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Analysis of El Kef blind test I
ELSEVIER
Marine Micropaleontology 29 (1997) 6% 103
MicroNotes
Analysis of El Kef blind test I Gerta Keller Geological and Geophysical Sciences, Princeton University, Princeton, NJ 08544-1003,
USA
Received 12 April 1996; accepted I2 June 1996
The blind sample test was designed to determine whether the observed species extinction pattern across the K/T boundary supports Smit’s (1982, 1990) scenario of all but one species suddenly extinct at the K/T boundary, or Keller’s (1988b) scenario of gradual extinctions with some species disappearing below and the majority at the K/T boundary with l/3 ranging into the Danian. The blind test can only resolve the controversy regarding the observed pattern of extinction, and not the controversy regarding the interpretation of this pattern. The Smit and Keller extinction models are shown in Figs. 11 and 12. Since Smit has not published a complete census list of Cretaceous taxa, his model is illustrated in Fig. 11 without species names. Keller’s (1988b) data are shown in Fig. 12 for six stratigraphic levels that are equivalent to the six blind test samples. The four testers were asked to collect species census data from the >63 pm size fraction, provide relative abundances of each species and the benthic/ planktic ratio based on population counts of 300 to 400 individuals in a random sample split. Unfortunately, not all testers used the same data gathering methods and as a result the relative species abundances and benthic/planktic ratio data differ by more than one magnitude. These data are excluded from this analysis. The blind sample test therefore rests upon taxic census data only. Taxic census data, however, have their own problems. They essentially vary from tester to tester based on taxonomic concepts.
This may result in different species names used for the same morphotypes among the four testers, or in several different species names given to morphotypes that some testers consider to be morphologic variants of the same species. The degree to which different taxonomic concepts influenced the taxic census data is seen by the number of species identified and by the common species names used. In Maastrichtian samples, the number of species identified by each of the blind testers are: Canudo 47, Olsson 45, Masters 52, and Orueetxebarria 59; in similar samples, Keller (1988b) identified 51 species. All four testers used the same species names for 14 to 16 species and three testers used the same species names for 10 to 16 species. Taxonomic agreement for the remaining species is low. This illustrates the fact that taxic census data of the four testers cannot be compared on a species by species basis. Even if the same species names are used, there is no guarantee that all testers applied that species name to the same morphotype. But, we can be reasonably sure that each tester applied each species name to a distinct and different morphotype. For these reasons, comparison of the patterns of extinction of all taxa is more instructive than comparing extinction of species by species using the same species names. There is no way taxonomic differences between testers can be identified and isolated in the data sets without getting them all together to sort out their taxonomic differences. But, doing so would defeat the
0377~8398/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. SSDI 0377-8398(96)00044-8
El Kef blind test/Marine
90
M
Micropaleontology
29 (1997) 65-103
Smit (1982,199O)
. 2-h
_. . - . ,.
.
_.
a1; p” 50” . .._..__..._.~“..._.... . _ o___ .._._.., _.* d T5 1
.. I
. -...__.. _ ,“#Y.
.]
.
.
..._._..... . ..I.._........_.....
_....... ...._
-i b-,
Fig. 11. Species extinction pattern of Smit (1982, 1990) showing the sudden extinction of all but one species at the K/T boundary. Note that Smit has not published species census data in support of his extinction pattern, therefore no species names can be listed.
purpose of the blind test which was to get a spectrum of outside views to compare and contrast with &nit’s (1982, 1990) and Keller’s (1988b) studies. The degree of variation between the testers is in itself an important and real variable of differences between workers and therefore must be preserved, if the test is to approximate the spectrum of opinions. The real test will be whether a common extinction pattern
emerges from these analyses. If so, then individual variations in taxonomic concepts are less relevant. If no common extinction pattern emerges, then taxonomic differences override the actual data. In this section, patterns of extinction, irrespective of species names, are discussed first, followed by analysis of the statistical similarity between the stratigraphic position of species identified by each tester.
