D. rex: The 2.2 kg Fungus That Rose After the Dinosaurs Fell

They examined four genetic regions: ITS, LSU, RPB2 and TUB2. These act as genetic markers that help establish evolutionary relationships. Comparing the resulting sequences against 202 fungal strains, the analysis consistently placed the new specimens on a distinct evolutionary branch within the fungal order Xylariales.

To be sure, the team sequenced whole genomes of three specimens and compared more than 300 conserved genes across 66 fungal samples. That confirmed the find warranted its own genus and family.

The genus name Dianjunus references the ancient Dian Kingdom of Yunnan. Rex is Latin for king — a nod to its extraordinary size.

"This remarkable species represents a new fungal family, Dianjunaceae, and provides a rare window into the evolution of extreme fungal body forms," said Indunil C. Senanayake, an associate professor at the Kunming Institute of Botany and one of the researchers.

The timing of its origins proved just as striking. Using genetic comparisons and established evolutionary timelines, the researchers estimate Dianjunaceae diverged about 65 million years ago, with a possible range of 50 to 80 million years. That places its emergence near the start of the Paleocene, just after the Cretaceous-Paleogene extinction event wiped out roughly 75 percent of Earth's plant and animal species.

"After the mass extinction, forests experienced dramatic changes, including the sudden availability of large amounts of dead plant material. For wood-associated decomposing fungi, such environmental disruption may have created new ecological opportunities," Senanayake said. That possibility remains a hypothesis for now.

Why does this fungus grow so large?

The biggest open question is what lets D. rex build enormous reproductive structures when its relatives stay small.

One explanation involves reproduction. The upper portions of its fruiting structures, called stromata, carry numerous tiny openings that release spores. On a forest floor blanketed with leaves and branches, spores released low down could easily get trapped. Growing tall might let spores escape the clutter and disperse wider. Building such massive structures costs substantial resources, but the trade-off appears to have worked.

The genetics offer another clue. D. rex has considerably fewer carbohydrate-active enzymes — proteins that help fungi break down complex plant materials — than many related wood-decaying species. Its genome also carries very few pectin lyases, enzymes involved in breaking down plant cell walls.

That profile supports a working hypothesis about its lifestyle. The researchers suspect the fungus first lives inside healthy trees without causing visible damage, then switches to consuming dead plant material once its host dies.

"Because fruiting bodies of D. rex were observed exclusively post-host mortality, these combined traits support a working hypothesis of an endophyte-to-saprotroph lifestyle transition," the researchers write in their study. In plain terms: quiet tenant first, decomposer later.

Could more giant fungi be waiting?

D. rex also exposes how much fungal life remains unknown. Yunnan has more than 6,000 documented fungal species, but researchers estimate the province could hold over 100,000. Fungi stay unnoticed partly because most of the organism lives underground or inside a host, and visible reproductive structures may appear only under particular conditions.

Fungi matter: they recycle nutrients, break down dead organic matter and maintain useful partnerships with plants.

"It's really important that we understand what they do. The first step in understanding what they do is describing them, acknowledging their existence, and then later trying to understand more details about their ecology," Peay said.

In short: a two-kilogram king of a vanished age was standing in Chinese forests all along, and nobody had named it until now.