Transposable elements (TEs) play an essential role in shaping eukaryotic genomes and in organismal diversification. It has been hypothesized that bursts of TEs may correspond to punctuated events of speciation (CArrier SubPopulation, Epi-Transposon, TE-Thrust hypotheses), thus it is expected that highly differentiated taxa bear highly active TEs in their genomes. To test this hypothesis, we designed two new parameters: the Relative Rate of Speciation (RRS) and the Density of Insertions (DI). These parameters measure, respectively, how much the taxa are undergoing an adaptive radiation and the magnitude of TE activity in their genomes. We call "hot" and "cold" those genomes with high and low DI, respectively. In this study, we test the association between RRS and DI ("Cold Genome Hypothesis") in Mammalian families and superorders. Furthermore, since the age of TEs can be inferred by calculating the distance from their respective consensus sequences, we subset TEs in different age classes in order to study the evolution of genomes at different time scales. Here, we consider "recent" TEs (divergence <1%) and "less recent" TEs (divergence <5%). Comparing the TE activity in 19 pairs of species belonging to different mammalian families and the average TE activity of sampled species from the four superorders of Placentalia, we assessed the correlation between taxa with high RRS and "hot genomes" and between taxa with low RRS and "cold genomes". Specifically, the density of recent insertions correlates with recent macroevolutionary events while the density of less recent insertions with old events. The results are strongly validated by statistical tests and they are coherent with the "Cold Genome Hypothesis". Our study supports the Punctuated Equilibria theory in both the phases of radiation and stasis, corroborating the hypothesis that Mammals evolved through punctuated mechanisms rather than through gradualistic ones.