Papers by Nan Crystal Arens
American Fern Journal, 2000
Geological Society of America Abstracts with Programs, 2018
Geological Society of America Abstracts with Programs, 2016
PALAIOS, 2016
Abstract The Soap Wash flora contains 152 angiosperm leaf fossils from the Albian–Cenomanian boun... more Abstract The Soap Wash flora contains 152 angiosperm leaf fossils from the Albian–Cenomanian boundary of central Utah. The flora exhibits many well-preserved examples of insect folivory for which the extent and type of damage on each leaf was recorded. Fewer than half of the specimens in the collection exhibit insect damage and most of the leaf damage affects less than five percent of the leaf area. Damage was distributed unevenly across the 18 leaf morphotypes described in the flora. Nineteen damage types were recognized. Most of the damage was of a generalized type, dominated by indiscriminate removal of leaf tissues (whole leaf feeding). We observed no correlation between leaf mass per area (MA) and the degree of damage by herbivores, which contrasts with Cenozoic and modern patterns. The low rate, modest area, and low diversity of damage type is similar to patterns reported through the Late Cretaceous and is consistent with a plateau in insect folivore diversity that persisted through the end of the Mesozoic. The preponderance of generalized damage types dominated by whole leaf feeding may reflect (1) the early stage in angiosperm radiation represented by this flora, and (2) the associated lag in folivore diversification and specialization.
Geological Society of America Special Papers, 2014
Many workers consider the Cretaceous-Paleogene extinction the archetypal catastrophic pulse event... more Many workers consider the Cretaceous-Paleogene extinction the archetypal catastrophic pulse event caused solely by the Chicxulub bolide impact. However, based on a global scale analysis of marine animals, the Cretaceous-Paleogene boundary is a candidate for an extinction enhanced by the coincidence of press and pulse disturbances. We make a preliminary test of key predictions of the press-pulse hypothesis using palynological data. We document a local palynological extinction of 21% at the Cretaceous-Paleogene boundary, which is consistent with extinction rates of 15% to 30% at other localities in the Hell Creek type area and throughout North America. We also fi nd a decline in the number of dicot angiosperm pollen taxa between −3.5 m and −2.5 m below the boundary. We document a low-palynospecies-richness interval between −1.4 m and −1.0 m that includes extirpation, but not extinction, of some palynospecies. These changes in species richness are not correlated with changes in depositional style or pollen preservation, indicating that they may represent a biological rather than entirely taphonomic signal. Review and reanalysis of previously published data from other localities in the western interior of North America suggest similar declines in species richness within approximately the same stratigraphic interval. However, many of the species absent during the low-species-richness interval reappeared before the Cretaceous-Paleogene boundary, suggesting changes in community structure and composition before the terminal Cretaceous event-a key prediction of the press-pulse hypothesis-rather than gradual extinction in the latest Cretaceous.
Geological Society of America Special Papers, 2014
Geological Society of America Special Papers, 2014
Questions of biotic and environmental change during deposition of the Upper Maastrichtian Hell Cr... more Questions of biotic and environmental change during deposition of the Upper Maastrichtian Hell Creek Formation require a robust and replicable system for intraformational correlation of fossil localities. In this paper, we present a carbon isotope chemostratigraphic curve based on terrestrial organic carbon. Data were taken from a complete measured section spanning the full 93 m of the Hell Creek Formation at our study site. Sedimentary beds were described at the centimeter scale, and samples for carbon isotope analysis were taken at ~10 cm intervals. Each sedimentary bed was analyzed in thin section, and grain-size data were assembled based on petrographic point counts. The well-documented Cretaceous-Paleogene boundary negative carbon isotope excursion, six negative carbon isotope excursions, and four tentative positive carbon isotope excursions provide chronostratigraphic tie points within the Hell Creek Formation. We used this curve to precisely correlate 12 additional fossilbearing localities from throughout the Hell Creek Formation across its type area. These correlations revealed signifi cant local variation in sediment accumulation rates, confi rming that simple stratigraphic position relative to the diachronous base and top of the Hell Creek Formation introduces signifi cant error in correlation.
Trends in Ecology & Evolution, 2005
Extinctions in the History of Life is a svelte, unpretentious little volume that might easily be ... more Extinctions in the History of Life is a svelte, unpretentious little volume that might easily be overlooked on the bookseller's table. Its goal (to synthesize the major findings of extinction research for an undergraduate audience) is as modest as the fading ammonite on the cover. However, editor Paul D. Taylor and an all-star cast of contributors have transcended synthesis to weave a cautionary tale about the modern biodiversity crisis.
