Luke Mander

Palaeoecology of the Late Triassic extinction event in the SW UK

A high-resolution palaeoecological study of the shelly invertebrate macrofauna across two marine Triassic–Jurassic boundary sections in the UK (St. Audries Bay and Lavernock Point) is presented. Loss of taxonomic richness occurs in the upper Westbury Formation to lower Lilstock Formation (late Rhaetian), but if sample size is taken into account there is little convincing evidence of a catastrophic marine extinction. There is, however, good evidence for significant palaeoecological change in the benthic marine ecosystem at this time. The immediate post-event recovery interval in the upper Lilstock Formation is characterized by assemblages of low abundance, low diversity, high dominance and low evenness. Body-sizes of taxa that survived the event and originated afterwards were low until the later Hettangian. Recovery to higher abundance, higher diversity and higher evenness is recorded in the Psiloceras planorbis Zone. Recovery of the benthic ecosystem in the aftermath of the Late Triassic event was disrupted by marine anoxia and shows additional similarities to the (much slower) recovery that followed the Late Permian event. The pattern of body-size changes recorded in the shelly fossil record closely matches that of the trace fossil record. Shell thickness trends do not support a biocalcification crisis during the Late Triassic biotic event.

An explanation for conflicting records of Triassic–Jurassic plant diversity

Macrofossils (mostly leaves) and sporomorphs (pollen and spores) preserve conflicting records of plant biodiversity during the end-Permian (P-Tr), Triassic–Jurassic (Tr-J), and end-Cretaceous (K-T) mass extinctions. Estimates of diversity loss based on macrofossils are typically much higher than estimates of diversity loss based on sporomorphs. Macrofossils from the Tr-J of East Greenland indicate that standing species richness declined by as much as 85% in the Late Triassic, whereas sporomorph records from the same region, and from elsewhere in Europe, reveal little evidence of such catastrophic diversity loss. To understand this major discrepancy, we have used a new high-resolution dataset of sporomorph assemblages from Astartekløft, East Greenland, to directly compare the macrofossil and sporomorph records of Tr-J plant biodiversity. Our results show that sporomorph assemblages from the Tr-J boundary interval are 10–12% less taxonomically diverse than sporomorph assemblages from the Late Triassic, and that vegetation composition changed rapidly in the boundary interval as a result of emigration and/or extirpation of taxa rather than immigration and/or origination of taxa. An analysis of the representation of different plant groups in the macrofossil and sporomorph records at Astartekløft reveals that reproductively specialized plants, including cycads, bennettites and the seed-fern Lepidopteris are almost absent from the sporomorph record. These results provide a means of reconciling the macrofossil and sporomorph records of Tr-J vegetation change, and may help to understand vegetation change during the P-Tr and K-T mass extinctions and around the Paleocene–Eocene Thermal Maximum.

Capturing the Surface Texture and Shape of Pollen: A Comparison of Microscopy Techniques

Research on the comparative morphology of pollen grains depends crucially on the application of appropriate microscopy techniques. Information on the performance of microscopy techniques can be used to inform that choice. We compared the ability of several microscopy techniques to provide information on the shape and surface texture of three pollen types with differing morphologies. These techniques are: widefield, apotome, confocal and two-photon microscopy (reflected light techniques), and brightfield and differential interference contrast microscopy (DIC) (transmitted light techniques). We also provide a first view of pollen using super-resolution microscopy. The three pollen types used to contrast the performance of each technique are: Croton hirtus (Euphorbiaceae), Mabea occidentalis (Euphorbiaceae) and Agropyron repens (Poaceae). No single microscopy technique provided an adequate picture of both the shape and surface texture of any of the three pollen types investigated here. The wavelength of incident light, photon-collection ability of the optical technique, signal-to-noise ratio, and the thickness and light absorption characteristics of the exine profoundly affect the recovery of morphological information by a given optical microscopy technique. Reflected light techniques, particularly confocal and two-photon microscopy, best capture pollen shape but provide limited information on very fine surface texture. In contrast, transmitted light techniques, particularly differential interference contrast microscopy, can resolve very fine surface texture but provide limited information on shape. Texture comprising sculptural elements that are spaced near the diffraction limit of light (∼250 nm; NDL) presents an acute challenge to optical microscopy. Super-resolution structured illumination microscopy provides data on the NDL texture of A. repens that is more comparable to textural data from scanning electron microscopy than any other optical microscopy technique investigated here. Maximizing the recovery of morphological information from pollen grains should lead to more robust classifications, and an increase in the taxonomic precision with which ancient vegetation can be reconstructed.

