Data from: Problems with using rock outcrop area as a paleontological sampling proxy: rock outcrop and exposure area compared with coastal proximity, topography, land use, and lithology.

Fossil specimens can be recovered easily only from exposed localities, so
rock exposure area should represent a better proxy for rock availability
than the frequently used outcrop (i.e., map) area. Data collected via
remote sensing and GIS show that map area does not consistently correlate
with exposure area in different regions. Proportional rock exposure is not
geographically consistent and is influenced by a number of variables that
are independent of outcrop area, including proximity to the coast,
elevation, bedrock age, land use and lithology. These variables appear to
be non-independent in their influence on rock exposure, and are not
consistent in their effects across continents. The inconsistency in the
correlation between outcrop and exposure area, and the variability in the
influence of different factors on rock exposure, suggests that using
outcrop area as a sampling proxy is poorly supported. The weaknesses in
using outcrop area as a sampling proxy, highlighted by the lack of
correlation with exposure area, suggest that a single accurate global
sampling proxy may never be attained and it is premature to assume that
paleodiversity curves can be corrected using such proxies. It is therefore
preferable to work on a regional scale, comparing regional fossil
collection data with a number of proxies representing all aspects of
sampling. The lack of correlation between outcrop and exposure area
suggests that the covariance detected between outcrop area and
paleodiversity might be better explained by a common-cause model, and that
geological megabiases may not have had as profound an effect on
paleodiversity curves as previously thought.