Comment on ‘‘Abrupt and Gradual Extinction Among Late Permian Land Vertebrates in the Karoo

In a groundbreaking paper, Ward et al. (1) provided both the stratigraphic context and a quantitative description of the terrestrial vertebrate fossil record across the PermianTriassic (P-T) boundary from the Karoo basin in South Africa. They drew three primary conclusions: (i) that there was an accelerated gradual decline in diversity prior to the P-T boundary (in the Dicynodon Zone); (ii) that this was followed by a pulse of extinction at, or near, the boundary itself; and (iii) that the taxa that first appear in the Triassic most likely originated in the Permian—that is, the origin of the Triassic fauna was not a response to vacated niches caused by the P-T extinction pulse. Thus, the authors conclude that there is little evidence for a single catastrophic extinction mechanism such as might be expected from a large bolide impact. These conclusions were based on the application of confidence intervals on the ends of the stratigraphic ranges, an approach designed to compensate for the incompleteness of the fossil record (2). Ward et al. (1) augmented their field data with qualitative data from outside their study area, which they used to extend the stratigraphic ranges of the taxa found within the study area. However, confidence interval lengths are a function both of the observed stratigraphic ranges and of the number of fossiliferous levels from which each taxon is known. Although Ward et al. (1) were able to extend the stratigraphic ranges of the taxa in their study, they were unable to provide a measure of how rich those extraneous fossil records are, and thus their confidence intervals are for the most part too long. Here, I have reanalyzed the data of Ward et al. (1), restricting my analysis to just their own high-precision data. This analysis suggests a rather different set of conclusions from those drawn by Ward et al. (1). First, for each taxon, I cannot reject the hypothesis that it became extinct before the P-T boundary (Table 1). In contrast, Ward et al. (1) argue that Pristerodon, Aelurognathus, and Diictodon became extinct before the deposition of Unit II, which marks the base of the Triassic (Fig. 1). Thus, the field data of Ward et al. do not support the claim that there was an accelerated background rate of extinction before the P-T boundary, although some taxa may have become extinct in the Permian. The apparent drop in diversity as one approaches the P-T boundary appears to be due to the Signor-Lipps effect (3). Second, the data are consistent with the simultaneous extinction of all seven taxa restricted to the Permian that are known from at least two fossiliferous horizons. The 91% confidence band on the position of the massextinction horizon (assuming that there was one) is very narrow, extending only 1 to 2 meters into Unit II (Fig. 1), although the width of this type of confidence band can be sensitive to new data. Thus, the data are consistent with a sudden catastrophic extinction, although there is insufficient data to rule out a wide range of gradual extinction scenarios. Figure S4 of Ward et al. shows a broader scale synoptic data set where many taxa disappear in the Dicynodon Zone well before the P-T boundary, which they use to support the notion of accelerated background rates before the P-T boundary. However, without quantitative data it is hard to test this scenario, so the question of whether there was an accelerated gradual decline (over normal turnover rates) leading up to the P-T boundary must remain open until data of the quality collected by Ward et al. (1) are collected more widely. However, noting the taxa held in common between their figure S4 and their high-precision field data, it appears that the Signor-Lipps effect extends through at least half of the Dicynodon Zone: The high-precision field data suggest that all the taxa that became extinct in this zone may have become extinct at the P-T boundary and not before—an observation that further undermines the argument for an accelerated background extinction rate close to the boundary. Turning to the eight taxa that first appear in the Triassic, the data do not strongly support the claim that these taxa originated in the Permian (that is, before the deposition of Unit II); indeed, the data provide stronger support for a Triassic origin of these taxa (Table 1). For example, if one assumes that these taxa all arose simultaneously, we can be confident at the 78% level that this mass origination occurred in the Triassic, with only 22% confidence that this horizon was in the Permian (Fig. 1). It is important to recognize that the use of confidence intervals on stratigraphic ranges here depends on two aspects of the data that have not been fully established. The first is accuracy of the relative positions of the fossil horizons depicted in the composite section. To make the problem tractable, Ward et al. (1) had to make the simplifying assumption that the relative rates of deposition between their correlation tie points was constant. The second simplifying assumption is that the fossils are distributed randomly within the true stratigraphic ranges of the taxa. Violations of this assumption may have a significant impact on the lengths of the computed confidence intervals. Bearing in mind these caveats, the data in Ward et al. (1) are not TECHNICAL COMMENT