Neutron tomography of <i>Austrosequoia novae-zeelandia</i>e comb. nov. (Late Cretaceous, Chatham Islands, New Zealand): implications for Sequoioideae phylogeny and biogeography

The Tupuangi Flora of the Chatham Islands, New Zealand, reveals a south polar forest ecosystem, and important biogeographical links between eastern and western Gondwana. We employed neutron tomography (NT) to image fossil Cupressaceae seed cones from the Upper Cretaceous (Cenomanian) strata of the Tupuangi Formation. This technique facilitated the non-destructive ‘virtual extraction’ of three-dimensional, coalified specimens, whilst they were still embedded within a large volume of supporting silicate sedimentary rock. This study is the first reported application of NT in palaeobotanical taxonomy, and the combination of virtual and manual extraction techniques enabled a more complete treatment than would otherwise be possible if taxonomic data were limited to only one of these approaches. The seed cones were identified as Austrosequoia novae-zeelandiae (Ettingshausen) Mays & Cantrill comb. nov. In this case, NT data supplemented the compression fossil data by providing details such as the three-dimensional measurements of the gross morphology, and accurate estimations of bract-scale complex number. Furthermore, this technique appears to show promise in differentiating between organic compounds within an individual specimen. However, anatomical details and fine-scale morphology were indiscernible due to present limitations in spatial resolution. Austrosequoia novae-zeelandiae is interpreted as a stem group of Sequoioideae; it shares synapomorphic seed cone characters with extant sequoioids (e.g. Sequoia and Sequoiadendron), and plesiomorphic stomatal structures and leaf morphology. Abundant epiphyllous fungi (Plochmopeltinites sp.; Microthyriaceae) were also identified on the leaf cuticles of A. novae-zeelandiae. The high abundance of Austrosequoia in the Tupuangi Flora supports a cupressaceous floral province at south polar latitudes during the early Late Cretaceous. Furthermore, this stem group of Sequoioideae in eastern Gondwana during the early Late Cretaceous suggests an alternative, south-to-north dispersal route of sequoioids before the final continental separation of eastern and western Gondwana.新西兰查塔姆群岛的图普安吉植物群（Tupuangi Flora）揭示了一处南极高纬度森林生态系统，以及东、西冈瓦纳之间重要的生物地理联系。本研究采用中子断层扫描（Neutron Tomography, NT）技术，对产自图普安吉组（Tupuangi Formation）上白垩统（森诺曼阶Cenomanian）地层的柏科（Cupressaceae）化石球果进行成像。该技术实现了三维煤化标本的无损“虚拟提取”，此时标本仍嵌附于大量承载的硅酸盐沉积岩中。本研究首次将NT技术应用于古植物分类学（palaeobotanical taxonomy）研究，结合虚拟提取与手工提取两种技术手段，相比仅采用单一方法获取分类学数据的情况，能够实现更完整的标本研究。经鉴定，该化石球果隶属于新西兰南方杉（Austrosequoia novae-zeelandiae (Ettingshausen) Mays & Cantrill comb. nov.）。在此研究中，NT数据弥补了压型化石（compression fossil）资料的不足，提供了整体形态的三维测量数据以及苞片-鳞叶复合体（bract-scale complex）数量的精准估算结果。此外，该技术在区分单份标本内的有机化合物方面展现出应用潜力。不过受限于当前的空间分辨率，解剖细节与精细形态结构仍无法被辨识。新西兰南方杉被认为是杉亚科（Sequoioideae）的茎类群，它与现生杉亚科类群（如红杉属Sequoia、巨杉属Sequoiadendron）共享球果的共有衍征，同时保留了祖征性的气孔结构与叶片形态。研究还在新西兰南方杉的叶片角质层（leaf cuticles）上发现了大量叶附生真菌（epiphyllous fungi）：Plochmopeltinites sp.以及小盾壳科（Microthyriaceae）类群。图普安吉植物群中南方杉属的高丰度，佐证了晚白垩世早期南极高纬度地区存在柏科植物区系。此外，晚白垩世早期东冈瓦纳地区出现的杉亚科茎类群，表明在东、西冈瓦纳最终大陆分离之前，杉亚科类群可能存在一条替代性的由南向北扩散路径。