Keller (1988)
2% I
..“3!
,...
.._..” ._...., _..” _ ..,.. _._ ....._...._. 6% .
. . .. .. . . . . . . . . . . . .
...”.......................... ......
Fig. 12. Species extinction pattern of Keller (1988b) at El Kef based on six sample intervals that are equivalent to the blind sample test.
El Kef blind test/Marine Micropaleontology 29 (1997) 65-103
1. Species extinction patterns
91
not published a species census list, we assume the best case scenario for his extinction pattern, namely that he found all the common Cretaceous taxa ranging up to the K/T boundary. Table 4 shows that the rank order correlations among the testers ranges from a high of 0.748 between Orue-etxebania and Canudo to a low of 0.301 between Masters and Canudo, with a mean value of 0.478 between all four blind test testers. In comparison, the mean correlation value between each blind tester and Keller’s (1988b) original data is 0.399. In contrast, the mean correlation value between each blind tester and Smit’s extinction scenario is only 0.272 and significantly lower than Keller’s (0.399) mean value. This means that the four testers’ results are similar to Keller’s (1988b) data and that none of them could reproduce Smit’s extinction pattern.
The species extinction patterns of the four testers are shown in Figs. 4, 5, 6 and 10. All four testers observed varying numbers of species disappearing below the K/T boundary, near or at the K/T boundary and ranging into the earliest Tertiary. None of the testers observed the simultaneous disappearance of all but one Cretaceous species at the K/T boundary. Canudo’s and Masters’ extinction patterns are very similar, showing 18% and 23% of the species disappearing at 75 cm and 15-20 cm below the K/T boundary, 42% at or near the K/T boundary, and 40% and 35% ranging above the boundary, respectively (Figs. 4 and 5). This extinction pattern is very similar to that of Keller (1988b), who shows 11% disappearing below the K/T boundary, 45% at or near the K/T boundary, and 44% ranging above the boundary (Fig. 2). Both Olsson and Orue-etxebarria also show 42% of the Cretaceous taxa ranging above the K/T boundary (Figs. 6 and 10). Their extinction patterns differ only below the K/T boundary, where they show nearly all of the remaining 58% of the species ranging up to the K/T boundary. Despite this difference, all four testers’ extinction patterns are much more similar to Keller’s (1988b) extinction pattern than to Smit’s (compare with Figs. 11 and 12). The level of similarity between the extinction patterns of the testers and those of Smit (1982, 1990) and Keller (1988b) can be determined based on Spear-man rank order correlations among the stratigraphic positions of common taxa (i.e., taxa identified by at least two testers) (Table 4). Since Smit has
2. Discussion The four blind testers’ trans-K/T boundary extinction patterns are remarkably similar, although significant differences remain, particularly in the number of species disappearing below the K/T boundary and in the biostratigraphic ranges of specific taxa. These differences are likely due to varying processing techniques, the size fraction analyzed, the amount of time spent searching for rare taxa and the taxonomic concepts used. For instance, although testers were asked to compile their data on the >63 pm size fraction, two of them analyzed the smaller (>38 pm) size fraction. The amount of time spent searching the entire washed sample residue for the presence
Table 4 Spearman rank order correlations among positions of common taxa (e.g., taxa identified by at least two testers) in various El Kef biostratigraphies Investigator
Canudo
Masters
Olsson
Canudo Masters Olsson Orue-etxebarria
O-e
0.301 0.522 0.748
1.000 0.489 0.337
1.000 0.473
1.oOO
Keller (1988) Smit (1982)
0.441 0.295
0.308 0.274
0.460 0.295
0.386 0.225
Summary statistics
Mean correlation among blind test participants Mean correlation with Keller (1988): Mean correlation with Smit:
Keller (1988)
Smit (1982)
1.000 0.225
1.oocl
l.ooo
0.478 0.399 0.272
92
El Kef blind test/Marine Micropaleontology 29 (1997) 65-103
