Selena Y. Smith

Premise: The modern distribution of the southern cypresses, Callitroideae (Cupressaceae), reflects a classic Gondwanan distribution. The initial diversification of Callitroideae is estimated to have occurred during the Cretaceous, yet fossils from this period are rare, hampering our understanding of its early evolutionary history. Methods: A permineralized cluster of callitroid seed cones has been recovered from the Late Cretaceous Santa Marta Formation on James Ross Island, Antarctica. The specimen was sectioned using the acetate peel technique. We conducted a comparative morphological and anatomical investigation of the fossil with living members of Callitroideae and tested its phylogenetic placement using Bayesian and parsimony inference methods. Results: Each cone has four opposite‐decussate ovuliferous complexes, a central columella, and subtending bifacially flattened shoots. This new fossil resembles living Austrocedrus in having ovuliferous complexes with a short, apically located bract tip and an abaxial row of vascular bundles. Phylogenetic reconstructions place the fossil sister to Austrocedrus chilensis , and we erect a new species, Austrocedrus vetus sp. nov. Conclusions: Austrocedrus is known by a handful of Cenozoic fossils from Australia and a single surviving taxon, A. chilensis , in the Southern Andes of Chile and Argentina. Austrocedrus vetus is the oldest evidence of the genus, extending its stratigraphic record by 40 Ma. Our findings indicate that Austrocedrus diverged by the Campanian and experienced stasis in seed cone morphology. Furthermore, this fossil is the first occurrence of Austrocedrus in Antarctica, demonstrating that the genus was an element of Antarctic forest ecosystems during the Late Cretaceous.

Summary: Through symbiotic associations with microbes, mosses play many important ecological roles in terrestrial ecosystems, such as facilitating trophic interactions and nitrogen cycling. However, little is known about interactions between mosses and microbes in past ecosystems. Here, we characterize a permineralized moss with epiphytic cyanobacteria and tubular hyphae from the Cretaceous of Antarctica. The fossil was identified in a calcium carbonate concretion from the Beta member of the Santa Marta Formation Cretaceous (Campanian) from James Ross Island. The fossil was studied using acetate peels and light microscopy. The fossil moss is assignable to Bryopsida. Cyanobacteria trichomes with heterocyst‐like cells are present in leaf axils and the perichaetium. In some leaf axils, epiphytic fungi are preserved. The apical perichaetium contains several archegonia; one contains an egg cell while another contains a possible embryo. Fossil epiphytic cyanobacteria demonstrate that moss associations were part of Antarctic polar forests during the Cretaceous. Recognizing their importance in modern ecosystems, moss–cyanobacteria associations likely contributed bioavailable nitrogen to the nutrient cycle of ancient polar forests, which would benefit organisms in different trophic levels. This work reveals long‐term stasis in commensal relationships between cyanobacteria and mosses and provides a minimum age for these relationships.

Premise: Mechanistic models using stomatal traits and leaf carbon isotope ratios to reconstruct atmospheric carbon dioxide (CO₂) concentrations (c_a) are important to understand the Phanerozoic paleoclimate. However, methods for preparing leaf cuticles to measure stomatal traits have not been standardized.Methods: Three people measured the stomatal density and index, guard cell length, guard cell pair width, and pore length of leaves from the same Ginkgo biloba, Quercus alba, and Zingiber mioga leaves growing at known CO₂ levels using four preparation methods: fluorescence on cleared leaves, nail polish, dental putty on fresh leaves, and dental putty on dried leaves.Results: There are significant differences between trait measurements from each method. Modeled c_a calculations are less sensitive to method than individual traits; however, the choice of assumed carbon isotope fractionation also impacted the accuracy of the results.Discussion: We show that there is not a significant difference between c_a estimates made using any of the four methods. Further study is needed on the fractionation due to carboxylation of ribulose bisphosphate (RuBP) in individual plant species before use as a paleo‐CO₂ barometer and to refine estimates based upon widely applied taxa (e.g., Ginkgo). Finally, we recommend that morphological measurements be made by multiple observers to reduce the effect of individual observational biases.

