Fossil evidence suggests a rapid biological radiation of placentals following the K-Pg event and peak diversification between 65-60 Ma. Early molecular studies however, found ancient estimates for the origin of placental mammals which pre-dated the K-Pg extinction significantly (35 million years before the event). Due to the nature of fossilisation, we should expect fossils to post-date the event itself, but this is not enough to account for the large discrepancy between the molecular and palaeontological time estimates.
Three different models were postulated in order to explain placental mammalian diversification with respect to the K-Pg mass extinction, but none have yet provided a coherent unambiguous theory. These are summarised in table 1.
The results are summarised in figure 1. Placental origins were estimated to be in the range of 88-99 Ma. This is a substantially later estimation compared to those stated by other molecular studies, and suggests that the diversification of placental orders occurred in the 20 Mya following the K-Pg event. For example Chiroptera (the order which includes bats) were estimated to have originated 59.1 Ma, Primates at 68.2 Ma, Lagomorpha (the order which include rabbits) at 66.8 Ma and Rodenta (includes mice, rats) was estimated to 64.4 Ma. These data are coherent with current paleontological knowledge and throw doubt on previous models that have been suggested. For example, Bininda-Emonds et al. published a paper in 2007, which was also based on molecular phylogenetic evidence; they estimated the origin of Chiroptera to be 74.9 Ma, Primates to be 87.7 Ma, Lagomorpha to be 66.8 Ma and Rodenta to be at 83.3 Ma. All of these estimations are before the K-Pg event and do not account for the chasm between molecular and paleontological time estimations. The authors suggested a ‘delayed rise’ model to account for the clear discrepancies. These current findings refute the delayed rise model and the explosive model but accord with the long fuse model. The debate goes on!