of rare species also significantly affects the results. In routine quantitative faunal studies, once the random split of 300-400 specimens has been picked and identified to species level, the remaining sample residue is searched for about 20-30 minutes for rare taxa which are not represented in the random sample split. In this search, species which are extremely rare (e.g., one or two specimens per entire sample residue) may not be found. Two testers reported several hours searching for each rare species. This may account for the difference in the number of species disappearing below the K/T boundary. Furthermore, over one-third of the last appearances of species are single specimen occurrences which raises the question of reworking. How confident can we be in calling the presence of one single isolated specimen the last living representative of that species? What is the likelihood that this one isolated specimen is reworked? How likely is it that the species survived longer (Signor-Lipps effect)? Calculated confidence intervals for these rare and sporadically occurring species span several meters into the early Paleocene. There seems to be no way that the precise extinction horizon of any rare and sporadically occurring species can be determined with confidence. In standard biostratigraphy, this problem is circumvented by using the last continuous occurrence of a species as its extinction datum. This practice generally avoids the problem of using potentially reworked specimens as biostratigraphic markers. However, the major source of variations between the testers and Keller (1988b) is the use of differing taxonomic concepts. Similar variations in the use of taxonomic concepts are likely present among all microfossil workers. The purpose of the blind test was not to evaluate these variations among the four testers, but to observe whether an extinction pattern similar to Smit’s or Keller’s can be reproduced in spite of them. In this task, the blind sample test has succeeded remarkably well with an extinction pattern similar to Keller’s and very unlike Smit’s. Even on the basis of taxonomy, each of the four testers’ taxonomic concepts are similar to Keller’s despite the 1980’s taxonomic concepts used by Keller as compared to the updated 1990’s concepts used by the testers. This is indicated by the high rank order correlation values of each tester with Keller’s data: Canudo 0.449, Masters 0.308, Olsson 0.460,
and Orue-etxebarria 0.386 (Table 4). In contrast, rank order correlation values with Smit’s best case taxonomy are very low (0.295 Canudo, 0.274 Masters, 0.295 Olsson, 0.225 Orue-etxebarria) and hence not reproducible (Table 4). These results show that Keller’s gradual extinction pattern is more strongly supported than Smit’s sudden extinction pattern by the four testers. Are the observed extinction patterns real or an artifact of sample size or reworking? The blind sample test was not designed to evaluate interpretations of the mass extinction patterns. Hence, it provides no support for either Smit’s sudden or Keller’s more gradual interpretation of the mass extinction tempo. It cannot determine whether the SignorLipps (1982) effect accounts for the pre-K/T boundary species disappearances and reworking accounts for the presence of Cretaceous species above the K/T boundary. A species last appearance is not necessarily its extinction; it could represent a local disappearance, or it may be too rare to be present in the sample size used (e.g., Signor-Lipps effect). An observed gradual extinction pattern may therefore differ from the actual extinction pattern. This principal is often misused by claiming that due to the Signor-Lipps effect, a gradual extinction pattern in reality masks a sudden catastrophic extinction. However, there is as little support for such an interpretation as for assuming that the last occurrence of a species necessarily represents its extinction. The true last occurrence of each species is likely to be random and not uniform up to an imaginary extinction horizon. The interval in which the true last occurrence of a species should be found can be estimated from stratigraphic confidence intervals. MacLeod (1996) has calculated confidence intervals based on the species range data of Keller at El Kef for the last 10 m of the Maastrichtian and early Danian (using 95% one-sided limits based on the method of Strauss and Sadler, 1989). The 95% confidence limits for all species exceeds the last observed occurrence by one to several meters. However, since the entire range of each species was not included, these confidence limits are gross overestimates of the true confidence intervals. L. Li (pers. commun., 1994) subsequently calculated confidence limits based on the total range of species which first appear during the Maastrichtian. The results are significantly bet-
El Kef blindrest/MarineMicropaleontology 29 (1997) 65-103
