This is consistent with water-column plankton studies that provide insight into the depth preferences of the three modern digitate species; in over 70% of observations digitates occurred in nets below 150 m, and down to 2000 m. The correlation between digitate species and subsurface habitats across multiple epochs suggests that elongated chambers were advantageous for survival in a deep mesopelagic habitat, where food is usually scarce. Increased abundance and diversity of digitates in association with some early and mid-Cretaceous oceanic anoxic events, in middle Eocene regions of coastal and equatorial upwelling, and occasionally in some modern upwelling regions, suggests an additional link with episodes of enhanced ocean productivity associated with expansion of the oxygen minimum zone (OMZ). We suggest that the primary function of digitate chambers was as a feeding specialization that increased effective shell size and food gathering efficiency, for survival in a usually food-poor environment, close to the OMZ. Episodes of increased digitate abundance and diversity indicate expansion of the deep-water ecologic opportunity under conditions that were unfavorable to other planktonic species. Our results provide evidence of iterative evolution reflecting common functional constraints on planktonic foraminifera shell morphology within similar subsurface habitats. They also highlight the potential of digitate species to act as indicators of deep watermasses, especially where there was expansion of the OMZ.
Occurrence data from the Paleobiology Database are used to analyze the waxing and waning of genera over time. Irrespective of whether we tabulate species richness, frequency of occurrence, geographic range, or other measures, the average rise and fall of genera is remarkably symmetrical. Genera tend already to be in a state of decline when they become extinct. Genera that last appear in the major mass extinction stages, however, are more frequently truncated while they are holding steady or even increasing. This need not imply that mass extinctions are qualitatively different from other events; it is consistent with the expected effects of simply increasing the magnitude of extinction. For reasons that are not completely clear, post-Paleozoic genera show less of a rise and fall on average and tend to be less symmetrical than do Paleozoic genera.
Geographic range size is one of the few traits that promoted survivorship during both mass and background extinctions, but the exact reason (or reasons) why a large geographic range confers extinction resistance remains unclear. Proposed explanations have focused on the roles of dispersal ability, climate tolerance, global abundance, and widespread ranges in predicting taxon longevities. This study uses biogeographic data for late Paleozoic brachiopod genera to test the relative contribution of these traits to genus longevities, using simple but accurate proxy measurements. The results demonstrate a strong positive relationship between genus longevity and geographic range size, which is robust to several potential errors. Further, latitudinal range, which predominantly reflects climate tolerance, was no more important than longitudinal range, which predominantly reflects dispersal ability, in predicting genus longevities. Rather, longevities were an outcome of the total number of occurrences, which estimates global abundance, and the advantages of widespread distribution, regardless of which particular traits were responsible for generating the total geographic range. The advantages of a large geographic range were apparent during both background and mass extinctions of late Paleozoic time. Although not statistically significant, there was a tendency for the greatest selectivity to occur in intervals with the lowest extinction rates. The correlation of genus longevity and geographic range size had a profound consequence for the secular pattern of global brachiopod diversity: because the diversity of genera with small geographic ranges was more volatile owing to their correspondingly shorter longevities, global diversity and mean geographic range size paralleled each other almost exactly. Given that the correlation between taxon longevity and geographic range size has also been demonstrated for other taxonomic groups and at other time intervals, these results suggest that global diversity curves compiled from taxonomic databases dominantly reflect changes in the diversity of genera with small geographic ranges.
The magnitude and pace of terrestrial plant extinction and macroecological change associated with the Triassic/Jurassic (Tr/J) mass extinction boundary have not been quantified using paleoecological data. However, tracking the diversity and ecology of primary producers provides an ideal surrogate with which to explore patterns of ecosystem stability, collapse, and recovery and to explicitly test for gradual versus catastrophic causal mechanisms of extinction.
We present an analysis of the vegetation dynamics in the Jameson Land Basin, East Greenland, spanning the Tr/J extinction event, from a census collected paleoecological data set of 4303 fossil leaf specimens, in an attempt to better constrain our understanding of the causes and consequences of the fourth greatest extinction event in earth history. Our analyses reveal (1) regional turnover of ecological dominants between Triassic and Jurassic plant communities, (2) marked structural changes in the vegetation as reflected by potential loss of a mid-canopy habit, and (3) decline in generic-level richness and evenness and change in ecological composition prior to the Tr/J boundary; all of these findings argue against a single catastrophic causal mechanism, such as a meteorite impact for Tr/J extinctions. We identify various key ecological and biological traits that increased extinction risk at the Tr/J boundary and corroborate predictions of meta-population theory or plant ecophysiological models. These include ecological rarity, complex reproductive biology, and large leaf size.
