Sprecher
Beschreibung
Needle-leaved conifer shoots are among the most abundant plant fossils found in the Upper Cretaceous sediments of southern Alberta, Canada, and typically are assigned to the subfamilies Taxodioideae or Sequoioideae (Cupressaceae sensu lato). Their taxonomic delineation, however, remains challenging due to the incompletely understood phenotypic plasticity of these taxa. Most Canadian Late Cretaceous ambers are thought to have been produced by cupressaceous trees of the fossil-genus ‘Parataxodium’ Arnold et Lowther based on amber inclusions and amber chemistry. This generic assignment is based solely on the amber inclusions, which are rare and incompletely preserved. Additionally, the taxonomy of the comparative material has been subject to a decades-long debate. We quantify the morphological variation of extant species of the Sequoioideae and Taxodioideae, the Grassy Lake amber inclusions, and the three-dimensional silicifications from the Horseshoe Canyon Formation originally used as comparative material for the amber inclusions. Furthermore, we compare the disparity between the fossils and extant species, and test for species delimitation. Our results show that extant taxa can be distinguished macromorphologically but the silicified shoots and amber fossils show greater variation, especially with regard to phyllotaxis. While the silicified fossils display remarkable stability in their micromorphology, allowing their delimitation, the amber inclusions displayed two distinct, hitherto overlooked, shoot morphologies assignable to Cupressaceae sensu lato. Combined with the lack of cuticular preservation, which prevents appropriate comparisons, these results invalidate an assignment of the amber inclusions to ‘Parataxodium’.