Patterns of diversity, extinction and origination in the Ordovician‐Devonian stropheodontacea

Abstract

Evolutionary patterns exhibited by ninety‐one genera and subgenera of Ordovician‐Devonian Stropheodontacea can be inferred using two measures of standing diversity, along with raw diversity, and three measures each of extinction rates, origination rates and turnover rates. Of the three measures defined for each type of rate, one counts taxa that are confined to the interval and two do not. Of the latter, one is defined on a pertotal taxa and one on a per‐standing‐diversity basis.

If it is assumed that the available data adequately represents the total range of morphological diversity and time distribution of the Stropheodontacea, then these measures can be treated as descriptive statistics and several patterns emerge: Diversity increases monotonically from Caradoc to Emsian and then decreases from Emsian to zero in the Famennian. Extinction rates increase from Wenlock to Frasnian. Origination rates show three to four peaks. Turnover rates show a decrease in the Early Silurian, a local peak in the Wenlock, and an increase from Pridoli to Frasnian.

Alternatively, if the observed data is treated as a sample of a larger population then 80% and 95% confidence limits indicate that several of the patterns (mostly involving Silurian intervals) are not statistically significant. Any assessment of the effect of errors on extinction rates should address false positives (taxa falsely regarded as becoming extinct in a time interval) not just false negatives (taxa falsely regarded as not becoming extinct in the interval). One way to evaluate error: assess likely ranges of reliability (measured as proportions of true positives and true negatives), sample sizes, and the true proportion of taxa that become extinct in an interval. Estimate the probability that 80% (95%) confidence limits on extinction rates contain the true proportion; if the probability is low then view any alleged patterns with skepticism. The stropheodontacean dataset passes this test.

Key words:

• Brachiopods
• Stropheodontacea
• Ordovician‐Devonian
• extinction
• origination
• diversity