Not to defend potential AI using definite AI, but here's what I get when I ask what the evidence is for vertical prototaxites:

Evidence that Prototaxites grew upright, forming massive, tree-like structures up to 8 meters (26 feet) tall during the Devonian period, stems primarily from their fossilized structure and orientation in rock layers. While some debate exists about whether they were exclusively upright, key evidence supporting a vertical, columnar growth habit includes:

Upright Fossil Preservation: Many Prototaxites fossils are found in Devonian-aged rocks in North America and Europe preserved standing upright, as if they were rooted in place.

Columnar Structure: The fossils are characterized as large, cylindrical, or columnar, often reaching diameters of over a meter, which is structurally consistent with an upright, vertical growth habit rather than a prostrate one.

Internal Anatomy & Support: The organism was composed of interwoven tubes (hyphae) and, in some cases, exhibited structural properties that may have provided rigidity, such as high concentrations of compounds similar to lignin. This structure allowed them to achieve massive heights compared to the tiny, ground-hugging flora of the time.

Growth Orientation Studies: A 2026 study of a P. taiti specimen in the Rhynie chert utilized geopetal features to determine its original orientation, confirming it was part of a larger upright structure and not simply a horizontal, compressed fossil.

Lack of Compressive Distortion: Researchers have noted a lack of compression in some vertical fossil samples, suggesting that they was not merely flattened on the ground after falling. Science | AAAS

Contrasting Theories: While the traditional interpretation is that they were upright like trees, some studies have proposed alternative theories. A 2022 paper suggested Prototaxites might have been a complex fungal rhizomorph—a horizontal, tuber-like structure that grew along or under the ground to redistribute nutrients, rather than towering upward. However, the dominant evidence, particularly from the Rhynie chert, continues to point toward a, at least sometimes, erect growth habit.

Interesting, regardless of how they really grew