ter with smaller (1 to 3 m) confidence intervals for species which are consistently present in samples throughout their stratigraphic ranges. However, for species which are rare and only sporadically present through the upper part of their range, confidence limits still exceed the last observed occurrence by 5 to 20 m. Since it is these rare and sporadically occurring species that disappeared below the K/T boundary, confidence limits provide no information as to their true extinction horizon. In fact, the broad confidence limits provide no support for interpreting the observed gradual extinction pattern as a shortterm catastrophic event. Are all but one or at most three of the Cretaceous species present above the K/T boundary due to reworking as suggested by Smit (1982, 1990), Liu and Olsson (1992) and Olsson and Liu (1993)? The assumption of survivorship of only one to three species is based on the bolide impact theory that predicts a catastrophic extinction pattern. Positive evidence for reworking is largely limited to the preservational state of Cretaceous foraminifers, whether they are abraded, discolored, or dissolution-resistant. Although some reworked species are present in any stratigraphic sequence, no systematic differences in preservational state have been observed between Maastrichtian and Danian occurrences of Cretaceous planktic foraminifers (Keller, 1988b, 198913;Liu and Olsson, 1992; MacLeod and Keller, 1994; MacLeod, in press). Thus, there is evidence for the presence of some reworked species in Danian sediments. But, there is no evidence to support the conclusion that all but one to at most three of the Cretaceous species consistently present in Danian sediments are reworked. Cretaceous species survivorship can be tested independently based on S13C isotopic values of foraminiferal tests, comparative morphology and comparative biogeography of species present in early Tertiary (Danian) sediments. Carbon-13 isotopic values of low latitude planktic foraminifera that lived in Danian sediments are 2 to 3 permil lighter than those that lived during the late Maastrichtian. Measurements of S13C values of a number of Cretaceous species present in Danian sediments show Danian isotopic signals (e.g., Heterohelix globulosa, H. striata, H. navarroensis, Guembelitria cretacea, G. trifolia, G. danica, Globigemelloides aspera, Chiloguembelina waiparaensis) and are thus
93
K/T boundary survivors (Barrera and Keller, 1990; Keller, 1993; Keller et al., 1993). Additional support of Cretaceous species survivorship is seen in comparative morphology and biogeography studies of K/T boundary sequences worldwide (MacLeod and Keller, 1991, 1994). These studies show consistent trends among groups of species with similar morphologies, and consistent trends in biostratigraphic ranges of Cretaceous species in the early Tertiary. Such global biogeographic trends cannot be explained by random reworking; they strongly support Cretaceous species survivorship. 3. Conclusions (1) The blind test was designed to evaluate the pattern of the K/T boundary mass extinction at the
El Kef stratotype, whether it was sudden or gradual. This test cannot evaluate between competing interpretations of the extinction pattern. (2) The blind test results of the four testers are influenced by sample processing methods, size fraction analyzed, amount of time spent searching for rare species, and differing taxonomic concepts used. All four testers used the same species names for only 14 to 16 species and three testers used the same species names for another 10 to 16 species out of a total of 45 to 59 species identified. This relatively low taxonomic agreement among the testers illustrates the fact that taxic census data cannot be compared on a species by species basis. Even if the same species names are used, there is no guarantee that all four testers applied the same species name to the same morphotype. (3) Despite the differing taxonomic concepts used by the four testers, the extinction patterns are remarkably similar with varying abundances disappearing below the K/T boundary, at or near the K/T boundary and ranging into the earliest Tertiary. Spearman rank order correlation of taxa identified by two or more blind test testers shows a mean value of 0.478. In contrast, the mean correlation value between each tester and Keller’s (1988b) original data is 0.399, and that of Smit’s (1982, 1990) is 0.272. This means that each of the extinction patterns of each of the testers is similar to Keller’s gradual extinction pattern and that Smit’s sudden extinction pattern could not be corroborated.