Recovery in terms of generic-level richness was quite rapid following Tr/J extinctions; however, species-level turnover in earliest Jurassic plant communities remained an order of magnitude higher than observed for the Triassic. We hypothesize, on the basis of evidence for geographically extensive macrofossil and palynological turnover across the entire Jameson Land Basin, that the nature and magnitude of paleoecological changes recorded in this study reflect wider vegetation change across the whole region. How exactly these changes in dominance patterns of plant primary production affected the entire ecosystem remains an important avenue of future research.
Leaf mass per area (MA) is a central ecological trait that is intercorrelated with leaf life span, photosynthetic rate, nutrient concentration, and palatability to herbivores. These coordinated variables form a globally convergent leaf economics spectrum, which represents a general continuum running from rapid resource acquisition to maximized resource retention. Leaf economics are little studied in ancient ecosystems because they cannot be directly measured from leaf fossils. Here we use a large extant data set (65 sites; 667 species-site pairs) to develop a new, easily measured scaling relationship between petiole width and leaf mass, normalized for leaf area; this enables MA estimation for fossil leaves from petiole width and leaf area, two variables that are commonly measurable in leaf compression floras. The calibration data are restricted to woody angiosperms exclusive of monocots, but a preliminary data set (25 species) suggests that broad-leaved gymnosperms exhibit a similar scaling. Application to two well-studied, classic Eocene floras demonstrates that MA can be quantified in fossil assemblages. First, our results are consistent with predictions from paleobotanical and paleoclimatic studies of these floras. We found exclusively low-MA species from Republic (Washington, U.S.A., 49 Ma), a humid, warm-temperate flora with a strong deciduous component among the angiosperms, and a wide MA range in a seasonally dry, warm-temperate flora from the Green River Formation at Bonanza (Utah, U.S.A, 47 Ma), presumed to comprise a mix of short and long leaf life spans. Second, reconstructed MA in the fossil species is negatively correlated with levels of insect herbivory, whether measured as the proportion of leaves with insect damage, the proportion of leaf area removed by herbivores, or the diversity of insect-damage morphotypes. These correlations are consistent with herbivory observations in extant floras and they reflect fundamental trade-offs in plant-herbivore associations. Our results indicate that several key aspects of plant and plant-animal ecology can now be quantified in the fossil record and demonstrate that herbivory has helped shape the evolution of leaf structure for millions of years.
Carbon and nitrogen stable isotopic compositions of fossil materials from Lake Messel (47 Ma) in Germany are used to investigate Eocene ecosystem dynamics. Autolithified soft tissues of terrestrial and aquatic vertebrate organisms, as well as plant compression fossils, contain organic material (20–50 wt% C, 1–6 wt% N), which appears to retain precursor compositions. Stable isotopic compositions (δ13C and δ15N) of Messel fossils are similar to those reported for components in modern lacustrine ecosystems. These data show trophically sensible enrichments relative to food sources, reflect multiple feeding strategies for each organism (e.g., omnivory, planctivory, piscivory), and differentiate between benthic and pelagic organic carbon sources. These chemical data broadly confirm existing Messel food web models based on coprolite and gut content analyses. δ13C values for the lacustrine shale range from −30.3 to −26.3‰, pointing to mixed terrestrial and aquatic origins for primary producers in the food web. δ13C values for primary consumers such as insects overlap with those for primary producers but are comparatively enriched in 15N. Secondary and higher consumers (fish, crocodiles, and frogs) are associated with even more positive δ15N values and show a more constrained range of δ13C values. Omnivory appears widespread in both low and high trophic level consumers. Hence, the stable isotopic compositions of Messel fossils are complex and overlap, and must be combined with paleontological investigations in order to be conclusive. This study represents the first comprehensive isotopic reconstruction, featuring tens of components, of an ecosystem of Eocene age. A thorough understanding of trophic structure in Eocene Lake Messel contributes to the global databank of ecological history.
Tyrannosaurus rex and other tyrannosaurid theropods exerted high bite forces, and large muscle attachments suggest that the tyrannosaurid neck was a concomitantly powerful component of the feeding apparatus. We examine accelerative and work-generating capacity (WGC) of neck muscles in adult Tyrannosaurus rex, using a 3-D vector-based method that incorporates aspects of muscle force generation, reconstruction of muscle morphology and moment arms, and rotational inertias of the head and neck. Under conservative assumptions, radial accelerations of the head by large superficial muscles (M. transversospinalis capitis, M. complexus, and M. longissimus capitis superficialis) enabled rapid gaze shifts and imparted high tangential velocities to food sufficient for inertial feeding. High WGC by these and deeper muscles under eccentric contraction indicate high efficacy for tearing flesh, especially with the head and neck in an extended posture. Sensitivity analyses suggest that assigned density of the antorbital region has substantial effects on calculated rotational inertia, and hence on the accuracy of results. However, even with high latitude for estimation errors, the results indicate that adult T. rex could strike rapidly at prey and engage in complexly modulated inertial feeding, as seen in extant archosaurs.