The working unit in most paleontological studies is one million years or, exceptionally, 100,000 years. Most neontological studies, however, last only a few years and very seldom extend over several decades. Changes taking place over millions of years and those taking place over just a few years are therefore well known. There is however a striking lack of information on changes taking place in the time span between those two extremes-that is, of the order of thousands and tens of thousands of years.

It is unfortunate that as a result of the postglacial marine transgression most of the marine sediments deposited in the interval are presently below sea level and are accessible to study only through material obtained by borings.

The southern part of the Boso Peninsula has been uplifted at least 20 to 30 m over the past 10,000 years and offers an excellent opportunity to study changes occurring over the time interval of several thousand years. Early Holocene sediments in the area were deposited during the Jomon (Flandrian) transgression; a period characterized by rapid sea level change, sedimentary facies shifts, and climatic variations. Changes in animal communities during this period of environmental fluctuation are of particular interest because of the availability of the time framework provided by C¹⁴ dating.

Because of their abundance in Holocene sediments from the southern part of Boso Peninsula, and the relative ease with which samples large enough for statistical analysis can be collected, ostracods were selected for this study.

Two aspects of Holocene geology of the area have already attracted attention for some time. One is the strikingly large diversity of hermatypic corals found in the uplifted sediments in the vicinity of Tateyama City (77 species of fossil corals in contrast with only 10 species found living in the present Tateyama Bay). It has been suggested that the climate optimum around 6,000 yr B.P. was responsible for the high diversity of the corals, and their decline has been related to the worsening climatic conditions around 4,000 yr B.P. The presumable effect of the climatic change on the associated molluscan fauna is much less pronounced (Nomura, 1932) and no substantial temperature changes were indicated by the foraminiferal faunas studied (Asano, 1936). The depth at which the corals lived and the reason of their irregular distribution throughout the area are still imperfectly understood. The study of the ostracod fauna within the larger time and space framework of the Holocene deposits in the entire southern Boso Peninsula might provide answers to some of these questions.

The second aspect of the Holocene geology of the area extensively studied in the past is the distribution and time of formation of the marine terraces. Because they are directly related to the relative sea level changes in the area, which were partly caused by tectonic movement, marine terraces offer important clues to the recent tectonic history of the area and activity of the Asian and Pacific crustal plates.

The elevation of the terraces is related to, but cannot be equated with, the position of sea level. The near absence of clear sea level indicators, such as peat intercalated with marine sediments, makes it difficult to determine the true position of sea level in spite of the abundant C¹⁴ datings available for the area. The distribution of ostracod assemblages is controlled by several factors, many of which are influenced by sea level variations; thus, changes in the ostracod assemblages may provide an indication of the relative sea level movement.

The bias introduced into the fossil record by differential preservation of its components is especially important in the case of microfossils, which are, due to their small size, particularly affected by destructive processes. The comparison of composition of the live and dead assemblages in the present Tateyama Bay and the fossil assemblages from similar environments in the Holocene sediments offer many insights into the changes that take place as a live assemblage is changed into a fossil assemblage.