On calibrating the completometer for the mammalian fossil record

Indrė Žliobaitė,Mikael Fortelius

doi:10.1017/pab.2021.22

Abstract

AbstractWe know that the fossil record is incomplete. But how incomplete? Here we very coarsely estimate the completeness of the mammalian record in the Miocene, assuming that the duration of a mammalian species is about 1 Myr and the species diversity has stayed constant and is structurally comparable to the taxonomic diversity today. The overall completeness under these assumptions appears to be around 4%, but there are large differences across taxonomic groups. We find that the fossil record of proboscideans and perissodactyls as we know it for the Miocene must be close to complete, while we might know less than 15% of the species of artiodactyl or carnivore fossil species and only about 1% of primate species of the Miocene. The record of small mammals appears much less complete than that of large mammals.

Highlights

May (1988) asked how many species there are on Earth today and, not surprisingly, showed that we know more of some groups of organisms than of others
The question of how many species have ever existed on Earth is even more challenging (Simpson 1952)
No matter how we look at this from various angles and with different assumptions, the estimates suggest that the global mammalian fossil record must still be quite incomplete

Summary

Introduction

May (1988) asked how many species there are on Earth today and, not surprisingly, showed that we know more of some groups of organisms than of others. Assuming that the relative structure of ecosystems globally by taxonomic orders stays the same,* the average species duration is 1 Myr, and the standing diversity stays constant over the Miocene, the last column of Table 2 gives estimates of completeness of the fossil record within each taxonomic group. The total progeny can be estimated via assumptions about diversity dynamics (Valentine 1970), it may as well come from phylogenetically explicit speciation models (Benton et al 2000) Apart from those three types of approaches, researchers have compared living species with the fossil record directly (Raup 1979; Valentine 1989), without extrapolating over time. Estimates of mammalian species duration are approximate averages, and changes in diversity over time, as well as relative proportions of different taxonomic groups in the past, are uncertain The degree of this incompleteness is poorly known. Are we sampling mainly from the savanna-like environments of the past? How much is happening out of sight, in the less fossiliferous environments or outside the active sedimentary basins? Will we even ever know?

Literature Cited

Main Variant with Constant Species Duration

An Alternative with Increasing or Decreasing Standing Diversity over Time

Findings

Complementary Sensitivity Analysis