Tendencies in paleontological practice when defining species, and consequences on biodiversity studies

Abstract

The exploration of evolutionary patterns over geological time has recently received new impetus from the development of morphological disparity as a new biodiversity metric alongside taxonomic diversity. Clade dynamics can be analyzed by comparing and contrasting these two metrics. Like any metrics based on sampling, quantification, and naming, taxonomic diversity and morphological disparity are not free from bias. The long use of taxonomic diversity has prompted many studies of bias and its effects, whereas bias affecting morphological disparity has only been summarily explored. This is particularly so for the effect of the growth of knowledge over paleontographical time (i.e., historical period of study) and its repercussions on morphological disparity and on the relative stability of disparity and diversity. Here, we analyze evolutionary patterns at a detailed scale. For a sample of Jurassic ammonites, morphological disparity increases faster than taxonomic diversity during paleontographical time. For disparity, we demonstrate two paleontological practices, based on comparison with a random numerical model: a tendency to identify and name extreme forms before intermediate ones for some morphological aspects (ornamentation), and a correlated tendency to underuse some other morphological aspects (shell geometry). However, the effects of the growth of knowledge over paleontographical time are only slightly marked on biodiversity curves through geological time, mainly because new discoveries and revalidations and/or invalidations of species are randomly distributed within the subzones during paleontographical time.