Trends in Ecology & Evolution, 2001
shift in attacks toward the rear, whereas the alternative hypothesis predicts the opposite. Corde... more shift in attacks toward the rear, whereas the alternative hypothesis predicts the opposite. Cordero suggests that experiments could be performed on butterfly species without false heads (but that are related to species with false heads) in which artificial heads are experimentally added, or in species with the false heads experimentally removed. We feel that such manipulations would be technically challenging, and the experiments difficult to design if manipulations are not to affect prey behaviour, and the target part of the body of a butterfly in a given attack is to be definitively identified. We suggest that an effective way to explore how adding a false head affects predator attack targets would be to use a predatory bird trained to peck at computergenerated images on a touch-sensitive screen. This technique is already well established in the study of prey visual detection by predators (e.g. Ref. 2) and might well lead to exciting and rapid advances in a system previously (but perhaps prematurely) considered to be well understood.
Trends in Ecology & Evolution, 2001
Trends in Ecology & Evolution, 2001
Trends in Ecology & Evolution, 2001
Trends in Ecology & Evolution, 2002
is famously supposed to have remarked that the Creator had an inordinate fondness for beetles. In... more is famously supposed to have remarked that the Creator had an inordinate fondness for beetles. In evolutionary biologists' minds, beetles have been basking in the warm glow of God's favour ever since: but are they really the chosen ones? Peter Mayhew [1] has now addressed this issue using data on species richness, taxon age and phylogenetic relatedness of hexapods (the Entognatha, e.g. Collembola, plus the insects) and concludes that the Creator's fondness had wider taxonomic bounds. Mayhew compared the species richness of sister clades and found that the Neoptera (insects with wing flexion) are significantly more diverse than the Entognatha plus the non-neopteran insects (dragonflies, damselflies, mayflies, silverfish and bristletails). The Neoptera comprise the Polyneoptera (e.g. grasshoppers, crickets, stick insects, earwigs, termites, mantids and cockroaches) and the Eumetabola [the Paraneoptera (true bugs, thrips and lice) and the Holometabola (insects with complete metamorphosis, such as lacewings, wasps, caddisflies,
Taxon, 1989
... microwave oven showed moderate to severe damage. The damage sustained by ace-tolysed and KOH-... more ... microwave oven showed moderate to severe damage. The damage sustained by ace-tolysed and KOH-treated pollen and spores was identical. Figure I contrasts air-dried grains of Polypodium aureum, Clivia, and Zea mays with Fig. ...
Reviews of Geophysics, 2008
Reconstruction of the carbon isotope composition of atmospheric CO 2 is critical to the understan... more Reconstruction of the carbon isotope composition of atmospheric CO 2 is critical to the understanding of longterm global carbon cycling. We have suggested that the d 13 C value of land plant carbon (d 13 C p) preserved in the geologic record should reflect the d 13 CO 2 at the time during which the plants grew (d 13 C a), based on a meta-analysis of modern plant data. Here we present the results of laboratory experiments designed to quantify the relationship between plant tissue d 13 C and d 13 CO 2 values under varying environmental conditions, including differential pCO 2 ranging from 1 to 3 times today's levels. As predicted, plants grown under elevated pCO 2 showed increased average biomass compared to controls grown at the same temperature. Across a very large range in d 13 C a (%24%) and pCO 2 (%740 ppmv) we observed a consistent correlation between d 13 C a and d 13 C p (p < 0.001). We show an average isotopic depletion of À25.4% for aboveground tissue and À23.2% for belowground tissue of Raphanus sativus L. relative to the composition of the atmosphere under which it formed. For aboveground and belowground tissue, grown at both $23°C and $29°C, correlation was strong and significant (r 2 ! 0.98 and p < 0.001); variation in pCO 2 level had little or no effect on this relationship. These results validate our initial conclusion that in the absence of environmental stress, plant d 13 C primarily reflects atmospheric d 13 CO 2 linearly across pCO 2 levels; the demonstrated excellent correlation in d 13 C a and d 13 C p suggests a high level of predictive power across varying environmental conditions.
