Relationship between palynoflora and megaflora

Even though leaves and palynomorphs are derived from the same source vegetation, their taxonomic resolution and taphonomic pathways are significantly different, such that they present different views of the same landscape. The strengths of palynology are limitless specimen numbers in the appropriate matrix, their ability to be retrieved from tiny rock chips such as well-cuttings and core samples, their ability to be sampled at very fine intervals, and their ability to travel into and be preserved in marine sediments. The weaknesses of palynology include a propensity of specimens to be reworked into younger strata by geologic processes, poor preservation in coarse or oxidized sediments, and relatively coarse taxonomic resolution. Fossil leaves provide high-resolution taxonomic data, climate proxies from physiognomy, and plant-insect interaction data from leaf damage. In addition, they are unlikely to be reworked into younger strata in a manner that is not immediately recognizable. The disadvantages of leaf records as opposed to palynological records are the relative difficulty of collecting leaves in great numbers and that stratigraphic resolution is rarely finer that a decimeter (usually a meter or more). However, leaves seem to travel less than palynomorphs and thus seem to map more tightly onto specific depositional environments. However, the majority of pollen grains also fall near their source plant (Kershaw and Strickland 1990), so it is possible to reconstruct local assemblages from both pollen and leaves.

In combination, leaves and palynomorphs have the potential to re-create excellent data concerning the occurrence of ancient plant taxa. Despite this, one of the great unresolved challenges is the correlation of pollen and leaf taxa. There are four main reasons for this discordance: (1) differential preservation of specific pollen or leaf taxa, (2) differential rates of leaf production by different plant habits (e.g., large trees vs. small herbs), (3) differential production of pollen and spores (e.g., wind vs. insect dispersal), and (4) differential taxonomic resolution of leaves and palynomorphs. It is well known that certain plant families have pollen grains with differential preservation potential and the same can be said of leaves. The family Lauraceae generally has tough and easily preserved leaves but easily degraded pollen. In contrast, herbaceous plants rarely shed leaves but often produce well-preserved pollen. In addition, certain plant habits (trees, vines) are more likely to shed greater numbers of leaves than are others (herbs and small shrubs), thus biasing the leaf fossil record toward arboreal forms in most cases. Pollen is abundantly produced by wind-dispersed plants but much less prolifically by insect-pollinated plants. These complex biases create a situation where the pollen and leaves in a particular stratum may be sampling different plant species and plant habits. For the Upper Cretaceous and Paleogene, this pattern is made more complicated by the fact that many of the taxa represent unde-scribed extinct families and genera and many of the leaves and pollen are not attributable to extant taxa or to each other. For all of these reasons, palynol-ogists and paleobotanists have difficulty comparing their data and reaching conclusions based on integration of their data sets.

An example germane to this book is the calculation of extinction percentages based on leaves versus those based on palynomorphs. The closer sample spacing and large number of specimens per slide would seem to indicate that palynology would give a more accurate estimate of the severity of an extinction or extirpation. This appears not to be the case, however, because leaves have much higher taxonomic resolution. In thoroughly sampled sections, megafloral diversity exceeds palynofloral diversity despite a much smaller actual specimen count.

A corollary of this observation is that a palynological estimate of extinction severity will be less than the megafloral estimate.

Very few K-T boundary sections worldwide have benefited from joint mega-and microfloral investigation. Most of the case studies and sites discussed in this book consist solely of palynological data.

Was this article helpful?

0 0

Post a comment