THREATENED SPECIES IN THE FOSSIL RECORD: THE INVISIBLE EXTINCTION

The anthropogenic decline in biodiversity over the last several centuries has been dubbed the “Sixth Extinction,” in line with the five major mass extinctions commonly recognized in the fossil record. Major efforts have been made to compare the current estimates of extinction rate, which is heavily based on terrestrial vertebrates, to rates in the fossil record, which are primarily calculated using marine invertebrates (Barnosky et al. 2011, Nature). Our alternative approach is to ask whether species that are either threatened or have recently gone extinct have a fossil record. That is, in the absence of the historic record, would the existence of these species be known in the future? This method of determining the “future fossil record” should allow a more reliable comparison of current and fossil record extinction metrics. We have applied this approach to examine terrestrial mammals. We also ask if particular characteristics (e.g., trophic level, taxonomic affiliation and body size) influence the tendency for animals to appear in the record. The current IUCN Red List of Threatened Species™ lists 714 species of mammals in the categories Extinct (E:75 sp.), Extinct in the Wild (EW:2 sp.), Critically Endangered (CR:194 sp.), and Endangered (EN:442 sp.). This list was compared with the fossil mammal record from three commonly used databases: the Paleobiology Database, Neotoma, and NOW. Body size data were obtained from the MOM v4.0 database. Of the 714 red-listed mammal species, only 89 (12%) appear in at least one of these databases. We find a strong signature of taxonomic affiliation, which may be size related. For example, while only 13 of 250 rodents (5%) have a record, 23 out of 64 cetartiodactyls (36%) do. The record improves somewhat if we use fossil genera; the total representation in the fossil record rises to 62%, with 49% of threatened rodent species having a congener in the fossil record. We also find a substantial overall size signature: the probability of taxa represented in the record being larger than those missing. The log-mass for species in the fossil record is much larger than that of the Recent species, reflecting the underrepresentation of small body size taxa such as chiropterans. The record of ancient extinctions may have been similarly biased.