be the beginning of a large research program. METHODS For our measure of biostratigraphic continuity, we have chosen to define it at the finest scale possible from PBDB data. This is because finer-scale data typically leads to clearer understanding, and more convincing conclusions. Future research can use coarser-scale data in those cases where the finest scale data are unavailable or unsatisfactorily rare. Biostratigraphic continuity is indicated when a particular taxon is found in two successive stratigraphic units within a particular geographic area. Thus biostratigraphic continuity uses taxonomic, stratigraphic, and geographic data. Since the finest-scale taxonomic identification of the greatest majority of PBDB fossils is the species, we use a species-level biostratigraphic continuity measure. Since the stage is the finest-scale global stratigraphic unit reported for the vast majority of Phanerozoic PBDB fossils, we also use a stage-level stratigraphic continuity measure in the Phanerozoic. In the case of Proterozoic and Archean fossils, the PBDB’s finest global stratigraphic resolution is the system and erathem, respectively. Thus the stratigraphic scales we use in the Archean and Proterozoic are the erathem and system, respectively. Requiring a taxon to be found in both the stratigraphic unit immediately below and above the boundary is a further refinement of scale for it omits taxa that are not found right up against the boundary. Finally, the PBDB reports fossil location with GPS coordinates and an estimate of GPS precision. Since the precision is most often given in qualitative terms (e.g., ‘minutes’ or ‘seconds’), an ‘average’ precision can only be estimated roughly, but it seems to be between 1 and 2 degrees of latitude and longitude. Thus, we use a biostratigraphic continuity measure of two degrees latitude and longitude. Our paleontological continuity measure, then, for the Phanerozoic is the number of species reported in both the stage below and the stage above a particular stage-stage boundary within 2 degrees longitude and latitude. We use the same measure for Archean and Proterozoic fossils, substituting (in place of stage) erathem and system, respectively. We call this the Number of Local, Stage boundary-Straddling Species (NLSSS). We determined the NLSSS for the 100 stage-stage boundaries of the Phanerozoic, the 10 system-system boundaries of the Proterozoic, and the 4 erathem-erathem boundaries of the Archean. For each boundary, we downloaded two files from the Paleobiology Database (PBDB; paleobiodb.org)—one for all the taxa reported in the global stratigraphic unit immediately below the boundary and one for all the taxa reported in the global stratigraphic unit immediately above the boundary. Each of these files was processed to the taxonomic level of species by deleting all records identified less precisely than the species level, and considering only at the species level those records identified more precisely than the species level. Each file was also processed to the proper stratigraphic level by deleting all records that located the fossil less precisely than the Phanerozoic stage (or Proterozoic system or Archean erathem). Some of the biostratigraphic data of the PBDB is given in regionally-defined stages, as opposed to globally-defined stages. Desiring to include as much data as possible, we accepted a regional stage boundary as equivalent to a globally-defined stage boundary when the radiometric ages of those boundaries was less than two million radiometric years apart. Given the fact that radiometric years are most probably an extreme exaggeration of real years, a difference of two million radiometric years is here considered small enough to suggest they are more or less simultaneous boundaries, even though they are found at different places on the earth. The average stage length through the Phanerozoic was 10.4 million radiometric years. The two million years was an arbitrary value less than one quarter that average stage length. We set this criterion less in the case of boundaries atop stages less than two million radiometric years long. For the two uppermost stage/stage boundaries, this criterion was set to zero, and for the remaining short stages, this criterion was set to one million radiometric years. Then, for each of the 114 globally-defined stratigraphic boundaries, the two files (the file with species below the boundary and the file with species above the boundary) were combined into a single file. From that file, all records were deleted for species found only on one side of the boundary. Then, for each species in that file, all records were deleted that were located more than two degrees latitude or longitude (greater than about 200-300 miles) from all other occurrences on the opposite side of the boundary. The records remaining in this file, then, are all records of species found on both sides of the boundary within two degrees longitude and latitude. The count of how many different species are found in this final file is the NLSSS for this stratigraphic boundary. RESULTS The results are given in Table 1. For brevity in referring to particular stratigraphic boundaries, each of the 114 stratigraphic boundaries was assigned a number from 1 for the oldest boundary to 114 for the most recent boundary (column A). For each boundary, the number of different species reported in the PBDB from the globally-defined stratigraphic unit below the boundary is given in column B, and the NLSSS for that boundary is given in column C. The percentage of species in the stratigraphic unit below that boundary that straddle the boundary is given in column F (as %NLSSS). Columns C and F are graphed in Figure 1. The results of our analysis are robust with respect to the most arbitrary decisions we made in our analysis. First, to include as many local biostratigraphic units as possible, we equated them with the closest global stratigraphic unit if their radiometric ages differed by as much as two million years. The results were not substantially changed by reducing that difference to one million radiometric years. Second, we assumed fossils were reported from the same location if they were reported in the PBDB within 2 degrees longitude or latitude. The results were not substantially changed by using 1, 3, or 4 degrees of longitude and latitude. A visual (qualitative) inspection of Figure 1 suggests five different biostratigraphic zones, as defined by NLSSS pattern: The Precambrian-Cambrian zone (boundaries 1-25) is characterized by very low to zero NLSSS values. This zone includes all the Archean erathem boundaries (1-5), all the Proterozoic system boundaries (5-15), and all the Cambrian stage boundaries (15-25). The only non-zero NLSSS values are boundaries 9-10, 15-16, and 21 (PBDB-reported radiometric year ages of 1800-1400, 541-529, and 500.5, respectively). The Ordovician-Mississippian zone (boundaries 25-49) is characterWISE and RICHARDSON Biostratigraphic continuity and earth history 2023 ICC 612
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