Microfossil tubes with internal thickenings, so-called banded tubes, are a conspicuous but enigmatic element of terrestrial palynomorph assemblages of Silurian to Early Devonian age. Their biological affinities and functions are controversial. Most researchers favor derivation from nematophytes, themselves an enigmatic group of organisms that are often considered to represent early terrestrial fungi-like organisms. Here we present the first transmission electron microscope (TEM) ultrastructural analysis of these banded tubes and show that their walls are homogeneous and lack preserved ultrastructure. Gross wall structure is extremely variable with respect to the nature of the thickenings, suggesting that the tubes were derived from a variety of sources or were highly variable within the source organism. Ultrastructurally, the tubes differ markedly from contemporaneous vascular plant tracheids. We discuss the affinities of the tubes based on the new ultrastructural and structural information, and we conclude that they most likely derive from nematophytes, heterotrophic organisms with fungal-like ecology, where they served as hyphal-like structures, anchoring the organism and transporting nutrients.
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