Returns first and last appearance datums
Returns the first and last appearance datums (FADs and LADs) for taxa in a stratigraphic column.
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an object of class strat.column.
depths from which to calculate the datums
a number of depths (levels) to skip, returning NA instead of values for FAD or LAD; defaults to 0.
FALSE (default) returns FADs less than or equal to LADs, as is the case when depths are measured up from the bottom of, e.g. a stratigraphic section; TRUE reverses FADs and LADs, as is the case when depths are measured from the surface in wells or cores.
not currently used.
depths is provided, it will clobber
a.datums returns a data frame with columns
lads, each of which is a numeric vector giving the FAD or LAD of each taxon in the stratigraphic column.
lads return respectively just the fads and just the lads in a single named numeric vector.
Currently no other notes.
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