Sanfordiacaulis densifolia: A tree with a preserved crown from the Early Carboniferous of New Brunswick, Canada.

The first trees appear in the fossil record during the Middle Devonian, about 393 million years ago, with the first woody trees appearing by about 385 million years ago. The evidence for these early trees is somewhat limited, comprising a variety of impressions in mud- and sandstones, as well as a few mineralized stumps and root-systems. Preserved fossils with root and crown structure attached to trunks are extremely rare. More common are preserved trunks, lacking their roots or crown structures, often in assemblages in which a few dozen trunks would accumulate over tens of millions of years, which give us a reasonable understanding of the early evolution of xylem systems and wood.

Intact trees become more common in the Early Carboniferous, from about 350 million years ago, when peat-forming forests first appeared, often leading to trees 5-7 m in height being preserved with attached rooting structures and sometimes also canopy branches. These trees often retain sufficient architecture to assign them to groups such as the Lycophytes, Pteridophytes, Equisetaleans, and seed-bearing Gymnosperms, even when the reproductive structures which define the groups are absent. This has enabled palaeobotanists to develop an idea of the diversity present in Late Palaeozoic forests, although our understanding of the ecological structure of these forests is much more restricted. 

In a paper published in the journal Current Biology on 2 February 2024, Robert Gastaldo of the Department of Geology at Colby College, and the Department of Paleobiology at the Smithsonian National Museum of Natural History, Patricia Gensel of the Department of Biology at the University of North Carolina, Ian Glasspool, again of the Department of Geology at Colby College, and of the Field Museum of Natural History, Steven Hinds of the Geological Surveys Branch at the New Brunswick Department of Natural Resources and Energy Development, Olivia King of the New Brunswick Museum and the Department of Geology at Saint Mary’s University, Duncan McLean of MB Stratigraphy Ltd, Adrian Park, also of the Geological Surveys Branch at the New Brunswick Department of Natural Resources and Energy Development, Matthew Stimson, also of the New Brunswick Museum and the Deptartment of Geology at Saint Mary’s University, and Timothy Stonesifer, again of the Department of Geology at Colby College, describe a new species of tree from the Early Carboniferous of New Brunswick, Canada, with several specimens preserving a three-dimensional crown structure.

The new species is named Sanfordiacaulis densifolia, where 'Sanfordiacaulis' is a combination of 'Sanford' which is the name of both the quarry where the specimens from which the new species is described where found, and the name of the owners of that quarry, and 'caulis' means stalk, while 'densifolia' refers to the dense foliage of the specimens. The species is described from five trees preserved in close proximity, within a block roughly 2.3 m long and 2 m wide; one of these trunks still remains in situ at the quarry, although there are plans to excavate it and place it in the New Brunswick Museum, where the other four specimens already reside.

Sanford Quarry locality, New Brunswick, Canada (A) Geologic map of Upper Devonian-Lower Carboniferous strata exposed around Norton (red dot) and the Sanford Quarry (yellow star; N 45.627786, W 65.691610). Scale in km. Inset: Canadian Maritime Provinces. (B) August 2023 quarry exposure where white arrow shows the location of primary tree crown. Matthew Stimson (yellow ellipse) for scale. Gastaldo et al. (2024).

The largest trunk is 2.25 m in length and 12-16 cm wide, lacking either a base or an apex, but a second specimen, with a trunk 7-8 cm in diameter, retains a crown. This is comprised of leaves which depart from the trunk at an acute angle, curve outward for about 5–6 cm, and then project up to 1.75 m in length distal to the trunk, being cut off by the edge of the block; the tips are not present. Leaves and leaf scars are arranged in a tight spiral about the stem.

Trunk and petiole features. NBM 22403/1 (A) An approximately 40 cm interval showing helically arranged, mudcast petiole bases in an estimated 1/13 phyllotaxy with coalified and mudcast petioles departing side of the trunk. Scale in cm and mm. (B) Finely striated and adaxially grooved petioles diverge at about 90° to the trunk (white arrows) beneath the apex; petioles are without secondaries. Scale in cm/mm. (C) Petioles diverge from trunk at about a 90° angle. Scale in cm and mm. (D) Divergence of petioles in dimensions reflecting their spiral arrangement. Strong longitudinal ridges mirror the petiole cross-sectional geometry, and striated petioles may exhibit transverse markings, similar to coal cleat, from tectonism. Scale in cm and mm. Gastaldo et al. (2024).

It was not possible to assign Sanfordiacaulis densifolia to any particular group of Plants; the general morphology with tightly packed leaves/leaf stems around a single trunk is a common one, found in numerous fossil extant and extinct Plant groups. However, it is possible to make some assumptions about the ecological role of the living tree. It is calculated to have stood about 2.65 m in height, in a forest that contained Pteridophyte and Pteridosperm trees which reached in excess of 20 m in height. This implies that it was an understory tree, growing beneath much taller trees. Modern trees with this habit often have dense crowns similar to that seen in Sanfordiacaulis densifolia, which enables them to capture as much light as possible in the darker, understory environment.

Actual and reconstructed tree heights and biostratigraphic ranges of Middle Devonian to Pennsylvanian trees Plants depicted based on fossils preserved with either trunks, trunks with attached crowns, or forest elements buried in growth position. Plant reconstructions are Cladoxylales: Calamophyton, Pseudosporochnus, and Eospermatopteris/Wattieza; Progymnosperms: Callixylon, Pitus, and Protopitys; Ferns: Megaphyton and Psaronius; Gymnosperms: Elkinsia, Medullosa, and Alethopteris zeilleri; Lycophytes: Lepidodendron sp., Lepidodendron lycopodioides, and Lepidophloios hallii; and Equisetales: Arthropitys bistrata. Horizontal scale in meters; log₁₀ vertical scale. Plants arranged in chronostratigraphic order according to geologic intervals in which they are reported. Trees that colonised landscapes at two successive geologic intervals are shown as overlapping the time scale. Hence, Callixylon and Pitus are known from both the Late Devonian and Early Mississippian; Medullosa, Alethopteris, and Psaronius are reported first in the middle Mississippian (Visean) and continue into the Pennsylvanian. Horizontal scale in 0.5 m; vertical log scale. Gastaldo et al. (2024).

The development of a stratified forest structure with a layer of understory trees would have had a profound impact on the Early Carboniferous environment. Stratified forests utilise light much more efficiently than non-stratified forests, absorbing more carbon dioxide, and creating a greater range of microhabitats and microclimates. This in turn allows for the existence of a much wider range of smaller organisms to inhabit these new environments. The development of an understory may also have helped the propagation of forest fires, as the understory can act as a ladder, helping fire to ascend from the ground into the canopy, further changing the emerging forest environment.

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