There is convincing evidence that supports the lycopsids as most closely related to the zosterophylls. Both groups are characterized by exarch protosteles and laterally produced reniform sporangia. In Crenaticaulis and Gosslingia, axillary tubercles are thought to represent rhizophore-like branches, similar to those of extant species of Selaginella, The absence of leaves in the zosterophylls, although an obvious difference between the two groups, can be explained in terms of the transitional status of the group. Although they do not have true leaves, the zosterophylls are known to bear various types of laterals. These range from unicellular to multicellular spines to multicellular teeth; the latter may have functioned to increase the photosynthetic surface of the stems. Generally, these appendages were randomly scattered over the stem surfaces, although in Crenaticaulis the large, toothlike outgrowths are arranged in two rows along the stems. In the putative early lycopsid Asteroxylon, traces extend through the cortex but do not enter the base of the enations. This tendency toward a definite arrangement and vascularization may constitute the initial stages in the subsequent evolution of the microphyll (Chapter 9).
The zosterophylls also demonstrate several stages in the evolution and organization of sporangia. These include forms such as Kaulangiophyton, in which sporangia are apparently helically arranged over the stem surface, to intermediate forms such as Gosslingia, in which the helically arranged sporangia are aggregated into definite spikes. A further modification might result in the arrangement seen in Rebuchia, where sporangia are organized into definite spikes, with the individual sporangia dorsiventral in arrangement (present on only one side of the axis). Niklas and Banks (1990) suggested that the zosterophylls can be separated into two groups based on the presence or absence of terminally located fertile units, as well as on the symmetry of the sporangial aggregation. These two groups can also be distinguished based on the presence of small flaps of tissue along the stems (enations), circinate tips on the axes, and, when preserved, the nature of the conducting strand. Based on their analysis, Niklas and Banks suggest that the lycopsids arose from a zosterophyllo-phyte-like group of plants, perhaps with a level of organization similar to Asteroxylon or Drepanophycus (Chapter 9).
Sporangial dehiscence within the zosterophyllophytes typically divides the sporangium into valves of equal or nearly equal size. As additional zosterophyllophyte taxa are described, a new perspective is emerging as to the diversity and spatial distribution of these interesting Paleozoic plants (Hao and Gensel, 2001; Kotyk et al., 2002; Wang and Hao, 2002). New taxa are providing an increasing data set of sporangial characters relating to dehiscence (e.g., in Crenaticaulis), where dehiscence results in two unequal valves. In addition, the discovery of new taxa is expanding our understanding of the diversity of sporangial aggregation, position and length of stalks, and pattern and orientation on the fertile axes. These characters may become more important as additional information on features of the fertile parts of these interesting plants become more fully known, and will no doubt result in an increased level of resolution relating to the phylogenetic position of these plants.
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