In 2007, isotopic analyses by a team including Hueber and Kevin Boyce of the University of Chicago concluded that Prototaxites was a giant fungus. They detected a highly variable range of values of carbon isotope ratios in a range of Prototaxites specimens; autotrophs (organisms such as plants and algae, that make a living via photosynthesis) living at the same time draw on the same (atmospheric) source of carbon; as organisms of the same type share the same chemical machinery, they reflect this atmospheric composition with a constant carbon isotope trace.
A study conducted by Hueber and Boyce's team in 2007 concluded that Prototaxites was a gigantic organism, likely a fungus. The analysis of carbon isotope ratios revealed variations among different specimens, suggesting that they had distinct ways of obtaining carbon from the atmosphere.