Marine Micropaleontology 29 (1997) 6% 103
MicroNotes
Analysis of El Kef blind test I Gerta Keller Geological and Geophysical Sciences, Princeton University, Princeton, NJ 08544-1003,
USA
Received 12 April 1996; accepted I2 June 1996
The blind sample test was designed to determine whether the observed species extinction pattern across the K/T boundary supports Smit’s (1982, 1990) scenario of all but one species suddenly extinct at the K/T boundary, or Keller’s (1988b) scenario of gradual extinctions with some species disappearing below and the majority at the K/T boundary with l/3 ranging into the Danian. The blind test can only resolve the controversy regarding the observed pattern of extinction, and not the controversy regarding the interpretation of this pattern. The Smit and Keller extinction models are shown in Figs. 11 and 12. Since Smit has not published a complete census list of Cretaceous taxa, his model is illustrated in Fig. 11 without species names. Keller’s (1988b) data are shown in Fig. 12 for six stratigraphic levels that are equivalent to the six blind test samples. The four testers were asked to collect species census data from the >63 pm size fraction, provide relative abundances of each species and the benthic/ planktic ratio based on population counts of 300 to 400 individuals in a random sample split. Unfortunately, not all testers used the same data gathering methods and as a result the relative species abundances and benthic/planktic ratio data differ by more than one magnitude. These data are excluded from this analysis. The blind sample test therefore rests upon taxic census data only. Taxic census data, however, have their own problems. They essentially vary from tester to tester based on taxonomic concepts.
This may result in different species names used for the same morphotypes among the four testers, or in several different species names given to morphotypes that some testers consider to be morphologic variants of the same species. The degree to which different taxonomic concepts influenced the taxic census data is seen by the number of species identified and by the common species names used. In Maastrichtian samples, the number of species identified by each of the blind testers are: Canudo 47, Olsson 45, Masters 52, and Orueetxebarria 59; in similar samples, Keller (1988b) identified 51 species. All four testers used the same species names for 14 to 16 species and three testers used the same species names for 10 to 16 species. Taxonomic agreement for the remaining species is low. This illustrates the fact that taxic census data of the four testers cannot be compared on a species by species basis. Even if the same species names are used, there is no guarantee that all testers applied that species name to the same morphotype. But, we can be reasonably sure that each tester applied each species name to a distinct and different morphotype. For these reasons, comparison of the patterns of extinction of all taxa is more instructive than comparing extinction of species by species using the same species names. There is no way taxonomic differences between testers can be identified and isolated in the data sets without getting them all together to sort out their taxonomic differences. But, doing so would defeat the
0377~8398/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. SSDI 0377-8398(96)00044-8
El Kef blind test/Marine
90
M
Micropaleontology
29 (1997) 65-103
Smit (1982,199O)
. 2-h
_. . - . ,.
.
_.
a1; p” 50” . .._..__..._.~“..._.... . _ o___ .._._.., _.* d T5 1
.. I
. -...__.. _ ,“#Y.
.]
.
.
..._._..... . ..I.._........_.....
_....... ...._
-i b-,
Fig. 11. Species extinction pattern of Smit (1982, 1990) showing the sudden extinction of all but one species at the K/T boundary. Note that Smit has not published species census data in support of his extinction pattern, therefore no species names can be listed.
purpose of the blind test which was to get a spectrum of outside views to compare and contrast with &nit’s (1982, 1990) and Keller’s (1988b) studies. The degree of variation between the testers is in itself an important and real variable of differences between workers and therefore must be preserved, if the test is to approximate the spectrum of opinions. The real test will be whether a common extinction pattern
emerges from these analyses. If so, then individual variations in taxonomic concepts are less relevant. If no common extinction pattern emerges, then taxonomic differences override the actual data. In this section, patterns of extinction, irrespective of species names, are discussed first, followed by analysis of the statistical similarity between the stratigraphic position of species identified by each tester.
