Andrew N. Drinnan

Gondwanan floristic and sedimentological trends during the Permian-Triassic transition: new evidence from the Amery Group, northern Prince Charles Mountains, East Antarctica

The Permian-Triassic boundary within the Amery Group of the Lambert Graben is placed at the contact between the Bainmedart Coal Measures and overlying Flagstone Bench Formation, based on the first regular occurrence of Lunatisporites pellucidus and the first appearance of Aratrisporites and Lepidopteris species. The Permian-Triassic boundary is marked by the extinction of glossopterid and cordaitalean gymnosperms, and by the disappearance or extreme decline of a range of gymnospermous and pteridophytic palynomorph groups. Earliest Triassic macrofloras and palynofloras of the Flagstone Bench Formation are dominated by peltasperms and lycophytes; corystosperms, conifers, and ferns become increasingly common elements of assemblages through the Lower Triassic part of the formation and dominate floras of the Upper Triassic strata. The sedimentary transition across this boundary is conformable but marked by a termination of coal deposits; overlying lowermost Triassic sediments contain only carbonaceous siltstones. Typical red-bed facies are not developed until at least 100 m above the base of the Flagstone Bench Formation, in strata containing ?Middle Triassic palynofloras. Across Gondwana the diachronous disappearance of coal deposits and appearance of red-beds is suggestive of a response to shifting climatic belts, resulting in progressively drier seasonal conditions at successively higher palaeolatitudes during the Late Permian to Middle Triassic. The abrupt and approximately synchronous replacement of plant groups at the Permian-Triassic boundary suggests that factors independent of, or additional to, climate change were responsible for the turnover in terrestrial floras.

Patterns of floral evolution in the early diversification of non-magnoliid dicotyledons (eudicots)

Recent cladistic analyses of angiosperms based on both morphological and molecular sequence data recognize a major clade of dicotyledons defined by triaperturate or triaperturate-derived pollen (non-magnoliids/eudicots). Evidence from morphology, as well as the atpB and rbcL genes (cpDNA), indicates that extant Ranunculidae (e.g., Papaverales, Lardizabalaceae, Berberidaceae, Menispermaceae, Ranunculaceae) as well as “lower” Hamamelididae [e.g., Eupteleaceae (allied to Ranunculidae), Hamamelidaceae, Myrothamnaceae, Platanaceae, Trochodendraceae] and several other families (e.g., Gunneraceae, Nelumbonaceae, Proteaceae, Sabiaceae) are basal in this group. The earliest records of diagnostic eudicot pollen are of mid-late Barremian age (c. 126myr BP) and by around the latest Albian (c. 97 myr BP) several basal eudicot groups (e.g., Trochodendrales, Platanaceae, Buxaceae, and perhaps Circaeasteraceae, Myrothamnaceae, and Nelumbonaceae) are recognizable in the fossil record. Possible Hamamelidaceae and perhaps Proteaceae are present by the Turonian (c. 90 myr BP). Among basal eudicots, flowers are generally bisexual although unisexual flowers are also common. In some groups (e.g., Myrothamnaceae, Buxaceae, certain Berberidaceae), delimitation of the flower is not always clear and there is a more or less gradual transition between tepals and inflorescence bracts. Plasticity in floral form at this level of angiosperm evolution is predominantly encompassed by dimerous and trimerous cyclic floral organization and transitions from one to the other are common. Spiral floral phyllotaxis of numerous stamens and carpels is more or less restricted to the Ranunculaceae. The basic condition of the perianth in eudicots appears to lack differentiation into sepals and petals, and petals appear to have arisen independently numerous times from stamens. Based on the generality of its systematic distribution, cyclic floral architecture is probably basic for eudicots as a whole, and at this level of angiosperm evolution flowers with numerous, helically-arranged stamens and/or carpels (e.g., many Ranunculaceae) almost certainly reflect processes of secondary multiplication that have occurred independently many times.

Lauraceous Flowers from the Potomac Group (Mid-Cretaceous) of Eastern North America

Fossil inflorescences and flowers of Mauldinia mirabilis gen. et sp. nov. are described from the early Cenomanian Elk Neck beds of northeastern Maryland, U.S.A., and are assigned to the family Lauraceae. Specimens are exceptionally well preserved and provide the earliest evidence of trimerous floral organization and endosperm in angiosperms. Inflorescences are compound and consist of elongated axes bearing distinctive, spirally arranged lateral, bilobed cladode-like units. Each lateral unit typically bears five sessile bisexual flowers on the adaxial surface. Flowers have a perianth of three small outer and three larger inner tepals, and an androecium of nine fertile stamens in three whorls. In addition, there is an inner fourth whorl of three dorsiventrally flattened staminode-like structures, and each of the three fertile inner stamens has an associated pair of staminode-like appendages with clavate-sagittate heads. Anthers dehisce by two valves that are hinged distally. The gynoecium consists of a superior, unilocular carpel with a single anatropous, pendent ovule. Floral organization in M. mirabilis is the same as that of many extant Lauraceae, and the unique inflorescence structure can also be interpreted in terms of inflorescence patterns in extant taxa. Mauldinia mirabilis is the first unequivocal documentation of Lauraceae from the Cretaceous and provides further evidence for considerable diversity among magnoliid dicotyledons at an early phase in angiosperm diversification.

