In plant communities today, leaf morphology is correlated with mean annual temperature, so paleobotanists use fossil leaf morphology to estimate the mean annual temperature of paleoclimates. However, the angiosperm fossil record contains an overabundance of samples fossilized near lakes or rivers where vines are especially common. Since vine leaves have a somewhat different association with temperature, use of data from vine-rich locations leads to mean average temperature estimates that are lower than actual recorded temperatures in modern plant communities. This potential bias in paleobotanical climate estimates is due to which type of bias in the fossil record?