The Intriguing Three-Dimensional Tree Fossils Reveal Early Carboniferous Tree Evolution

Sanfordiacaulis densifolia（Image source：Tim Stonesifer/Gastaldo RA et al， CC BY 4.0 ）

The earliest vascular plants appeared in the late Silurian, around 420 million years ago. Trees with true woody structures evolved later, emerging during the middle to late Devonian roughly 380 million years ago. During that period, several ancient plant groups already formed tall tree-like organisms. Examples include Wattieza, Archaeopteris, and Calamophyton. These plants could reach heights comparable to modern woody gymnosperms and flowering trees. Fossils of Wattieza, for instance, indicate individuals that may have grown between eight and ten meters tall.

Evidence for ancient trees often comes from indirect traces left in the sediment. Some fossils preserve impressions left in ancient soils, while others consist of mineralized stumps. Before the Carboniferous, however, such discoveries were extremely rare. Many fossilized trunks lack roots or crowns, leaving an incomplete picture of the original plant. Only later, toward the latter part of the early Carboniferous, do tree fossils become more common and better preserved, often retaining both root systems and crowns. At roughly the same time, abundant fossils of spore-bearing plants and early seed plants provide sufficient data to reconstruct what late Paleozoic forests may have looked like.

From the middle–late Devonian into the earliest Carboniferous—particularly during the Tournaisian stage—tree fossils remain scarce. Because of this gap, the structure of forests during that interval has remained poorly understood. Recently, however, a remarkable discovery was made at Sanford Quarry in New Brunswick, Canada. Five Tournaisian tree fossils were excavated there, several preserving parts of their crowns. This exceptional preservation allows scientists to reconstruct aspects of the plant's overall morphology.

The five trees were found close together and represent some of the most complete early Carboniferous tree fossils yet discovered. Much of their three-dimensional structure remains intact. The trunk of the best-preserved specimen measures roughly sixteen centimeters in diameter near the crown and about twelve centimeters near the base. The preserved trunk length is approximately 2.25 meters. Unfortunately, the root system was not preserved, making it impossible to determine the tree's full height with certainty.

One of the most striking features of these trees lies in their unusual leaf arrangement. The petioles are organized in a spiral pattern along the upper portion of the trunk, extending over roughly the top 0.75 meters. Each leaf extends outward as much as 1.75 meters, and the crown carries an extraordinary number of leaves—nearly 250 in total. The dense outward spread of these leaves creates a structure resembling a giant umbrella.

The new species has been named Sanfordiacaulis densifolia. The genus name refers to Sanford Quarry, where the fossils were discovered, while the species name means "dense-leaved," describing the tightly packed foliage.

The unusual architecture of this tree reflects the evolutionary experimentation that characterized early forest ecosystems. Throughout plant evolution, different structural strategies appeared as lineages explored ways to gain ecological advantages. In the case of Sanfordiacaulis, the extremely dense and expansive crown likely functioned to maximize light capture. Trees of this size probably occupied the forest subcanopy layer—beneath taller canopy trees but above ground-level vegetation. In such a position, sunlight filtering down from above would be limited. Expanding the crown into a broad umbrella-like structure would allow the plant to intercept more of that scattered light. Modern plants rarely exhibit such an extreme architecture, suggesting that this evolutionary strategy eventually disappeared over time.

Sanfordiacaulis densifolia（Image source：Gastaldo RA et al， CC BY 4.0 ）

From a systematic perspective, the classification of this tree remains uncertain. The fossils do not preserve reproductive structures or detailed internal anatomy, both of which are essential for identifying its evolutionary relationships. As a result, it could represent either a spore-bearing plant or an early seed plant. Nevertheless, several morphological features provide clues. The spiral arrangement of compound leaves, the structure of the leaf axes, and the restriction of leaf attachment to the upper trunk resemble characteristics seen in ferns. Because of this, researchers currently consider it likely that Sanfordiacaulis densifolia was related to fern lineages, similar to modern tree ferns that display comparable growth patterns.

Previous research suggests that layered forest structures—consisting of canopy trees, subcanopy vegetation, and ground plants—began developing during the middle Devonian and became increasingly complex through the early Carboniferous, particularly during the Mississippian. The new fossils provide strong evidence that the subcanopy layer had already become ecologically significant during the Tournaisian. This indicates that vertical forest stratification was evolving earlier than previously recognized.

Preserving complete trees in the fossil record is extremely rare. Whole plants are seldom buried intact, and most fossil discoveries represent only fragments of the original organism. The discovery of Sanfordiacaulis densifolia therefore represents a remarkable window into early forest ecosystems. Despite the limited data available from this period, these fossils provide valuable new insights into how early trees experimented with different growth forms as forests became increasingly complex during the Paleozoic.

Author: Shui-Ye You

Reference:

Gastaldo RA et al. (2024). Enigmatic fossil plants with three-dimensional, arborescent-growth architecture from the earliest Carboniferous of New Brunswick, Canada. Curr Biol.

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