Plant, Cell & Environment, 2007
Angiosperms first appeared during the Early Cretaceous, and within 30 million years they reigned ... more Angiosperms first appeared during the Early Cretaceous, and within 30 million years they reigned over many floras worldwide. Associated with this rise to prominence, angiosperms produced a spectrum of reproductive and vegetative innovations, which produced a cascade of ecological consequences that altered the ecology and biogeochemistry of the planet. The pace, pattern and phylogenetic systematics of the Cretaceous angiosperm diversification are broadly sketched out. However, the ecophysiology and environmental interactions that energized the early angiosperm radiation remain unresolved. This constrains our ability to diagnose the selective pressures and habitat contexts responsible for the evolution of fundamental angiosperm features, such as flowers, rapid growth, xylem vessels and net-veined leaves, which in association with environmental opportunities, drove waves of phylogenetic and ecological diversification. Here, we consider our current understanding of early angiosperm ecophysiology. We focus on comparative patterns of ecophysiological evolution, emphasizing carbon-and water-use traits, by merging recent molecular phylogenetic studies with physiological studies focused on extant basal angiosperms. In doing so, we discuss how early angiosperms established a roothold in preexisting Mesozoic plant communities, and how these events canalized subsequent bursts of angiosperm diversification during the Aptian-Albian.
Paleobiology, 2008
Previous discussions of mass extinction mechanisms generally focused on circumstances unique to e... more Previous discussions of mass extinction mechanisms generally focused on circumstances unique to each event. However, some have proposed that extensive volcanism combined with bolide impact may offer a general mechanism of mass extinction. To test this hypothesis we compared generic extinction percentages for 73 stages or substages of the Mesozoic and Cenozoic. We found that the highest frequency of intervals with elevated extinction occurred when continental flood basalt volcanism and bolide impact co-occurred. In contrast, neither volcanism nor impact alone yielded statistically elevated extinction frequencies. Although the magnitude of extinction was uncorrelated with the size of the associated flood basalt or impact structure, crater diameter did correlate with extinction percentage when volcanism and impact coincided. Despite this result, case-by-case analysis showed that the volcanism-impact hypothesis alone cannot explain all intervals of elevated extinction. Continental flood...
Paleobiology, 2000
Atmospheric carbon dioxide is the raw material for the biosphere. Therefore, changes in the carbo... more Atmospheric carbon dioxide is the raw material for the biosphere. Therefore, changes in the carbon isotopic composition of the atmosphere will influence the terrestrial δ13C signals we interpret. However, reconstructing the atmospheric δ13C value in the geologic past has proven challenging. Land plants sample the isotopic composition of CO2 during photosynthesis. We use a model of carbon isotopic fractionation during C3 photosynthesis, in combination with a meta–data set (519 measurements from 176 species), to show that the δ13C value of atmospheric CO2 can be reconstructed from the isotopic composition of plant tissue. Over a range of pCO2 (198–1300 ppmv), the δ13C value of plant tissue does not vary systematically with atmospheric carbon dioxide concentration. However, environmental factors, such as water stress, can influence the δ13C value of leaf tissue. These factors explained a relatively small portion of variation in the δ13C value of plant tissue in our data set and emerged...
PALAIOS, 2000
A Ϫ1.5‰ to Ϫ2‰ carbon isotope excursion immediately above the clay layer that defines the Cretace... more A Ϫ1.5‰ to Ϫ2‰ carbon isotope excursion immediately above the clay layer that defines the Cretaceous-Tertiary (K/ T) boundary has been reported in marine sediments world wide. This paper reports a similar Ϫ1.5‰ to Ϫ2.8‰ carbon isotope excursion recorded by C3 land plants from three temporally-controlled, stratigraphically-constrained terrestrial sections in the Western Interior of North America (Garfield County, Montana, and Slope County, North Dakota). Carbon isotope measurements of bulk sedimentary organic carbon were well-correlated with those of isolated plant cuticle, suggesting that the terrestrial organic carbon signature in these sediments parallels that of plant cuticle. Carbon isotope signatures were also independent of rock type and depositional environment, showing that the carbon isotope signature of plants, although altered, is not biased taphonomically. Because the signature in terrestrial facies records the isotope composition of paleoatmospheric CO 2 , this record-combined with that from marine sections-offers additional insight into changes in carbon cycling underlying the K/T negative carbon isotope excursion. For example, radiometric age determinations from the Hell Creek Road locality in Montana bracket the atmospheric carbon isotopic recovery between 65.00 Ϯ 0.05 Ma and 65.16 Ϯ 0.04 Ma. This reflects a more rapid recovery for the terrestrial biosphere than for that of the marine realm, perhaps due to lower extinction rates in land plants than in marine primary producers.
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Papers by Nan Crystal Arens