Commentary: The missing sabertooth baculum—At what point might the absence of evidence reasonably be considered evidence of absence?

Abstract

Most carnivorans and all modern felids have ossified bacula; however, no machairodont baculum has ever been identified. This is true despite the many fairly complete skeletons found around the world of several sabertooth taxa. Although the bacula of modern felids are much smaller than those of canoids (even the least weasel's baculum is longer than the tiger's barely 1 cm baculum!), among the 166,000 bones found at the Rancho La Brea Tar Pits (RLB) of perhaps 3000 Smilodon fatalis individuals there are other small and delicate bones—including clavicles, hyoids, and tiny ossicles—from that taxon. Furthermore, the matrix from that site found around the large fossils is painstakingly sorted under microscopes, resulting in the identification of thousands of microfossils. Despite these concerted efforts, including the posting of images of modern felid bacula near the RLB fossil lab to help form potential search parameters for those sorting the matrix, the search continues for this elusive bone. It is possible that RLB's unique “pit wear”—abrasion related to the notable seismic activity in Southern California—has pulverized this bone that may have been less dense than the other small bones that are found at the site. Parsimoniously, machairodonts should have bacula, but our failure to identify a sabertooth baculum in the richest fossil carnivoran locality in the world naggingly forces us to consider whether, at some point, we have to accept this stubborn absence of evidence as legitimate evidence of absence.