Isolated tubes represent the third type of fragmentary fossil found as early as the Late Ordovician (Vavrdova, 1988). One study reports two types of tubes with annular-helical thickenings from the Mesoproterozoic (~1-1.6Ga) of northern China (Yin et al., 2004). Most tubes can be separated into two groups. In one group are narrow tubes, ~8-20pm in diameter and about 50 pm long, which are generally smooth, aseptate, and unbranched. The second group has tubes that are longer (~200pm long) with annular-helical thickenings on the inner surface. Some have a papilliform or bulb-shaped tip. Burgess and Edwards (1991) developed an artificial classification system for isolated tubes and filaments found in Ordovician-Lower Devonian rocks, and suggested that many belong to the nematophytes (Lang, 1937). The attempt to define the stratigraphic and morphological extent of these tubes may ultimately prove an important component in understanding their biology. Some tubes are like those found in certain green algae, especially members of the Dasycladales; others suggest the tubes were produced by a variety of different organisms.
Although the biological affinities of the cuticle-like sheets, isolated spores, and tubes continue to remain speculative, they all suggest features that are analogous to those of the land plants found in slightly younger rocks (Edwards et al., 1998a). The presence of cuticles on aerial plant parts and spores enclosed in a wall made of sporopollenin are primary antidesiccation features found in vascular plants and a few bryophytes today. Although the Ordovician-Silurian tubes do not precisely fit the definition of a tracheid or any other known type of putative water-conducting cell, they nevertheless show structural and functional equivalents in organisms that were adapting to a desiccating environment, a major feature in the transition to the land (discussed below).
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