Keller (1988)
2% I
..“3!
,...
.._..” ._...., _..” _ ..,.. _._ ....._...._. 6% .
. . .. .. . . . . . . . . . . . .
...”.......................... ......
Fig. 12. Species extinction pattern of Keller (1988b) at El Kef based on six sample intervals that are equivalent to the blind sample test.
El Kef blind test/Marine Micropaleontology 29 (1997) 65-103
1. Species extinction patterns
91
not published a species census list, we assume the best case scenario for his extinction pattern, namely that he found all the common Cretaceous taxa ranging up to the K/T boundary. Table 4 shows that the rank order correlations among the testers ranges from a high of 0.748 between Orue-etxebania and Canudo to a low of 0.301 between Masters and Canudo, with a mean value of 0.478 between all four blind test testers. In comparison, the mean correlation value between each blind tester and Keller’s (1988b) original data is 0.399. In contrast, the mean correlation value between each blind tester and Smit’s extinction scenario is only 0.272 and significantly lower than Keller’s (0.399) mean value. This means that the four testers’ results are similar to Keller’s (1988b) data and that none of them could reproduce Smit’s extinction pattern.
The species extinction patterns of the four testers are shown in Figs. 4, 5, 6 and 10. All four testers observed varying numbers of species disappearing below the K/T boundary, near or at the K/T boundary and ranging into the earliest Tertiary. None of the testers observed the simultaneous disappearance of all but one Cretaceous species at the K/T boundary. Canudo’s and Masters’ extinction patterns are very similar, showing 18% and 23% of the species disappearing at 75 cm and 15-20 cm below the K/T boundary, 42% at or near the K/T boundary, and 40% and 35% ranging above the boundary, respectively (Figs. 4 and 5). This extinction pattern is very similar to that of Keller (1988b), who shows 11% disappearing below the K/T boundary, 45% at or near the K/T boundary, and 44% ranging above the boundary (Fig. 2). Both Olsson and Orue-etxebarria also show 42% of the Cretaceous taxa ranging above the K/T boundary (Figs. 6 and 10). Their extinction patterns differ only below the K/T boundary, where they show nearly all of the remaining 58% of the species ranging up to the K/T boundary. Despite this difference, all four testers’ extinction patterns are much more similar to Keller’s (1988b) extinction pattern than to Smit’s (compare with Figs. 11 and 12). The level of similarity between the extinction patterns of the testers and those of Smit (1982, 1990) and Keller (1988b) can be determined based on Spear-man rank order correlations among the stratigraphic positions of common taxa (i.e., taxa identified by at least two testers) (Table 4). Since Smit has
2. Discussion The four blind testers’ trans-K/T boundary extinction patterns are remarkably similar, although significant differences remain, particularly in the number of species disappearing below the K/T boundary and in the biostratigraphic ranges of specific taxa. These differences are likely due to varying processing techniques, the size fraction analyzed, the amount of time spent searching for rare taxa and the taxonomic concepts used. For instance, although testers were asked to compile their data on the >63 pm size fraction, two of them analyzed the smaller (>38 pm) size fraction. The amount of time spent searching the entire washed sample residue for the presence
Table 4 Spearman rank order correlations among positions of common taxa (e.g., taxa identified by at least two testers) in various El Kef biostratigraphies Investigator
Canudo
Masters
Olsson
Canudo Masters Olsson Orue-etxebarria
O-e
0.301 0.522 0.748
1.000 0.489 0.337
1.000 0.473
1.oOO
Keller (1988) Smit (1982)
0.441 0.295
0.308 0.274
0.460 0.295
0.386 0.225
Summary statistics
Mean correlation among blind test participants Mean correlation with Keller (1988): Mean correlation with Smit:
Keller (1988)
Smit (1982)
1.000 0.225
1.oocl
l.ooo
0.478 0.399 0.272
92
El Kef blind test/Marine Micropaleontology 29 (1997) 65-103