Classification of grass pollen through the quantitative analysis of surface ornamentation and texture.

Taxonomic identification of pollen and spores uses inherently qualitative descriptions of morphology. Consequently, identifications are restricted to categories that can be reliably classified by multiple analysts, resulting in the coarse taxonomic resolution of the pollen and spore record. Grass pollen represents an archetypal example; it is not routinely identified below family level. To address this issue, we developed quantitative morphometric methods to characterize surface ornamentation and classify grass pollen grains. This produces a means of quantifying morphological features that are traditionally described qualitatively. We used scanning electron microscopy to image 240 specimens of pollen from 12 species within the grass family (Poaceae). We classified these species by developing algorithmic features that quantify the size and density of sculptural elements on the pollen surface, and measure the complexity of the ornamentation they form. These features yielded a classification accuracy of 77.5%. In comparison, a texture descriptor based on modelling the statistical distribution of brightness values in image patches yielded a classification accuracy of 85.8%, and seven human subjects achieved accuracies between 68.33 and 81.67%. The algorithmic features we developed directly relate to biologically meaningful features of grass pollen morphology, and could facilitate direct interpretation of unsupervised classification results from fossil material.This material is based upon work supported by the National Science Foundation under NSF DBI-1052997 to S.W.P., NSF DBI-102942 to W.M. and NSF DBI-1053036 to C.C.F. L.M. was partly supported by a Marie Curie International Incoming Fellowship within the 7th European Community Framework Programme (PIIF-GA-2012-328245).

Palynostratigraphy and vegetation history of the Triassic–Jurassic transition in East Greenland

We present a palynological study of a terrestrial Triassic–Jurassic (Tr–J; c. 200 Ma) boundary section at Astartekløft, East Greenland. We have generated a new palynostratigraphic scheme and vegetation history for this locality, and have integrated these with existing carbon isotope records. Samples for palynological analysis were collected from precisely the same stratigraphic horizons as plant macrofossils and samples used for geochemical analyses. Our results highlight four local sporomorph assemblage zones that are compositionally distinct from each other at Astartekløft. The extremely low abundance of Classopollis pollen in all samples, and the pronounced decline of Ricciisporites tuberculatus during the Late Rhaetian are notable features of the sporomorph record of Tr–J vegetation at Astartekløft. Correlation of Astartekløft and a marine Tr–J boundary section at St Audrie’s Bay, UK, provides no support for the idea that extinction and diversity loss in terrestrial ecosystems preceded biotic change in marine ecosystems at the Tr–J. Instead, the available data support suggestions that the onset of the Tr–J biotic crisis was synchronous in terrestrial and marine environments. Peak extinction among plants at Astartekløft occurred relatively late in the sequence of events across the Tr–J, and may represent a response to long-term cumulative effects of volcanism at this time. Supplementary material Plates of selected sporomorphs recovered from Astartekløft and a full pollen diagram are available at www.geolsoc.org.uk/SUP18553.

High potential for weathering and climate effects of non-vascular vegetation in the Late Ordovician

It has been hypothesized that predecessors of todays bryophytes significantly increased global chemical weathering in the Late Ordovician, thus reducing atmospheric CO2 concentration and contributing to climate cooling and an interval of glaciations. Studies that try to quantify the enhancement of weathering by non-vascular vegetation, however, are usually limited to small areas and low numbers of species, which hampers extrapolating to the global scale and to past climatic conditions. Here we present a spatially explicit modelling approach to simulate global weathering by non-vascular vegetation in the Late Ordovician. We estimate a potential global weathering flux of 2.8 (km3 rock) yr−1, defined here as volume of primary minerals affected by chemical transformation. This is around three times larger than todays global chemical weathering flux. Moreover, we find that simulated weathering is highly sensitive to atmospheric CO2 concentration. This implies a strong negative feedback between weathering by non-vascular vegetation and Ordovician climate.

Aberrant Classopollis pollen reveals evidence for unreduced (2n) pollen in the conifer family Cheirolepidiaceae during the Triassic–Jurassic transition

Polyploidy (or whole-genome doubling) is a key mechanism for plant speciation leading to new evolutionary lineages. Several lines of evidence show that most species among flowering plants had polyploidy ancestry, but it is virtually unknown for conifers. Here, we study variability in pollen tetrad morphology and the size of the conifer pollen type Classopollis extracted from sediments of the Triassic–Jurassic transition, 200 Ma. Classopollis producing Cheirolepidiaceae were one of the most dominant and diverse groups of conifers during the Mesozoic. We show that aberrant pollen Classopollis tetrads, triads and dyads, and the large variation in pollen size indicates the presence of unreduced (2n) pollen, which is one of the main mechanisms in modern polyploid formation. Polyploid speciation may explain the high variability of growth forms and adaptation of these conifers to different environments and their resistance to extreme growth conditions. We suggest that polyploidy may have also reduced the extinction risk of these conifers during the End-Triassic biotic crisis.