Premise: The ecological conditions that constrain plants to an environmental niche are assumed to be constant through time. While the fossil record has been used previously to test for niche conservatism of woody flowering plants, additional studies are needed in other plant groups especially since they can provide insight with paleoclimatic reconstructions, high biodiversity in modern terrestrial ecosystems, and significant contributions to agriculture.Methods: We tested climatic niche conservatism across time by characterizing the climatic niches of living herbaceous ginger plants (Zingiberaceae) and woody dawn redwood (Metasequoia) against paleoniches reconstructed based on fossil distribution data and paleoclimatic models.Results: Despite few fossil Zingiberaceae occurrences in the latitudinal tropics, unlike living Zingiberaceae, extinct Zingiberaceae likely experienced paratropical conditions in the higher latitudes, especially in the Cretaceous and Paleogene. The living and fossil distributions of Metasequoia largely remain in the upper latitudes of the northern hemisphere. The Zingiberaceae shifted from an initial subtropical climatic paleoniche in the Cretaceous, toward a temperate regime in the late Cenozoic; Metasequoia occupied a more consistent climatic niche over the same time intervals.Conclusions: Because of the inconsistent climatic niches of Zingiberaceae over geologic time, we are less confident of using them for taxonomic‐based paleoclimatic reconstruction methods like nearest living relative, which assume a consistent climatic niche between extant and extinct relatives; we argue that the consistent climatic niche of Metasequoia is more appropriate for these reconstructions. Niche conservatism cannot be assumed between extant and extinct plants and should be tested further in groups used for paleoclimatic reconstructions.

While stromatolites, and to a lesser extent thrombolites, have been extensively studied in order to unravel Precambrian (>539 Ma) biological evolution, studies of clastic‐dominated microbially induced sedimentary structures (MISS) are relatively scarce. The lack of a consolidated record of clastic microbialites creates questions about how much (and what) information on depositional and taphonomic settings can be gleaned from these fossils. We used μCT scanning, a non‐destructive X‐ray‐based 3D imaging method, to reconstruct morphologies of ancient MISS and mat textures in two previously described coastal Archaean samples from the ~3.48 Ga Dresser Formation, Pilbara, Western Australia. The aim of this study was to test the ability of μCT scanning to visualize and make 3D measurements that can be used to interpret the biotic–environmental interactions. Fossil MISS including mat laminae with carpet‐like textures in one sample and mat rip‐up chips in the second sample were investigated. Compiled δ¹³C and δ³⁴S analyses of specimens from the Dresser Fm. are consistent with a taxonomically diverse community that could be capable of forming such MISS. 3D measurements of fossil microbial mat chips indicate significant biostabilization and suggest formation in flow velocities >25 cm s⁻¹. Given the stratigraphic location of these chips in a low‐flow lagoonal layer, we conclude that these chips formed due to tidal influence, as these assumed velocities are consistent with recent modeling of Archaean tides. The success of μCT scanning in documenting these microbialite features validates this technique both as a first step analysis for rare samples prior to the use of more destructive techniques and as a valuable tool for gaining insight into microbialite taphonomy.

Premise: We studied the 3D morphology of a small, well‐preserved cone from the Pennsylvanian Mazon Creek Lagerstätte to characterize its structure and determine its systematic affinity. Previously tentatively assigned to the enigmatic Tetraphyllostrobus, we show that it differs in key respects from that genus as described.Methods: We systematically compared the new fossil with relevant Paleozoic cone genera and employed advanced imaging techniques, including scanning electron microscopy, Airyscan confocal super‐resolution microscopy, optical microscopy, and X‐ray microcomputed tomography to visualize and reconstruct the fossil cone in 3D.Results: The analyses demonstrate unequivocally that the sporophylls of the new Mazon Creek cone are arranged in whorls of six and have characters typical of Sphenophyllales, including epidermal cells with undulatory margins and in situ spores assignable to Columinisporites. The combination of characters, including sporophyll arrangement, anatomy, and spore type, supports the establishment of Hexaphyllostrobus kostorhysii gen. et sp. nov. within Sphenophyllales. Furthermore, we show that Tetraphyllostrobus, although originally described as possessing smooth monolete spores, actually possesses Columinisporites‐type spores, indicating that it, too, was most likely a sphenophyll.Conclusions: The recognition of Hexaphyllostrobus contributes to our knowledge of Pennsylvanian sphenophyll diversity, and in particular increases the number of species with in situ Columinisporites‐type spores. Attribution of Hexaphyllostrobus to Sphenophyllales calls into question current interpretations of Tetraphyllostrobus suggesting that future research on better‐preserved macrofossil material may demonstrate a sphenophyllalean relationship.