The Currency and Tempo of Extinction

This study examines estimates of extinction rates for the current purported biotic crisis and from the fossil record. Studies that compare current and geological extinctions sometimes use metrics that confound different sources of error and reflect different features of extinction processes. The per taxon extinction rate is a standard measure in paleontology that avoids some of the pitfalls of alternative approaches. Extinction rates reported in the conservation literature are rarely accompanied by measures of uncertainty, despite many elements of the calculations being subject to considerable error. We quantify some of the most important sources of uncertainty and carry them through the arithmetic of extinction rate calculations using fuzzy numbers. The results emphasize that estimates of current and future rates rely heavily on assumptions about the tempo of extinction and on extrapolations among taxa. Available data are unlikely to be useful in measuring magnitudes or trends in current extinction rates.

Some Morphological Features of Wollemi Pine (Wollemia nobilis: Araucariaceae) and Their Comparison to Cretaceous Plant Fossils

Morphological details of Wollemia nobilis (Wollemi pine) are described and illustrated, and compared with the extant genera Agathis and Araucaria and with selected araucarian fossils from the Cretaceous of Australia. Adult and juvenile shoots of Wollemia differ in leaf arrangement, leaf shape, and cuticular features; in these features they are most similar to Araucaria. The cone scales have a long, distal spine reminiscent of Araucaria section Eutacta, but the winged seeds that are ontogenetically free from, and shed independently from, the cone scale are similar to Agathis. Shoots with variable leaf types, Araucaria-like cone scales, and Agathis-like winged seeds are found in several plant fossil assemblages from the Cretaceous of Australia; these fossil conifers, which had been recognized as araucarian, can now be favorably compared with Wollemia. Pollen of Wollemia is indistinguishable from the fossil pollen form-genus Dilwynites, which has a fossil record extending back to the Late Cretaceous in Australia and New Zealand. Reexamination of Mesozoic and Tertiary paleofloras will most probably reveal an important contribution of Wollemia to the fossil record of Araucariaceae.

Fagaceous Flowers, Fruits, and Cupules from the Campanian (Late Cretaceous) of Central Georgia, USA

A new genus of fossil angiosperms (Protofagacea allonensis gen. et sp. nov.) is established for staminate flowers with associated fruits and cupules from the Campanian (Late Cretaceous) Buffalo Creek Member of the Gaillard Formation in central Georgia, U.S.A. Staminate flowers are typically borne in sevenflowered dichasia (more rarely three- or five-flowered) subtended by three series of bracts. Flowers have six small imbricate tepals in two cycles of three, 12 stamens in two cycles of six, and a vestigial gynoecium with three styles surrounded by a mass of simple trichomes. Pollen is very small, prolate, tricolporate, and reticulate to microfoveolate. Associated fruits are triangular or lenticular in cross section and the triangular fruits bear six short tepals at the apex. Fossil pollen identical to that produced by the staminate flowers is attached to the apex of the fruits. Associated cupules are pedunculate and four-lobed and bear three or more fruits. The cupule lobes bear three series of bracts similar to those subtending the staminate dichasia. Scars on the internal surface indicate that each cupule contained a central fruit that was elliptical in cross section and two lateral fruits that were triangular in cross section. Comparisons of P. allonensis with extant taxa clearly indicate a relationship to extant Fagaceae sensu lato, based particularly on the presence of the cupule, the form of the fruits, and the morphology of staminate inflorescences and flowers. However, pollen morphology (probably plesiomorphic) differs from that of extant taxa, and detailed resolution of the affinities of Protofagacea will require a clearer understanding of relationships among extant Fagaceae sensu stricto and Nothofagaceae.

Reproductive structures of an extinct platanoid from the Early Cretaceous (latest Albian) of eastern North America

Abstract Two new species of platanoid reproductive structure are described from the Bull Mountain locality in the Patapsco Formation (Potomac Group) of northeastern Maryland, USA. Pistillate inflorescences and infructescences ( Platanocarpus elkneckensis sp. nov.) consist of flowers and fruits in sessile globose heads that are borne on an elongate axis. Individual pistillate flowers consist of five free carpels surrounded by prominent tepals. Staminate inflorescences, flowers and isolated stamens are assigned to Hamatia elkneckensis gen. et sp. nov. Staminate flowers are borne in a globose head with a small number of stamens (five?) per flower. Stamens consist of very short filaments, long anthers with strongly valvate dehiscence and an apically expanded connective. The connective expansion is frequently very well-developed, hook-like and extends down the ventral surface of the stamen. Anthers contain small, tricolporate, reticulate pollen. Association evidence, similarity of inflorescence structure and the occurrence of Hamatia -type pollen on flowers, carpels and fruits of Platanocarpus elkneckensis suggests that the staminate and pistillate material was produced by a single species of plant. The “ Hamatia -plant” provides further evidence of pentamerous floral structure in mid-Cretaceous platanoids and documents the occurrence of unequivocal tricolporate pollen in the platanoid complex.

