Microfossil Abundance
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Only three beds yielded abundance data from the Lower Kellwasser. Interestingly, bed WCL-13, the same bed with abundant large, smooth leiospheres, also had the highest ratio of fossils per mg of macerate. Beds sampled below the Kellwasser horizon were not fossiliferous.
Similarly, bed WCU- -17, sampled below the Upper Kellwasser horizon, had low fossil abundance compared to other beds within and above the Upper Kellwasser horizon. All beds except WCU- -17 have similar ratios of fossils per mg macerate; however, when scaled to fossils per mg of rock, there is an increasing trend in fossil abundance that starts below the Upper Kellwasser horizon and continues into the overlying Dunkirk Shale.
Microfossil Morphologies
The Walnut Creek (LK
and UK) assemblages consist of entirely of leiospheres. All leiosphere
specimens collected from the Kellwasser horizons sampled in this work fall
under the umbrella of genus Leiosphaeridia, first described by Eisenack (1958) and modified by Downie and Sarjeant (1963). According to Lindgren (1981), Leiosphaeridia includes all fossil leiospheres that cannot be grouped with genus Tasmanites (an extinct genus classified
as belonging to the prasinophyte group of green algae). The vesicle walls of Tasmanites are characterized by radially
arranged pores, whereas the vesicle walls of Leiosphaeridia are without ultrastructure (Kjellström, 1968). Species within Leiosphaeridia can be further differentiated by internal vesicle
wall structure; however, such analysis is beyond the scope of this study. All Leiosphaeridia species remain classified
as acritarchs, meaning that they cannot be definitively grouped with a
phylogenetically true clade.
Within the assemblage of leiospheres collected, there is
some variation. Vesicle thickness, approximated by coloration under a
transmission light microscope, varies from thin (as in image K, Figure __)
to thick (as in image O, Figure __).
Vesicle texture varies from smooth (most specimens) to rough (as in images G,
L, and P, Figure__).
The leiospheres are sometimes perfectly round, ovular, or compressed in
appearance. Most leiospheres fall between 35 µm and 110 µm in diameter,
although some are much larger, reaching 200 µm in diameter. Larger leiospheres
more commonly have smooth, thin-walled vesicles (as in images A, B, D, and E, Figure 4‑1).
Most vesicles were whole, although some, especially among the larger
leiospheres, were fragmented. Split structures, as in image D (Figure__)
are sometimes present in vesicles of all sizes. In modern prasinophycean algae
(the group with which leiospheres are most often affiliated), similar split
structures on reproductive phycoma cells result from the release of offspring (Tappan, 1980).
Consequently, in fossil leiospheres these splits are often interpreted as
excystment or dehiscent structures.