Classification Of The Spore Of Leavigatosporites Sp

Figure 5.8 Diagram showing variations in iridium concentration (black dots) and fern-spore percentages (open diamonds) at the Starkville North K-T boundary locality in the Raton Basin, Colorado (from Tschudy et al. 1984). Reprinted by permission.

plants. The earliest visitors to the caldera found no plants living on it. A botanist arriving in 1886 found that some plants had returned and was impressed that most of them were ferns. After a few years, immigrant species from nearby islands evidently reestablished the former, diverse communities, although Richards noted that seeds, spores, and underground parts of plants could also have survived in ravines on the devastated island. Such areas could have served as refugia from which the communities were reestablished. Thus, the scenario suggested by the presence of fern-spore spikes just above the K-T boundary at widely separated locations contributed much to an understanding of the effects of the terminal Cretaceous event on plant communities.

Fleming and Nichols (1990) codified the nature of fern-spore spike assemblages at the K-T boundary and their differences from other palynological assemblages rich in fern spores found elsewhere in the stratigraphic record. They proposed that for an assemblage to qualify as a fern-spore abundance anomaly or spike, its composition must be 70-100% fern spores of a single species occurring within an interval 0-15 cm above the K-T boundary (Figure 5.9). Fleming and Nichols accepted Tschudy's interpretation of the significance of such assemblages - that they represent surviving species that ubiquitously colonized earliest Paleocene landscapes.

Robert Tschudy's original interpretation of the nature and significance of fern-spore spikes did not go unchallenged. Arthur Sweet of the Geological Survey of Canada and his colleagues at first questioned the existence of fernspore spikes because they observed none at K-T boundary localities in Canada. Eventually they detected fern-spore spikes at Canadian localities using microstratigraphic sampling and they acknowledged the occurrence of anomalous spore percentages, but disputed the interpretation of the fern spores as representing colonizing vegetation on a devastated landscape.

Sweet and Braman (1992) described occurrences and relative abundances of groups of various palynomorphs (pollen, spores, and algal cysts) through strati-graphic intervals at twelve Canadian K-T boundary localities. They reported geographic differences in which fern taxa established numerical dominance shortly after the extinction event, and they expanded the concept by identifying three species of angiosperms that substituted as pioneer species (they preferred to call them "opportunistic species'') in place of ferns at some localities (Figure 5.10). They asserted that the particular species (fern or angiosperm) that briefly assumed dominance in the local flora was determined by which species were prevalent in the local area just prior to the extinction event, which appeared to be related to paleolatitude. This is an interesting expansion of the concept of pioneer species flourishing after the extinction, but with regard to angiosperms - which reproduce via seeds instead of spores - it has less theoretical support from botany than does the original explanation of the phenomenon published by Tschudy et al. (1984). Nonetheless, the work of Sweet and Braman (1992) shows a pattern of paleolatitudinal variation in which species are the first to colonize the local landscape following the extinction event. Sweet and Braman noted that ferns tended to be the pioneer or "opportunistic" species in the southern part of the Western Interior region and a few species of angio-sperms assumed that role in the northern part.

A final viewpoint offered on post-extinction, fern-dominated plant communities requires comment. In their paper on the megaflora of the Raton Basin discussed in Section 7.2, Wolfe and Upchurch (1987a) tended to shift emphasis

Figure 5.9 Views of the fern-spore spike. a - low-power magnification, b - highpower magnification. All specimens are Cyathidites diaphana except the bean-shaped spore (Laevigatosporites sp.) at center right in the low-power view. Both the high abundance and low diversity of spores characteristic of the fern-spore spike are illustrated. Specimens of C. diaphana are 25-30 micrometers in diameter.

Figure 5.9 Views of the fern-spore spike. a - low-power magnification, b - highpower magnification. All specimens are Cyathidites diaphana except the bean-shaped spore (Laevigatosporites sp.) at center right in the low-power view. Both the high abundance and low diversity of spores characteristic of the fern-spore spike are illustrated. Specimens of C. diaphana are 25-30 micrometers in diameter.

Figure 5.10 Map showing latitudinal variation in dominant taxa of the fern-spore spikes and angiosperm-pollen-dominated assemblages just above the K-T boundary (from Sweet and Braman 1992). Reprinted by permission of Elsevier.

Figure 5.11 Fern spores. a - Cyathidites diaphana, b - Laevigatosporites sp., c - Stenochlaena tenuifolia. Cyathidites and Laevigatosporites characterize fern-spore spikes from New Mexico to Saskatchewan; spores of Stenochlaena bear no resemblance to either, despite its having been identified as the fern spike genus on the basis of megafossils by Wolfe and Upchurch (1987a). Illustration of Stenochlaena from Erdtman (1957). Specimens ''a'' and ''b'' 30 micrometers, ''c'' 45 micrometers.

Figure 5.11 Fern spores. a - Cyathidites diaphana, b - Laevigatosporites sp., c - Stenochlaena tenuifolia. Cyathidites and Laevigatosporites characterize fern-spore spikes from New Mexico to Saskatchewan; spores of Stenochlaena bear no resemblance to either, despite its having been identified as the fern spike genus on the basis of megafossils by Wolfe and Upchurch (1987a). Illustration of Stenochlaena from Erdtman (1957). Specimens ''a'' and ''b'' 30 micrometers, ''c'' 45 micrometers.

of the nature of the fern-spore spike interval toward a megafloral perspective. They reported a dominance of fern fronds at three Paleocene localities and referred to it as a "fern spike'' interval. They stated that the fronds they collected represent an extinct genus related to the extant fern genus Stenochlaena, a primary colonizer in the modern vegetation of Indomalaya and Africa. Palynological records from the fern-spore-spike interval do not support this interpretation, however appealing it might be. Two species of fern spores representing two different fern genera, Cyathidites and Laevigatosporites (Figure 5.11a, b), characterize the fern-spore spikes in the Raton Basin and elsewhere in North America, and neither of these genera includes species that resemble the distinctive morphology of spores of Stenochlaena (Figure 5.11c).

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