Occurrence of a particular group of organism depend on the strategy they adopt for survival and the ability to cope up with the existing environmental stress. The horizon is infested by single genus that means bio-diversity is less. Definitely the genus successfully adopted the environmental stress while the other genera could not. It is not clear whether in the upper horizon this particular is absent or not. Abrupt extinction of a group of organism is not uncommon.
Yes, I observed two different genera at two different horizons. There is no occurrence of lower hhorizon biota in upper and vice- versa. Random orientation in both horizon. Lower horizon fauna highly affected by weathering. Almost all show cast or fragile deterioted, lost hardness of shell and appear powdery at same places. At places show mold impressions. However, in upper horizon fauna well preserved in silly clay in full body from. Most of them have broken she'll form posterior side and appers as if they might have been prey for others, as both the valves are intact and attached as in living position.
Tarun Koley is correct; environmental constraints probably account for monospecific assemblages in most cases. As to the reason that one bed might have one genus and another bed a different genus, that may be due to extinction or evolution of the older organisms, but it may also be due to inherent patchiness of the assemblages. That is, if one species has achieved a dense population in an area, it may exclude other species from entering and competing. For example, if kudzu has already covered an area with its vines, then it may be difficult for other species of plants such as wisteria to colonize the area. Conversely, kudzu might find it difficult to colonize areas already covered with wisteria. This hypothesis predicts that if you investigate different sites at the same horizon, then they may have different monospecific assemblages ("patches").
All of the above may apply. Also freshwater and brackish environments frequently show low diversity and may be dominated by a single bivalve. Before thinking about ecology one must be able to discount; 1) diagenesis - for example loss of aragonite taxa might leave a monogeneric calcitic fauna, and 2) bottom conditions, for example strong currents may winnow out smaller and lighter taxa or conversely accumulate smaller and lighter taxa producing skewed death assemblages.
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