TIME OF ORIGIN OF MESONYCHIA

There are approximately 56 independently sampled sites yielding mesonychian condylarths distributed on the three northern continents (Table 3). These range in age from Rupelian early Oligocene (t₂, ca. 33 Ma) to Torrejonian middle Paleocene (t₃, ca. 63 Ma), for an observed temporal range t₂ - t₃ = 30 m.y. Likelihood calculation of the time of origin of mesonychians is outlined in the work sheet of Table 4 and shown graphically in Figure 6. Critical likelihoods l = 0.5 and l = 0.05 correspond to hypothesized times of origin at 63.83 and 66.68 Ma, respectively, with the latter, an extension of about 3.7 m.y. beyond the earliest stratigraphic record, being the likelihood equivalent of a 95% confidence limit.

The likelihood function calculated for the time of origin of Mesonychia is shown in Figure 7. The 50% confidence limit at 63.83 Ma falls within the early Paleocene, and the 95% confidence limit at 66.68 Ma falls within the very latest Cretaceous close to the Cretaceous-Tertiary boundary at 66.5 on the Haq et al. (1987) time scale used here. Considering what we know of their fossil record, mesonychians evidently originated at or near the Cretaceous-Tertiary boundary. Any discovery older than the known temporal range of mesonychians would of course extend both the range and the ages associated with all critical likelihoods.