Early Land Plant Evolution

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Many paleobotanists regard the upper part of the Silurian as the point in geologic time when the first plants with organized conducting tissue appear. Others have suggested that vascular plants occur in strata as old as the Cambrian (Kryshtofovich, 1953). Some of these pre-Devonian fossils have subsequently been demonstrated to be the remains of nonvascular plants or even animals (Theron et al., 1990). In other instances, reinterpretation of the age of the rocks containing the fossils has negated reports of early vascular plants. The early lycopsid, Baragwanathia, is an unusual case in this regard. (Garratt, 1978) It was initially described from Upper Silurian compressions from Australia (Lang and Cookson, 1935) and the age was based on the occurrence of the graptolite Monograptus. Baragwanathia (FIG. 8.1) will be discussed in more detail in Chapter 9, but it is most certainly a vascular plant, with annular-helical tracheids forming the conducting strand. The age of this plant has been debated in the literature ever since as to whether the rocks are truly Upper Silurian or Lower Devonian. Subsequent studies have confirmed the Late Silurian (Ludlow) age (Rickards, 2000). The age has

Denise Levertov, About Marriage been hotly debated (Thomas, 1984) because Baragwanathia represents a relatively complex vascular plant at a point in geologic time when all other vascular plants were comparatively simple. This suggests that either vascular plants evolved far earlier or that tracheids perhaps evolved more than once during the terrestrialization of the earth. As we learn more about early land plants, including those with well-defined conducting tissues, it is becoming clear that a number of these early plants did not possess vascular tissue like that in extant vascular plants. Although identifying plants with vascular tissue early in the geologic record is important, what is perhaps equally significant is understanding the nature of these cells.

HIGHER TAXA IN THIS CHAPTER:

Rhyniophytes (Silurian-Devonian) Zosterophyllophytes (Silurian-Devonian) Trimerophytes (Silurian-Devonian)

Baragwanathia Longifolia Fossil
figure 8.1 Baragwanathia longifolia (Silurian). Bar = 2 cm.
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