of rare species also significantly affects the results. In routine quantitative faunal studies, once the random split of 300-400 specimens has been picked and identified to species level, the remaining sample residue is searched for about 20-30 minutes for rare taxa which are not represented in the random sample split. In this search, species which are extremely rare (e.g., one or two specimens per entire sample residue) may not be found. Two testers reported several hours searching for each rare species. This may account for the difference in the number of species disappearing below the K/T boundary. Furthermore, over one-third of the last appearances of species are single specimen occurrences which raises the question of reworking. How confident can we be in calling the presence of one single isolated specimen the last living representative of that species? What is the likelihood that this one isolated specimen is reworked? How likely is it that the species survived longer (Signor-Lipps effect)? Calculated confidence intervals for these rare and sporadically occurring species span several meters into the early Paleocene. There seems to be no way that the precise extinction horizon of any rare and sporadically occurring species can be determined with confidence. In standard biostratigraphy, this problem is circumvented by using the last continuous occurrence of a species as its extinction datum. This practice generally avoids the problem of using potentially reworked specimens as biostratigraphic markers. However, the major source of variations between the testers and Keller (1988b) is the use of differing taxonomic concepts. Similar variations in the use of taxonomic concepts are likely present among all microfossil workers. The purpose of the blind test was not to evaluate these variations among the four testers, but to observe whether an extinction pattern similar to Smit’s or Keller’s can be reproduced in spite of them. In this task, the blind sample test has succeeded remarkably well with an extinction pattern similar to Keller’s and very unlike Smit’s. Even on the basis of taxonomy, each of the four testers’ taxonomic concepts are similar to Keller’s despite the 1980’s taxonomic concepts used by Keller as compared to the updated 1990’s concepts used by the testers. This is indicated by the high rank order correlation values of each tester with Keller’s data: Canudo 0.449, Masters 0.308, Olsson 0.460,
and Orue-etxebarria 0.386 (Table 4). In contrast, rank order correlation values with Smit’s best case taxonomy are very low (0.295 Canudo, 0.274 Masters, 0.295 Olsson, 0.225 Orue-etxebarria) and hence not reproducible (Table 4). These results show that Keller’s gradual extinction pattern is more strongly supported than Smit’s sudden extinction pattern by the four testers. Are the observed extinction patterns real or an artifact of sample size or reworking? The blind sample test was not designed to evaluate interpretations of the mass extinction patterns. Hence, it provides no support for either Smit’s sudden or Keller’s more gradual interpretation of the mass extinction tempo. It cannot determine whether the SignorLipps (1982) effect accounts for the pre-K/T boundary species disappearances and reworking accounts for the presence of Cretaceous species above the K/T boundary. A species last appearance is not necessarily its extinction; it could represent a local disappearance, or it may be too rare to be present in the sample size used (e.g., Signor-Lipps effect). An observed gradual extinction pattern may therefore differ from the actual extinction pattern. This principal is often misused by claiming that due to the Signor-Lipps effect, a gradual extinction pattern in reality masks a sudden catastrophic extinction. However, there is as little support for such an interpretation as for assuming that the last occurrence of a species necessarily represents its extinction. The true last occurrence of each species is likely to be random and not uniform up to an imaginary extinction horizon. The interval in which the true last occurrence of a species should be found can be estimated from stratigraphic confidence intervals. MacLeod (1996) has calculated confidence intervals based on the species range data of Keller at El Kef for the last 10 m of the Maastrichtian and early Danian (using 95% one-sided limits based on the method of Strauss and Sadler, 1989). The 95% confidence limits for all species exceeds the last observed occurrence by one to several meters. However, since the entire range of each species was not included, these confidence limits are gross overestimates of the true confidence intervals. L. Li (pers. commun., 1994) subsequently calculated confidence limits based on the total range of species which first appear during the Maastrichtian. The results are significantly bet-
El Kef blindrest/MarineMicropaleontology 29 (1997) 65-103
