Exceptional anatomical preservation of Jurassic Austrohamia leaves from Patagonia reveals distinctive features of ancient Cupressaceae

Abstract

Premise: The Mesozoic fossil record of Cupressaceae is widespread and abundant, indicating that members of the family were dominant components of ancient ecosystems and that several cupressaceous lineages had higher species diversity in the past than they do today. Despite this extensive record, anatomical evidence remains critical to understanding the evolution of the group. This study provides new insights into the early evolutionary history of Cupressaceae and their adaptations to Jurassic environments through exceptionally preserved fossils from Patagonia. Methods: Silicified leaves and twigs were collected from chert deposits at the Claudia and Cañadón Nahuel localities (La Matilde Formation, Santa Cruz Province). Samples were cut, polished, and analyzed using light microscopy and image‐stacking techniques. Morphological, cuticular, and anatomical characters were compared with extant and fossil Cupressaceae to assess affinities. Results: The leaves are dorsiventrally flattened and helically arranged with a semiplanar orientation. Anatomical features include a single abaxial resin canal and transfusion tissue surrounding the vascular bundle adaxially and expanding laterally into two rounded lobes. Cuticular traits include abaxial, monocyclic stomata with subsidiary cells resembling adjacent ordinary epidermal cells. These features align with Austrohamia and indicate affinity with Cupressaceae s.l. Conclusions: The combination of vegetative, cuticular, and anatomical features supports assigning these Jurassic Patagonian fossils to Austrohamia and suggests a position near the root of Cupressaceae s.l. Their remarkable histological preservation provides new anatomical and biostratigraphic data, enhancing understanding of the early evolution of this family.