New evidence of the architecture and affinity of fossil trees from the Jurassic Purbeck Forest of southern England

Abstract

We document the habit and affinity of the most complete Mesozoic Era tree to be excavated in the UK. The fossil was found in situ in a palaeosol of the Upper Jurassic Purbeck Group of southern England (Tithonian: ca. 150–145 million years). It comprises over 100 permineralized (silicified) pieces that represent a rooted stump and fallen trunk, together weighing more than two tonnes. This exceptional specimen was excavated in a manner that retained the original associations among its parts, providing a unique insight into the overall habit and mode of growth. A laser scanning approach was used to facilitate the investigation, producing the largest 3D reconstruction of a plant fossil. Anatomical details reveal that the wood belongs to the fossil-genus Agathoxylon. Despite an estimated growth age of more than 200 years, the tree was of modest size, not greatly exceeding 12 m in height. The main trunk bifurcated, developing into a decurrent, spreading crown. Its habit differed significantly from most modern arborescent conifers, which have pole-like central trunks and narrow, conical crowns, and from known growth forms in the Cheirolepidiaceae, an important extinct group of Mesozoic conifers. These findings extend our knowledge of the growth architecture of Jurassic conifers, which were prominent and diverse elements of seasonally arid, low to mid-latitude coastal communities during the Late Mesozoic.