On the Taxonomic Resolution of Pollen and Spore Records of Earth’s Vegetation

Premise of research. Pollen and spores (sporomorphs) are a valuable record of plant life and have provided information on subjects ranging from the nature and timing of evolutionary events to the relationship between vegetation and climate. However, sporomorphs can be morphologically similar at the species, genus, or family level. Studies of extinct plant groups in pre-Quaternary time often include dispersed sporomorph taxa whose parent plant is known only to the class level. Consequently, sporomorph records of vegetation suffer from limited taxonomic resolution and typically record information about plant life at a taxonomic rank above species. Methodology. In this article, we review the causes of low taxonomic resolution, highlight examples where this has hampered the study of vegetation, and discuss the strategies researchers have developed to overcome the low taxonomic resolution of the sporomorph record. Based on this review, we offer our views on how greater taxonomic precision might be attained in future work. Pivotal results. Low taxonomic resolution results from a combination of several factors, including inadequate reference collections, the absence of sporomorphs in situ in fossilized reproductive structures, and damage following fossilization. A primary cause is the difficulty of accurately describing the very small morphological differences between species using descriptive terminology, which results in palynologists classifying sporomorphs conservatively at the genus or family level to ensure that classifications are reproducible between samples and between researchers. Conclusions. In our view, the most promising approach to the problem of low taxonomic resolution is a combination of high-resolution imaging and computational image analysis. In particular, we encourage palynologists to explore the utility of microscopy techniques that aim to recover morphological information from below the diffraction limit of light and to employ computational image analyses to consistently quantify small morphological differences between species.

The Ultrastructure and Botanical Affinity of the Problematic Mid-Mesozoic Palynomorph Ricciisporites tuberculatus Lundblad

Ricciisporites tuberculatus Lundblad is a prominently sculptured palynomorph that is permanently united in tetrads. This palynomorph has a wide geographic distribution and reaches a stratigraphically important acme in the Late Triassic of Europe, Greenland, and Arctic Canada. This palynomorph has not yet been found in situ in a fossilized reproductive structure, and consequently its botanical affinity is unclear. Some authors have suggested that R. tuberculatus was produced by a seed plant, but others have suggested that this palynomorph was produced by a liverwort comparable to Riccia. In order to clarify the botanical affinity of R. tuberculatus, we have analyzed its morphology using SEM and TEM. Individual grains within the tetrad are equipped with a single distal colpus, and the ultrastructure of the palynomorph wall is characterized by a granular inner sexine and an electron-dense laminated nexine. These morphological features ally R. tuberculatus to the gymnosperms. The combination of strongly ornamented monocolpate pollen grains permanently united in tetrahedral tetrads is unusual and to the best of our knowledge has not been reported previously in a gymnosperm. This may represent an extinct strategy to promote the fertilization of several archegonia in an ovule leading to simple polyembryony.

Taxonomic resolution of the Triassic–Jurassic sporomorph record in East Greenland

Sporomorphs (pollen and spores) provide valuable information about vegetation history over a range of temporal and spatial scales. However, sporomorphs can be morphologically invariant among species within genera, and among genera within certain families. In some cases, the parent plant of a sporomorph is unknown. These factors blur the relationship between sporomorph assemblages and the source vegetation, and reduce the taxonomic precision of vegetation reconstructions based on sporomorphs. This study investigates the taxonomic precision with which sporomorphs record vegetation across the Triassic–Jurassic transition (Tr–J) at Astartekløft, East Greenland. Results indicate that reconstructions of Tr–J vegetation at Astartekløft based on sporomorphs are hampered by considerable taxonomic imprecision. Something is known of the botanical affinity of almost all sporomorphs at Astartekløft at the class level, but just 50% of sporomorph taxa have a known botanical affinity at the family level. Additionally, ~23% of all sporomorph taxa at Astartekløft have affinities to more than one parent plant class, and ~36% of sporomorph taxa have affinities to more than one parent plant family. This taxonomic imprecision should be accounted for when interpreting percentage diagrams of sporomorph taxa across the Tr–J.