Author: Gustavo A. Ballen and Sandra Reinales
Date: 2025-08-19

# Stratigraphic intervals

#### Gustavo A. Ballen and Sandra Reinales

#### 2025-08-19

Some methods are available for estimating the confidence intervals on the extreme points of a stratigraphic interval, i.e., an estimate for the point of appearance and disappearance of a given taxon in geologic time. These methods are relevant here since we can think of a biogeographic events in the same way as taxa appear and become extinct. This approach is lacking in historical biogeography as far as we know, and seems to fit the main goal of determining the final point of connection between biogeographic areas. Marshall (2010)1 summarised different approaches for estimating confidence intervals under certain sampling regimes, assumptions and confidence levels, ranging from classical to Bayesian estimators. The package `tbea` implements two of these methods, the classical (Strauss and Sadler, 1989)2 and the distribution-free estimator of Marshall (1994)3.

The first method provides estimators where one- (\(\theta_{1}\)) or two-parameter (\(\theta_{1}, \theta_{2}\)) cases are constructed adding and subtracting a portion \(\alpha\) of the magnitude of the observed stratigraphic interval \([y, z]\). Such constant depends on the confidence level wanted (e.g., \(95%\)).

The second method of distribution-free confidence intervals was developed without assuming any particular distribution of underlying gap sizes, instead, it calculates quantiles of gap size between occurrence points for an ordered vector of gap sizes. The major cost is that such intervals are larger that classical CIs and on the other hand, require much more data for constructing intervals of high confidence (e.g., \(95%\)).

The method makes use of the binomial distribution in order to find the quantiles that satisfy the condition of being smaller that \(\gamma\) and from these picking the largest (in the case of the lower bound) and the smallest (for the upper bound). Marshall (1994) mentioned that inference using his method is only valid as long as there is no trend in gap size (i.e., a trend of increase or decrease in gap size as we go along the time data). It is possible to use the Mann-Kendall test for monotonic trend to test such assumption. Because the method works with quantiles of the sorted gap sizes, these are first calculated from the sorted vector of times, but then internally sorted when using the method “Marshall94”; however, whether there is any monotonic trend must be assessed on the unsorted version. We use the Mann-Kendall test on the times vector prior to applying the function:

    # load the packages
    library(Kendall)
    library(tbea)

    # load the data
    data(andes)
    andes <- andes$ages

    # remove missing data
    andes <- andes[complete.cases(andes)]

    # remove outliers
    andes <- sort(andes[which(andes < 10)])
    gapsizes <- sapply(2:length(andes), FUN=function(x) andes[x] - andes[x-1])

    # run the Mann-Kendall test
    MannKendall(gapsizes)

    ## tau = 0.119, 2-sided pvalue =0.27337

We fail to reject the null hypothesis of no monotonic trend in the gap sizes. Therefore we may apply the “Marshall94” method for bracketing the confidence interval.

These two methods are available in the function `stratCI` which allows to pick one of two methods: “StraussSadler89” or “Marshall94”. We can use the same data from the previous block in order to illustrate the usage of the function `stratCI`:

    # calculate the older theta that is an extension beyond
    # the oldest (first from past to present) occurrence
    # using the method Strauss-Sadler89
    stratCI(times=andes, method="Strauss-Sadler89",
            nparams="one.par", C=0.95, endpoint="first")

    ##     minObs     maxObs     maxEst          H     alphaR      alpha
    ##  0.2749000  6.9500000  7.4435055 43.0000000  0.4935055  0.0739323

    # calculate the lower and upper bounds on the confidence interval with a quantile 0.8
    stratCI(times=andes, method="Marshall94",
            confidence=0.95, quantile=0.8)

    ## [1] 7.0701 7.2147

    stratCI(times=andes, method="Marshall94",
            confidence=0.95, quantile=0.95)

    ## Warning in .upperBound(confidence, length(gapsizes), quantile): Impossible to
    ## calculate upper bound

    ## [1] 7.14   NA

As Marshall (1994) already noted, the method requires a large number of gaps to calculate both bounds for the \(95%\) confidence interval; for our dataset with just \(42\) gaps (i.e., \(43\) times), we can find both bounds at the \(80%\) confidence level (or lower), and thus claim that the lower and upper bounds for the \(95%\) confidence interval are \(7.07-7.2\) Ma. When we raise the confidence level to \(95%\), we can only infer the lower bound, thus being able just to claim that the \(95%\) confidence interval is at least as old as \(7.14\) Ma. The higher the confidence level for finding the bounds on the \(95%\) confidence interval, the better, in a similar way as calculating a \(95%\) confidence interval is better than calculating a \(70%\) confidence interval.

We can plot these extensions after the oldest occurrence and compare the results of both methods:

    # calculate the confidence interval on the oldest time using the
    # Strauss-Sadler89 method
    straussSadlerAndes <- stratCI(times=andes, method="Strauss-Sadler89",
                                  nparams="one.par", C=0.95, endpoint="first")

    # calculate the confidence interval under the distribution-free approach
    marshallAndes <- stratCI(times=andes, method="Marshall94",
                             confidence=0.95, quantile=0.8)

    # plot the estimations
    hist(andes, probability=TRUE,
         col=rgb(red=0, green=0, blue=1, alpha=0.3),
         xlim=c(0, 10), main="CIs based on stratigraphic intervals",
         xlab="Age (Ma)")

    # plot the lines  for the classical CI estimator of Strauss and Sadler
    arrows(x0=straussSadlerAndes["maxObs"], y0=0.025,
           x1=straussSadlerAndes["maxEst"], y1=0.025,
           code=3, angle=90, length=0.1, lwd=3, col="darkblue")

    # plot the lines for the CI estimator of Marshall 94
    arrows(x0=marshallAndes[1], y0=0.05,
           x1=marshallAndes[2], y1=0.05,
           code=3, angle=90, length=0.1, lwd=3, col="darkgreen")

    # plot a legend
    legend(x="topright", legend=c("Strauss and Sadler \'89", "Marshall \'94"),
           col=c("darkblue", "darkgreen"), lty=1, lwd=3)

1. Marshall, C.R. (2010). Using confidence intervals to quantify the uncertainty in the end-points of stratigraphic ranges. Quantitative Methods in Paleobiology, 16:291-316.↩︎

2. Strauss, D. and Sadler, P.M. (1989). Classical confidence intervals and Bayesian probability estimates for ends of local taxon ranges. Mathematical Geology, 21(4):411-427.↩︎

3. Marshall, C.R. (1994). Confidence intervals on stratigraphic ranges: Partial relaxation of the assumption of randomly distributed fossil horizons. Paleobiology, 20(4):459-469.↩︎