ter with smaller (1 to 3 m) confidence intervals for species which are consistently present in samples throughout their stratigraphic ranges. However, for species which are rare and only sporadically present through the upper part of their range, confidence limits still exceed the last observed occurrence by 5 to 20 m. Since it is these rare and sporadically occurring species that disappeared below the K/T boundary, confidence limits provide no information as to their true extinction horizon. In fact, the broad confidence limits provide no support for interpreting the observed gradual extinction pattern as a shortterm catastrophic event. Are all but one or at most three of the Cretaceous species present above the K/T boundary due to reworking as suggested by Smit (1982, 1990), Liu and Olsson (1992) and Olsson and Liu (1993)? The assumption of survivorship of only one to three species is based on the bolide impact theory that predicts a catastrophic extinction pattern. Positive evidence for reworking is largely limited to the preservational state of Cretaceous foraminifers, whether they are abraded, discolored, or dissolution-resistant. Although some reworked species are present in any stratigraphic sequence, no systematic differences in preservational state have been observed between Maastrichtian and Danian occurrences of Cretaceous planktic foraminifers (Keller, 1988b, 198913;Liu and Olsson, 1992; MacLeod and Keller, 1994; MacLeod, in press). Thus, there is evidence for the presence of some reworked species in Danian sediments. But, there is no evidence to support the conclusion that all but one to at most three of the Cretaceous species consistently present in Danian sediments are reworked. Cretaceous species survivorship can be tested independently based on S13C isotopic values of foraminiferal tests, comparative morphology and comparative biogeography of species present in early Tertiary (Danian) sediments. Carbon-13 isotopic values of low latitude planktic foraminifera that lived in Danian sediments are 2 to 3 permil lighter than those that lived during the late Maastrichtian. Measurements of S13C values of a number of Cretaceous species present in Danian sediments show Danian isotopic signals (e.g., Heterohelix globulosa, H. striata, H. navarroensis, Guembelitria cretacea, G. trifolia, G. danica, Globigemelloides aspera, Chiloguembelina waiparaensis) and are thus
93
K/T boundary survivors (Barrera and Keller, 1990; Keller, 1993; Keller et al., 1993). Additional support of Cretaceous species survivorship is seen in comparative morphology and biogeography studies of K/T boundary sequences worldwide (MacLeod and Keller, 1991, 1994). These studies show consistent trends among groups of species with similar morphologies, and consistent trends in biostratigraphic ranges of Cretaceous species in the early Tertiary. Such global biogeographic trends cannot be explained by random reworking; they strongly support Cretaceous species survivorship. 3. Conclusions (1) The blind test was designed to evaluate the pattern of the K/T boundary mass extinction at the
El Kef stratotype, whether it was sudden or gradual. This test cannot evaluate between competing interpretations of the extinction pattern. (2) The blind test results of the four testers are influenced by sample processing methods, size fraction analyzed, amount of time spent searching for rare species, and differing taxonomic concepts used. All four testers used the same species names for only 14 to 16 species and three testers used the same species names for another 10 to 16 species out of a total of 45 to 59 species identified. This relatively low taxonomic agreement among the testers illustrates the fact that taxic census data cannot be compared on a species by species basis. Even if the same species names are used, there is no guarantee that all four testers applied the same species name to the same morphotype. (3) Despite the differing taxonomic concepts used by the four testers, the extinction patterns are remarkably similar with varying abundances disappearing below the K/T boundary, at or near the K/T boundary and ranging into the earliest Tertiary. Spearman rank order correlation of taxa identified by two or more blind test testers shows a mean value of 0.478. In contrast, the mean correlation value between each tester and Keller’s (1988b) original data is 0.399, and that of Smit’s (1982, 1990) is 0.272. This means that each of the extinction patterns of each of the testers is similar to Keller’s gradual extinction pattern and that Smit’s sudden extinction pattern could not be corroborated.