Prototaxites fossils are structurally and chemically distinct from extinct and extant Fungi

was the first giant organism to live on the terrestrial surface, represented by columnar fossils of up to eight meters from the Early Devonian. However, its systematic affinity has been debated for over 165 years. There are now two remaining viable hypotheses: was either a fungus, or a member of an entirely extinct lineage. Here, we investigate the affinity of by contrasting its organization and molecular composition with that of Fungi. We report that fossils of from the 407-million-year-old Rhynie chert were chemically distinct from contemporaneous Fungi and structurally distinct from all known Fungi. This finding casts doubt upon the fungal affinity of , instead suggesting that this enigmatic organism is best assigned to an entirely extinct eukaryotic lineage. fossils are distinct from Fungi, suggesting that they represent an extinct lineage of eukaryotic life.

Funder

Funder name	Awards
UK Research and Innovation	MR/T018585/1, MR/Y03399X/1
Engineering and Physical Sciences Research Council	EP/Y037138/1
Leverhulme Trust	ECF-2023-202
Royal Society	NIF\R1\211589
Royal Society	NIF\R1\211589
Royal Society	RGS\R2\212063
European Research Council	1101114969
Human Frontier Science Program	RGP006/2024
Science Foundation Ireland	22/PATH-S/10692
Philip Leverhulme Prize	PLP-2023-324
NERC E4 Doctoral Training Partnership
NERC E4 Doctoral Training Partnership
Human Frontier Science Program	RGP006/2024