Leaf stomata facilitate the exchange of water and CO₂ during photosynthetic gas exchange. The shape, size, and density of leaf pores have not been constant over geologic time, and each morphological trait has potentially been impacted by changing environmental and climatic conditions, especially by changes in the concentration of atmospheric carbon dioxide. As such, stomatal parameters have been used in simple regressions to reconstruct ancient carbon dioxide, as well as incorporated into more complex gas-exchange models that also leverage plant carbon isotope ecology. Most of these proxy relationships are measured on chemically cleared leaves, although newer techniques such as creating stomatal impressions are being increasingly employed. Additionally, many of the proxy relationships use angiosperms with broad leaves, which have been increasingly abundant in the last 130 million years but are absent from the fossil record before this. We focus on the methodology to define stomatal parameters for paleo-CO₂ studies using two separate methodologies (one corrosive, one non-destructive) to prepare leaves on both scale- and broad-leaves collected from herbaria with known global atmospheric CO₂ levels. We find that the corrosive and non-corrosive methodologies give similar values for stomatal density, but that measurements of stomatal sizes, particularly guard cell width (GCW), for the two methodologies are not comparable. Using those measurements to reconstruct CO₂ via the gas exchange model, we found that reconstructed CO₂ based on stomatal impressions (due to inaccurate measurements in GCW) far exceeded measured CO₂ for modern plants. This bias was observed in both coniferous (scale-shaped) and angiosperm (broad) leaves. Thus, we advise that applications of gas exchange models use cleared leaves rather than impressions.

Summary: Cunoniaceae are important elements of rainforests across the Southern Hemisphere. Many of these flowering plants are considered Paleo‐Antarctic Rainforest Lineages that had a Gondwanan distribution since the Paleocene. Fossils of several modern genera within the family, such as Ceratopetalum, have indicated biogeographical connections between South America and Australia in the Cenozoic. Here, we report a dramatic geographical range extension for Ceratopetalum, and Cunoniaceae as a whole, based on two exceptionally preserved fossil winged fruits from Campanian (c. 82–80 Ma old) deposits on Sucia Island, Washington, USA. The fossils were studied using physical sectioning, light microscopy, micro‐computed tomography scanning and multiple phylogenetic analyses. The fossil fruits share diagnostic characters with Ceratopetalum such as the presence of four to five persistent calyx lobes, a prominent nectary disk, persistent stamens, a semi‐inferior ovary and two persistent styles. Based on morphological comparisons with fruits of extant species and support from phylogenetic analyses, the fossils are assigned to a new species Ceratopetalum suciensis. These fossils are the first unequivocal evidence of crown Cunoniaceae from the Cretaceous of North America, indicating a more complicated biogeographical history for this important Gondwanan family.

Los análisis filogenéticos de coníferas que incluyen caracteres morfológicos se ven limitados por la falta de información en la caracterización anatómica de conos ovulíferos, una consecuencia de la dificultad de seccionar conos en estadio maduro, ya que se encuentran esclerificados. En este trabajo, comparamos el poder de resolución de tres métodos—cortes delgados de parafina, cortes delgados petrográficos y microtomografía computada de rayos X (micro-CT). Utilizamos a Taxodium como caso de estudio, y en base a los resultados obtenidos recomendamos complementar todos estos métodos, así como también presentamos un protocolo para la obtención de cortes de parafina para ejemplares con tejidos esclerificados. Las secciones delgadas de parafina, mientras que proveen una alta resolución anatómica, solo pueden ser utilizadas en ejemplares pequeños, es una técnica laboriosa y se ve dificultada por los tejidos duros. Los cortes delgados petrográficos son más rápidos y efectivos en ejemplares grandes, pero su resolución anatómica es baja y su aplicación se limita a ejemplares sin tejido fresco. El micro-CT, cuando está disponible, es rápido, produce cortes de alta resolución sin limitantes en el tamaño del ejemplar, y permite realizar cortes digitales y modelos 3D precisos; sin embargo, para poder identificar las estructuras anatómicas es preciso complementar con cortes obtenidos mediante alguno de los otros dos métodos. Estos métodos, aunque se superponen, proveen información anatómica con diferentes escalas de detalle. Por lo tanto, es ideal combinar estos tres métodos para así obtener información anatómica y morfológica de detalle de alta calidad a cualquier escala.