Distribution of cell wall components in Sphagnum hyaline cells and in liverwort and hornwort elaters

Abstract Spiral secondary walls are found in hyaline cells of Sphagnum, in the elaters of most liverworts, and in elaters of the hornwort Megaceros. Recent studies on these cells suggest that cytoskeletal and ultrastructural processes involved in cell differentiation and secondary wall formation are similar in bryophytes and vascular plant tracheary elements. To examine differences in wall structure, primary and secondary wall constituents of the hyaline cells of Sphagnum novo-zelandicum and elaters of the liverwort Radula buccinifera and the hornwort Megaceros gracilis were analyzed by immunohistochemical and chemical methods. Anti-arabinogalactan–protein antibodies, JIM8 and JIM13, labeled the central fibrillar secondary wall layer of Megaceros elaters and the walls of Sphagnum leaf cells, but did not label the walls of Radula elaters. The CCRC-M7 antibody, which detects an arabinosylated (1→6)-linked β-galactan epitope, exclusively labeled hyaline cells in Sphagnum leaves and the secondary walls of Radula elaters. Anti-pectin antibodies, LM5 and JIM5, labeled the primary wall in Megaceros elaters. LM5 also labeled the central layer of the secondary wall but only during formation. In Radula elaters, JIM5 and another anti-pectin antibody, JIM7, labeled the primary wall. The distribution of arabinogalactan–proteins and pectic polysaccharides restricted to specific wall types and stages of development provides evidence for the developmental and functional regulation of cell wall composition in bryophytes. Monosaccharide-linkage analysis of Sphagnum leaf cell walls suggests they contain polysaccharides similar to those of higher plants. The most abundant linkage was 4-Glc, typical of cellulose, but there was also evidence for xyloglucans, 4-linked mannans, 4-linked xylans and rhamnogalacturonan-type polysaccharides.

Palaeontology of Devonian thermal spring deposits, Drummond Basin, Australia

Molecular phylogenetic studies of extant organisms have shown that those branches of the Bacteria and Archaea that lie closest to the “last common ancestor” of all life are occupied by hyperthermophiles. At the same time, the search for former life on Mars has focussed on thermal spring deposits. For these reasons there is interest in the palaeobiology of ancient thermal spring deposits on Earth. Many such deposits are known but very few have been studied by palaeobiologists. The Devonian sinters of the Drummond Basin, Australia, rank with the Rhynie cherts of Scotland as the oldest well established examples of fossil subaerial hot springs. The Drummond Basin sinters are closely comparable with modem examples in Yellowstone National Park, Wyoming, and elsewhere. It is possible to recognise a range of palaeoenvironments from high temperature vents through former hot-water channelways and terraces to ambient temperature marsh deposits. Cyanobacterial stromatolites and microfossils are abundant in those pala...

Intraspecific Variation of Taeniate Bisaccate Pollen Within Permian Glossopterid Sporangia, from the Prince Charles Mountains, Antarctica

Permineralized sporangia from Late Permian sediments of the Amery Group in the Prince Charles Mountains, East Antarctica, are assigned to Arberiella sp. cf. A. africana Pant and Nautiyal. These sporangia contain between 2000 and 3000 taeniate, saccate pollen grains that are predominantly haploxylonoid bisaccate and referable to the palynotaxon Protohaploxypinus limpidus (Balme and Hennelly) Balme and Playford. However, the sporangia also contain greater than 4% of diploxylonoid bisaccate forms comparable to Striatopodocarpidites cancellatus (Balme and Hennelly) Hart 1963, together with sporadic monosaccate and trisaccate grains that, if found dispersed, would be assigned to several different pollen form genera. Morphometric analysis of in situ bisaccate pollen grains and taeniate bisaccate pollen in the dispersed palynoflora indicates that in situ grains occupy only the smaller end of the total size range. The tendency for in situ grains to cluster into two different size groups may reflect differential predispersal expansion of the corpus. The in situ pollen grains are variable in most qualitative and quantitative features used for taxonomic discrimination of dispersed taeniate bisaccate pollen, and this may lead to unreliable estimates of Late Permian floristic diversity if an overly restrictive species delimitation scheme is used.