Neutron tomography of Austrosequoia novae-zeelandiae 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）展现了一个南极圈森林生态系统，同时揭示了冈瓦纳大陆（Gondwana）东西两部分间至关重要的生物地理关联。本研究采用中子断层扫描（Neutron Tomography，NT）技术，对采自图普安吉组上白垩统（森诺曼阶）地层的柏科（Cupressaceae）化石球果进行成像。该技术可在化石标本仍嵌于大量硅酸盐沉积围岩的状态下，实现对三维煤化标本的非破坏性“虚拟提取”。本研究首次将NT技术应用于古植物分类学研究，结合虚拟提取与手工提取两种技术路径，相比仅采用单一方法所能获取的分类学数据，能够实现更为完整的标本研究与类群鉴定。经鉴定，该化石球果属于新组合种*Austrosequoia novae-zeelandiae* (Ettingshausen) Mays & Cantrill comb. nov.。本研究中，NT数据补充了压型化石（compression fossil）数据，可提供整体形态的三维测量值、苞鳞复合体数量的精确估算等关键细节。此外，该技术在区分单一个体标本内的有机化合物方面展现出良好的应用前景。但受当前空间分辨率的限制，标本的解剖细节与精细形态仍无法被有效识别。研究认为*Austrosequoia novae-zeelandiae*属于杉亚科（Sequoioideae）的干群类群：它与现生杉亚科类群（如红杉属*Sequoia*、巨杉属*Sequoiadendron*）共享共衍征的球果特征，同时保留了祖征的气孔结构与叶片形态。在*A. novae-zeelandiae*的叶片角质层上，还发现了丰富的叶生真菌，包括*Plochmopeltinites* sp.与小盾壳科（Microthyriaceae）类群。图普安吉植物群中*Austrosequoia*的高丰度，支持了晚白垩世早期南极圈纬度带存在柏科植物区系的推论。此外，晚白垩世早期东冈瓦纳地区出现的该杉亚科干群类群，暗示在东、西冈瓦纳大陆最终分离之前，杉亚科类群可能存在一条由南向北的扩散路径。