Ancient Prototaxites Reclassified as Unique, Extinct Branch of Life

New research has re-evaluated the classification of Prototaxites, a large ancient organism that lived approximately 400 million years ago.

The study suggests Prototaxites may represent a unique, extinct branch of life distinct from any known modern or previously described group of organisms.

Historical Classification Challenges

Prototaxites fossils, first discovered over 165 years ago, have historically posed challenges for biological classification. Geologist John William Dawson initially identified fossils found in Quebec, Canada, in the 1850s, suggesting they were rotting trees and leading to the name 'first conifer.' Early theories also proposed Prototaxites could be a form of algae or a lichen-like mass.

In 2001, paleontologist Francis Hueber proposed that Prototaxites was a type of enormous fungus. This hypothesis received support in 2017 following an analysis of a fossil fragment from a smaller species, P. taiti, which claimed to identify textures resembling fertile structures of modern Ascomycota fungi. This organism did not appear to photosynthesize and likely consumed environmental carbon sources.

Recent Research Findings and Reclassification

A recent study, co-led by University of Edinburgh paleobotanist Alexander Hetherington and palaeontologist Corentin Loron, focused on three P. taiti fossil fragments from the Rhynie chert in Scotland. The research utilized microscopic anatomy and chemical analysis of the organism's tubular structures to investigate various candidate biological groups:

• Fungi: The study found Prototaxites fossils to be chemically distinct from fossilized fungi found in similar conditions at the site. Fungi fossils contained breakdown products of chitin and glucan, which were absent in Prototaxites. Additionally, unique anatomical connections and complex branching patterns within dark spherical spots, potentially for gas, nutrient, or water exchange, differed from all known fungi.
• Plants or Algae: These were deemed unlikely based on the organism's chemical composition.
• Lichen-like mixture: This classification was ruled out by its anatomy.
• Animals: An animal classification was dismissed due to the presence of cell walls.

The researchers concluded that the morphology and molecular fingerprint of P. taiti are distinct from fungi and other organisms, making it difficult to assign Prototaxites to any extant lineage. Based on this investigation, the study suggests Prototaxites should be considered a member of a previously undescribed, entirely extinct group of eukaryotes.

Characteristics of Prototaxites

Prototaxites existed during the late Silurian period, approximately 400 million years ago. These organisms grew into tall, wide, and branchless towers, reaching heights of up to 8 meters (26 to 30 feet).

Expert Perspectives and Future Considerations

While the fungal classification is not universally accepted, Kevin Boyce, a professor at Stanford University, noted that Prototaxites' age predates many complex multicellular examples in fungi, which could make direct comparisons challenging. The study acknowledges that further reviews could potentially lead to a reconsideration of Prototaxites' classification, including the possibility of returning the group to ancient fungi.

This research was published in the journal Science.