User:NikelsenH/First and last fossil occurrence

In Paleontology, the first fossil occurrence and the last fossil occurrence is the highest or the lowest fossil of a species, or more general, a taxon in a stratigraphic column or section. First and last fossil occurrences are fundamental to biostratigraphy and paleoecology, where they allow to trace speciation rates and extinction rates, which makes them an important indicator of mass extinctions.

Definition
The first fossil occurrence, short first occurrence, of a taxon is the stratigraphically lowest occurrence of a fossil of said taxon in a section. Similarily, the last fossil occurrence, short last occurrence, of a taxon is the stratigraphically lowest occurrence of a fossil of said taxon in a section.

Note that with this definition, inverted folds can theoretically have higher (in the sense of distance from sea level) first occurrences than last occurrences, since the whole stratigraphy is turned upside down.

Ecological significance of first and last occurrences
First and last occurrences are taken as indicators for the appearance and the disappearance of a taxon. However the last occurrence of a taxon will always underestimate the true disappearance of the taxon. This effect is called the Signor-Lipps effect and if based on the fact that preservation is rare and the last individual of a taxon will in general not be preserved. The same holds for first occurrences, which will always overestimate the appearance of the taxon. This effect is named the Jaanusson effect or the Sppil–Rongis effect (Signor-Lipps spelled backwards).

With the Signor-Lipps effect, a curved pattern of last fossil occurrences at a mass extinction is expected. PIC!! However is rarely observed. Recent studies based on simulations and empirical data have shown that this cannot be expected, since first and last occurrences are strongly influenced by changes in deposition rates (Condensation (geology)) and changes in facies. Low deposition rates generate peaks in first and last fossil occurrences per stratigraphic height, since they contain "more time" and therefore automatically more exticntions. Changes in facies reflect a change in habitat, leading taxa to migrate with their preferred habitat and therefore going extinct locally. Due to both effects, first and last fossil occurrences change predictably within a sequence stratigraphic setting.