Quantitative Stratigraphy of Sedimentary Sequences
281
The deterministic algorithm by Rubel and Pak (1984) and the corresponding
computer programme DISTR (Pak 1989) have been used to estimate the total
ranges of the studied ostracode species and to arrange their appearances/disappearances into an ordinal time scale. This algorithm extends the local ranges of
each species, according to their observed coexistence with other species, which
in turn are arranged by their persistent mutual positions in all the studied sections. It should be emphasized that not all taxa studied can be arranged into a
single succession. This is due to lacking and contradictory relations between the
taxa in real data sets. This results, in practice, in the presence of multiple possible time scales for a single data set.
In the latest modification of the algorithm, preferences given to certain formal properties of the taxa (see Rubel and Sarv 1996) allow the formation of the
possible successions to be controlled. A succession can be constructed preferably of taxa which occur in a greater number of sections. As a rule, such a succession will contain comparatively fewer taxa, which have often relatively long
ranges. The scale formed by such long-ranging species is less detailed than the
scales created under different preset conditions. If preference is given to taxa
having less contradictory and uncertain relations to the others, the succession
usually contains more taxa with relatively short ranges, being arranged into
more numerous unitary associations. In this case, the average and particular frequencies of the taxa included into the scale are lower, which restricts the possibilities to locate the corresponding datum planes in sections.
7
Ostracode Successions
Aspects of evolution of ostracodes in the Ordovician of the East Baltic have been
discussed in several papers (Sarv 1968; Hints et a1. 1989; Meidla and Sarv 1990;
SidaraviCiene 1992). In the development of this group, the Oandu crisis is the
most remarkable event in the post-Tremadoc Ordovician, resulting in the formation of a uniform late Ordovician ostracode fauna (Meidla 1996).
In the present chapter, the results obtained with the DISTR programme under
different strategies, are discussed. The statistics on the results is given in Table 1.
The scale (succession) with the greatest number of taxa and datum planes includes only 38% of all taxa studied, while 54% of taxa have been included at least
into one of these successions. However, all three successions describe the temporal alteration of origins and extinctions of taxa in the same way - all have a remarkable change in their middle part. It is expressed, first of all, by a reflection
of the radiation of taxa, marking the beginning of the second period in the development of the biotic history of the Baltoscandian Palaeobasin and coinciding
with the Oanduan migration (Jaanusson 1976) or Oandu crisis (Kaljo et a1.1995
1996).
A principal chart of total ranges of ostracodes constructed by the algorithm
is shown in Fig. 4.The dynamics of appearances and disappearances of species,
in the three successions constructed, can be summarized in a diagram in Fig. 5.
281
The deterministic algorithm by Rubel and Pak (1984) and the corresponding
computer programme DISTR (Pak 1989) have been used to estimate the total
ranges of the studied ostracode species and to arrange their appearances/disappearances into an ordinal time scale. This algorithm extends the local ranges of
each species, according to their observed coexistence with other species, which
in turn are arranged by their persistent mutual positions in all the studied sections. It should be emphasized that not all taxa studied can be arranged into a
single succession. This is due to lacking and contradictory relations between the
taxa in real data sets. This results, in practice, in the presence of multiple possible time scales for a single data set.
In the latest modification of the algorithm, preferences given to certain formal properties of the taxa (see Rubel and Sarv 1996) allow the formation of the
possible successions to be controlled. A succession can be constructed preferably of taxa which occur in a greater number of sections. As a rule, such a succession will contain comparatively fewer taxa, which have often relatively long
ranges. The scale formed by such long-ranging species is less detailed than the
scales created under different preset conditions. If preference is given to taxa
having less contradictory and uncertain relations to the others, the succession
usually contains more taxa with relatively short ranges, being arranged into
more numerous unitary associations. In this case, the average and particular frequencies of the taxa included into the scale are lower, which restricts the possibilities to locate the corresponding datum planes in sections.
7
Ostracode Successions
Aspects of evolution of ostracodes in the Ordovician of the East Baltic have been
discussed in several papers (Sarv 1968; Hints et a1. 1989; Meidla and Sarv 1990;
SidaraviCiene 1992). In the development of this group, the Oandu crisis is the
most remarkable event in the post-Tremadoc Ordovician, resulting in the formation of a uniform late Ordovician ostracode fauna (Meidla 1996).
In the present chapter, the results obtained with the DISTR programme under
different strategies, are discussed. The statistics on the results is given in Table 1.
The scale (succession) with the greatest number of taxa and datum planes includes only 38% of all taxa studied, while 54% of taxa have been included at least
into one of these successions. However, all three successions describe the temporal alteration of origins and extinctions of taxa in the same way - all have a remarkable change in their middle part. It is expressed, first of all, by a reflection
of the radiation of taxa, marking the beginning of the second period in the development of the biotic history of the Baltoscandian Palaeobasin and coinciding
with the Oanduan migration (Jaanusson 1976) or Oandu crisis (Kaljo et a1.1995
1996).
A principal chart of total ranges of ostracodes constructed by the algorithm
is shown in Fig. 4.The dynamics of appearances and disappearances of species,
in the three successions constructed, can be summarized in a diagram in Fig. 5.